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In Reply to: RE: 180hz hum from amplifier posted by rkeman on September 28, 2014 at 16:11:05
Parasound used a bunch of bypass caps in their power supply. They were white in color IIRC and all over the place. Not the world's greatest caps they did happen to go before the big electrolytics. Meter the caps and if they show a short check the bypass caps first.
If bad, then just clip the bad one out.
Follow Ups:
What part of the circuit would be responsible for primarily 180Hz hum? 60Hz and 120Hz hum, I understand. Interesting.
Thanks!
I would be open to the possibility that the on line tone generator method of determining frequency is flawed.
I'm matching the hum to the tone generator audio only by ear. The "beat frequencies" bracketed 180 Hz distinctly.
DWPC,
while that is a neat way to identify a tone, my guess would be the tone is closer to 120hz, which would indicate its coming from the rectified 60 hz AC line.
On the 1500 as well as other models, Parasound used some caps which exhibited Capacitor Plague (the premature drying out of the electrolite solution). If you know how to solder, would suggest you start by replacing all the caps in the input stage power supply first. If that does not solve, it then move to output stage power supply.
I would suggest using either the Nichicon or Panasonic hi ripple type caps available from digikey or mouser.
AFter that your amp should be good for another 20+ years.
best
60 plus 120, the first harmonic, +180hz lading me to suspect the bypass caps are the culprit. I've had issues with the bypass caps before.
Can I identify the bypass caps visually? I have no schematic (not that it would do me much good if I had it!).
Edits: 10/01/14
In the photo you've shown, the bypass caps are the smaller yellow circular caps next to the main filter caps. Under each of those square heat sinks between the two filter caps are the bridge rectifiers.
While one of the other posters thinks the bypass caps may be the issue, the likelyhood of that is much less than having a bad electrolytic cap, especially since Parasound used some caps which exhibited capacitor plague, and Parasound used metalized film caps in these bypass locations which seldom fail unless really hit with a 4x or more voltage. But if the voltage spike had happened, the spike would have taken out other parts first.
If you really want to do something, you may switch your DVM to mV AC and see if you can measure any AC Voltage on those cap leads. You should see nothing but DC at these points. YOu will need to do this with the amp powered up, so be careful and don't short anything out. If you want to be really safe, shut the amp off, attach the volt meter lead to the cap lead then power the amp back on. Take your measurement, shut the amp off and repeat this process. If you need to replace any of the caps, the entire circuit board will need to be removed from the amp chassis.
I have worked on several of these amps, they are not hard to work on, but you really can't do much until the main circuit board is removed from the amp.
at some point, you may want to seek out a qualified service person, or send the amp back to parasound and have them repair it.
happy listening.
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