Eico 753 User manual

753/Tri-Band SSB/AM/CW Transceiver
753 - Rev.I

GENERAL DESCRIPTION
The EICO Model 753 SSB/AM/CW Transceiver incorporates a filter type single sideband trans-
mitter and a single conversion superheterodyne receiver for operation on the 80, 40 and 20 meter
amateur bands. It is equally suited for mobile and fixed station use.
In fixed station use, the Model 751 AC Supply/Speaker Console (117 VAC, 60 cps) is required
for its operation. In mobile use, the Model 752 DC Supply (12 VDC , positive or negative ground) is
required. Both supplies have been designed integrally with the 753 and provide the requisite filtering
and regulation. They can not be supplanted by supplies of other manufacture if the design capabilities
of the 753 are to be obtained.
FEATURES
SPECIFICATIONS
o Full band coverage on 80, 40 and 20 meters. o Receiver off set tuning (up to ±lOkc) without
altering transmitter frequency. o Built-in VOX. o Panel selected VOX, PTT & STANDBY. o High
level dynamic ALC to prevent flat-topping or splatter and permit the use of a linear amplifier. o Auto-
matic carrier level adjustment on CW and AM. o Dual ratio ball drive per mits single knob 6:1 rapid
tuning and 30:1fine tuning (over 10 degrees of scale). o Position of hairline adjustable on panel.
o Illuminated S-meter/PA Cathode Current Meter and tuning dial. o Fast attack, slow decay AGC.
o Grid-block break-in CW keying. o Product detector for SSB and CW, triode detector for AM. o TR
relay with auxiliary contacts for use with high power linear amplifier. o Includes mobile mounting
bracket.
ADDITIONAL SPECIFICATIONS
FREQUENCY COVERAGE: 3490-4010kc, 6990-7310kc, 13890-14410kc. SSB EMISSIONS: LSB 80
and 40 meters, USB 20 meters. RF POWE R INPUT: 200 watts SSB PE P and CW, lOO watts AM. RF
POWER OUTPUT: 120 watts SSB PEP and CW, 30 watts AM. OUTPUT PI NETWORK MATCHING
RANGE: 40-80 ohms. SSB GENE RATION: 5. 2 Mc crystal lattice filter; bandwidth 2. 7kc at 6db.
STABILITY: 400 cps af ter warm-up. SUPPRESSION: Carrier-50db; unwanted sideband-40 db.
RECEIVER: Sensitivity luv for lOdb S/N ratio: selectivity 2. 7kc at 6db; audio output over 2 watts
(3. 2 ohms). PANEL CONTROLS & CONNECTORS: Tuning, Band Selector , AF Gain, RF Gain, MIC
Gain with calibrator switch at extreme CCW rotation, Hairline Set (capped), Mode (SSB, AM, CW,
Tune), Function (Off , Standby, PTT, VOX), Carrier Balance, Exciter Tune, PA Tune, PA Load,
Receiver Offset Tune, MIC input, phone jack. REAR CONTROLS & CONNECTORS: VOX Threshold,
VOX delay, VOX sensitivity, Anti-VOX sensitivity, PA Bias adjust, S-Meter zero adjust, power
socket, external relay, antenna connector , key jack, accessory calibrator socket. ME TERING: PA
cathode on transmit, S-Meter on receive. SIZE (HWD): 5-13/16" x 14-1/4" x 11-1/4". POWE R
REQUIREMENTS: 750 VDC at 300ma, 250 VDC at 170ma, -100 VDC at 5ma, 12. 6 VAC at 3. 8 amps.
INSTALLATION PROCEDURES
COPYRIGHT © 1965 EICO ElECTRONIC INSTRUMENT COMPANY, In<.
1
WARNING
LETHAL HIGH VOLTAGES ARE PRESENT WITHIN
THIS EQUIPMENT. BE VERY CARE FUL TO
AVOID CONTACT WITH THESE HIGH VOLTAGES
IF FOR ANY REASON THE EQUIPMENT IS OPERATED
OUTSIDE OF ITS CABINET.

UNPACKING
This equipment has been thoroughly tested and inspected before packing. If you find visible
damage upon unpacking, notify the dealer at once. If the unit was shipped, you must file a claim with
the carrier, since only you can recover for shipping damages. Your dealer and EICO will cooperate.
VENTILATION
In all installations, consideration should be given to adequate ventilation in placing the unit.
Free air circulation through the perforated cabinet should not be obstructed at any time. If heat
removal by convective air movement is prevented or curtailed by cabinet obstruction, an excessive
temperature risewill occur which can damage components. Inany installation, avoid placing the unit
in a warm location (near a radiator or heating unit) or in a hot air stream.
ANTE NNAS
A suitable antenna, pr esenting 40 to 80 ohms resistive load should be connected to theANTE NNA
connector at the rear. Install the antenna as reco mmended by the antenna manufactur er . A type
PL259 connector is r equired for coupling the antenna cable to the ANTE NNA connector . In mobile
installations, the antenna may be mounted on the trunk lid, or on a rear fender . Note that a mobile
antenna tuned to resonance may have a radiation resistance below the 40-80 ohm matching range
provided by the 753. In this case, it will be requ ired to introduce additional capacitance across the
antenna cable, pref erably at a point in the antenna cable close t0 wher e it connects to the transceiver .
EXTE RNAL RELAY CONTROL TERMINALS
The EXTERNAL RELAY control terminals 1 and 2 at the rear comprise a SPST switch (rated at
1 ampere) to per mit control of accessory equipment, in conjunction with selection of transmit or
receive operation. Terminal 1 is chassis ground; ter minal 2 is floating during receive, and connected
to terminal 1 during transmit.
For example, a DPDT coaxial relay (or a pair of SPST coaxial relays} connected to EXTE RNAL
RE LAY ter minals 1 and 2 may be used to enable operation of a high power linear amplifier with the
Model 753. The required connections are shown in Fig. 1, including an additional SPST switch to
per mit either low power "straight-through" operation with the Model 753 or high power operation with
the linear.
MOBILE MOUNTING
The mobile mounting bracket supplied per mits the unit to be installed underneath the automobile
dashboard, and locked in place in the most suitably angled position for convenient operation. Further-
more, the unit may easily be released and removed in a few moments without tools. This facilitates
servicing, or , if desired, double use of the Model 753 as both a mobile and f ixed station. Fig. 2
provides all the requir ed infor mation for installing the mounting bracket and for attaching the trans-
ceiver to the bracket.
MOBILE SPEAKER INSTALLA TION
In mobile installations, it is usually most practical to use the automobile radio speaker. To do
this, mount a DPDT switch underneath the dash, and use it to switch the speaker terminals to either
the automobile radio or the speaker output of the Model 753. DO NOT use the automobile chassis as
a ground return for the speaker, as an undesirable ground loop may be formed and result in an abnormal
degree of "whine".
2

