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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 10th Apr 2018, 6:51 pm   #1
Tractorfan
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Smile Uher audio amp oddity.

Hi,
With the forum's (fora?) help. I managed to resolve a motor driver fault on my Uher 4000-L.
Today I noticed another fault with the audio amplifier. The volume was very low, but not distorted. I took the amp out and tried replacing a couple of electrolytic caps. This seemed to restore full volume (or so I thought), but it soon returned to its low level. It appears, however, that if isolate the AC187K heatsink from the chassis, it works well, but putting it back in contact diminishes the volume once more. I can't measure any obvious short from the transistor to the heatsink (one of those aluminium block jobs), nor can I see any obvious bad connections or dry joints.
I will try mounting the heatsink on a mica insulated washer and see what happens.
It's a pity that it's not the complimentary AC188, as I have a spare one of those. Curses!
Cheers, Pete.
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Old 11th Apr 2018, 9:41 am   #2
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Default Re: Uher audio amp oddity.

I would use a scope to see if whatever you are doing
isn't causing an ultrasonic oscillation. Such can sap power.
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Old 11th Apr 2018, 10:27 am   #3
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Default Re: Uher audio amp oddity.

Classic symptoms of a leaky transistor.
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Old 11th Apr 2018, 11:43 am   #4
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Default Re: Uher audio amp oddity.

Or the onset of tin whiskers?
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Old 11th Apr 2018, 4:26 pm   #5
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Default Re: Uher audio amp oddity.

Years ago when I serviced Phillips dictation products some AC187 transistors sometimes shorted "base to case". I did the same as you have done here, isolating the case from the heatsink, and all worked well again.
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Old 11th Apr 2018, 8:51 pm   #6
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Smile Re: Uher audio amp oddity.

Hi,
I read somewhere that 'tin whiskers' could be vaporised by strapping the three leads together, then zapping the transistor with a charged up elec cap between the leads and the case. Is this likely to work? I think it was tried with some AF117s.
Failing that, I'll stick to the isolated heat sink idea.
Thanks for your replies so far.
Cheers, Pete.
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Old 13th Apr 2018, 12:34 pm   #7
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Default Re: Uher audio amp oddity.

Yes, it's a common fix; I've done on several occasions with success on the driver transistors in Tandberg models 12 and 1200X as well as in a Philips machine. The remedy is not permanent as the whiskers are bound to grow back at some point but at that time it can be repeated. Isolating the transistor also works, but again might be temporary if another whisker grows to short the case to another electrode.
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Old 16th Apr 2018, 11:07 pm   #8
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Default Re: Uher audio amp oddity.

I am contemplating replacing the output amplifier boards on my 4400 with a modern low wattage amp board.There are several on a certain auction site that will fit in the same spot. Haven't got round to it yet as the machine is really used just as a curiosity and played through the HiFi. Results are remarkably good, not that far behind my rebuilt Revox A77 mk IV. The Uher has had all the electrolytics replaced btw, well worth doing.
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Old 13th May 2018, 7:19 pm   #9
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Smile Re: Uher audio amp oddity.

Hi,
After a break away, and thanks to a rainy grey Spring afternoon, I managed to get back to the audio amplifier. I suspected that one of the output transistors had grown 'tin whiskers' because when I isolated its heat sink from the chassis the volume came back up to normal.
This afternoon I tried zapping the transistor with a 1000uF cap charged to about 18 volts between the case and the three leads which were connected together.
Alas, it didn't work, so I've resigned myself to keeping it isolated via a mica piece with a touch of silicon compound and a small cable tie. So far, so good.
Mind you, I can't see any continuity reading between any of the leads and the case, so it might not be tin whiskers after all.
Oh well, as long as it keeps working.
Cheers, Pete.
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