Bass Combo Mark 3 Series (260C) loud humming

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pjzzz
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Bass Combo Mark 3 Series (260C) loud humming

Post by pjzzz » Wed Mar 15, 2023 11:06 am

I own a Mark 3 Combo (130 Watts) that I bought in 1980 and had in storage for more than 15 years.
Powered it up slowly last week with my laboratory isolating transformer and was welcomed by super loud 50Hz humming noise.
I noticed no excessive rising of Amperage, so I suppose there are no shorts to ground.
It's an 260C series poweramp marked 080778 260C JCS on the back of the PCB.

This it what I've tried so far.
- Checked speaker output, and measured no DC present.
- Uncoupled tone control PCB from poweramp PCB, and the 50Hz is still present.
- Tested all 4 power transistors (de-soldered from PCB) as good with a MK-168 component tester.
- Tested 2 driver transistors Q5 and Q9 (de-soldered from PCB) as good with a MK-168 component tester.
- All 10W 0.33 and one 0.1 Ohm cement resistors measure the correct value.
- Switched TL074 over from tone control board to power amp, no difference.
- Replaced all electrolytic caps including the two 2.2uF/50V NP caps (C10, C11) with good new ones I had in on stock, no difference.

I'm a little stuck right now and would appreciate help fixing this issue.

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pjzzz
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Re: Bass Combo Mark 3 Series (260C) loud humming

Post by pjzzz » Thu Mar 16, 2023 9:35 am

FIXED.

I totally forgot that my Peavey needs to be connected to a mains socket with proper earthing. :oops:
After plugging it into an earthed 230V wall socket and plugging in my bass, I was stunned by how much power this old combo can produce.
It's working like never before, and with all the new components I’m sure it will do so for the next 40 years to come. :D

PS I also cleaned the magnet and the voice coil from the BW 15" speaker because as a well known fact most of those old BW's have the “sticky foam disease”.
I've removed a lot of hardened molten old foam between the voice coil and magnet and the speaker is moving freely again.

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dak
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Re: Bass Combo Mark 3 Series (260C) loud humming

Post by dak » Thu Mar 16, 2023 4:07 pm

pjzzz wrote:
Thu Mar 16, 2023 9:35 am
FIXED.

I totally forgot that my Peavey needs to be connected to a mains socket with proper earthing. :oops:
"Proper earthing" is not to be underestimated. I have a tendency to hum fests because of an overland electricity line a few hundred yards away. It was sort of a revelation when I figured out that I had significant resistance between earth and return, and it turned out that the earthing clamp in the central heating room was broken and loosely hanging on a pipe. Getting it fixed helped.

Normally amps help killing you: this one helped diagnosing the danger.

P_duffer
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Re: Bass Combo Mark 3 Series (260C) loud humming

Post by P_duffer » Sun Dec 17, 2023 8:46 am

I also have the same issue with my 260c combo. It had been left on for an extended time and we were awakened in the middle of the night by a loud humming noise. Have not been able to get it looked at and is it even worth it.
What could be the most likely issue with the amp?

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