PA-7E right channel cut-off for seconds - diyAudio
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Old 30th November 2015, 07:52 AM   #1
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Default PA-7E right channel cut-off for seconds

Hi all
I'm new here, please excuse my rather poor swiss-english.
My PA-7E worked fine for the last few years (with EV Sentry 500) but now after playing for about 15 minutes, i had a right channel cut-off (right LED (red) was on). My PA-7E already has these aditional 1x1" PCBs. After a first try with the original crap-trimpots i replaced them with 20 turn 200Ohm and 5kOhm and could easily set the bias correctly, i have steady 40mV after 1-2 hours with cover closed. The right channel temp. (heatsink) is about 43C (109F), this is slightly higher than the left channel (40C /104F).
B U T, when playing now, after ca. 10 minutes, i still have have a right channel cut-off, but only for a very short moment (1 second, next time maybe 5 seconds), the red LED does NOT light. Before disassembling too much, my question would be: where to start best with the fault finding ?
Your help is very much appreciated, thanks in advance for every input to get my baby healthy again. Regards Walter
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Old 30th November 2015, 08:09 AM   #2
Mooly is offline Mooly  United Kingdom
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I'm not familiar with the amp or its circuitry but the two things to look at...

1/ Monitor the DC offset at the output before any speaker relays etc and see what it does when the amp trips.

2/ The symptoms seem to describe either a dry joint somewhere or a transistor failing intermittently open circuit (usually conformed by seeing a massive rise in the base/emitter voltage)
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Old 30th November 2015, 02:48 PM   #3
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Quote:
Originally Posted by Mooly View Post
...Monitor the DC offset...
Hi Mooly
Thanks for your quick reply.
Measuring the DC offset will get difficult without an oscilloscope, i think....
I have a multimeter, of course and this one to check th transistors:
Peak Electronic Design Limited - Atlas DCA - Semiconductor Analyser - Model DCA55
and a second from peak to check capacitors.
Regards Walter
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Old 30th November 2015, 05:21 PM   #4
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I don't have a PA-7E but have looked at the service manual.

When you changed the trimpots, did you follow the instructions from the service manual when setting the bias?

And it is the PA-7E (200 wpc) version you have and not the PA-7E II (225 wpc) , the latter doesn't have DC-offset adjustment (tho it's called DC Balance adjustment in the service manual.).

The red leds you mentioned must be the clipping indicators, one per channel.

For all I can see there is no indicators for the protection circuit more than that you'll hear the click from the relays when they goes off or on.
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Old 30th November 2015, 06:19 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bongobongo View Post
Hi Mooly
Thanks for your quick reply.
Measuring the DC offset will get difficult without an oscilloscope, i think....
I have a multimeter, of course and this one to check th transistors:
Peak Electronic Design Limited - Atlas DCA - Semiconductor Analyser - Model DCA55
and a second from peak to check capacitors.
Regards Walter
A meter is fine for normal offset and bias measurement. The problem with the transistors is that the failure mode can be intermittent, even the very act of physically moving it or unsoldering it can cause it to 'heal' for an indeterminate time. I'm not saying that this is definitely what the problem is but its a failure mode I've encountered time and again in consumer electronics. The only way to find it is to carefully measure DC voltages when the amp is in the faulty state.

Quote:
Originally Posted by Mannegizen View Post
I don't have a PA-7E but have looked at the service manual.....................The red leds you mentioned must be the clipping indicators, one per channel.
Interesting. Clipping LED's could well detect when the output approaches either rail, and that would tie in with a DC offset fault condition.
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Old 30th November 2015, 06:20 PM   #6
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Hi Mannegizen
Yes, inputs shorted, dc at speaker output balanced to around zero mV and bias set to 40mV, i did this at least 6-7 times until i had it a steady 40mV and 0 mV at the ouput, with cover and after power on for at least 1 1/2 hours. output relay works fine, also soldered out the thermal switch and tested it in the oven; works fine. And yes, the red led is the clipping indicator. This clipping led (right channel) was red all the time and there was no signal at the out also all the time when i first had this issue. Only After setting the bias it changed as described to cut-off for seconds only and with no clipping led on. Regards Walter
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Old 30th November 2015, 06:30 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bongobongo View Post
dc at speaker output balanced to around zero mV
Under fault conditions, you have to measure the DC before any speaker relay. If there is DC offset, the relay opens and the speaker voltage will be zero but the voltage on the other side of the relay could be high.
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Old 30th November 2015, 06:34 PM   #8
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Originally Posted by Mooly View Post
....The only way to find it is to carefully measure DC voltages when the amp is in the faulty state.....
Hi Mooly
That's indeed a problem; the faulty state is for a few seconds only, to be at the wrong place in the right time (ooh, that's Dr.John almost...) is almost predictable i think. If one you experienced guys has not THE idea of where to start with checking (unsoldering) components, i have no other chance than to propably start with the small additional protection board (?)
Regards Walter
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Old 30th November 2015, 06:38 PM   #9
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Originally Posted by Mooly View Post
....Under fault conditions, you have to measure the DC before any speaker relay......
I agree Mooly, will do this as a first step and come back then.
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Old 30th November 2015, 06:48 PM   #10
Mooly is offline Mooly  United Kingdom
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Yes, the first step is to see if the problem is a DC offset issue.

I've never seen one of these amps, and haven't even seen the circuit of one and so I don't want to lead you up the wrong path but that said these kind of faults are likely to be either a dry somewhere or a transistor that runs hot which could typically be the VAS (voltage amplifier stage) of the circuit.

If the fault is intermittent and you can't get it to do it then you sometimes have to second guess the cause and replace the suspect parts 'on spec'.

So look for semiconductors that run hot, look for areas on the board that may be discoloured through heat... these are the parts and areas that are going to be most suspect.
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