CBS laboratories FM Volumax 410 Manual

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FM Volumax
FM Stereo Volumax
Automatic peak controller
OPERATING AND MAINTENANCE INSTRUCTIONS
~ LABORATORIES
Stamford, Connecticut
A Division
of Co lumbia Broadcasting System, Inc .

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~ LABORATORIES
TABLE OF CO NTENTS
SECTION I - lNTRODUCTlON
~eNo.
l-1
General
1-1
1-2 Warranty
1-1
1-3
Factory Servi ce and Repair
1-2
1-4
Specifications
1-2
SECTION ll -INSTALLATION INSTRUCTIONS
2-1
Unpacking
2-1
2-2 Power Supply 2-1
2-3
Installation
2-2
2- 4 Electrical Connectio ns 2-3
SECTION ID - SET-IJP PROCEDURES
3-1 Gene ral
3-1
3-2 Stereophonic Operation
3-2
3-3 Pr c,of or Performance Measurements
3-4
SECTION IV -PRINCI PLES OF OPERATION
4-1 General
4-1
4-2
Circuit Operati on
4-2
SECTION V - MAINTENANCE
5-1
Electrical Alignment 5-1
5-2
Troubleshooting 5-2
5-3
Transient Characteristics
5-4
5-4
50 Microsecond Operallon
5-5
SECTION VJ - PARTS LIST
6-1 Input Board Assembly
6-1
6-2 Power Supply Board
6-1
6-3 output Board Assembly
6-2
6-4
AGC Board Assembly *I
6-2
6-5
AGC Board Ass emb ly # 2
6-3
6-6
Control Board 6-3
6-7
Miscellan eous Parts
6-4
Revised May, 1967

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~ LABORATORIES
LIST OF ILLUSTRATIONS
Figur e 1-1
The FM Voluma.x Automatic Peak Controller
1-1
Figure 2-1
Circuit Board Layout
2-1
Figure 2- 2 600 ohm, 16 dB Attenuation Network
2-2
Figure 4-1
FM Voluma.x Amplitude R esponse
4-3
Figw· e 4-2 FM Volumax Block Diagram
4-4
Figure 5-1 FM Voluma.x Sc hematic Diagram
5-6
ii

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1-1 GENERAL
SECTION I
INTRODUCTION
~ LABORATORIES
This instruction book co nt ains complete maintenance and operating instructions for the Model
410 and Model 411 (Stereo) FM Volumax. The i nstrum ent has been designed for the FM and TV
broadcaster and will enable him to use his full modulation capability with no risk of overmodu -
lation or distortion. Used with SCA systems, it prevents crosstalk caused by overmodulation
into adjacent channels. It replaces any li miter or c lipper that may now be in use and should
be used with well-controlled input levels such as those provided by the CBS L aboratories'
Audimax automatic level control.
The various problems associated with FM and TV broadcasting and their solutions are dis-
cussed at length in Section IV of this book.
Figure 1-1 The FM Volumax Automatic Peak Contro ll er
l-2 WARRANTY
A
standard warranty card with a return post card has been included with your FM Volumax.
Fill out the card and return it to CBS Laboratories as soon as possible in o rd er to validat e
your warranty.
Your new FM Volumax is warranted ag ainst elect r ical a nd mechanical defects for a period of
one year after you register th e warranty. Refer to th e warranty card for th e specifics on our
resp o nsibilities and yours.
1-1

