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Old 21st June 2011, 11:20 PM   #1
batsong is offline batsong  United States
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Default Blown Bluesbreaker

Hi. Having a problem with a Marshall reissue JTM45 Bluesbreaker combo.
What I am getting is very very little signal until volume '8' and then a buzzing harsh distortion with an almost ring modulator type quality. The volume is not all that loud. No other abnormal humming, rustling or buzzing.
I've noticed that the 2nd and 3rd preamp tubes hum loudly if place my hand near the glass, and #2 with buzz loudly if I grab the glass.
Seems like the voltages from the PS are right, so I don't know about my original idea of filter cap failure. Maybe trans or choke? Speakers are ok.
I would like to isolate the pre to see if it is there or in the power amp, but not sure how.
Any help would be greatly appreciated.
-Gabe

Schematic:
http://www.raw-sewage.net/images/bb62ri-schm1.jpg
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Old 21st June 2011, 11:37 PM   #2
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Gabe - to test power amp section by itself, disconnect the wire from the treble pot wiper and use that as the input - make sure the DC blocking cap C10 remains in place.

Does tremlo intensity knob position change the described behaviour at all?

Cheers,
Ian
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Old 22nd June 2011, 12:57 AM   #3
batsong is offline batsong  United States
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Thanks, Ian. I'll look into isolating the power amp right now.
Trem only does what it does to signal, no change for the better.
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Old 22nd June 2011, 01:08 AM   #4
batsong is offline batsong  United States
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Power section ok. Must be in the pre.
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Old 22nd June 2011, 02:12 AM   #5
batsong is offline batsong  United States
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Just remembered-when amp failed, I found that the vibrato tube was d.e.a.d. I don't know if this was concurrent with my problem though, as I have not had the vibrato connected for a long time.
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Old 22nd June 2011, 04:11 AM   #6
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Is it the same for both CH1 and CH2 - if so points to V2 circuitry.
else its in V1 circuitry.
Don't forget simple stuff which could cause this sort of behaviour like a volume pot which is open circuit somwhere along its input to output track such that the wiper position has to be turned up far enough to bypass the track break. I've seen this happen before.
Cheers,
Ian
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Old 22nd June 2011, 03:02 PM   #7
batsong is offline batsong  United States
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Thanks Ian. You pointed me in the right direction. It was the 100k resistor across V2.
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