I
I
L _
ANTENNA
"STRAIGHT-THROUGH"
o ORELAY
HIGH POWER LINEAR COIL
----------- VOLTAGE
2
EXTERNAL
JACK
{MODEL 753) RELAY
{MODEL 753)
FIGURE 1
WARNING
AVOID PLACING THE
TRANSCEIVER IN
OR NEAR HOT AIR
STREAM.
FIGURE2
..,.
- --
--
TO REAR
BUMPER
MOUNTED
ANTENNA
-
.....
-
1--- -- -7ANOL£ --ioa1NG1
THUMB MOUNTING IPLATE
I
SC EW BRACKET
LOCK
WASHER
I MODEL 753
I l.2RN.::: E - --------,
'l/ DETAIL A ,.
DETAIL OF MOUNTING BRACKET ATTACHMENT
TO EACH SIDE OF TRANSCEIVER COVER.
INDEXING ARRANGEMENT PERMITS ADJUST-
MENT OF UNIT TO ANY CONVENIENT ANGLE
FOR OPERATION.
I
L__ _ _ _ WEUoED TO
cA__•1r T OF UNIT
I
--- - _ _ _J
3
IN
HIGH POWER
LINEAR
AMPLIFIER
OUT

MICROPHONE CONNECTIONS
An Amphenol Type MC2M plug, or the equivalent, must be used for connecting the microphone
to the panel MIC receptacle. The microphone may be either a high impedance dynamic (preferred) or
a ceramic type. Since all necessary audio bandwidth restriction is incorporated in the transceiver
circuitry, a wide-range, high quality dynamic mike may be used and is most desirable. The required
wiring to the MC2M plug is shown in Fig. 3.
FIGURE 3
CAUTION
r ---- -,
R I
i / \] !
L- -- -
Interchanging of the microphone connections by
incorrect wiring to the microphone plug will
cause a negative bias voltage to be applied to
the microphone cartridge, resulting in possibly
serious damage.
OPERATION
IMPORTANT: Before operation of the transceiver is attempted, a proper installation must be com-
pleted. This includes connection of a suitable antenna (40-80 ohms) to the ANTENNA jack, a correctly
wired microphone to the MIC connector (or a key to the KEY jack at the rear), and a proper power
cable from the power supply to the POWER plug. The FUNCTION switch must be set at OFF during
the installation.
SSB OPERATION
Preset panel controls as follows: (abbreviations: cw - clockwise, ccw - counterclockwise)
MODE switch. . . . • . .. . .... . ..... . .at SSB
MIC GAIN. .. . .. . . .... . . • . .. . fully ccw, but not past detent
CARRIER BALANCE ... • ... .. at12o'clock
RF GAIN.... .. .... ....... .• .at 9 o'clock
AF GAIN ......... . ...... . .. .fully cw
BAND selector.. • . ........ ..at proper band
MAINTUNING ..............at desiredfrequency
RCVR OFFSET. . ..... . .. . ..atcenterline
PA TUNE . ....... .. .. .. ...•at 12 o'clock
PA LOAD. . . .. ....... . • . . . . fully ccw
EXCITER TUNE . ... . . ..... .at 12 o'clock
FUNCTION switch..... . . ... atSTBY for 5 minute warmup, and then to P-T-T
RECEIVER TUNE-UP FOR SSB OPERATION
1. Adjust PA TUNE and EXCITER TUNE for maximum audible signal strength or background
noise. These adjustments result in approximate transmit tuning since they are common to receiver
and transmitter.
2. Adjust AF GAIN for a normal listening level.
TRANSMITTER TUNE-UP FOR SSB OPERATION
1. Set MODE switch at TUNE and adjust EXCITER TUNE for a maximum meter reading.
2. Adjust PA TUNE for a minimum meter reading (dip) to obtain correct tuning of the output PI
network.
4

3a. If microphone is equipped with PTT switch, return MODE switch to SSB. Depress PTT
button on mike and adjust the CARRIER BALANCE control for a minimum meter reading.
NOTE
If this is the first time the 753 is placed in
operation, set the BIAS adjust on the rear
apron so that the minimum meter reading
obtained with the CARRIER BALANCE
control in 3a above is 50mA.
3b. If the microphone is not equipped with a PTT switch, return the MODE switch to SSB. Set
the FUNCTION switch at VOX. Leave the MIC GAIN fully ccw (but not past the detent).
NOTE
If this is the first time the 753 is placed in
operation, now adjust the rear panel VOX
controls as described in the NOTE of Step 2
in the subsequent MICROPHONE GAIN
adjustment.
While holding the relay in the transmit condition with a steady tone or sound into the microphone,
set the panel CARRIER BALANCE control as described in step 3a (and the rear apron BIAS adjust,
as given in the NOTE following 3a, if this is the first time the 753 is placed in operation).
4. Reset the MODE switch at TUNE and adjust EXCITE R tune and PA TUNE as described in
steps 1 and 2. Slowly turn the PA LOAD control cw and "dip" meter reading with PA TUNE control
until the meter reads between 250 and 300ma. Reset MODE switch to SSB. This completes proper
tuning and loading of the transmitter.
NOT
s
E
(>('Oh j - I '4- f D v
f
;l1t.l
Tune-up procedures should not be prolonged
more than Wseconds in any one adjustment
period to avoid excessive heating of compo-
nents in out-of-resonance conditions.
ADJUSTMENT OF MICROPHONE GAIN
1. For push-to-talk operation, set FUNCTION switch at PTT. Depress mike PTT· button and
turn MIC GAIN control cw while speaking until the average meter reading is between 100 and 150ma.
As the MIC GAIN is advanced further cw, a point will be reached where the average meter reading
with uninterrupted speech will hover around 150ma. When this occurs, peaks .will actually be in the
300ma region. If the MIC GAIN is advanced past this point, the average meter reading will not in-
crease because of the ALC action. Do not turn the MIC GAIN clockwise past the point where there
is no increase of the average meter reading on normal uninterrupted speech, as doing so will not
increase output power but simply result in excessive gain and unnecessarily increased noise.
2. For VOX operation, turn the FUNC TION switch to VOX. Set the MIC GAIN control fully ccw
(but not past the detent).
NOTE
If this is the first time the 753 is placed in
operation, now adjust the rear apron VOX
controls as follows (unless they have already
been adjusted as required in the NOTE of
step 3b, in the previous TRANSMITTER
TUNE-UP FOR SSB OPERATION procedure).
5