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~LAB O RAT O RI E S
l-3 FACTORY SERVICE AND REPAIR
If you should exper ience difficulty in installing or operating the FM Volumax, please contact
the Professional Products De partment at CBS Laboratories in Stamford, Conn. Within the
continental
limits of the United States, ca ll collect area code {203) 327-2000.
1-4 SPECIFICATIONS
Dimensions:
Fits standard 19-inch rack, 3-1/2" high and 9-5/8" deep .
Frequency R esponse: Below threshold of limiting:
Flat within 1 dB, 50 H z* to 1 5,000 Hz.
Above
threshold: Variable depending on level.
Al maximum level, response follows 75-microsecond
de-emphasis cu rv e within 1 dB.
Harmon ic Dist ortion: Less than 1% from 50 to 15 ,000 Hz thro ughout entire
control range.
Input Level: -24 to +8 dBm
Maximum
Peak Output L evel: +2 1dBm
Input
and Output Impedance: 600 ohms balan ce d or unbalanced ( 150 ohms available on request) .
Noise Level: Below -70 dB re + 21 dBm output
Maximum Gain: 50
dB
Attack Tim e: Less than l microsecond or 2 millisecon ds depending on program waveform
Recovery Time: Low Frequency Loop: 140 milliseconds
High Frequency Loop: 10 milliseconds
Maximum Operating Temperature: 55° C
Power Required: 15 watts at 115-230 V AC, 50/60 Hz.
* Hz, abbreviation for Hert z, has been adopted as the new engin eering term for cycles per
second. KHz is 1000 cps.
1-2

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2-1 UNPACKING
SECTION
II
INSTALLATION INSTRUCTIONS
~ LABORATORIES
Examine your FM Volumax carefully for evidence of possible shipping damage. If the equip-
ment is damag e d, file a c laim with the carrier and notify us immediately. If future trans-
portation of the instrum e nt is anticipated, save the shipping carton and the packing material.
2-2 POWER SUPPLY ( See Figure 2-1)
The transformer in the power supply of the FM Volumax allows the selection of either 115 or
230 volt operation. The unit is shipped for 115 volt operation. If 230 v olt operation is re -
quired , remove the jumpers on the power supply board between lugs 1 to 2 and 3 to 4. Re-
connect a sing le jumper between lugs 2 and 3. Repla ce the fuse with a type 3AG rated 0.10 Amp.
R222
-- --,
Output Board
AGC Board #2
AGC
Board #1 I
Control Board
Power Supply Bao rd
Figure 2-1 Circuit Board Layout
2-1

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~ LABOflATOfllES
2-3 INSTALLATION
FM Volumax is designed to be mounted in a standard 19"-wide rack. It requires 3-1/2" panel
space and is slightly less than 10" deep. As with all transistorized equipment, the unit must
be installed in a reasonably well-ventilated position, with no high heat-producin g equipment
benea th it.
NOTE:
Ambient temperatur e should not exceed 130° F.
FM Volumax is normally installed at the transmitter, immediately preceding the transmitter
audio input terminals. H owever, it may be installed at the studio ahead of the program line to
the transmitter, when the phase-amplitude characteristic between studio and transmitt er are
known to be uniform under all climatic conditions and telephone company equipment changes.
For o ptimum operation, the FM Volumax shou ld be fed uniform average program levels and --
as in the c as e with all limit ers -- shou ld not be used to perform a gain-ri ding function. The
use of CBS Laboratories' Audimax is r ec ommended wit h the FM Vo lum ax. Any wave shaping
or phase scramblin g devices shou ld be used ahead of the instrument.
The FM Voluma.x has sufficient gain to correct for the long-line and equal izer l osses incurred
when the transmitter site is remotely loca t ed, and input l evels as low as -24 dBm can be
tol erated. Howev er, if input levels excee din g +8 dB m are used, the range of attenuation on the
input pad will not be sufficient. Such is the case when a CBS Labor at ories' tube-type Audimax
is connected directly to the FM Volumax input.
If a CBS Laborator i es' M odel 440, 441 or 443 Audi max is used to feed the FM Volumax, the
atte nuator circuit shown in Figure 2-2 must be inserted in the line betw ee n the units. An
atte n uator is not necessary wh en using the Model 444 Audimax m.
~""----~---M/IMIVW.W-- --
430 n 430 n
200n
Figure 2-2 600 o hm , 16 dB Attenua ti on Network
2-2

