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Old 19th February 2014, 03:22 PM   #1
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Default Marshall 8100 buzz

Hi,

I have problem with Marshall 8100 guitar amplifier. It buzzes instantly when I turn the amp on, the buzz does not react to any knobs, so it must be poweramp issue. I have already replaced main filter caps, but buzz is still there. Output transistor are biased at 20mA, DC offset is 16mV. Everythong seems to be working, amp does play when guitar plugged in.

I think it might be some ground loop issue, but it is quite a loud buzz and I don´t think that Marshall would release amplifier woth ground loop issue.

Any ideas what should I check?
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Old 20th February 2014, 07:49 AM   #2
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The buzz doesn´t react to VR13 pot, so it is definately poweramp issue. I have removed IC8 and buzz went away. I have replaced TR4,5,6,7, but buzz is still there.

I don´t think the + - 15V supply is noisy, because preamp doesn´t buzz at all.
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Old 20th February 2014, 08:52 AM   #3
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Is the +-15V supply smooth?
If you lift C18 is the hum still there? Is the HT smooth? (High Voltage!) Be careful as the output transistors have a gain of more than 500 and can damage very quickly!
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Old 20th February 2014, 09:06 AM   #4
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+-15V supply is propably smooth, do not have scope, but I have replaced 1000uF caps and voltages are fine.
HT has new power supply caps too.

Disconnecting C18 doesn´t change anything, it is the same as turning down the pot VR13.

I suspect +-15V supply, but if it would be noisy then preamp should buzz too, right? But the preamp doesn´t buzz, it is completely silent.
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Old 20th February 2014, 10:03 AM   #5
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I would think in that case that you may have a ground issue. Poor or corroded steel to aluminium tag may be?
Is the buzz at mains frequency, twice mains frequency or random?
Mains frequency is usually ground related as it random but twice frequency is bridge rectifier issues. (You probably know this already, If you do I don't wish to annoy you).
It would be so much easier if you had access to an oscilloscope. Are C20 and C55 OK?
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Old 20th February 2014, 10:07 AM   #6
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It seems like 100 Hz buzz. But rectifier bridge is replaced with new one and it still buzzes. C20 and C55 are new too, no change.
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Old 20th February 2014, 10:09 AM   #7
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Try running a shorting link from the centre point of C61 and C62 (0Volt grounding point) to the ground side of R26.
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Old 20th February 2014, 10:13 AM   #8
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If the hum stops when the output from pin 7 of IC8b then it must either be induced into the chip on pin 6 to produce hum on pin 7 or there is something strange going on with the power supply. I take it the hum stops immediately the mains is switched off. The chassis is grounded to your mains supply? I am clutching at straws here now, without seeing the amp.
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Old 20th February 2014, 10:20 AM   #9
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Quote:
Originally Posted by JonSnell Electronic View Post
Try running a shorting link from the centre point of C61 and C62 (0Volt grounding point) to the ground side of R26.
No change.
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Old 20th February 2014, 10:22 AM   #10
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Quote:
Originally Posted by JonSnell Electronic View Post
If the hum stops when the output from pin 7 of IC8b then it must either be induced into the chip on pin 6 to produce hum on pin 7 or there is something strange going on with the power supply. I take it the hum stops immediately the mains is switched off. The chassis is grounded to your mains supply? I am clutching at straws here now, without seeing the amp.
Amp chassis is grounded directly to safety ground. PCB is grounded to the chassis via 10R + 100nF + oarallel diodes.

Yes, hum stops immediately when mains is switched off.
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