GENERAL RADIO COMPANY 620-A User manual

GENERAL RADIO COMPANY
FIGURE 1.
FIGURE 2.

GENERAL RADIO COMPANY
OPERATING INSTRUCTIONS
FOR
TYPE 620-A
HETERODYNE FREQUENCY
METER AND CALIBRATOR
PART l
DESCRIPTION
PURPOSE The Type 620-A Heterodyne Fre-
quenGy Meter and Calibrator is
designed particularly for frequency meas-
urements over a
very wide range, 300 kilo-
cycles to 300 megacycles.
ADVANTAGES 1'he heterodyne frequency me -
ter is direct-reading over
its fundamental frequency range, which
makes it very convenient and simple to usa
Provision is made for checking the ca li-
bration of the heterodyne frequency meter
in terms of a piezo- electric oscillator.
Because of the direct -reading feature it
is very easy to utilize harmonic methods
for extending the frequency range to both
low
er and higher frequencies.
PRINCIPLES OF OPERATION The instrument
contains the
heterodyne frequency meter, covering the
frequency range from 10 to 20 megacycles
in 10 steps ;a one-megacycle piezo-electric
oscillator tor checking the calibration of
the heterodyne
oscillator; a detector and
audio-frequency
amplifier for obtaining
beats between the heterodyne and the cali-
brator or between the heterodyne and un-
known. The heterodyne may be set to any
frequency
in the range from 10 to 20 mega-
cycles and any error corrected by ref-
erence to the calibrator. By harmonic
methods,
frequencies between 300 kc and
300 Me are readily measured.
DIRECT-READING SCALE The condenser dial
is graduated to
read fractions of a megacycle directly,
the smallest division on the scale corre-
sponding to 0.01 megacycle (10 kc). The
auxiliary scale on the driving shaft effec-
tively subdivides each main scale division
into 10 parts . Each auxiliary scale divi-
sion is, therefore, 1 kilocycle. The fre-
quency for any scale setting is given by
the
sum of the coil switch reading and the
reading or the dial . For example, in Fig-
ure l and Figure 2 the frequency is 14. 496
Me.
PART 2
INSTALLAT-I nN
SHIPPING LIST With each Type 620 -A Het-
erodyne Frequency Meter
and
Calibrator are packed the following
accessories and spa re parts:
1 - Instruction Book
1 - 115-volt Attachment Cord
2 - Mazda 40 Lamps (6-volt)
1- Box Fuses (0.2 amp.)~For
115
_volt
1- Box Fuses (1 .0 amp .) operatio n.
l- Box Fuses (3.0 amp.)
1- Box Fuses (0.2 amp.) For
23
0-volt
1- Box Fuses (0.5 amp.)
l- Box Fuses (3.0 amp.) operation.
3 - RCA Type 955 "Acorn" Triodes
1 -
RCA Type 954 "Acorn" Tetrode
1 -
RCA Type 84 Rectifier
2 - Multipoint Connectors
TUBES
Install as follows:
955 in oscillator socket (on end of
coil assembly)
-1-
955 in detector socket~
955 in amplifier socket (on top of
954 in crystal oscilla- sub-shelf)
tor socket
84 in rectifier socket (on main shelf)
Note: In mounting the 954 crystal oscil-
lator tube, be certain that the plate ter-
minal enters the connector in the block
below
the tube s9cket.
POWER SUPPLY For a-c operation connect
the instrument to the 115-
volt 50 - 60 cycle mains by means of the
attachment cord provided. To turn instru-
ment on, throw POWER switch to "A-C".
For battery operation, connect bat-
tery leads to multipoint connector (at the
right, seen from the front of the instru-
ment) . Connect 6-volt filament battery to
terminals 7 and 8 (polarity is not impor-
tant); connect 180-volt plate battery,
positive to terminal 11, negative to ter-
minal 12 (which is also grounded).

GENERAL RADIO COMPANY
COUPLING Coupling connections are pro-
vided on the front panel and
also through the left-hand (seen from
front? multipoint connector on the rear of
the instrument . For most work a short
wire connected to the ungrounded terminal
and led in the vicinity of an oscillator
will provide adequate coupling. In cases
where permanent wiring is desired , the con-
nection at the rear may be utilized to ad-
vantage.