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~ LABORATORIE 9
The standard FM Volumax is designed to work in conju ncti on with a 75- microsecond trans-
mitter pre-emphasis network. H owever, if operation into a 50-mi cr oseco nd netw ork is re-
qu ired, such as th e CC IR standard, modify the instrum ent as outlined in Section V of this
manual.
F or ste reophonic transm ission, th e Model 411 FM Volumax is used. Thi s instrument co ns ists
of two Mod el 410 FM V ol umax co upled mechanically and elect r ically in a single fram e. For
field conversion of a monaural FM Vo lu max Mo del 410 to stereo, o rder a second Mod el 41 0 and
a Stereophonic Cable
Assembly # 200354 directly from CBS Lab oratories. Connect the two
units with the
ca bl e assemb ly and adjust R227 on both unit s so that the gai n reduction met ers
read full scale in the ab sence of signal.
In stall your FM Volumax in a standar d 19-inch moun tin g ra ck but do not depend on the mounting
hardwar e for grounding. In some installations, an excessive amount of RF pickup may be
pres e nt on the input l ea ds to the Vo lumax , causing tmdesirable gain reduction and distortion.
Car ef ul grounding t echn iques will cure this problem . Be sure a good bond exists between the
Volumax metal chassis and the equipment rack. L ead shields should be connec ted at only one
end a nd should n ot be use d for gro und strapping. Additionally, if the transmitter site is rem ote
from the stu dio, make certain that the TELCO service entry bo x is appr opria tely grounded.
2
-4 ELECTRICAL CONNECTIONS
The input and output leads shou ld be connected to the five-termi n al strip at the rear of the
unit c hassis. T ermi nal s 1 and 2 are for the input, and terminals 4 and 5 are for the output.
The unit may be co nnected for balanced or unbalanced opera ti on. Te rminal 3 is the c hassis
gro und.
The
FM Volumax is normally designed for 600 ohm operation. H owever, it may be set up for
150 ohm operation by usin g special input and output pads and by strapping tr ansformers T2 and
T3. If you require 150 ohm ope rati on and did not reques t it when placin g your order, refer to
the schematic diagram in Section V and co nta ct CBS Laboratories for the required parts.
2-3

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3-1 GENERAL
SECTION
ill
SET-UP PROCEDURES
~ LA ■ ORATOR l &s
All of the set-up and operating contro ls are located behind the front panel of the unit. With all
the input, output, and ground connections properly made ( Refer to Section II ) tur n the power
switch ON and proceed as follows to set up a Model 410 FM Voluma.x for monophonic operation:
a. Rotate both the 1NPUT L EVEL and OUTPUT LEVEL cont ro ls fully
counter-clockwise.
b. Feed a 400 Hz signal into the FM Vo l umax. Adjust the INPUT LEVEL
co n trol, or the signal generator, so that the meter on the FM Volumax
is deflected to the red / green junction.
c. Depress the MODULATION SET swit ch and, holding it in this position ,
turn the OUTPUT LEVEL control clockwise to produce 10 0 %, or any
maximum
desired, modulation. This step provides a precise setting
of th e modul ation level with tone signals. H owever, it should be recogn i zed
that phase-shifts in transmitting equipment may cause previo u sly limited
complex program waveforms to slightly exceed the 10o% point. A slight
r e-adjustment of the OUTPUT LEVEL control may be necessary if modu-
lati on appears excessive.
NO TE:
Th e phase-shift problem may be reduced at installations
where it is easy to disable or remove the pre - emphasis
circuit from the transmitter . If this is done, the de -
emphasis network at the output of the FM Volumax must
also be disabled. This can be accomplished at the out-
put board by disconnecting C204 and placing a jumper
across L200.
d. Rel ease the MODULATION SET switch and remove the 400 Hz signal.
e. Feed typical lev e l-controlled program into the instrument.
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~ LABOflATORI ES
f. Adjust th e INPUT LEVEL control to cause the GAIN REDUCTION
meter to operate in the gree n or normal area with occasio 11al peaks
deflecting into the red r eg ion.
NOTE:
ll should be noted that the design calibration of the
meter is, of ne cess ity, based 011 subjective consider-
ations. Not all users will want to operate with the
same d egree of limiting action. A setting for heavy
limitin g action will produce higher average modulation,
but with a
potential loss of brillian ce at high levels .
In gene ral , FM br oadcasters with wide-range pro-
gram min g will re -adjust the input level to operate
with less limiting action than FM or TV broadcasters
who normally transmit narrow-band material .
3-2 STEREOP HONIC OPERATION
For FM stereophonic br oa d cas ting, two FM Volumax units are used , one for each channel. To
prevent any possible change of stereophonic perspective , the two units are coupled to p1 ·ovide
trackin g of ga in redu ctio n. Thi s is a ccomplished by interconnecting Control Board points F and
I, between the two unit s. If this modific ati on is performed in the field, R227 in each unit must
be r ea dj ust ed to provide full- scale de fl ec tion of both met e rs with no input signal. In the FM
St ereo Volumax Model 4U, the two units are co upled by a plug-in cable assembly at the rear.
This ca ble may be disc onne cted for independent operation if that is d es ir e d.
On the c urr e nt model of the M ode l 410 FM Volumax (monophonic) pr o visi on ha s b ee n made so
that an FM sta ti on which is co n verting to stereo can simply add another Model 410 to its sys tem
rath er than buy a new M ode l 411. H owever, when this is done, the two Model 410 units must be
re -ad j ust ed so that th eir ga in reduction meters track. R efe r to Se ct ion II f or the proper pro-
ce dur e.
3-2

