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Old 05-15-2018, 12:25 AM
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Zenith 25DC56 lost audio

This is the Chromacolor that I got at an estate sale in April of 2016. I had it repaired in May of that year, and it has been working perfectly since then. Just yesterday, for no reason, I turned the set on to watch The Simpsons, and the audio was almost totally quiet. I found that if I turned the volume control up rapidly, I could get some weak, scratchy audio for a second or two, then it would fade away. I tried that again tonight, but there was even less audio, if any at all. I checked the obvious first, checking the speaker connections, then I tapped around on the audio module that sticks up and was able to get some loud humming when I tapped certain places with my screwdriver. I then tried moving the IC to reseat it in the sockets, and that didn't do any good either. I have another set with this chassis I could pull the IC out of, but it's inaccessible at the moment.
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Old 05-15-2018, 07:03 AM
zeno zeno is online now
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Most common on these & hybrids was the output transistor.
Usually would make popping noises first. Can use 2 different
pks's that are interchangeable. OEM's were a kit that also had a VDR
for improved reliability. IC's also went sometimes 221-45 ?? easier
to change too. IIRC gotta go under for output.

73 Zeno
LFOD !
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Old 05-15-2018, 10:16 AM
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Later today I'll try to snap a pic of the audio section of the schematic. Voltage measurements should be telling.
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Old 05-15-2018, 11:35 AM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by zeno View Post
Most common on these & hybrids was the output transistor.
Yuppers. On all makes and models that used the high voltage type transistor. Used to know the Sylvania and RCA numbers by heart, but can't recall them now. Been a lotta years.
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Old 05-15-2018, 02:43 PM
zeno zeno is online now
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ECG124 or ECG157 IIRC.
Boy is it getting hard to remember.

73 Zeno
LFOD !
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Old 05-15-2018, 06:43 PM
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Quote:
Originally Posted by zeno View Post
Most common on these & hybrids was the output transistor.
Usually would make popping noises first. Can use 2 different
pks's that are interchangeable. OEM's were a kit that also had a VDR
for improved reliability. IC's also went sometimes 221-45 ?? easier
to change too. IIRC gotta go under for output.

73 Zeno
LFOD !
Yes! That was one thing I had to deal with on this set. It was like it was an extremely dirty control. It actually got better before it broke, to the point where the popping went completely away.
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Old 05-15-2018, 09:41 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by zeno View Post
ECG124 or ECG157 IIRC.
Boy is it getting hard to remember.

73 Zeno
LFOD !
ECG124 sure rings a bell. Also RCA's SK3021.
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Old 05-16-2018, 12:46 AM
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Here is the audio portion of the schematic. The output came out a bit blurry...I'll retake those tomorrow if you want.
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Old 05-16-2018, 01:44 AM
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How did YOU end up with a schematic for MY television??? These are the questions we should be asking.
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Old 05-16-2018, 10:23 AM
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Quote:
Originally Posted by TUD1 View Post
How did YOU end up with a schematic for MY television??? These are the questions we should be asking.
IIRC my Avanti is also a 25DC56...Also as a Zenith person when the chance to own two books, one containing all their color schematics from the first through the end of the upright modular era, the other containing some of their full manuals for some color TV chassis I could not resist. I cannot pass up the chance to obtain service literature when available and in my budget.
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Old 05-16-2018, 12:55 PM
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Unfortunately, this set will have to be placed on the back burner. My CTC-28 died last night for absolutely no reason, and that's what I'm going to have to worry about for the time being.
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