Set the rear apron VOX controls as follows:
VOX-SENSITIVITY ........ .. . .. . . . ..fully cw (max. sensitivity)
VOX- THRESHOLD. . . ... • ..... .fully cw (guaranteed to return to receive)
VOX-DE LAY. ...... • .... . .... .fully ccw (minimum delay)
ANTI-VOX. . ... . ... . ... ... ....fully cw (maximum anti-vox)
Repetitively speak a few syllables into the microphone to trigger the VOX to "on" as often as
necessary, while adjusting the VOX-DELA Y control until the desired release time af ter speaking is
obtained.
Now, turn the VOX-SENSITIVITY control ccw to a point where the relaytrips easily while speak-
ing into the microphone at normal level and distance, yet does not trip on normal background noise.
In a mobile installation, this adjustment should be made for "close-talking", in order that the nor mal
automobile and wind noises not cause the VOX to trigger.
Next, place the microphone in its normal position of use, and set the A F GAIN control for a
normal listening level from the speaker as normally located. Then turn the ANTI- VOX control ccw
(reducing ANTI-VOX gain) as far as it may go without permitting the VOX to be triggered to "on" by
the sound from the speaker reaching the microphone. From this point, back off cw about 30 degrees
of rotation to insure adequate anti-vox gain to handle voice peaks.
Now reset VOX-THRESHOLD to fully ccw position. Trigger the VOX to "on" by speaking one or
two syllables into the microphone, and then turn the VOX-THRESHOLD control cw until the r elay re-
leases. As there may be some "chatter" at this critical point of the VOX- THRESHOLD control, turn
it just slightly further cw than the critical point.
While holding the relay in the transmit condition by speaking steadily into the microphone, adjust
the MIC GAIN control exactly as described in step 1above. In a mobile installation, make this adjust-
ment for "close-talking", in order that the normal automobile noises not cause the VOX to trigger.
3. The transceiver is now tuned and ready to operate. Re-adjust EXCITER TUNE and PA TUNE
if the operating frequency is shifted substantially.
MAIN TUNING CONTROL
The MAIN TUNING control has a unique dual ratio ball drive, permitting both 6:1rapid tuningand
30: 1 fine tuning. Fine tuning over 10 degrees of scale is obtained in the opposite direction of knob
rotation used in rapid tuning. For this reason, turn slightly past the desired frequency in rapid tuning
to assure that the exact desired tuning f requency will be in the range of the fine tuning.
RECEIVE R OFFSET TUNING
Permits fine tuning of the receiver ± lOkc without disturbing the transmitting frequency or the
initial calibration of the frequency dial. No offset from the transmitting frequency (and the frequency
dial setting) is obtained with the RECEIVER OFFSET control setat the center line. Rotation cw from
the center line lowers the receiving frequency, while ccw rotation from the center line raises the
receiving frequency.
CW OPERATION
1. It is assumed that the r ear apron BIAS adjust has been set as descr ibed in TRANSMITTE R
TUNE -UP FOR SSB OPE RATION. For break- in operation, it is also assu med that all VOX adjust-
ments at the rear have been set as described in the note of step 2 of MIC ROPHONE GAIN ADJUST-
MENT.
2. Insert key plug in KEY jack on rear apron.
6

3. Set BAND SELECTOR & MAIN TUNING to desired band and frequency.
4. Set MODE switch at TUNE and adjust transmitter as described in steps 1 and 2 of TRANS-
MITTER TUNE-UP FOR SSB OPERA TION. The CARRIE R BALANCE setting has no eff ect in CW
operation.
5. For manual operation, set the MODE switch at CW and the FUNCTION switch at PTT. For
the duration of a transmission with manual operation, either the microphone PTT button must be held
depressed, or an external PTT switch wired to the microphone must be set at the transmit position.
When the microphone PTT button is released, the receive condition is obtained.
6. For break-in operation, set the MODE switch at CW and the FUNCTION switch to VOX.
Depressing the key automatically switches the VOX relay from receive to transmit, and the relay will
hold at transmit until sending is stopped. The rear apron VOX-DELA Y control may be readjusted to
set the relay "hold-in" time as the operator desires.
NOTE
In CW operation, the usual practice is to transmit "zero beat" with
the station being worked, which would seem to require the RCVR
OFFSET tuning control be set at the center line (no offset). How-
ever, a small fixed offset, determined as follows, can be used to
obtain a comfortable, consistent listening note in receive, while
retaining exactly "zero beat" transmission. First, set the RCVR
OFFSET tuning control at the center line (zero offset) and tune in a
CW station to "zero beat" with the MAIN TUNING control. Since
there is zero receiver offset, you will transmit "zero beat" with
the station. Now tur n the RCVR OFFSET tuning control in the
direction of the strong side of "zero beat" to a point where a
comfortable listening note is heard. This is the small, f ixed off-
set referred to above, which should be retained thoughout CW
operation. Now, if you tune in any CW station subsequently with
the MAIN TUNING control only (RC VR OFFSET remains as set
previously), so as to produce the same comfortable listening note,
your transmitter will then be tuned exactly "zero beat" with the
received CW station.
AM OPERATION
1. The following procedure is based on the assumption that the 753 has been setupfor SSBopera-
tion, and that it is now intended to operate AM.
2. Set the FUNCTION switch at either VOX or PTT, depending on the desired type of operation.
3. Set BAND selector and MAIN TUNING at desired band and frequency.
4. Set MODE switch at TUNE and adjust transmitter for a meter reading of 250-300ma under ''tune''
conditions.
5. Set MODE switch at AM, and turn MIC GAIN fully ccw (but not past detent). Now trip the
relay to the transmit condition, either as described in step 1 (PTT operation) or step 2 (VOX operation)
of MICROPHONE GAIN ADJUSTMENT.
6. If the CARRIE R BALANCE has been adjusted previously as described for SSB operation, it
is not necessary to readjust this control for AM operation. If the meter reading is below 150ma in
transmit with the MIC GAIN fully ccw, a more critical adjustment of the EXCITE R TUNE may be
necessary. This adjustment may be made with the MODE switch at AM, and the EXCITER TUNE
control adjusted for a peak meter reading (around 150ma).
7

