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Old 18th Jan 2021, 8:02 pm   #1
samjmann
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Default Sony ST-88. FM Tuner fault

I've never worked on what many call one of these 'cute' little tuners before.
This is the original version I believe, there is no suffix after the 88.

The fault is that it will go silent on FM. The tuning meter goes to zero, then a second or so later it bursts back into life. Tuning and sensitivity both return back to normal. When its working the FM alignment is very good. The stereo decoder works on weak signals. The AM section is unaffected by all of this.

On the circuit snipit Q103 (FM osc), this is the only stage where there is a significant change in the DC conditions when the unit faults.

Q103 collector load resistor has normaly 0.6v across it (270 ohm), this is approx 5mA in normal working.

Q103 Fault conditions. R103 current goes up to 7.3mA. The base of Q103 falls from 0.6v to almost zero...

All I can think is the transistor is going leaky intermittently. (2SC710).

The problem is getting to it to change it. It's buried in a little box next to the tuning gang with L103 adjacent to it. What I don't want to do is change the transistor to find the osc frequency way off. I suppose it would then mean a tweak of the osc coil to pull it back to calibration. I'm not sure if this coil would move after all this time.

Q103 is the transistor next to the red coil in the last picture.

I haven't got a FM sig gen to set it up with either... Also it looks as if some of
the components in the osc can have been lacquered....

Any thoughts on doing this? Anyone worked on these in the past?
To get to it I'll have to the whole 'box' open to get access to transistor!!

Thanks for any help. SJM.
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Old 18th Jan 2021, 8:36 pm   #2
Ted Kendall
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Default Re: Sony ST-88. FM Tuner fault

I think it was the similar ST80 which had the F suffix. I've had trouble with Sony kit of similar age where signal came and went with accompanying crackle. I found a softish black deposit on the transistor leads, some of which had spread to the transistor body. Careful scraping and cleaning with IPA fixed the problem. I suppose some conformal coating on the leads would stop the problem coming back.
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Old 18th Jan 2021, 9:11 pm   #3
samjmann
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Default Re: Sony ST-88. FM Tuner fault

Thanks Ted, I'll look carefully for what you've mentioned.

I've got a replacement transistor on order, I only want to open the thing up the once. I can't understand how the transistor would break down on and off every few seconds. A tiny spot of freezer stopped it working for a good 5 mins, it started back on it's own afterwards.

When I first plugged the unit in, it ran for 4 hours before showing any fault. Now it's cycling every 10 secs or so.

SJM.
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Old 18th Jan 2021, 9:15 pm   #4
ms660
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Default Re: Sony ST-88. FM Tuner fault

Quote:
Originally Posted by Ted Kendall View Post
I think it was the similar ST80 which had the F suffix. I've had trouble with Sony kit of similar age where signal came and went with accompanying crackle. I found a softish black deposit on the transistor leads, some of which had spread to the transistor body. Careful scraping and cleaning with IPA fixed the problem. I suppose some conformal coating on the leads would stop the problem coming back.
The black death, it was happening back in the 1970's when I worked on Sony stuff, not restricted to Sony transistors either.

Lawrence.
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Old 19th Jan 2021, 5:36 pm   #5
Phil G8DIY
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Default Re: Sony ST-88. FM Tuner fault

I used to work for Sony. Most products had to be disassembled to repair a Japanese trait.
We used freezer and a soldering iron to heat and cool if we suspected a component.
It is a long time since I worked on one of these. Whilst transistors generally were not too bad. The capacitors however used to give a lot of crackling or other intermittent issues.
Not certain if this model had one but many had a ceramic IF filter that used to die.

Phil
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Old 19th Jan 2021, 7:56 pm   #6
samjmann
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Default Re: Sony ST-88. FM Tuner fault

Thanks Phil, I think the Japanese at that time thought that as their products seemed to be so inherently reliable, access was not a major priority...!

The clue in this seems to be the disappearance of the osc base volatge, and the increase in collector current at the same time. I'm going to soak the transistor in electronic cleaning solvent and leave it a few hours to evaporate. Hopefully this might break down some of the 'deposit' that may be on the transistors legs. I appreciate that this is just a temporary measure, and not a complete fix.

Regards, SJM.

P.S. Yes Phil, it's got 3 or 4 ceramic filters in it!!
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Old 21st Jan 2021, 5:27 pm   #7
Phil G8DIY
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Default Re: Sony ST-88. FM Tuner fault

If I recall a BC107 or BC171 were close equiv.
C112 is worth a look as well.
I don't have any old transistors of that type left in my odds box - shame
Phil
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Old 26th Jan 2021, 10:14 pm   #8
samjmann
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Default Re: Sony ST-88. FM Tuner fault

Quick update:

Replacing the oscillator transistor seems to have worked. Have left on for 3 hours and it's stayed on tune without AFC and no drift.

It was quite a job to get apart, carefully separating the tuning dial from the tuning gang to get to the oscillator section. Reassembly was almost as bad. I wondered if the calibration would change. But Radio 2 and Smooth FM 106.7 came in where they should.

The old transistor was covered in the 'Black Death' as Lawrence and Ted suggested. Top marks gentlemen!

Thanks everyone for all the advice.

SJM
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