Peavey PA100 curing hum

[BobV]

Just picked on of these up for some small gigs. Problem is, there is a hum. As I increase volume/gain, either on the individual or master controls, on all four channels, the hum gets more pronounced. This is the model with only an on/off switch, not the dual switch. Suggestions for reducing/eliminating the hum, please, thanks. Bob V

[Enzo]

Oooh, I had forgotten this old one.  So it is quiet with the master at zero?  Or is it always there, just louder or softer?

My first reaction of course is the power supply lacks filtration.  You have a 40 year old amp, and there is a good chance the big electrolytic caps are dried out.  So look at the main power supply and see how much ripple is on it.  I don't have my schematic handy (You can get that from Peavey customer service) but at that age I am thinking we have a single ended transistor preamp.  The power for the power amp is stepped down for the preamp.  So if the main supply is all hummy, so then will be the lower voltage supplies.

[DrGonz78]

I got this one. Here it is... oh wait... It is already loaded up here on the SSguitar.com site.

http://www.ssguitar.com/index.php?topic=2909.0

[BobV]

Enzo;

The hum goes away when the individual gains and/or master gain are turned down. It is not always present. Also, last night I switched wall plugs and found that the hum was about 95% gone - even at a higher volume, it was barely present. So I think it does have something to do with the power supply and/or grounding. It has a very nice sound and everything works on the system.

DrGonz78;

Thanks for the reference. I read through that thread before I posted. I don't have the same issues as that fellow, but there were some tips in there I could use, thanks.


Guys,

Is there anything I can do or something I can get to make sure that if I use this in a club and get a poorly grounded outlet, I can eliminate the hum?

Bob V

[Enzo]

If changing the wall plug makes the hum go away, then the amp is likely fine.   The term "power supply" refers to circuits inside the amp that change the mains 120vAC into other voltages the circuits use.  Generally we do not refer to the wall outlets as power supply.

Does the unit have a three wire grounded power cord and plug?  Verify the ground pin has continuity with the chassis.  What did you mean no dual switch?  Does the power switch have three positions - center off, and on towards either end?  Or is it just on/off?  If it is center off, then the two ON ends are both ON, but they do reverse the mains polarity.  This can sometimes reduce hum.

When you are on the road - meaning any gig not in your home - carry a cheap outlet tester.  They are available at any hardware store, home center, harbor freight, etc.  Here is an example:
http://www.harborfreight.com/electric-receptacle-tester-32906.html

It will instantly tell you if the outlet is miswired or missing a ground.

[DrGonz78]

DrGonz78; Thanks for the reference. I read through that thread before I posted. I don't have the same issues as that fellow, but there were some tips in there I could use, thanks.

I only posted that link for other members here to be able to download the schematic.