7. Turn the MIC GAIN control cw until the meter is just on the verge of flickering on voice peaks,
but no further. This will require a lower setting of the MIC GAIN control than is used in SSB opera-
tion.
8. The 753 is now properly set up for AM operation. Least distortion on AM reception will
usually be obtained when tuned slightly to one side of "peak".
ADDITIONAL NOTES ON CONTROLS
MODE SWITCH
The rear apron KEY jack is internally disconnected at the SSB and AM positions of the MODE
switch, thus allowing a key to be plugged into the KEY jack in all modes of operation. Note that in
manual CW operation (FUNCTION switch at PTT), closing the key without depressing the PTT button
(thus remaining at receive) may possibly result in an annoying audible oscillation. Of course, no
transmission occurs. In break-in CW operation ( FUNCTION switch at VOX), closing the key auto-
matically causes the VOX relay to trigger to "on" and provides the desired break-in. In the TUNE
position of the MODE switch, the keying circuit is internally grounded to place the transceiver in the
transmit condition. Simultaneously, one of the 7360 balanced modulator deflection plates is grounded
to provide greater than normal drive of the final amplifier for tune up. It must be noted that, with an
antenna connected, full power CW transmission will occur at the TUNE position and cause undesired
interf erence on the frequency in use. Therefore, perfor m the tune-up procedure as quickly as possible.
PA TUNE & PA LOAD CONTROLS
Note that the PA TUNE and PA LOAD controls are inter-dependent. As the PA LOAD is turned
cw (increasing the meter reading), repetitive readjustment of the PA TUNE control for a "dip" in the
meter reading is required. At a meter reading of 250-300ma, proper loading to 200 watts peak input
is obtained. Do not exceed a loading current of 300ma, as additional input power from a 750V supply
will not provide increased output power because of decreasing efficiency of the 753 final amplifier.
In making these adjustments, it will be possible to observe whether the conditions for proper
operation of the linear final amplifier are obtained; namely, proper idling current adjustment, and
enough drive for adequate peak plate current in the off -resonance condition. Presu ming proper set-
ting of the rear apron BIAS control as given in OPERATION, and proper adjustment of the EXCITE R
tune control, a maximum off -resonance plate cur rent greater than 350ma should be read on either
side of the "dip" point obtained with the PA TUNE control. This value of off -resonance plate current
may not be achieved if AC line voltage is below the rated input of 115 VAC with the Model 751 AC
supply or below 12. 0 VDC input with the Model 752 Mobile supply; also if, for any reason, there is
low plate supply voltage to the final amplifier at full load, low driver B+ voltage, incorrectly opera-
ting exciter stages providing low drive, or def ective final amplifier tubes. If low off -resonance plate
current is obtained because of low AC line supply or battery voltage, operation will not be impaired
as long as the PA LOAD control is not adjusted to produce excessive loading conditions. Do not, in
any circumstance, advance the PA LOAD control so that the plate current at the resonant dip exceeds
85% of the off-resonance plate current observed by detuning the PA TUNE control.
If the "dip" point reading exceeds 300ma with the PA LOAD control turned fully ccw, then antenna
load impedance is less than 40 ohms. If the "dip" point reading remains less than 300ma with the PA
LOAD control turned fully cw, then antenna load impedance is over 80 ohms. If either of these condi-
tions obtain, it will be necessary to adjust the antenna impedance to present a proper load at the
transceiver antenna ter minals. This may be done by means of impedance matching networks, or, in
some cases, by adjusting the length of the transmission line to obtain a terminal impedance within
the matching range. The remedy for a particular case (impedance below the matching range with a
mobile antenna) is described under Antennas in the Installation section. Unless proper matching an:d
loading is obtained, excessive distortion will occur in SSB operation.
8

CARRIE R BALANCE
Once the CARRIE R BALANCE control is properly adjusted as described for SSB operation, it is
to be lef t in this position for all subsequent SSB, AM, and CW work. Partial carr ier unbalance is
obtained automatically at the AM position of the MODE switch by connection of resistor R42 between
one of the 7360 balanced modulator tube deflection plates and ground. This partial unbalance should
be exactly that required to set up the transceiver for AM transmission, provided that the CARRIE R
BALANCE control is properly adjusted as described for SSB operation. In the CW position of the
of the MODE switch, one deflection plate of the 7360 balanced modulator tube is automatically grounded
to provide complete carrier unbalance. Thus, in CW operation, the CARRIE R BALANCE control
setting has no effect.
CALIBRATION AC TIVATION
The 753 provides power and input connections for a lOOkc crystal oscillator employing a tube with
a 12. 6 volt heater. A 4-pin plug (EBY 11'9721-71) must be wired to the calibrator connection cable as
follows:
pin 1: 12. 6 volt heater
pin 2: Ground
pin 3: Calibrator signal output
pin 4: B+
The 4-pin plug may then be inserted in the CALIBRATOR socket on the rear apron. To activate
the calibrator , the MIC GAIN control is turned fully ccw past the detent.
AF GAIN CONTROL
May be used as a volume control in listening with either a loudspeaker or headphones. However ,
most operators pref er to run the A F GAIN wide open (fully cw) for SSB and CW reception, and use the
RF GAIN control as a volume control.
RF GAIN CONTROL
May be set in accordance with the operator 's preferences, with, however , the understanding that
the S-Meter reading is affected by the setting. Nor mal S-Meter operation is obtained with this control
turned fully cw.
S-METER ZERO ADJ USTMENT
Set the MODE switch at SSB and the RF GAIN fully ccw. Adjust the S-METER ZERO control on
the rear apron for a zero S-Meter reading. The S-Meter will now read received signal strength or
background noise.
All other control adjustments and uses are described fully in the Operation section.
CIRCUIT DESCRIPTION
TRANSMITTING
Two stages of microphone amplification are provided by V lA and VlB. A sample of the speech
wavefor m is taken from the plate of V lA, before the MIC GAIN control, and applied to V3A, the VOX
amplifier . A description of the VOX circuit is given in the Receiving section.
Balanced modulation is eff ected with the 7 360 beam deflection tube, V2, which has as its grid in-
put signal the output of the 5. 2 mc crystal-controlled carrier oscillator , V4A. Audio from V lB is
applied between the deflection plates, and is R-C coupled through C7 and R9. The DC bias on the
9

