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
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
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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
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
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
FINGER ?????
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
wakibaki said:
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
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.
I finally got a chance to take out the curcuit board (lots of wires to take off and label) and I found a few parts that need replacing. Their are 8 big (3") holes in the top of the case (I guess for cooling), and it looks like something must have fallen through one of them and hit one of the capacitors on the board. It has a big dent in the side. right next to it were 2 polysyrene caps in series and they have one lead broken off the board. Their's still about 1/2" of lead left so they might be OK to just reslder back on. The electrolytic thats damaged has another just like it (330 mfd 50v 85 degree C) so I'll just replace both. Each one is shunted by a series of 3 film caps, a larger polypropylene, a smaller polypropylene and an even smaller polystyrene. These look OK so I'll just remove them from the two existing caps and put them onto the new ones. I was going to use the Nichicon KZ series caps. Also, theirs a grouping of 8 small round transsters on the board (2 groups of 4). They each have a star shaped heatsink on them but one is missing the heatsink and I imagine I should put one on. As I haven't built anything in years, I was wondering if anyone could reconnend a good source of thes parts in the US. I also started to upgrade my Hafler DH-101 preamp and need parts for that (caps, resisters, gold RCA jacks, wire, etc.) I couln't find any one place that had all the parts I need with an easy to order interface online. Thanks, Bob
Thanks or the info. None of them have the Nichicon KZ 330 uf 50v in stock but one has the 100v version so I can wait, or just order the 100v. Does anyone know why my uncle would have used 3 film caps of different values to bypass the electros instead of just one? There's a .47 uf polypropylene, a .01 uf polypropylene, and a 100pf polystyrene. He even had the power supply caps bypassed (Eight 8200 uf 150v caps in the supply for each amp). They only read 75-76 v across them so I guess that the 150v is overkill. Thanks again, Bob
Replacing Power Supply Caps
I see that many are saying that in amps of this age, it's a good idea to replace all the lytic caps. If everything seems to be working and sounds fine, is this still necessary. This wouldn't be a problem as their are only 4 on the board of each amp. But their are 4 more spots where banks of parralleled polypropelene caps are used to get to the higher values and I see that these would be VERY expensive to replace as they are made up of 3 very large caps each times 2 amps. Would I have a problem just replacing the lytics and leaving these alone? And what about the supply caps. As I said, there's 8 - 8200 uF 150 volt caps in each amp. If I had to replace them, could I just use 2 of these in each amp ( 190275506845 ) Just paste that number into the Ebay search bar. They're 32,000 uF, 160 V Nichicons. This would allow me to use much smaller cases, since there would only be one larger value cap for each rail, would it matter if their not matched, as I know the tolerance on these is pretty wide, especially the over tolerance. Thanks, Bob
I see that many are saying that in amps of this age, it's a good idea to replace all the lytic caps. If everything seems to be working and sounds fine, is this still necessary. This wouldn't be a problem as their are only 4 on the board of each amp. But their are 4 more spots where banks of parralleled polypropelene caps are used to get to the higher values and I see that these would be VERY expensive to replace as they are made up of 3 very large caps each times 2 amps. Would I have a problem just replacing the lytics and leaving these alone? And what about the supply caps. As I said, there's 8 - 8200 uF 150 volt caps in each amp. If I had to replace them, could I just use 2 of these in each amp ( 190275506845 ) Just paste that number into the Ebay search bar. They're 32,000 uF, 160 V Nichicons. This would allow me to use much smaller cases, since there would only be one larger value cap for each rail, would it matter if their not matched, as I know the tolerance on these is pretty wide, especially the over tolerance. Thanks, Bob
Replacing all electrolytics is a good idea even if "it works". Dried up electrolytics will eventually cause trobule, and may well be degrading the performance of the amp.
Using multiple bypass caps the way your uncle has is overkill - especially the 100pF polystyrenes! You could go with good quality electrolytics bypassed with 100n-470n polyester or ceramics just fine. Capacitor technology has also improved a whole lot since the amp was first made. Are you sure the 100pF polystyrenes are being used as supply bypass capacitors, and not compensation capacitors?
Find out what power supply rails your amps are really using. It sounds like 80V or 100V power supply capacitors would be ample rating. The more capacitance here the better.
You could indeed use the Nichicon capacitors you mention in the supply.
Using multiple bypass caps the way your uncle has is overkill - especially the 100pF polystyrenes! You could go with good quality electrolytics bypassed with 100n-470n polyester or ceramics just fine. Capacitor technology has also improved a whole lot since the amp was first made. Are you sure the 100pF polystyrenes are being used as supply bypass capacitors, and not compensation capacitors?
Find out what power supply rails your amps are really using. It sounds like 80V or 100V power supply capacitors would be ample rating. The more capacitance here the better.
You could indeed use the Nichicon capacitors you mention in the supply.
Thanks Jaycee. Would there be any particular brands of caps that are well liked for audio power supplies and any that should be avoided. I ordered Nichicon Muse KZ series caps to replace the 4 electolylics on each of the curcuit boards. I'll probably leave the film cap banks alone on the boards. Is it OK to leave the original bridge rectifier or is that replaced with the power supply caps? Thanks. Sorry for all the questions, but it's been a while since I've built anything. I'm not familiar with a lot of the brands I see out there now. Thanks again, Bob
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