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~ L AII O II ATORIE S
To set up the Model 411 FM Volumax, or two Model 410 units , for stereophonic operation, make
sure that all of the input, output, and ground connections have been properly made (Refer to
Section Il}
and proceed as follows:
a. R otate the INPUT LEVEL and OUTPUT LEVEL controls on both the
left and right channels fully counter-clockwise.
b. Feed a 400 Hz signal into the left channel FM Volumax only. Adjust
that channel's INPUT LEVEL cont rol so th at the meters on both
channels
are deflected to the red/green junctions.
c. Depr ess the left channel MODULATION SET switch and, holding it
in this position, turn the OUTPUT LEVEL control clockwise to
produce the
maximum desired modulation for that channel. Remove
the 400 Hz signal from the left channel.
NOTE:
This switch is spring-loaded. If one man is doing
proof of
performance or installation, a wooden
brace 1-5 /8 inches long can be used to hold the
switch in the down position.
d,
Perform steps a. through c. above, on the right channel.
e. Feed a 400 Hz signal to the l eft channel again and adjust the INPUT
LEVEL
control to produce a meter deflection approximately 1/4-inch
into the green region on the meter face.
f. Rem ove the 400 Hz signal from th e left channe l, apply it to th e right
channel and perform st ep e., above, on that channel.
g. A final check of output hum levels should be made. lithe hum in
either channel is higher when both wiits are operating together ,
as compared with uncoupled operation, reverse the AC power plug
of one of the units.
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~ LA ■ OIIA.TOIIIES
A slight re-adjustment of the INPUT LEVEL control may be necessary to accommodate a
particular station's normal programming . It should be noted here that the meter on the FM
Volumax is necessarily based on subjective cons id erations and that all users will not want to
operate with the same degree of limiting action. If the FM Volumax is set up for heavy lim-
iting action, hig her average modulation will result , but with a potential loss of brilliance at
high l evels. In genera l, FM broadcasters with wide -ran ge programming will operate with less
limiting action than FM or TV broadcasters, who normally transmit narrow-band material.
To re-adjust for normal programming, app ly a typical level monophonic signal to one channel
at a time. This signal should be representative of the station's normal program material.
Then adjust the INPUT LEVEL control to produce meter deflection in the green region.
Occasional high level, bri ght passages should drive the meter into the red region.
3-3 PROOF OF PERFORMANCE MEASUREMENTS
Generally accepted procedures for proof of performance tests usually call for making these
measurements "without compression." In the FM Volumax, the compression action can be
disabled by depressing the MODULATION SET switch. HoweverJ in so doing. care must be
taken to e nsure that th e audio pr og ram amplifier section of this device is not operated at lev e ls
significantly different from normal operation.
Although other methods may also be acceptable, the following suggestions are made:
a. Frequen cy Response:
l. With the MODULATION SET switch depressed, feed excessive lev el at
100 Hz to produce a clipped waveform at the output of the FM Volumax.
If the clipping level is at less than 100% modulation, turn up the OUTPUT
LEVEL control so that clipping occurs slightly above the 100% point.
2. Reduce the input for 100% modulation.
No clipping should occur.
3, M easure the system frequency response by noting variations required
at the audio channel input for constant modulation at specified modu-
lati on levels . Ti 1e resulting response characteristic should be lhe
in verse of foe standard 75-mi crosecon d pre-emphasis curve.
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~ LABORATORIES
(Do n ot attempt to feed constant maximum level at frequenc i es
above 1 kHz. The combinationof Voluma.x pre-emphasis,
lim i ted amplifier headroom, zener clipping, and the de-
emphasis network will produce a falling characteristic and
considerable distortion , even with the AGC action disabled.)
b. Distortion Measurements:
Rev. 5/67
H armon ic distortion in the FM Voluma.x has been mini mized for
operation in the normal limiting mode. It may n ot be qu i te as
low when the unit is operated at high level without compression,
but nevertheless , should be within required specificat i ons.
Distortion may be measured with the MODULATI ON S ET switch
depressed after the OUTPUT LE VEL cont rol has been adjusted
as described above.
3-5