dynamically grounded plate is adjusted with R20 to control the balance of the modulator. This balance
is automatically upset in the AM mode by connection of R42 from one deflection plate to ground. Out-
put is taken across the two anodes of the 7360, and coupled to the four pole crystal filter by means of
T l.
Amplif ication at 5. 2mc is provided by the common IF amplifier, V5, which follows the crystal
filter output. The bias on the grid of this stage is modified by the ALC control voltage, which acts to
decrease the stage gain when the peak input power to the final approaches the 200 watt level. Both the
output of the common IF amplifier and the output of the VFO, Ql and Q2, are applied to the transmitter
mixer tube, V12, whose output circuit is tuned to the frequency in use. A dual section tuning capacitor ,
C93, tunes this stage and the following driver plate circuit.
Two VFO ranges are employed. The 8. 7 to 9. 2Mc range provides for both 80 and 20 meter opera-
tion, and the 12. 2 to 12. 5Mc range yields 40 meter operation. On 80 meters, the transmitter mixer
selects the diff erence frequency between the 9Mc VFO and the 5. 2Mc modulated signal from the com-
mon IF amplifier. These same two signals are added for 20 meter transmission. The mixer subtracts
the 5. 2Mc IF output from the 12Mc VFO signal on 40 meters.
CW grid-block keying is accomplished in the transmitter mixer stage. In the CW mode this stage
is biased to cutoff with approximately 50 volts under key-up conditions. A relatively low plate voltage
is employed on the mixer stage, the diff erence between this voltage and the +250 volt supply being taken
up across R89. When the key is up and the stage is cut off , there is no current through R89, so that
at these times the full 250 volts appears at the plates of Vl2. R90 and R95 form a voltage divider
between the plate voltage of V12 and the -100 volt bias supply voltage. When the key is up and there
is 250 volts at the plates of V12, the voltage at the junction between R90 and R95 is some small posi-
tive value. As soon as the key is depressed and V l2 draws cur rent, the voltage at this junction becomes
negative. This change from positive to negative voltage when the key is depressed forms the trigger
signal which provides CW break-in operation through the means of the VOX circuit.
Af ter additional amplification by the driver stage V l3, the signal is applied to the power amplifier ,
V l4 and V15. The screen current in these tubes rises sharply as the control-grid voltage nears zero.
Thus, since the screen is supplied through dropping resistor R l ll from the +250 volt supply, the
screen voltage drops when the grid voltage rises. This variation is detected by diode CR5, and then
filtered before application to the control grid of the common IF amplif ier . The gain of the common
IF stage will vary depending on the output of this detector , and will be lowered when the final tends to
operate near its non-linear region above zero grid voltage. Zero signal grid voltage for the final
stage is set to approximately -55 volts by means of the BIAS adjust control, R108.
RECEIVING
The same PI-network that matches the output of the transmitter to the antenna is used to couple
the antenna to the receiver input amplifier , V16. Conversion of the amplified input to the IF frequency
of 5. 2Mc is accomplished in the receiver mixer , Vll, which is f ed by both the RF stage and the VFO.
The plate circuit of Vll is tuned to 5. 2Mc with the same transfor mer as is the balanced modulator.
After passage through the crystal filter , three stages of IF amplification are provided by V5, V6, and
V7. At the output of V7, a diode detector, CR3 and CR4, develops the AGC control voltage which is
applied to the grids of V5, V6, Vl6 and Vll, as well as to the S-Meter amplifier, V4B.
One section of V8 functions as a product detector in the CW and SSB modes. Signal from the
third IF stage, V7, is applied to the grid of V8A while the output of the 5. 2Mc crystal oscillator, V4A,
is injected at the cathode. For AM reception, the cathode of V8A is grounded directly, causing this
stage to function as a grid-leak detector. The 5. 2Mc oscillator is disabled for AM reception. V8B
provides audio frequency voltage amplification, and V9 yields the AF output to the speaker or head-
phones.
A RECEIVER OFFSE T tuning control, R48, permits the receiving frequency tobe offset from the
transmit frequency by up to ± lOkc. It operates by controlling the bias on a voltage variable capacitor
(varicap), CR6, across the VFO tuning capacitor in the receive condition. In the transmit condition,
10

