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Old 6th December 2011, 05:27 AM   #1
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Default Amplifier mains hum help

I've got a sold state guitar amp with a bad mains ground hum. It was working fine when I tested it before I bought it and even when I got it home for a bit. Then I cranked it up and bam, got a very loud 60hz hum louder than the input signal. I have concluded it's not a thermal issue. The main power supply caps 'look' OK. The hum level does change with the master volume knob, but at the half way point it starts to go back down again, so it's quieter at 0, louder at 5, then quieter (same as 0) at 10.

Any advice on this?
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Old 6th December 2011, 06:43 AM   #2
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Old 6th December 2011, 08:14 AM   #3
Enzo is offline Enzo  United States
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Pictures don;t tell us much. WHy not let us in on what the amp is? Marshall, Hartke, Fender, Peavey, GK, etc? Make and model.

Since the volume control affects the hum, does the amp make the hum with NOTHING plugged into the input jack?

The master volume affects the hum, but do any of the other gain controls affect it? And do the tone controls affect the tone of the hum? If the amp has channels and channel switching, do all the chanels hum or just one?
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Old 6th December 2011, 08:18 AM   #4
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Quote:
Originally Posted by Enzo View Post
Pictures don;t tell us much. WHy not let us in on what the amp is? Marshall, Hartke, Fender, Peavey, GK, etc? Make and model.

Since the volume control affects the hum, does the amp make the hum with NOTHING plugged into the input jack?

The master volume affects the hum, but do any of the other gain controls affect it? And do the tone controls affect the tone of the hum? If the amp has channels and channel switching, do all the chanels hum or just one?
It's a Randall RH200 2x12 Combo amp.

Yes, the amp hums with nothing plugged into the input jack.

The preamp is not a part of the problem, meaning it does not change based on the preamp settings of either of the channels. With both of the preamp volumes all the way down doesn't make a difference. It's definitely in the power amp section of the amplifier.

Thanks for asking and for any advice or things to try.
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Old 6th December 2011, 08:49 AM   #5
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Try unpluging the input cable from the amplifier board . The one at far left of the pcb in the picccy . That will isolate the preamp totaly . if it still hums it will most definatley the power amp.

Regards Ian
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Old 6th December 2011, 09:22 AM   #6
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Did it. When the input from the preamp is unplugged from the power amp, the hum is quieter but still there. I unhooked it while it was also on to compare the difference. The master volume knob is before this input, which makes sense now, but for some reason I thought it would be more part of the power amp.

I'm not sure if this makes the preamp guilty or not. I had all the volume levels on the preamp channels turned all the way down and it still hummed more with the preamp plugged in. Does that make it the source of the ground problem?
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Old 6th December 2011, 09:27 AM   #7
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I unplugged the power going to the preamp and the hum is still there. That kind of makes the preamp look less like the culprit to me..
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Old 6th December 2011, 11:20 AM   #8
AndrewT is offline AndrewT  Scotland
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Fit a dummy Rs to your input plug. Try 10r to 100r.
Plug this dummy Rs into the amp and power up.
Has the hum changed?
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regards Andrew T.
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Old 7th December 2011, 02:34 AM   #9
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Quote:
Originally Posted by AndrewT View Post
Fit a dummy Rs to your input plug. Try 10r to 100r.
Plug this dummy Rs into the amp and power up.
Has the hum changed?
Do you mean the main input jack on the front panel of the amp or the input plug on the power amp board?
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Old 7th December 2011, 05:08 AM   #10
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Hi,
If you have a voltmeter read the rail voltage with the meter voltage selector in AC. The voltage should read close to zero. If it's read close to zero the power supply is OKAY.
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