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Post subject: SIGNAL BLEED THROUGH "THE TWIN"
Posted: Mon Oct 05, 2009 9:20 pm
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Please help if you can - I have a THE TWIN blackknob snakeskin. Signal is leaking through from the input jack (any of the four and no matter what switch positions) to the speakers. When I input into the POWER AMP IN - it is quiet. When I pull all of the pre-amp tubes, I still get the signal bleeding through. Turning tone stack pots do not have an effect when all volumes are off.

Any ideas?

Bleeding through some caps to ground?

Thanks


martinbima@@@mts.net (please remove 2 of the @@'s)


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Posted: Mon Oct 05, 2009 10:04 pm
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Very interesting, Mbima. Are you saying that you get a guitar signal to the speakers, without any pre amp tubes in the amp? Or is it a type of noise, like a loud hum or static? Does this happen only when you plug the guitar into the amp? How does the amp sound with nothing plugged in and the volume control(one at a time) turned up to about 6 (all tone controls at 5 ) ? ART

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Posted: Thu Oct 08, 2009 9:03 am
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I use a loop pedal for a guitar-level signal input. I can pull V7 (clean side preamp) and the clean signal gets through. I can pull V1 (dirty side preamp) and the clean signal gets through. It bypasses the tone stacks and reverb. However, if I pull both preamp tubes, it is quiet. Somehow, the signal is going from the input to the pre-PI 12AX7 tube. I have been grounding and disconnecting for 3 days - I am starting to dream about it.

It is either going to the power supply "C" or through ground.


Thanks for any help.


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Posted: Thu Oct 08, 2009 7:32 pm
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Mbima, looked at the schematics a bit and we can see that V3a is the normal signal source, then paths are provided through the effects loop, the level control, and the associated circuit to V3b. So the amp is allowing a signal through to the grid of V3a , am I right so far? Have you tried plugging another pre amp into the power amp in jack , just to verify that all is well from that point on? I'll print out the schem and give it more attention, PDF files are a little tough for these old eyes. ART

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Posted: Thu Oct 08, 2009 9:37 pm
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Hi Art - thanks for the reply !!!!

THE TWIN I am looking at is the four-input, reverb equiped (no trem). It is FENDER's drawing number 26059.

The signal is getting in between V7A and V9A.

I can ground pin 1 on V7 and it is quiet.
I ground pin 2 on V7 and it still bleeds.
I can ground anywhere past pin 1 on V7 to the Channel 1 wire junction and it is quiet.

just wierd.


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Posted: Fri Oct 09, 2009 5:58 am
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I pulled out the VACTROL chain last night. All it did was turn off the lights for channel 1/2. Sound kept bleeding through.

I pulled V7 (clean pre-amp) and it still bled.

I pulled V1 (dirty pre-amp) and it still bled.

Pulled them both and it was quiet.

Checked resistance between the C+ pins (1 and 6) on V7 - 200 ohms - what it should be.

still hacking away at it.


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Posted: Sat Oct 10, 2009 11:04 am
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I would expect to see 200Kohm, not 200. Can you check the 100kohm plate load resistors? Can you test the filter caps for leakage? ART

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Posted: Sat Oct 10, 2009 2:48 pm
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yes - 200k is what I meant - sorry.

I will check the filter caps this week. I had to reassemble this weekend for a show. I am going to do a PPIMV when some parts come in.



Thanks !!!


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Post subject: SUCCESS !!!
Posted: Fri Oct 16, 2009 5:31 am
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Changed out all of the high-voltage caps - all is back to normal. Most likely the de-coupling caps but they are all new now.

Thanks for your replies .....

Hoepfully in 10 years I can remember why the sound is bleeding through again.


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Posted: Fri Oct 16, 2009 2:02 pm
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That's good news, Mbima ! ART

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