On operating the equipment, particu -
larly in the immediate vicinity of trans -
mitters , excessive coupling or stray pick -
up is indicated by the pl ate current drop-
ping
to low values when the transmitter
is turned on . The coupling should be re -
duced, or a change in location of the in -
strument shoul d be made , if the change in
plate current is more than a few tenths
of a milliampere.
If excessive pickup at the fundamen-
tal frequency is obtained and a change in
location is not feasib le, me asu re ments may
st1ll be satisfactorily,made by using nar -
monics instead of the fundamental.
TELEPHONES Telephone receivers may be
pl ugged in at the front pan-
el , either at the telephone jack or using
a
GR plug on the jack-top binding posts .
For permanent
wiring the connections at
the rear multipoint connector may be used
(terminals 10 - ll of left-hand plug, seen
from
front of instrument) .
PART 3
FREQUENCY MEASUREMENTS
CHECKING CALIBRATION · When it is des ired
to check the het -
erodyne frequency meter calibration on any
range, simply throw
the CAL switch to "O N ".
The crystal oscillator plate current is in-
dicated by the meter when the switch below
the meter is thrown to the CAL position .
If the crystal does not oscil l ate , the
plate current meter will read about 7 mil-
liamperes; when the crystal oscil l ates ,
the· reading is about 1.5 mi lli amperes .
At each end of t~condenser range a
loud
beat against the calibrator will be
heard, corresponding
to a heterodyne fre -
quency of an integral number of megacycles.
For example,
on Coi l 17, such loud points
are found at o and 1.0 on the dial corres -
ponding
to 17.0 and 18.0 megacycles .
Intermediate points , where the beats
are not as loud as the principal beats men -
tioned above, occur at severa l points
along the scale . These intermediate points
occur where a harmonic of the heterodyne
beats with a crystal harmonic . For exam-
ple , the second harmonic gives a zero beat
setting at l/2 scale , or 0.5 Me ; the third
harmonic gives zero beat points at l /3 and
2/3 scale , corresponding to 0.33 and 0. 66
Me ; the fourth harr.onic gives points at
0. 25 , 0. 50 and 0. 75 Me and so on . Figure
3 shows where
these harmonic points are ob-
tained, the number of the harmon i c and the
exact frequency, for harmonics up to the
lOth.
It should be noted here that the dial
readings for these harmonic points are~
same on all coi l ranges, which makes it
very conven ie nt in checking or in inter -
polating .
The difference in reading between the
heterodyne and the crystal calibrator fre -
quency at any point where zero beat may be
obtained is the error of the heterodyne.
While
instructions are given below for re-
aligning the heterodyne frequency meter,
it will be found more convenient to simpl y
correct the reading of the heterodyne to
compensate for the erro r, unless the error
is unusually large .
MEASURING AN UNKNOWN
FREQUENCY
For example su p-
pose zero beat
against an unknown
fre quency is obtained giving an indicated
frequency of 16 ,613 Me , The cali br a tor is
then turned on, and the he terody ne checked
at the near est convenient harmo nic point,
say at 16,500 Me, Let the check re ad ing
turn out to be 16,482 Me, sh owing that the
hete rody ne reading is low by 16, 500 -
16.482 = 0.018 Me . Conse<;ently th e in-
~ frequency of the unkn own i. s low by
0,01 8 Me, so the true frequency is 16,613
+ 0.01 8 = 16 , 631 Me .
Similarly, if the heterodyne reading
were hi gh , the true freque nc y would be the
indicated frequency minus the het erodyne
correction .
SETTING HETERODYNE TO
A DESI RED FREQUENCY
In the case where
it is desired to
se t a local o sc il-
l at or to a specific frequ ency, the hetero-
dyne may be set to this frequency in ad-
vance. The osc illator may then be varied
until zero be at is obtained. For examp le,
suppose it is desired to set an oscillator
to 16.631 Me. The heterodyne is checked
against th e calibrator at the nearest con-
venient harmonic, say 16,500 Me. Let the
check re ading be 16 .4 82 M c,sho wing that
the he terod yne r ea ding is low by 16,500 -
16.482 = 0.01 8 Me. Consequently, the het-
erodyne . must be set ~by this amount
-2-

. GENERAL RADIO COMPANY
!rom the desired frequency, or at 16.631 -
0.018 = 16.613 Me to obtain the desired
frequency.