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4-1 GENERAL
SECTION N
PRINCI PLES OF OPERATION
~ LABORATORIE S
FM Vo lu max is functionally similar to conventional peak limiters in that it prevents over -
modulation
of a radio frequency carrier by instantaneous program peaks. How ever, the FM
Volumax overcomes the handicaps inherent in peak limiters and also controls high level,
high frequency
signals which, because of pre - emphasis, could cause overmodulation.
Significant
differences in peak factors may exist between various program waves of the same
average level as indicated by a s tandard VU meter. When program with a high peak factor is
fed into a conventional limiter, it wi ll normally be passed at a lower gain than other program
waves with a smaller peak factor, even though both programs may have the same average level.
This discrepancy in volume level prevents the optimum utilization of transmitter capability
and handica ps the broadcaster who is trying to achieve maximum average modulation.
Another limitation
in the operation of a conventional peak limiter exists during the interval
immediately following a train of program waves with high peak factor. It is during this
period that the limiter is recovering from a condition of redu ced gain. Thus, for a second or
two, program will be transmitted at l ess than opt imum gain. Ji recovery time of the limiter
is reduced so that gain is in cr eased more rapidly during this period, then the familiar
"pumping" effect will be heard.
A thi rd problem peculiar to FM broadcasting, including TV audio, is the effect of pre-emphasis
on high level, high frequency signals. The standard 75 microsecond pre-emphasis curve lifts
3 KHz
signals by 5 dB and 10 KHz signals by 14 dB. Consequently, if a high frequency compon-
ent is present in the program signal at an amplitude similar to that of the mid and low fr e-
quency components,
th e pre-emphasis network will raise the lev el of the high frequency com-
ponent cons id erably above that of the majority of the program. Therefore, to prevent over-
modulation, the average level must be kept sufficiently low so that an adequate margin exists
to accommodate the occas ional high level burst of high frequency energy. The most common
sources of such signals are cymbal crashes, muted trumpets and sibilant speech.
4-1

