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Old 6th July 2009, 02:04 PM   #1
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Default Power amp with no output at speakers

Hi Everyone. I'm new to this forum and it's been over 20 years since I've built any audio equipment. I have 2 mono power amps that were designed and built for me about 30 years ago by my uncle. I actually did most of the building with him telling and showing me what to do. They have been in storage for about 12 years and I just got them out to test them. They were rated at 400 Watts into 8 ohms and would easily run my AR-9 speakers (4 ohm) all day long. When I tested them, one worked perfectly but the other one produces no sound at all. I tried each one through the same channel of the preamp with the same wires to elliminate that as a cause. I read the voltage just before it goes into the circuit boards and both amps matched, so the power supply seems to be working fine. I checked all fuses and all have continuity. I took some readings in the curcuit board and found voltage and current at many of the places their should be, yet no output at the speaker terminal. What might cause no sound at all? If something were blown. I would think you would get at least some distorted sound. But I get total silence. These were really nice sounding amps and I'd love to have them both working again as as I just finished restoring my AR-9's. Their are 12 very large mosfet transisters in each amp. Is their a way to test their output? I've ben reading through many of the posts looking for answers, but can't seem to find the right one. Thanks
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Old 6th July 2009, 02:32 PM   #2
h_a is offline h_a  Europe
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Quote:
yet no output at the speaker terminal
Bad solder connection or broken wire to speaker terminal?

Have fun, Hannes
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Old 6th July 2009, 02:56 PM   #3
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You should see the power supply rail voltages at one of the pins (drain) of the MOSFETS. If you can see this voltage there, then look for some drive at the gate pin with a source connected, or inject a bit of signal (you can probably get a bit of hum off your finger), enough to tell whether the MOSFETS are dead. If the amp is totally quiet then the output txistors are probably dead. If you can hear something then you need to look further back up the chain

w
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Old 6th July 2009, 03:44 PM   #4
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A few things that come to mind include---
Does the heatsink of the dead amp become warm like the good one? If not the output devices are not drawing any quiescent current and maybe there is a biasing problem. the bias voltage will appear between the gates of the MOSFETs in one half of the output stages and the other half. Compare the good amp with the bad.

Does the amp have a speaker protection system (relay) between the amp output and the speaker terminal? Maybe there is a fault in the protection circuitry or it is just doing its job by staying open circuit because there is a DC offset at amp out. If the latter is the case you will have to find the reason for the offset and fix it. Some designs share the protection circuit between the two channels of a stereo amp, in which case the above advice is irrelevant.

Your initial efforts should be directed at satisfying yourself that there is no DC at amp out as this is a general indicator of failed components anywhere in the feedback loop. Any DC voltage should be less than 10mV otherwise you could be putting your speakers at risk. If this is in order look for things such as the input wiring having fallen off or an open circuit input coupling capacitor.

Keith Taylor
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Old 6th July 2009, 07:21 PM   #5
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Hi everone. Thanks for the quick responses with your help. I checked the voltages and both amps seem to match. I'm probably going to have to remove the circuit board to check for a broken connection or hairline crack on the pcb. On the good amp there's pretty much no problem with DC offset voltage (under 1 mV), but on the bad one I get no sound and no readings. Not sure if I understand how to actually test the transisters to see if all 12 are good. If voltage and current is going through them, does that indicate their OK? I know they were fine when the amp went into storage. Not sure if time alone would make them bad without use. I know the amps were moved around a lot over the years to different storage locations so it very well could be a connection problem. It's possible a solder connection popped loose from all the bouncing around during all the moves.. I'll probably have to wait until Sunday to take everything apart when I have some more time. Again thanks for the help. Bob
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Old 7th July 2009, 01:48 AM   #6
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Quote:
Originally posted by Keith Taylor
Does the amp have a speaker protection system (relay) between the amp output and the speaker terminal? Maybe there is a fault in the protection circuitry or it is just doing its job by staying open circuit because there is a DC offset at amp out. If the latter is the case you will have to find the reason for the offset and fix it. Some designs share the protection circuit between the two channels of a stereo amp, in which case the above advice is irrelevant.

I second the notion of the speaker protection relay(s) (if present) being dirty.
I recently had an issue with my Sansui amp,after lots of fiddling,I found it to be dirty relay contacts,after cleaning all is well!
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Old 7th July 2009, 06:14 AM   #7
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Please don;t use the term "no reading." That doesnt tell us what you read. Zero volts? Open circuit? Your meter display went blank?
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Old 7th July 2009, 05:00 PM   #8
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Default FINGER ?????

Quote:
Originally posted by wakibaki
You should see the power supply rail voltages at one of the pins (drain) of the MOSFETS. If you can see this voltage there, then look for some drive at the gate pin with a source connected, or inject a bit of signal (you can probably get a bit of hum off your finger), enough to tell whether the MOSFETS are dead. If the amp is totally quiet then the output txistors are probably dead. If you can hear something then you need to look further back up the chain

w

THIS BIT OF HUM .....AS YOU DISCRIBE IT....coming from a finger is enough to blow a pair of otherwise good speakers to the moon

----if this is a home made amp
---- if the input is too free
----if there is no potensiometer for the gain
-----if the gain is too much


it could be even more things ...this is very bad practice for an amplifier that is supposed to be 400 w...

regards sakis
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Old 7th July 2009, 05:28 PM   #9
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Hi Enzo. I basically meant that when I try to read DC offset at the speaker terminls with no signal source connected, on the good amp it starts at around 1.8 mA when I first switch on the amp and then after about 15 - 20 ninutes settles to around 1 mA. On the amp thats not working, the needle does not move at all so I'm not getting any reading at all. Sorry for the confusion. Their is no relay curcuit for the speakers. Their are some fuses, but I checked them and their all OK. That's what leads me to believe either a solder connection on the board popped loose or their may be a hairline crack somewhere on the pcb. Like I said, they were moved around a lot over the years and there's no way of telling how carefully the movers were with them. I'll remove the board on Sunday and check it out and hopefully will find the problem.
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Old 7th July 2009, 09:17 PM   #10
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Hopefully, you're not really trying to measure current at the speaker terminals.
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