Similarly, if the heterodyne reading
were high,
the heterodyne must be set high
from ~ desired frequency by the amount
or
the correction .
USE OF HARMONICS The principal use for
harmonic methods lies
in extending the range of the heterodyne
freQuency meter to higher or lower freQUen·
cies. In t:tns equipment every effort has
been
made to make this harmonic extension
as simple and reliable as possible .
EXTENSION TO HIGHER
FREQUENCIES
In Figure 4. are
shown, on a logar-
ithmic· scale, the
frequency ranges covered by each harmonic
of the heterodyne frequency meter from 1
to 20 . Each range is 5hown as a horizon-
tal line, the length5 of the lines being
consoant, sinca the percentage frequency
coverage
of each range is the same.
On each line are marked 10 intervals
corresponding to the 10 coil ranges of the
instrument. The interpretation of these
coil range marks will agree with the en~
graving of the coil switch if the first is
called 10, the second 11, and so on up to
19. The last mark is then 20, which repre-
sents the highest frequency which can be
reached on Coil 19.
For any frequency, simply
move upward
to the point where the desired frequency
line crosses the harmonic range line. At
the intersection read off the number of
the harmonic (which is the number of the
line), the coil range and an approximate
indication of the condenser scale reading .
For example,
let the desired frequency be
25 Me. Entering at 25 on the frequency
scale- move upward meeting line 2 at Coil
12, condenser approximately 1/2 . The de-
sired frequency is then obtained when the
heterodyne is set to Coil 12, with the con -
denser at about half - scale, ~sing the sec-
ond harmonic. The harmonic number being
known (in this case 2), the exact setting
of the heterodyne is 25/2 = 12.50 Me .
In extending the range to higher fre -
quencies,
it should be noted from Figure 4
that the gain toward higher frequencies 1s
obtained only at the high frequency ends
of·the harmonic ranges. Consequently, to
use the lowest harmonic in a given high
frequency measurement, always use
as high
a fundamental frequency
as possible . In
searching for an unknown high frequency
always
start with the heterodyne set on
Coil 19 with the condenser at 1.0 (20 Me)
and progress toward lower frequencies un-
til zero beat with the unknown frequency
is picked up . For example, suppose a fre-
quency near 60 Me is to be measured. En-
terin~ Figure 4 at 60 on the frequency
scale, an intersection on line 3, Coil 19,
condenser
maximum is found. Progressing
further upward, an intersection on line 4,
Coil 15, condenser zero is obtained. Simi-
larly, line 5, Coil 12, condenser zero,
line 6, Coil 10, condenser 0. Any one of
these settings gives a harmonic at 60 Me,
being, respectively, the 3rd, 4th, 5th and
6th.
The lowest harmonic would in this
case be the third. The unknown frequency
is then 3 times the frequency of the het-
erodyne. (In this case 3 x (19+1.0) -60.
If the harmonic beat against the unknown
fell at 19.78 Me (instead of just 20 Me)
the harmonic frequency would be 3 x 19.78
= 59.34 Me.
If no idea is had of the value of an
unknown frequency, the procedure is to
start at the high frequency end of the het-
erodyne range and note the successive set-
tings of harmonic beats as the frequency
of the heterodyne is progressively reduced.
Then with the coil and condenser settings
of the highest frequency point, enter a
harmonic
line, and search for the next low-
er settings on the line immediacely above
the one entered . If agreement is obtained,
the proper line was entered ; if not , move
up or down a line and try again. If more
than two settings of the heterodyne were
obtained , agreement should be obtained for
every line crossed. This is simply another
way of stating thao the s~ccessive funda-
mental
frequencies of the heterodyne, each
multiplied by the correct harmonic number
must give the same answer for the unknown
frequency. A quick test on the figure will
generally fix the correct harmonic num-
bers much more quickly than numerical tri -
als. ·
EXTENSION TO LOWER
FREQUENCIES
When measuring .fre-
quencies below the
fundamental range
of
the heterodyne frequency meter, the actual
measurement is made at a harmonic of the
unknown frequency. If a sufficiently
strong signal of the unknown frequency is
applied to the COUPLING terminals, harmon-
ics will be generated in the detector
itself . With weak signals, however, an
external means of harmonic generation must
be provided. In Figure 5 are shown the
sub-harmonic frequency ranges
of the het-
erodyne frequency meter. The general de-
scription of the chart follows that al-
ready given for Figure 4 F'rom this chart
the appropriate submultiple number (sub-
harmonic number),
the coil switch setting
and approximate condenser setting may be
found for frequencies below the range of
the heterodyne frequency meter. For exam -
ple, suppose the settings of the hetero-
dyne are desired for a frequency of 4800
-3-

GENERAL RADIO COMPANY
kc. Entering at 4800 kc on the frequency
scale, move downward meeting the •3rd sub-
harmonic
line at Coil 14, condenser l/3,
and, next below, line 4 at Coil 19, conden-
ser l/4. The actual settings would be
4800 x 3 = 14,400 kc or 14.40 Me and 4800
x 4 = 19,200 kc or 19.20 Me.