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~ LABORATORIES
FM Volumax solves these problems in a precise and esthetically acceptab le manner . It
prevents over modulation of the carrier, while providing a hi gh level signal with no audible
distortion.
FM Volumax limits audio l eve l in three steps:
1. Limiting action operates on the low and middle frequency ran ge of the
program signal.
2. An automatically variable equalizer, the Dynamic Frequ ency Compensator,
operates on the high frequency range of the pro g ram signal. This action
is weighted by a pre-emphasis network that anticipates the effect of trans-
mitter pre-emphasis.
3. Final instantaneous diode limiting prevents overmodulation by initial trans-
ient peaks.
4-2 CffiCUIT OPERATION ( See Figures 4-1 and 4-2 )
Input
signal is applied through the input attenuator R-8 and input transformer T-2. Push-pull
amplifiers Q4-5, rectifier CRU- 12 and the variolosser network CR7- 10 constitute an AGC
l
oop which provides the fir st limiting action. Gain variation is achieved by changing the shunt
impedance of the variolosser diodes by the c ontrol signal fed through R24. High frequency
ro
ll-off in the rectifier driver amplifier Ql0-13 raises the effective threshold of limiting of
frequencies above 2 KHz. .
The first AGC amplifier maintains low frequency output at the correct level, but its attack
time of two millise co nds precludes operation on short duration peaks. In the event that
program material causes gain reduction in this loop, recovery is quit e rapid to prevent a
condition
of unnecessarily low gain. A recovery time of 140 milliseconds is used, coinci din g
with
th e syllabic rate. At this speed no audible pumping will occur, provided th e specified
limits of gain reduction are not exceeded.
In orde r to anticipate the effect of trans mitter pre-emphasis, high frequency signals are n ext
passed through a pre-emphasis stage Q200- 201 . Following this is a second AGC loop consis tin g
of
amplifiers Q202-203, rectifier CR200-201, 2U-212 and the Dynamic Frequen cy Compensator
4 -2

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~ L ABORATORIE 9
A400388. Unlike the variolosser of the first limiting section which adjusts the amplitude of
all frequency components of a complex waveform by the same amount, the Dynamic Frequency
Compensator
is a voltage- controlled equalizer which operates only on frequencies above 2 KHz.
Rectifier time constants for the l atter section are 2 and 10 milliseconds for "attack" and
"recovery" times respectively. H owever, it should be pointed out that these terms do not
describe normal gain reduction but rat h er changing response characteristics of the Dynamic
Frequency Compensator.
Program peaks which do not significantly contribute to the average power level of the signal,
and are therefore passed without gain reduction by either the vario l osser or the Dynamic Fre-
quency Compensator, are limited in amplitude finally by diodes CR208-209 at the output of
amplifier Q6-9. Instantaneous action is employed here to provide an absolute amplitude limit
for the passing signal, thus assuring that no over modulation of the carrier will occur. Follow-
ing
the output transformer T3, a de-emphasis network, complimentary to the pre -emphasis
stage Q200-201, is used to restore the low level frequency to a flat condition.
"
;!IQ 10:) :t $ 10001. t iO&QlU
HIU'lUthC't l fll CVQ £\ fl .Ac SECO#ID
Figure 4-1 FM Volumax Amplitude Response
Figure 4-1 shows the steady-state amplitude vs frequency characteristic of the FM Volumax.
For input levels 20 dB below the limiter threshold, the input-output curve is per!ectly flat.
Therefor e, any input frequency at this level is passed through a completely flat amplifier
before emerging to feed the transmitter . At the opposite extreme, consider a signal above the
threshold. The output
is in heavy gain reduction and the frequency response of the maximum
output
curve is the inverse of the response of the 75 microsecond pre-emphasis network.
Consequently,
the subsequent passage of the signal through the pre-emphasis network at the
transmitter will raise it to precisely 100 % modulation at all frequencies and no overmodulation
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~ LABORATORIES
will result . At an intermediate level, such as 10 dB below the threshold, the output will be
flat up to the frequency where pre-emphasis would normally cause overmodulation ( 7 KHz in
this case). Above this frequency, the curve will follow the balance of the inverse 75 micro -
secon d curve as shown in the diagram. Similarl y, at other pr ogram levels, only those fre-
que ncies which would ca u se overmodulation of the carrier after pre-emphasis are reduced.
Therefore, the resultant amount of limitin g -fr e qu en cy com pen sation of the FM Volumax is a
threshold function co nt ro ll ed by the ThlPUT LEVEL att enuat or. At the highest input levels
some reduction of high frequencies may be apparent. During quieter program intervals, or
if the input level is reduced, a lesser redu ct ion w ill occur . Control action is monitored by
the front panel meter . As shown in th e block diagram, resistors R220 and R221 pass portions
of the low
and high frequency control signals , respectively. Th e meter thus indicates both
con
trol actions, but no scale cali br ation can adequately describe the situation for all high
frequencies as well as for the low frequency co ntr ol.
tl,l'l/1 LYVt'l-
"tttnu, 11 ,,
, • .-r ,,,to ► ,,,
en t • > u,
__/
oarrr•~T I rvn
.......... ,, .. ,
""' r-n11,,,..-,~u
0"1fl,lUU(
rn1.11" ' '""
c.,,1111"-'""_.t,,1
n 221
'
$,111 t~
l ,1,n lll11· :
CR !13,11, .1&.' t
r.____
Figure 4 -2 . FM Volumax Block Dia g ram
tHwtt """Im "t
c· .. 111 •u1 1\1,11•
Q :UM 1(1<;,
110. lll
R:1 tU Jll• f"
t·~ iw.10,.
tll !It.
4-4

