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Sick Marshall Studio 15...

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4/19/2003 12:17 AM

Sick Marshall Studio 15...

I've never worked on a PCB amp before and I need some recommendations on a sick Marshall Studio 15...

Symptoms:

1. With all the knobs dialed up to 10, it's not as loud as my vibrochamp and it's still completely clean sounding, (no breakup even though the gain is set to 10).

2. I'm hearing radio stations through the amp when it's turned up. (This doesn't happen on any of my Fender amps in the same location on the same outlet).

3. The previous owner "claimed" that it worked fine for him before he shipped it to me.

What I've tried:

1. Changed out the original Sovtek 12AX7's for a couple old Nationals, which significantly raised the volume, but only to the level described above (think clean champ tone with volume on 6).

2. Changed out the 6v6's, but this made no perceptible difference.

3. Tried plugging headphones into the headphone jack and unplugging the speaker. This sounded just like it did when I was playing through the speaker, so I think this eliminates the speaker and associated wiring as the potential culprit.

4. Plugged the speaker into the headphone jack (I was told that this would allow attenuated operation), but when I do this I get no perceptible sound out of the amp at all.

Any idea's on where the problem might be? Barring that, does anyone know a good tech that works on Marshalls? Also, would an authorized Marshall Repair facility be likely to work on a discontinued amp that's this old?

4/19/2003 12:20 AM
Deltablues	Oh... It's also fairly noisy when you crank everything to 10. Not like a floating ground hum, just excessive hiss and general noise.

4/19/2003 3:00 AM

Hi Larry,

I think if you are getting familiar with the way the controls effect the noise and performance of the amp right now, try removing V1 and test it then try again removing V2 only. You can't expect the guitar to come through, but if you hear a difference in the noise behavior it might help to localize the problem to a certain tube.

With all the tubes in, if the noise seems to be normal but the signal is just weak, and all the knobs seem to be active the problem could be at the very beginning of the audio circuit. Maybe even the input jack, looking at the drawing this is an unusual one where a ground is made but not just the input cord at the plug tip, it also has a ground from C11 to the ring of the phone jack which is supposed to move free when you plug in the cord, and even if a little noise (or signal) gets in the main input jack, unless that (cheap plastic) secondary jack connection is fully disconnected, it will kill everything right after the first triode has amplified it. This makes for real low background noise when there's nothing plugged in but its also a potential place for failue which can kill your sound, that does not exist on a traditional amp. Or for partial failure which could partially kill your sound. Still this should be a nice amp, I know you're going to like it.

Anyway, that's my ideas (the easy ones

), some of these guys are bound to be really familiar with this amp, I'm really confident you'll get this thing fixed.

If anyone is not familiar, take a look for the drawing at

http://www.schematicheaven.com

and just look under the marshall section for the Studio 15. Larry keeps a nice site there, I think we can all appreciate the benefit that Deltabluesman provides for anybody who wants schematics of any kind.

Thanks,
Mike

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