' In measuring an unknown frequency ly-
ing below the range of the heterodyne, al-
ways start with the heterodyne set at the
lowest frequency (10.00 Me) and progress
toward higher frequencies until zero beat
with the unknown is obtained, to make use
of the lowest subharmonic number in a giv-
en measurement.
If no idea of the value of an unknown
frequency is had, the procedure is to
start at the low frequency end of the het-
erodyne range and note the successive
points at which zero beat can be obtained
against the unknown, as the heterodyne
frequency
is progressively increased. Then
with the coil and condenser settings of
the lowest frequency point, enter on a sub-
harmonic
line and search for the next high-
~ settings on the line immediately below
the one entered. If agreement is obtained
the proper line was entered; if not, move
up or down a line and try again. If more
than two settings of the heterodyne were
obtained, agreement should be obtained for
every line crossed. This is simply another
way of stating that the successive funda-
mental
frequencies of the heterodyne, each
divided by
the correct subharmonic number
must give the same value for the unknown
frequency. A quick test on the figure
will generallY ~ive the correct subharmon-
ic numbers much more quickly than numeri-
cal trials.
PROCEDURE FOR VERY
WW FREQ.UENCIES
For frequencies be-
low l Me it will be
found
that more than
one zero
beat point will be found on a giv-
en coil range. If the readings at two
successive points, corrected for any fre-
quency meter errors, be noted, then the
difference between them is approximately
the· unknown fundamental frequency. With
this approximate value of the unknown fre -
quency,
it is easy to determine the number
of the harmonic for any of the zero beat
points wtlich may be heard on the given
coil or any other coil.
When the unknown frequency is very
low, several zero beat points will be
found in one coil range. In this case,
taking the difference between two success -
ive
zero-beat settings can only give an
approximate value
for the unknown frequen-
cy. It is sufficiently good, however, to
permit the harmonic numbers to be deter-
mined without ambiguity. Knowing the num-
bers of the harmonics corresponding to any
zero
beat point, it is possible to improve
-4 -
the accuracy of measurement by taking the
difference in frequency of two zero beat
points which are separated by several har -
monics ,
then dividing this difference by
the number of harmonic intervals between
the two observed zero beat points (or the
difference in the harmonic numbers of the
two zero beat points).
For example, a low frequency is being
measured and on Coil 10 zero beat points
are observed at 0.127, 0.374, 0.621 and
0.863 on the condenser scale . The differ-
ence between successive harmonics is 0. 247
Me (247 kc) which is the approximate value
of the unknown frequency. The frequency
of the first zero beat point is 10.127 Me
which, divided by 0.247 Me, the approxi-
mate fundamental,
gives 10 .127/0.247 = 41
for the harmonic number of this point .
Then the harmonic numbers of the other
points are, by inspection, 42, 43 and 44 ,
respectively.
The fundamental frequency may then be
determined by dividing the frequency of
any zero beat point by its harmonic number,
10.127/41, 10.374/42, 10 .621/43, 10.868/44
etc. giving 0. 247 Me or 247 kc for the un -
known frequency; or the unknown frequency
may be found from the difference in fre -
quency-of any two zero beat points, thus:
1044868 - l04i27 ~ 0 . ~41 =0.247 Me (247kc) .
INSTRUCTIONS FOR ALIGNING
HETERODYNE
Factory ad-
justments of
the values of
inductance, fixed capacity and variable
capacity are made to make the calibration
agree with the dial readings . Ordinarily
the error of the heterodyne will be small
enough
that for use over long periods of
time no readjustment should be required .
If it becomes necessary to realign the het -
erodyne,
the following procedure should be
u~ect. .
