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Old 14th December 2007, 08:50 PM   #1
Kolloi is offline Kolloi  United Kingdom
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Unhappy Class AB amp - left channel completely dead. Please help!

Hi everyone

I have a late-70s JVC tuner, which has an amplifier section. This is a stereo class AB amp based on transistors, putting out 15 watts per channel.

Anyway, the problem is this: the left channel isn't putting out any signal AT ALL. No hiss, no crackle, no hum, no weird stuff. Just nothing at all. There's nothing coming out. This problem happened overnight, and I didn't notice it (the speakers are close together) until I had been listening for about half and hour today. There were no warning signs or signs of failure at all, before this happened.

So, I opened it up and I find that there are 2 small capacitors of equal rating, 25V 100uF, and one of them is leaking from the bottom and the other isn't. I ordered some new ones this evening to replace them both, but is this likely to be the sole cause of the problem? I.e. will the amp start working normally when they are replaced?

I really fail to understand how one channel can just stop working without any warning signs whatsoever after 30 years of perfect operation. What's more, there are 7 fuses, all original, inside the amp section and not a single one has blown, which I find very odd. Anyone have any ideas?

Thanks in advance
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Old 14th December 2007, 08:57 PM   #2
Netlist is offline Netlist  Belgium
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Call your self lucky after 30 years.

Could be anything, really, but look for the obvious first:
-Switches on the front, producing bad contacts in one channel.
-A loose speaker wire.
-Bad solderings. Spend some time on this, in particular the parts that suffer from thermal stress.
-Last but not least, a bad speaker. Switch left and right speakers to make sure.

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Old 14th December 2007, 09:51 PM   #3
AKN is offline AKN  Sweden
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one of them is leaking from the bottom
kolloi, you cleaned the PCB thoroughly? Important as electrolytic is highly conductive.
/ Anders
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Old 14th December 2007, 09:55 PM   #4
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Default Hi!

Model, Kolloi, model ?

Regards zeoN_Rider
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Old 14th December 2007, 11:12 PM   #5
Kolloi is offline Kolloi  United Kingdom
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Good news! It's working!

The culprit was a loose connection on the circuitboard - there was a capacitor in the bass boost/cut section that needed wriggling only once, then it worked again.

The model is JVC SR-50 (black). I use it as a general purpose amp, and never actually use the radio receiver. It's connected to two large Sony speakers from 1972, and a few other newer no-name speakers (midrange and tweeters) which I've rehoused inside cabinets I made myself from multi-ply and stuffed with insulation. There are probably too many speakers attached, but it's never seemed to have any ill effects on the unit. When the left channel quit, I thought that maybe it was because it had been overworked by the number of speakers, but luckily it was nothing of the sort.

Thanks for the helpful suggestions.
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