OVER A LL RECE IVER SENSITIVITY MEASUREMENT
An o erall m easurem ent of sen siti ity is made as fo l
low s:
1. Connect an AM signal generator to the input term inals
of the re cei er ch assis after rem o ing the short 300 ohm
lead which conn ects to the antenna input strip on the back of
the cabinet. To m atch the generator to the re cei er input,
a resisti e m atching netw ork should be used. In the case of
a generator with a 50 ohm output im pedance, for exam ple,
place a 100 ohm resisto r in series with the output term inal
of the generator and a 150 ohm re sisto r in se ries with the
ground term inal.
2. F rom cathode of picture tube to ch assis connect a c a li
brated oscillo s co p e.
NOTE: T o calibrate scop e, conn ect it a cro ss 6.3 olt fila
ment supply. The p eak -to-peak amplitude on the screen w ill
then be approxim ately 18V (6. 3x2. 8).
3. Set contrast con trol for m axim um sen siti ity.
4. Tune signal generator to the ideo c a rrie r frequ ency of
the channel being checked. G enerator signal should be 30%
m odulated at 400 cy cles. The signal fro m the generator to
produce 20 olts peak -to-peak at picture tube cathode should
be less than 25 m icro olts on channels 2 to 6 and less than
75 m icro olts on channels 7 to 13.
CIRCUIT DESCRIPTION
The circu it description appearing in the TS-89 ser ice
manual applies to the T S-172 chassis with the follow ing ex
ceptions:
1. C -17 , the RF to m ixer coupling condenser, is 6 m m f in
stead of 8 m m f.
2. In som e sets, V -6 (6A L 5) the ideo d etector, is replaced
by a 1N60 germ anium crystal.
3. C apacitor C-41 and resisto r R -32, a part of the cathode
com pensating netw ork on the T S -89, between the ground end
of the con trast con trol and the low est tap^ha e been rem o ed.
L -17 is changed to a red -g reen dot coil.
4. In som e sets, a 6CB6 tube is substituted for the 6AH6 in
the ideo am p lifier stage (V -7 ). The screen re s isto r, R -35
changes from 33K to 22K with this substitution. Another
substitution for the 6AH6 is a 6BC5. The screen resisto r
R -35 is changed to 22K with this substitution also.
5. An additional RC netw ork, consisting of R-100 (470K)
and C -92 (100 m mf) has been added in the clipp er input c ir
cuit. This network, w hich has a short time constant, helps
to m inim ize the effect of noise pulses at the clipp er grid.
I6. In som e sets, a 6SN7 tube is substituted for the 12AU7
ertical sw eep generator (V-13). The second m ulti ibrator
grid re sisto r, R -63 , changes from 330K to. 100K with this
substitution.
7. In som e sets a 6AS5 tube is substituted for the 6W6 e r
tical output tube. No com ponent changes are required with
this substitution.
8. In som e sets a 6H6 tube is substituted for the 6AL5 h or
izontal phase detector V-15. No com ponent changes are r e
quired with this substitution.
9. The dam ping re sistor a cro ss oscilla tor coil L -4E was
changed from 2200 ohm s to 3900 ohm s to stabilize the o s c il
lator on the low channels.
10. In som e sets, 6AG5 tubes are substituted for 6AU6
tubes in the first and second IF am plifier stages. L -ll, the
first IF coil, L-12, the low frequency trap, L-15, the s ec
ond IF co il, and L-14, the high frequency trap, are changed
in alue with this substitution. See parts list. R-13, the
dam ping resisto r acro ss T -l , changes from 6800 ohm s to
4700 ohm s. R-14, the first IF stage cathode res isto r, is
bypassed with a 1500 m mf capacitor and R-14 and R-19, the
second IF stage cathode re s isto r, change from 47 to 56
ohm s with this substitution.
11. In som e sets, a 6AG5 is substituted for the 6AU6 in the
audio d ri e r-lim iter stage. This is an exact replacem ent
and requ ires no circu it or com ponent changes; how e er,
substitution of one for the other in the field requires r e
touching the alignm ent of the p rim ary of the discrim in ator
tra n sform er for best perform a n ce.
12. R-15, R -20, C -24, and C -29 are rem o ed . R-14 and
R-19, the cathode resisto rs for V -3 and V -4 , are grounded
directly. The su pp ressors are also grounded.
13. The ideo am plifier B plus supply was changed from
the 240 olt bus to the 265 olt bus and the audio am p lifier
B plus supply was changed from the 265 olt bus to the 310
olt bus. The bootstrap cap a citor C -83 , which was conn ect
ed to the 240 olt bus is now conn ected to the 310 olt bus.
14. In later sets, the horizontal output tube screen re sisto r
R -94 , is returned to the load side of the fuse F -l .
15. It has been found that certain pow er tra n sform ers
raise the "B " oltage 35 to 45 olts too high. In order to
afford a m easure of p rotection for the 6BQ6 and 6W4 in
these instan ces, a resistor R-104 has been placed in se ries
with the speaker field.
This condition arises only with certain issu e "O ", "A ",
or "B " tra n sform ers (issue designation follow s the part
num ber stam ped on tra n sform ers). In issue "C " tra n sform
ers, which are currently being produced, this d ifficulty has
been rem edied and the re sistor is no longer n ecessa ry .
In future replacem ents of tra nsform ers, a ch eck should
be m ade of "B " oltage. If it is found to be low after the
new transform er has been installed, rem o e the re sistor
mentioned abo e. If "B " oltage is found to be higher than
norm al, insert co rr e ct R-104 resistor in se ries with speak
er field, as shown in schem atic diagram . C orre ct part
num bers for these resisto rs w ill be found in R E PL A C E
MENT PARTS LIST.
The tube substitutions w ere necessa ry because of the
critica l shortage in som e tube types. These substitutions
do not affect the operation or circu it theory.
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Scanned by mbear2k - Jan 2012