If the calibration shifts upward on
the dial, while the span remains essen-
tially constant (that is , if the calibra -
tion reads, say, 2 divisions high at 0 and
also 2 divisions high at 1.0 on the dial)
adjust the zero capacity for the particu -
lar range involved, increasing the capaci-
ty to bring the calibration into agreement
with the dial . (If the calibration is low,
decrease the capacity .) As a precaution ,
after making this adjustment, check·the
span to see that calibration agrees with
dial reading .
~f the span is not correct , even
though
the calibration may agree with the
dial readings at one end or the other of'
the dial, proceed as follows : If the span
is too small (that is , one megacycle occu-
pies-a larger portion of the dial than the
space between 0 and 1.0) decrease the in -
ductance
for the particular range involved,
•

GENERAL RADIO COMPANY
by turning the hex-head adjusting screw at
the rear of the coil. If the core is
plain metal, turn slightly to the right to
decrease inductance. If ohe core is pow-
dered iron, turn slightly to the left ~
decrease inductance. After adjusting in-
ductance, check ·the calibration, and ad-
just capacity, if necessary , as instructed
in preceding paragraph.
If the span is too large (that is,
one megacycle occupies a smaller portion
of the dial than the' space between 0 and
1.0) increase the inductance for the par-
ticular range involved, by turning the hex-
head
adjusting- screw at the rear of the
coil. If the core is plain metal, turn
slightly to the left to increase induc-
tance . If the core is powdered iron, turn
slightly to the right to increase induc-
tance . After adjusting inductance, check
the
calibration, and adjust-capacity, if
necessary, as instructed above .
The positions of the coils and con -
densers
for adjustnent of the various
ranges are shown in Figure 6.
APPLICATIONS The Type 620 -A Heterodyne
Frequency Meter
and Cali -
brator has such a wide field of usefulness
that no complete instructions for applica-
tion of the instrument in any given prob-
lem can be given. The following sugges-
tions may prove helpful , in those cases
which
are similar to the ones outlined ,
and
may suggest suitable procedures in
other problems .
For measuring high frequency
trans -
mitters , where the heterodyne is located
in the transmitting room , or at least in
the immediate vicinity of the transmitter ,
generally no special coupling means is
necessary, A short wire connected to the
upper "coupling" post on the panel of the
heterodyne will increase the pickup con-
siderably ,
For
low frequency transmitters , par-
ticularly where harmonic suppressor cir -
cuits are employed, or where the trans-
mitter is very well shielded , it may be
necessary to extend the coupling wire in -
side of the transmitter shield to obtain
sufficient pickup, particula r ly if a high
harmonic must be used in t he measuremen t.
-5-
Another method of obtaining a signal
strong enough to be measured is to use an
aUY.iliary oscillator whose fundamental is
set to a harmonic of the transmitter fre-
quency, and to measure a harmonic of this
oscillator .
In measuring ultra-high frequency
transmitte rs located at some distance from
the heterodyne, an exposed coupling
wi re
or small antenna has made it possible to
measure the frequency of a transmitter at
a distance of nearly one-half mile. The
distance which may be covered in such meas -
urements
will depend greatly upon the par-
ticular locality , the frequency and the
power of transmitter .
In checking automobile transmitte rs
and receive rs, it has been found conven-
ient to put up a coupling grid or zig -zag
over the space
in which the cars may be
parked du ring test . The coupling grid is
connected to the upper coupling post of
the heterodyne, which may be located in an
adjoinin g room or build i ng , For conven-
ience, a long cord connected
to the tele -
phone binding posts may be led back in t o
the
area where the cars are parked and
terminated
in a loud speaker or telephones.
The operator adjusting the transmitte rs
then simply adjusts for zero beat .
For checking
oscillators of low har -
monic content as well as relatively low
output voltage , such as signal generators
(which also normally have a very low out-
put impedance)
best results are obtained
by coupling
the "high" side of the output
to the upper coup ling post. The attenua -
tor sett i ng should be for maximum voltage
and maximum impedance. Shielded ca bl es,
or other high- capacity devices should not
be used, In those cases whe re a coupli ng
rod antenna is provided in the signal gen-
erator , connected to a point of compara-
tivel y high vol tage , the coupling wire of
the hete rodyne ma y be brought near or
wrapped around the rod. In
some cases,
particularl y at low frequencies , it is
necessar y to use an auxiliary oscillator
or oscill ating receiver set to the signa l
generator as a source in order to obtain
sufficien t harmon i c output or suffi ~i ent
output vo lt age.