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5-1 ELECTRICAL AL IGNMENT
SECTION V
MAINTENANCE
~ LABORATORIES
The following steps are suggested as a comprehensive alignment pr oce dure. Such alignment
is usually performed to compensate for normal component tolerances. It should not be neces-
sary to completely re - align your unit unless certain component changes are made. Ho wever ,
an occasional meter realignment or distortion improvement may be required. For these pur-
poses steps I, 7 and 19 through 21 may be performed separately. ( See Figure 2-1 for potentio-
meter locations. )
l. Terminate the unit with a 600 ohm load and drive it with a 600 ohtn signal source.
Set INPUT and OUTPUT LEVEL controls fully cl ockwise. Allow unit to warm up
for 15 minutes before commenc ing el ectrical alignment.
2. Apply a 100 Hz signal at 1. 6 millivolts .
3. Adjust R219 for output of 0. 5 volts.
4. Switch to 10 KHz, maintaining constant input. Adjust R222 for output of 0. 5 v ol ts .
5.
Recheck output at 100 Hz and readjust R219 if necessary.
6. Check outpul at 1KHz. It should be 0. 5 volts± 1dB.
7. Adjust R227 for full-scale ( to the right ) meter defle c tion. ( This meter adjust-
ment may als o be made with no signal input. )
8. With input frequency of 100 Hz, increase input l eve l to ca us e meter to read at
red/green j uncti on.
9. Adjust R54 for maximum outp ut vo ltage.
10 . Rotate R62 fully clockwise.
11. While observing output at 100 Hz on an oscilloscope, increase input to determine
output level at verge of clipping. This should be 8. 5 volts ± 0. 5 dB . Re cord this
lev el for reference in Step 12.
12 . Alternately ad just R62 and input signal to achieve an output voltage 2 dB below the
o utput of Step 11, with the meter reading at the red / green j un ction.
13. D epress th e MODULATION SET switch and c heck for symmetrical clipping. Release.
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~ LABORATORIES
14. Increase input, forcing meter into red area. Clipping should not occur before
meter goes o ff-scale.
15 . Reduce input at 100 Hz 4 dB below that which produces a readin g at the red /g reen
jwtction. Note output voltage for St ep 17 .
16. Switch to 3 KHz. Place lOX oscilloscope probe on Point I with vertical sensitivity
at 100 mv / Cm, sweep speed 50 microseconds/Cm. Adjust R254 for equal ampli-
tude of alternate cycles. Verify that both sides of the rectifier are workin g by
noting
pulse separatio n. It sh ould be 150 microseconds.
17. Adjust R262 for output voltage 6 dB below Step 15 output ± 0. 5 dB.
18. Check 8 KH z, o utput should be 11. 5 dB below Step 16 output± 0. 5 dB.
19. Switch to 1 KHz and set input level to cause readi ng at the center of the · green
region. Adjust R22 for minimum distortion.
20. Switch to 5 KHz and adjust input signal for a reading at the center of the green
r egio n. Adjust R232 for minimum distortion.
21. Change frequency for 50 Hz, set input for center of th e green re gion, and adjust
R36 for minimum distortion. ( It may also be necessary to make a slight readjust-
ment of R54 . )
5-2 TROUBLESHOOTING
CAUTION:
Make sure all power is off before removing or in-
serting any transistors. If soldering of any com-
p onent is necessary, avoid overheating by using
standard heat-sink soldering practices.
Tr ou ble shooting of any ap pare nt malfunction of th e FM Volumax should begin with a check of
th e power supply. DC vo ltage, as measured with a VTVM, should fall between the upper and
lo wer limits as shown in Figure 5-1. Accidental shorting of the ± 20 V supply cou ld cause Ql
to develop a co ll ector -t o-emitter short ther e by impressing an unregulated +28 V at point D.
If this transistor is replac ed, be sure to install the heat sink on the new transistor. Check
for open filter capacitors in the e vent of excessive 120 Hz hum. If the power supply checks
5-2