the bias on CR6 is taken from a fixed center-tap on R48, so that the frequency is independent of the
setting of the RECEIVER OFFSE T tuning control. The r eceiver and transmitter will operate on the
same frequency when the control is set at mid-rotation (center line on panel).
VOX CIRCUIT
A six pole relay controls the voltage and biases to the various stages of the transceiver , and
deter mines whether the unit is in the receive or transmit condition. Tr iode V3B has, as its plate
load, the coil of the relay. When V3B is cut off , the relay is unenergized and the condition is "receive".
Cutoff is maintained by a negative bias which is applied through R26 to the grid of V3B.
When an audio signal is applied to the VOX amplifier V3A, from the first microphone amplifier
V lA , V3A tends to draw less plate current. This causes the plate voltage at V3A to rise, thus firing
the neon bulb PL 3 and causing the voltage of the grid of V3B to rise. This brings V3B into conduction,
and the relay closes. The transceiver is now in the transmit condition. Af ter the voice peak has
passed, PL3 extinguishes and there is no longer any electr ical connection between V3B and any other
part of the VOX circuit. The grid voltage of V3B now tends to return to its original negative value,
and does so with a time constant which depends upon R26, C 19, and the setting of the VOX-DE LAY
control, R32. With the arm of R32 set to the plate end of the control, C19 will have a "virtual"
capacitance of approximately its own value times the gain of the stage V3B. As the arm of the control
is moved away from the plate end, the eff ective capacitance in the grid circuit is reduced; when it is
at the ground side of the control, the circuit capacitance is merely the value of C 19. This change of
eff ective grid circuit capacitance with stage gain is known as "Miller eff ect".
When the voltage at the grid of V3B decays to a suff iciently high negative value, the relay retur ns
to the "off" condition and the unit is in "receive" again. The voltage at the grid of V3A is adjusted so
that PL3 is just on the verge of firing when there is no speech input. This assures that the sensitivity
of the VOX circuit is always at a maximum. This bias is adjusted with R24, the VOX- THRESHOLD
control. If a speaker is being used there is the possibility that the microphone will pick up the audio
from the speaker and cause the VOX to trip. The anti-vox detector , CR l and CR2, desensitizes the
VOX amplifier by converting a sample of the receiver output signal into a positive bias which br ings
V3A into a heavier state of conduction. The amount of anti-vox injection is controlled with the ANTI-
VOX sensitivity control, A
J.).
In order to prevent transient voltages, which occur during relay switching, from accidentally
tr iggering the VOX circuit, the relay control tube is cut off for a short period of time immediately
after the unit switches to receive. A high negative pulse is f ed through PL4 to the grid of V3B, and
its duration is such that it keeps V3B cut off (and, thereby, the unit in the receive condition) until the
switching transients have decayed.
MAINTENANCE
GENE RAL
Your transceiver should require little service except for nor mal tube replacement. Do not sub-
stitute for tube types used in this transceiver. Factory-wired transceivers have been completely
aligned and adjusted at the factory before shipment. Transceivers built from the kit must be aligned
as described in the Alignment Procedure given. (Note that the crystal filter supplied in the kit has
been pre-adjusted and aligned at the factory) .
CABINET REMOVAL
To remove the instrument from the cabinet, first disconnect it from its power supply, and then
remove four #6 sheet metal screws at the rear and seven #6 sheet metal screws on the bottom. Unit
can be removed from the front of the cabinet.
11

ALIGNMENT PROCEDURE
Before proceeding with the alignment, the transceiver should be removed from the cabinet and
the cover should be removed from the VFO compartment. In removing the latter , it will be necessary
to temporarily remove the fibre extension shaf t from the P. A. tuning capacitor. The shaf t should be
reinserted after the VFO cover is removed.
A 40 to 80 ohm dummy load capable of dissipating at least 125 watts should be available. A 100
watt light bult may be substituted for the dummy load if necessary. Some means of measuring relative
H. F. power output to the dummy antenna should also be provided. An R. F. ammeter or an in-line
watt meter will be helpful toward this end.
Three reference signals from a well calibrated VFO or crystal oscillator will be necessary.
These are 7. OOMc, 7. 30Mc, and 3. 80Mc. VFO alignment should not be attempted unless the signal
source is well calibrated. If a calibrated communications receiver is available, it may be used to
set the external source. In any case, a crystal oscillator is to be pref erred.
RECEIVE R ALIGNMENT
a) IF Strip
1. The transceiver should be connected to the EICO 751 (or 752) power supply. There
should be no microphone connected to the unit at this time. Before turning the unit on, set the controls
as follows:
MIC GAIN .. . .. . . • . ....... .fully ccw (but not past detent)
RF GAIN. . . . . . ..... . .... . . fully ccw
AF GAIN. ... . ..... . . ... . .. fully ccw
CARRIER BALANCE ..... .. . . . mid-rotation
MODE SWITCH ..... .. .. • .. .SSB
FUNCTIONSWITCH. . . ..... .OFF
MAIN TUNING FREQUENCY. .. . .. . . . . 7. OOMc
PA TUNE . . . . . .... . • . ..... .Mid-rotation
BAND SWITCH. .... •• .. . .. .40 meters
PA LOAD.. .... . ........... fully ccw
RCVR OFFSET ..... .. ..... .mid-rotation (at centerline)
EXCITER TUNE .......... . . mid-rotation
NEUTRALIZING CAPACITOR C107. . . ... . ... fully un-meshed
2. Connect a small capacitor (between 20 and 50pf) from pin 3 of V4 to pin 7 of V l7. This
is a temporary measure and is best done with a pair of clip leads.
it at PTT.
3. Set the FUNC TION switch at STANDBY, and after one or two minutes of warmup, reset
4. Advance the RF GAIN control to the fully cw position, and adjust the slugs of IF coils
T3, T4, and T5 for maximu m S-Meter reading.
5. Return the RF gain control to its fully ccw position, and turn the FUNCTION switch to
OFF. Disconnect the clip leads installed in step (2).
This completes the IF strip alignment.
b) VFO
1. The controls are set as in step 1of part (a).
2. Set both trimmer capacitors C40 and C42, on the VFO board, to mid-rotation.
12