GENERAL RADIO COMPANY
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FIGURE 3
- 6-

GENERAL RADIO COMPANY
FIGURE 4.
-7-
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GENERAL RADIO COMPANY
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FIGURE 5.
- 8-

GENERAL RADIO COMPANY
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FIGURE 6. Location ot Coils and Condensers
- 9-
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FIGURE 7. Wiring Diagram for Type 620 -A Heterodyne Frequency Meter and Crystal Calibrator

GENERAL RADIO COMPANY
CONDENSERS
g=~ ~} 620-350
C-3 =
20 uuf
C-4 =5 uuf
C-5 = 0.0002 uf
C-6 =0.0002 uf
C-7 =0.0002 uf
C-8 = 1.0 uf
C-9 -}
C-lO : 420 uuf
C-11 = 0.01 Uf
C-12 = 5 uuf
C-13 = 5 uuf
C-14 = 200 uuf
C-15 = 0.01 uf
C-16 = 250 uuf
C-17 =10 uuf ± 20%
C-18 =0.0001 uf
C-19 =0.0001 uf
C-20 = 0.01 Uf
C-21 = 0.001 ut
C-22 = 0.001 uf
C-23 = 0.01 L!f
C-24 = 0.01 uf
C-100 = 75 uuf
c-101 =75 uuf
C-102 = 75 uuf
C-103 =100 uuf
C-104 = 100 uuf
C-105 = 50 uut
c-106 = 50 uut
C-107 = 50 uuf
C-108 = 50 uuf
C-109 =50 uuf
PARTS LIST
T-1 = Type 954
T-2 = Type
955
T-3 = Type 955
T-4 = Type 955
T-5 = Type 84
PATENr NOTICE
RESISTORS
R-1 = 0.5 M..O.
R-2 = 3 M..n.
R-3 = 50, OOOil.
R-4 = 1000..0.
R-5 = 4 Mn
R-6 = 4000.!1
R-7 = 7ooon
R-8 = 18,000..n.
L-1
L-2
L-3
L-4
L-100
L-101
L-102
L-103
L-104
L-105
i.-106
L-107
L-108
L-109
INDUCTORS
= Type 620-36
= Nat•l 100 Choke
= Nat•l 100 ehoke
= Nat•l 100 Choke
~ Type 620-310
= Type 620-311
= Type 620-312
- Type 620-313
=Type 620-314
= Type 620-315
=Type 620-316
=Type 620-317
= Type 620-318
=Type 620-319
SWITCHES
S-1 = Type 510-450
S-2 = Type 139-335
S-3 = Type 339A
S-4 =Type 139-333
This instrument is manufactured under the following u. s. Patents
and license agreements:
Patents of the American Telephone and Telegraph Company, solely
for utilization in research, investigation, measurement, testing, in-
struction and development work in pure and applied science.
Patents and patent applications of Dr. G. W. Pierce pertaining to
piezo-electric crystals and their associated circuits.
Patent 1,542,995.
Patent 2,012,497.
-11-

GENERAL RADIO COMPANY
SERVICE AHD liAINTENANCE NOTES
for the
TYPE 620-A
TYPE 620-A HETERODYNE FREQUENCY METER AND CALIBRATOR
NOTE
The following information will be found useful for all
Type 620-A Heterodyne Frequency Meters and Calibrators, but is
especially pre~red for application to those instruments with
serial numbers of 244 and higher, or those with lower serial num-
bers which have been reconditioned at the factory and now have
the ve~nier dial on the panel. It is recommended that the earlier
instruments which have not been brought up to date be returned for
reconditioning and recalibration if trouble should develop. A
rough estimate of the cost can . be supplied upon request, provided
that the serial number of the instrument is given.
1.0 GENERAL MAINTENANCE
1.1 From time to time it may become necessary to replace the
det~ctor and amplifier tubes when the higher harmonics
become weak. It would be advisable to remove the cover
from the main tuning condenser and see that the contacts
and brushes on the rotor are clean at this time, and also
to see that the AC-BAT switch contacts are clean. Crocus
cloth is recommended for cleaning these surfaces.