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~ LA BO RAT O RIES
out satisfactorily, pr oceed with the following checks for possi ble trouble.
In the extreme case -- no output at a ll -- ch eck input and output connections to the printed
circuit boards for possible open leads. If this visual inspection does not uncover any defects,
stage-by-stage signa l tracing of the input and output boards ls n ecessary.
With the input and output l evel controls fully cl ockwise, feed in a 1 KH z 10 millivolt signal.
Th e signal at points P and Ron the input board should be about 0. 20 volts measured with a
VTVM
to groun d. U this vo lt age appears to be incorrect, l oca liz e the trouble by remoVing the
harness leads fr om P and R thereby eliminati ng any l oa ding effects by the Control and AGC
# 1 boar d s. V oltage at these points should now be 0, 6 to 0. 7 volts . Obtaining the correct
voltage at these points would indicate a malfunction of either the con tro l board or the AGC 4f. 1
board. H owever, if this measuremen t does not check, then the input board must be investi-
gated fu rther.
With the input maintained at 10 millivo lt s, the following signal voltages should be present:
Q2 and Q3 co ll ectors - 70 millivolts; Q4 and Q5 b ases - 10 millivo lt s. With correc t voltages
at Q2 and Q3, but incorrect at Q4 and Q5, remove harness l ead from point E and observe the
increase in lev el at Q4 and Q5 bases. U the increase is more than 4 dB , the quiescent control
vo l tage is incorrect. Th is may be due to an i ncorrect setting of R219 or a failur e of Q204 or
Q205.
If transistors Q4 or Q5 a.re defective, replace them with a pair of transistor s of beta matched
to within 20%. If it is n ot practica l to ma tch b etas, select replacement transistors such tha t
the
collector DC vo lt ages of Q4 and Q5 differ by less than 1 volt.
Diodes CR7
and CR8 a re matched as are CR9 and C RlO. To match a replacement diode, feed
1
mi lli ampere from a regulated power supply through a high resistance in se r ies with the diode.
Forward v oltage drop across th e diode under test must be within 10 millivolts of the ot her diode
under the same condi tions.
With the input at IO millivolts and 1 KHz, the signal at points P2 and R2 should be approx -
5-3