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Mote 1st October 2012 08:33 AM

JBL CS50.4 one channel clipping
I have a JBL CS50.4 amplifier with one channel clipping.

When I received the amplifier it would go into protect, the muting transistor was shorted. Also two of the output transistors were shorted.

There are problems with two channels, lets start with this:
When input signal is 100Hz sine, then on speaker terminal I have a square.
That is with filters set to flat, bass boost off, volume to minimum. When I turn down the volume on signal source I get just DC on the speaker terminals, sometimes 7V, sometimes 22V (rail voltage). I do not know what makes it change from 7V to 22V.

I have tried replacing transistors circled in red with transistors from working channel with no success:
There is a clean sine on the bases of Q135 and Q136 (Circles hide the designations, sorry).
Square is already on the drivers of the outputs, not only on outputs. There are both negative and positive rail voltages present.
I have tried lifting muting and protect jumpers, no effect.

Perry Babin 1st October 2012 09:24 AM

Do you read ~0 ohms between the emitters of the output transistors?

Does one ed of R144 connect directly to the emitter resistors?

Does the other end connect directly to the bases of Q137 and Q138?

Mote 1st October 2012 06:22 PM

Does one end of R144 connect directly to the emitter resistors?

One trace was broken, fixed it. Now that channel produces clean and loud audio.
I will have to put the other parts back in the other channels and see where I am at with the other problems and then we can troubleshoot the amp further, if neccessary.

I have been searching for the problem for a week and you found it with two questions, amazing. I would have seen the problem sooner but those dark brown boards with black traces are a nightmare if there is a issue like this.

Mote 2nd October 2012 06:47 PM

Three channels are fine now but one channel still has a problem: outputs got hot very fast. One set started smoking within seconds. Then I replaced drivers and tried another set, they did not get hot at all, voltages on their legs: 0V, 22V, 0V. Tried listening music through that channel, volume was lower than on the other channel which we repaired earlier, although amp was set up the same way. Also the audio was a bit distorted, then I wiggled RCA plugs a bit and the audio became clean and louder. Then I noticed that outputs are getting hot again (amp was out of the sink, so maybe it was normal). I think that there is intermittent contact on one of the RCA shields.

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