3. Apply an unmodulated 7. OOMc signal to the ANTE NNA connector on the rear apron.
This signal should be quite weak (approximately lOOuV), which may require very loose coupling between
the signal generator or oscillator to the transceiver.
it to PTT.
4. Set the FUNCTION switch at STANDBY, and after one or two minutes of warmup, turn
5. Turn the RF GAIN control to the fully cw position, and then turn up the AF GAIN control
until normal receiver noise is heard.
6. With the MAIN TUNING control set to 7. OOMc, adjust the slug of VFO coil L4until zero
beat is heard in the loudspeaker. The S-Meter reading may be peaked with the EXCITE R TUNE con-
trol and with the PA TUNE control. If it is not possible to peak the signal with the EXCITER TUNE
control, set this control to mid-rotation and adjust the slug of coil L ll to peak the meter reading.
The RF GAIN control should be adjusted to yield an S-9 reading.
7. Apply a 7. 30Mc input signal to theANTENNA terminal, as per the procedure in step(3).
8. Turn the MAIN TUNING control toward the highendof the40meter banduntil the applied
signal is heard. If it is heard at an indicated frequency that is higher than 7. 30Mc, unmesh the plates
of C40 by several degrees of rotation, and repeat steps (3) through (8). It will not be necessary to
make further adjustments on L ll at this time.
If the signal is heard at an indicated frequency of less than 7. 30Mc, mesh the plates of
C40 by several degrees of rotation and proceed as above. This procedure is continued until the signal
is heard at exactly 7. 30Mc on the dial. Do not proceed to step (9) until this condition is achieved.
9. Set the FUNCTION switch at STANDBY, and the BAND switch at 80 meters.
10. Set the MAIN TUNING control at 3. 80Mc, and leave the RF GAIN and AF GAIN controls
in the positions they had upon the completion of step (6).
11. Apply an unmodulated signal of 3. 80 Mc to the ANTENNA connector.
12. Set the FUNCTION switch at PTT, and adjust trimmer capacitor C42 until zero-beat
is heard. The signal should be peaked on the S-Meter by setting the EXCITE R TUNE control to mid-
rotation and adjusting the slug of LlO.
13. Replace the VFO cover and secure it with four #6 self -tapping screws. This completes
the VFO alignment. There should be a negligible change in dial readings due to the presence of the
VFO cover. If it is desired to correct for this change, the hairline may be reset from the front
panel.
c) TRANSMITTE R ALIGNME NT
1. The controls should be setas in step (1) of part (a) of the preceding RECEIVER ALIGN -
MENT, except for the following:
BAND SWITCH. ... ... . . .... 20 meters
MOD_E SWITCH. ............ CW
MAIN TUNING FREQUENCY.. .. . .. . .. 14.15Mc
NEUTRALIZING CAPACITOR C107. ....... to about 20% of full mesh
2. Connect a suitable dummy antenna to the ANTENNA connector, and an output power
meter (R F ammeter, etc. ) to the connecting transmission line. Connect a microphone with a PTT
button (or an external PTT switch) to the MIC connector. Do not connect key.
13

::a
Vf2
V9
6A05 VII
6BE6
CI07
V3
7199
vr
60068
,.,
§60·,· .""
r
-
Vf4
..../'#-
C5
L .••
c
60068
va
Vf6
m
12AX7 6BA6
.i:..
Vl3
12BY7A
vro
•
·.
-·--···-
.. ...
12AZ7A
082
VI
12AX7
TS T4 T3
vs
6BA6
V6 V7
6BA6 6BA6
-

Ll2 LII LIO
FIGURE5
C40
C42 L4

3. Set the FUNC TION switch at STANDBY, and, af ter one or two minutes of war m-up,
reset the FUNC TION switch at PTT. Depress the microphone PTT button, and adjust the rear apron
BIAS control for a meter reading of 50mA.
4. Release the microphone PTT button. It will not be used again in this procedure
5. Set the MODE switch at TUNE .
CAUTION
Do not allow the unit to remain in the "tune" condition
for more than about 20 seconds at a time, to avoid
excessive heating of components in out-of -resonance
conditions. If more time is required to make an
adjustment, allow the final to cool by setting the MODE
switch back at CW for about 30 seconds before continuing.
6. Adjust L9 for a maximu m meter reading, and proceed immediately to step (7).
7. Adjust the PA TUNE control for a minimum meter reading. The plate current should
fall below 200ma, since the PA LOAD control is already at its maximu m ccw position.
8. Adjust coil Ll2 for a peak meter reading.
9. The neutraliz ing capacitor, C 107 must be adjusted so that maximu m output power is
obtained at minimum plate current (dip). Slowly rotate the PA TUNE control and simultaneously note
the relative output power and the plate current. If maximu m output does not occur at plate current
dip, make a slight readjustment of the neutraliz ing capacitor and repeat the above test. After proper
neutralization is achieved, repeat steps (6), (7), and (8) above.
10. Set FUNCTION switch at STANDBY, and then set the BAND switch at 40 meters, and
the MAIN TUNING control at 7. 15Mc.
11. Re-set the FUNC TION switch at PTT, and adjust L8 for a maximum meter reading.
12. Adjust the PA TUNE control for plate current dip.
13. Adjust L ll for maximum meter reading.
14. Set the FUNCTION switch at STANDBY. Then set the BAND switch at 80 meters, and
the MAIN TUNING control at 3.80 Mc.
15. Re-set the FUNC TION switch at PTT, and then adjust L7 for a maximu m meter reading.
16. Adjust the PA TUNE control for a plate current dip.
17. Adjust LlO for maximu m meter reading.
This completes the transmitter alignment.
CRYSTAL FILTER
NOTE: The crystal filter in this transceiver has been pre-adjusted and aligned at the factory.
At most, only one slight adjustment will be necessary, if at all. The following procedure
should be employed only if the sound of received SSB signals appears to be unnatural or
distorted.
16

