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Old 29th March 2013, 03:07 AM   #21
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Given the valves are new:
If the sound is sagging with volume then I would check the power supply electrolytics.
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Old 29th March 2013, 03:09 AM   #22
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I will check the AC ripple tomorrow and report back.
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Old 29th March 2013, 07:13 PM   #23
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Took voltage measurements of the smaller filter caps, and here is the results

250VDC & 10mVAC (Should be 300VDC, but I have a 100W bulb in line with the power cord.)

170VDC & 15mVAC (Should be 200VDC, but I have a 100W bulb in line with the power cord.)

Can't measure the main 220mfd 300V caps because I can't reach the other side of the caps unless I desolder a bunch of wires and pull the board and jumper everything back in thanks to the MB's terrible design.

I don't think I really want to do the repair, but I do want to at least find the fault.
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Old 30th March 2013, 05:07 PM   #24
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I checked all the power tubes and preamp tubes by putting them in my Epiphone Valve Junior. I tapped on each tube (both power and preamp) with a pen, and no static or ringing noises. In short, all the tubes work.

So we can rule out the tubes.
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Old 30th March 2013, 05:12 PM   #25
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by ouimetnick View Post
I checked all the power tubes and preamp tubes by putting them in my Epiphone Valve Junior. I tapped on each tube (both power and preamp) with a pen, and no static or ringing noises. In short, all the tubes work.

So we can rule out the tubes.
Excellent idea
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Old 31st March 2013, 01:55 AM   #26
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Looking at the schematic, it seems the speaker switch disconnects the preamp from the power amp.. Not the effects loop. So I could solder a wire to the input of the power amp and see if music makes it through cleanly, and isolate the issue? Or am I crazy?

In fact switching the SILENT RECORDING switch to MUTE cuts all noise, but it doesn't appear to disconnect the speaker or place a dummy load. Instead it looks like it is breaking the connection connecting the preamp to the power amp.

The manual says "This switch mutes all of the signal at the power section driver stage, removing the need for a speaker load IN THIS POSITION ONLY!

So you can run a tube amp w/o a load on the output transformer provided you do NOT have an input source? (in this case, the preamp)
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Old 30th April 2013, 01:34 AM   #27
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Still messing with this PITA. I decided to replace all electrolytic capacitors and diodes. I know shotgunning isn't recommended. As Enzo states "Its like throwing sponges at a rainstorm"

I didn't have high hopes that doing what I did would fix the issue, and sure enough it doesn't.

I know I've said it before, but the is a PITA!! Have to remove all pots, desolder nearly 20 different wires, and maneuver the board out with the stupid plastic nylon standoffs.

Besides sending it to Mesa Boogie.. any other suggestions?
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Old 2nd May 2013, 02:44 PM   #28
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after checking out your you tube vid and the schematic and my failing memory i would be suspicious of the relays and their supply the fact that you get no sound on one channel and the weird fart noise on power down i remember this as clues in previous repairs that lead me to replacing relays only to discover bad zeners and bad 16 v filters in the switching matrix supply
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Old 2nd May 2013, 04:16 PM   #29
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Quote:
Originally Posted by turk 182 View Post
ouimetnick
after checking out your you tube vid and the schematic and my failing memory i would be suspicious of the relays and their supply the fact that you get no sound on one channel and the weird fart noise on power down i remember this as clues in previous repairs that lead me to replacing relays only to discover bad zeners and bad 16 v filters in the switching matrix supply
I replaced all the diodes. what do those relays actually do?
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Old 2nd May 2013, 06:31 PM   #30
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one of the relays is channel switching the other is selecting which channel the reverb is applied to.
i was wondering do you have the footswitch if i recall this amps lead channel won't work without it.
or was it something about push pull controls being set right, might be another model i'm thinking of ...

the matrix circuit in these can be finicky
did you in fact replace all the diodes because not all are the same type! which ones did you replace?

Last edited by turk 182; 2nd May 2013 at 06:35 PM.
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