2.0 LINE FUSES BLOWING OUT
2.1 Remove all tubes and check with new fuses. If fuses hold,
check the tubes and then reinstall the rectifier and the
other tubes, one by one, until the defective tube or de-
fective tube circuit has been located.
2.2 If removing all tubes does not clear the trouble, dis-
connect the output circuits of the power transformer and
repeat. If fuses now hold, replace connections one by
one
until trouble is located.
2.3 If fuses still blow with all output circuits disconnected
from transformer, this unit should be removed from the
instrument and returned to the factory for replacement.
3.0 PLATE FUSE BLOWING OUT·
3.1 Remove all tubes, switch calibrator circuit off, and try
new fuse.
3.2 If fuse holds, check all tubes and replace them one by
one
until trouble is located. Switch calibrator circuit
on.
3.3 If fuse does not hold, disconnect output of Type 527-B
Filter Unit at terminal No. J. If fuse still blows, the
filter unit is defective and should be returned to the
factory for replacement.
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GENERAL RADIO COMPANY ••••••••••••••••••••••••••••••••••••• TYPE 620-A
4.0 NO SIGNAL IN PHONES
4-3
4·4
4· 5
. 4-7
Check or replace all tubes and see that they make. positive
contact in their sockets. Be sure that the acorn tubes are
mounted right side up.
If the plate-current milliammeter reads between 1.5 and 2.5
with the switch in · either the RET or CAL positions, both
tubes are oscillating normally. Should this read 6 or 7
milliamperes, it would indicate that the tube is not oscil-
lating and the circuit should be carefully checked. The
tube voltages should be-checked in accordance with the Tube
Data.
Apply an audio signal to the plate circuit " of the T-4 tube.
If this is heard in the phones, apply the signal to the grid
of T-4, then the plate of T-3 and the grid of T-3. This
procedure checks the detector and amplifier circuits.
Apply an r-f voltage to the coupling terminal and check if
a beat note can be obtained in the phones when the cali-
brator circuit is switched off and the main dial is turned.
See if a beat can be obtained against the crystal oscillator
by varying the frequency of the r-f signal.
Snap the switches back and forth several times to see if an
erratic contact may be causing the non-oscillating condition.
Also rap the panel to check for some loose joint, contact,
or defective tube.
If the heterodyne oscillator does not osc~llator, check the
L-3 and L-4 chokes to see that they are making contact in
their mountings and for open circuit •
If the crystal oscillator is inoperative, check resistors
R-5 and R-6 to determine that they , are making good contact
in their mounting clips. Choke L-2 should also be checked.
If the plate-current milliammeter is sticky, it should be
returned for repair or replacement.
5.0 CALIBRATION
l
Complete instructions for realigning the heterodyne frequency
meter will be found in the instruction book. Please note
that it is not practical to set the calibration of the coils
so that they agree perfectly with the dial. Small shifts
may be expected whenever the coil selector switch is ~oved
to a different point and back again. It was for this reason
that the crystal calibrator circuit was provided, and for
very accurate results the dial should be checked against the
calibrator every time the coil switch is changed.
-13-

GENERAL RADIO COMPANY ••••••••••••.••••...•••••••••••••• TYPE 620-A
If errors begin to appear in the total span of any coil,
or if the difference between the dial reading and the
calibrator check points becomes consistently large, the
coil should be realigned in accordance with the instruc-
tions.
TUBE DATA
These data were measured in'the Calibrating Laboratory of
the General Radio Company using Model 772 Weston Analyzer for the
a-c and d-e measurements of all voltages.
Values as given were obtained from a standard Type 620-A
Heterodyne Frequency Meter and Calibrator that was being calibrated
for stock. Where operating voltages are . not critical, variations
as great as 20% may be expected.
T-1
T-2
T-.3
T-4
T-5
*Output
Function
Piezo-Elec-
tric Oscil-
954
lator
Heterodyne
955
Oscillator
955
Detector
955
Amplifier
84
Rectifier*
Heater
(60 I"J ac)
6.5 v
6.5 v
6.5 v
6.5 v
6.5 v
Plate
to
Cathode
187 v
20.3 v
20.3 v
198 v
185 v
(cathode to ground) = 165 volts d-e.
-14-
Screen
to
Cathode
197 v
Control
Grid
Bias
-4.7 v
-4.7 v
7
1