1. Set the transceiver up in the receive condition and apply an input signal to the ANTENNA
connector. The signal may be on any frequency in any of the three bands covered. Tune the trans-
ceiver to this signal and set the RF GAIN control so that, at maximum, the S-Meter reads S-9.
2. Upon tuning slowly past the signal, the S-Meter should indicate two peaks, both on the same
side of zero-beat. If the RF gain control has been set so that, at each peak, the reading is S-9, then
at zero beat the meter should read between S3 and S4. If this is not the case, or if both peaks do not
yield equal S-readings, proceed to step ( 3). Otherwise, the filter is in proper alignment and no adjust-
ments should be made.
3. Tune the receiver to a point midway between the two peaks, and adjust the slug of T2 for
maximum S-Meter reading. Repeat step (2) above. If the filter is still not in alignment, offset the
slug of T2 slightly, and repeat the measurement. The setting of this coil will be quite critical and
its adjustment must be made carefully. If it is not possible to achieve a reasonable pass-band by
adjusting T2, set T2 for a maximum S-Meter reading midway between the two peaks and proceed to
step (4).
4. Repeat the procedure in step (3), only this time adjust the slug of transformer Tl on the crystal
filter assembly. Exercise caution in making this adjustment since too great a misalignment of this
coil will cause the pass=band to widen, thereby reducing selectivity and sideband suppression. It will
be possible to properly align the filter by repeating steps (3) and (4) several times until the desired
pass-band is obtained. No adjustment of L l will be necessary.
DC VOLTAGE AND RESISTANCE CHART
The following chart lists the approximate DC voltages and resistances to ground at each of the
ter minals of those tube sockets which are chassis mounted, and is off ered an an aid in troubleshooting.
Reasonable deviations from the values given should be expected and cause no concern, since compo-
nent values will vary within their allowable tolerances to produce such deviations.
Three tables are listed. The first, Column A, lists the DC resistance to ground from each
tube socket pin. These values are to be measured with no external connections to the transceiver,
meaning that the power supply, speaker , antenna, microphone, etc. , are all disconnected. For the
DC voltages, Columns B and C, only the power supply is connected to the unit. The readings given
are those to be expected when using the EICO Model 751 AC power supply under normal ( 1l7V) line
voltage conditions. In all cases, the front panel controls are to be set as indicated below:
Column A:
FUNCTIONSWITCH. ............. . . .OFF
MODE SWITCH. .. . ..... . ...... . .... SSB
CARRIER BALANCE .. . ....... . .... .mid-rotation
PA TUNE ... ... ... . . . ............. .mid-rotation
EXCITER TUNE . ................... mid-rotation
RCVR OFFSET ..................... mid-rotation
MIC GAIN ......... .. ... .. .. .. . . ....fully ccw (but not past detent)
RF GAIN .... ........ . • ..... . .... ...fully ccw
AF GAIN ... . .. ... . ........ . ........ fully ccw
PA LOAD. . .. ... .. .. ............... fully ccw
Column B:
Same as for Column A, except FUNCTION switch is set at PTT
Column C:
Same as for Column B, except that a microphone with PTT switch is connected, and
the PTT button is depressed; also, the MODE switch is set at CW.
Voltage checks are made with a vacuum-tube voltmeter having an input impedance at the probe
of 11 meghoms. The heater voltages on the var ious tubes are listed as AC , since the 751 AC power
supply is being used to power the unit for measur ement purposes.
17

VOLTAGE AND RESISTANCE CHART
TUBE
PIN
A
RESISTANCE
TO
CHASSIS
B
DC VOLTAGE
WITH RESPEC T
TO CHASSIS
c
DC VOLTAGE
WITH RESPECT
TO CHASSIS
Vl
1
475K
90V
90V
2
lOOK
0
0
3
2.2K
lV
0
4
0
12.6 AC
12.6 AC
5
6
7
0
lOOK
75K
12. 6 AC
265 v
-42V
12.6 AC
265 v
-100 v
8
lK
0
0
9
0
6. 3 AC
6. 3 AC
V2
1
2
3
1. 8K
BOK
600K
0
265V
-42 v
0
225 v
0
4
0
12.6 AC
12.6 AC
5
6
7
8
9
0
50K
50K
55K
55K
6. 3 AC
265 v
265 v
35 v
35 v
6. 3 AC
200 v
200 v
25 v
0
V3
1
22K
265 v
75 v
2
3
500K
1Meg
8 V
13 v
8 V
13 v
4
0
6. 3 AC
6.3 AC
5
0
12.6 AC
12. 6 AC
6
10on
0
0
7
00
0
0
8
0
0
0
9
20 Meg
Do not measure
0
V4
1
28K
265 v
0
2
20K
250 v
220 v
3
15K
40 v
200 v
4
0
12.6 AC
12.6 AC
5
0
6. 3 AC
6. 3 AC
6
7
8
9
0
470K
1.4K
600K*
0
-1. 7 v
13 v
0
0
0
0
-100 v
V9
1
2
1.5 Meg
27on
0
11 v
-90 v
0
3
0
12.6 AC
12.6 AC
4
5
6
7
0
9. 5K
20K
1.5 Meg
6. 3 AC
250 v
240 v
0
6. 3 AC
270 v
270 v
-90 v
VlO
1
6.5K
110 v
110 v
2
3
0
00
0
-
0
-
4
5
0
6.5K
0
110 v
0
110 v
6
00
-
-
7
0
0
0
18

TUBE
PIN
A
B
c
Vll
1
2
620K
0
- 2 v
0
-95 v
0
3
4
5
6
7
0
0
12K
55K
1Meg*
12. 6 AC
6. 3 AC
275 v
75 v
0
12.6 AC
6. 3 AC
275 v
0
-90 v
Vl2
1
25K
260 v
265 v
2
55K
-50 v
-50 v
3
5600
0
0
4
0
0
0
5
6
0
25K
0
260 v
0
265 v
7
55K
-50 v
-50V
8
5600
0
0
9
0
6. 3 AC
6. 3 AC
Vl3
1
2
0
130K
0
-50 v
0
-4 v
3
0
0
0
4
0
0
0
5
0
12.6 AC
12.6 AC
6
7
8
0
lOK
2.2K
6. 3 AC
275 v
0
6. 3 AC
210 v
240 v
9
0
0
0
Vl4
1
00
0
0
2
0
6. 3 AC
6. 3 AC
3
4
5
00
4700
85K
0
0
-90 v
0
270 v
-60 v
6
00
0
0
7
8
0
7. 50
0
0
0
. 37 v
Vl5
1
00
0
0
2
3
4
5
6
0
00
4700
85K
00
6.3 AC
0
0
-90 v
0
6. 3 AC
0
270 v
-60 v
0
7
8
0
7. 50
12.6 AC
0
12.6 AC
. 37 v
Vl6
1
2
3
4
5
6
1. 8 Meg
0
0
0
lOK
BO K
0
0
0
6. 3 AC
280 v
220 v
-90 v
0
0
6. 3 AC
210 v
0
7
lOK
23 v
0
*These resistance measurements are made with the + voltmeter lead on the tube pin and
the -lead on chassis. The resistance depends upon one or more diodes in the circuit
and will change if the meter leads are reversed.
19