Zenith Y504B - Page 13

Tuning the receivers on the frequency modulation band INTEGRATED CIRCUIT (I C ) 221-32
SstS'XXyTnX XJ VaXSeXp.* T, »,T “ • ° 22 « 2 to ■»«. K".it -J
quench RfoddatlM stations This is noXTaS
disappear as the station is tuned in. After a station is voltac^ di n „M £» -r • i \ c o <7
o°ver te k unH th^- ^t and L This wB1 determine if the proper voltage - ’ are
over it until the point of quietest reception and best be i ng applied to this I.C. S
tone quality is found. Correct tuning is indicated by the
disappearance of background noise. These voltages as a group can vary ±5%. A further test
to definitely determine if the I.C. is functioning proper-
’ ly is to insert a 12Q millivolt peak-to-peak 10.7 MHz
signal at Terminal 5 of T4, 4th I.F. transformer. If your
SPEAKER PHASING generator cannot supply 120 MV of signal, its output
t . .. . . x x , ... can be fed into previous I.F. stages that will act as
nUtd ° to ‘ ZT C i° d SP6a J 63 ? C ° n ’ ^PHfiers to supply 120MV at Terminal #5 of T4. With
nntarit . d 1, term T ° n speaker , s f ° r P r0 P er this input voltage, which will be insufficient to cause
polarity within each speaker group. It is also then most limiting in the I.C., one should expect to obtain 1 volt
important that the speaker S roa ps be in phase with peak-to-peak of 10.7 MHz signal at pin 2 of T5, ratio
rorn A her - One ax c elleat method is to play a monaural detector transformer, measured with a high frequency
,nX o^t? SC d U ? Aut .° matlC Balance Contro1 ’ low capacity scope having an input resistance of no
in the customers operating guide. less than one megohm . / hig inp P ut . output signal
Under these conditions the sound should appear to i n addition to the voltages should be sufficient for the
come from a point midway between the two speaker technician to determine if this I.C. is functioning prop-
groups. If the sound comes from any other point than erly. l f not > 1* can be replaced with a new 221-32 I.C.
midpoint, then one speaker group is out of phase with n n r ; r n„if rliot,,™, • i j
lh. other and you should cheek polarity. One of the 0 X ^^"the iS ? network S™," i "ff
easiest methods of checking polarity within the speaker not inte nded that any attempt be made to substitute the
t g h7 P snZXr m 7 e d t 1 y - Pl T 6 V aCr ° SS man V components, since this from the physical sense
iT k u d t , ermlnals : AU the s P. eaker cones is impossible. Should the technician find that the I.C
should simultaneously move in the same direction. is not f unct ioning properly, the only practical repaid
solution is to replace it with a new one.
POWER AMPLIFIERS AND ,F ALIGNMENT - CHASSIS 18YT20,
D . ♦ Lu- - 20YT20, 23YT124, 26YT20 AND 26YT21.
Power transistors and their circuits are unique in oper- A1 . , c ., , . ....
ation, therefore, repair procedure differs from those Alignment of these chassis will, in most cases, not be
steps followed when repairing tube type circuits. necessary unless an RF or IF transformer is replaced
or lf someone has tampered with the adjustments.
1. Each channel of the push-pull amplifiers use a pair D £ - j < , , .
of matched power transistors in the final output because of the wide band pass required in the multi 
stage. Therefore, should one transistor fail, both P ** ™ tUner ’ ls . daSlrableasa an ™ S1 8 nal gen
transistors must be replaced simultaneously, since ® ha ? ng a 400 KC with a sweep rate
they will not perform properly unless matched. wuS pr ™ 30 ° SClllo f c °P e when all S nin g
both the IF and RF FM portions of this receiver. It is
2. When a power transistor using an insulator between not only necessary to obtain maximum amplitude in the
the transistor and the heat sink is replaced, the IF amplifier stages, but also necessary to maintain
heat sink should also be replaced. Be certain to symmetry. To help achieve this symmetry, it is desir-
apply Dow Corning #340 heat conductive grease able to have 10.6, 10.7 and 10.8 megacycle markers in
between the transistor and the insulator. obtaining IF curve symmetry.
Also between the insulator and the chassis. The ™ , .. , r .
Dow Corning grease can be obtained in 1 C.C.quan- The 5° ndens u er mentioned further on in the alignment
tities by ordering part #205-51 procedure should be as small as possible and the
ground lead of the generator must be connected to the
3. On some chassis place the heat conductive grease chassis at the base of the socket, where the signal is
in the detent of the chassis, all around the transistor being injected. Should the signal be injected at some
and also into the detent in the combination heat point other than a socket, then the ground lead should
sink and retaining bracket. be connected to ground as closely as possible to this
point.
4 ‘ Soe ” kl Tri 16 th6Se ampHfierS With ° ut th6ir pr ° per alignment procedures, the signal generator output
p a r ° a ’ should be kept just high enough to obtain an indica-
5. Do not short out the audio output of either channel ^ on * This is most necessary, since on some chassis
when the amplifier is Qperating. we have a zero time constant limiter which will clip
cl cl . L £ .1 z i i . the signals if their magnitude is too great, resulting in
6. Should a power transistor fail (short) be certain to erroneous waveforms.
replace the emitter resistors for the specific channel.
Also be certain to check the condition of the A - On chassis 18YT20, 20YT20, 23YT124, 26YT20
silicon diode rectifiers. and 26YT21 connect scope or V.T.V.M. to junction
7 0 x . . £ xl _ . * 2 £ . of 10 ohm resistor and .001 mfd capacitor Test
7. Remove transistors from their sockets before doing Point rm P I
any solderin, to the socket t„ B s. “ .X oXXXoXetXXSXer* 6 ' 1 ,O
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