Still need help with this VM..

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TM1
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Still need help with this VM..

Postby TM1 » 12 Oct 2017, 13:10

So the problem is in the effects loop. I don't normally work on solid state circuitry. But looking at the schematic it just has some FET's and diodes. Shit, I wouldn't know where to get any of those FET's as I strictly do Valve/tube stuff, so any help as to where in that circuit for the loop would be appreciated! Too bad they didn't do the loop like the one in the reissue series or the one in the Astoria's. It would've been fixed weeks ago..
Thanks
DB

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Re: Still need help with this VM..

Postby SteveD » 12 Oct 2017, 13:33

If you put a signal directly into the loop's return, is it still bad? If not, then the problem is likely FET3. This can be verified by measuring the standing DC voltage across R23. Be careful as this circuit is running off around 200V! Hence the high working voltages of C16 and C17.
Still my guitar gently weeps

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Re: Still need help with this VM..

Postby TM1 » 21 Oct 2017, 18:43

Signal looks good going into the return. I have 140v on R23.
FET: do you have a number for that FET? I'll have to order them from Mouser Electronics here in the States.
Cheers Steve!

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Re: Still need help with this VM..

Postby Anitoli » 22 Oct 2017, 03:17

It is a TN2404KL and good luck, this is the only source that may have them:

https://www.alibaba.com/product-detail/ ... 67053.html

You might be better off looking at the data sheet and finding a subsitute.

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Re: Still need help with this VM..

Postby SteveD » 22 Oct 2017, 06:29

TM1 wrote:Signal looks good going into the return. I have 140v on R23.
FET: do you have a number for that FET? I'll have to order them from Mouser Electronics here in the States.
Cheers Steve!


140V across R23 is ok actually.

Are you quite certain there isn't a failed component or solder joint around the tone stack area?
Still my guitar gently weeps

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Re: Still need help with this VM..

Postby TM1 » 23 Oct 2017, 13:25

Steve; The amp works fine until the loop is engaged. I've been in a number of VM's over the years and pretty sure I know how they work. Just never have anyone who used the loop. But, before I discovered the loop was on I certainly thought that the problem was in the tone stack. Cheers!

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Re: Still need help with this VM..

Postby TM1 » 23 Oct 2017, 13:27

Anitoli wrote:It is a TN2404KL and good luck, this is the only source that may have them:

https://www.alibaba.com/product-detail/ ... 67053.html

You might be better off looking at the data sheet and finding a subsitute.

Yeah, Mouser doesn't carry them.. wish they did. The other FET is a 1202. Can't even find a cross for that or even anyone who's got a source..
Thanks!!

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Re: Still need help with this VM..

Postby SteveD » 23 Oct 2017, 15:56

TM1 wrote: The other FET is a 1202. Can't even find a cross for that or even anyone who's got a source..
Thanks!!


It's actually a common J202 n channel jfet. That won't be the problem if the signal is fine when directly plugged into the return.

The problem has to be fet 3 and/or R23 as the bias chain R21/R22 may still give a deceptively normal looking reading in the send stage.

If you can't get a TN2404 maybe try something like a ZVN4424A.
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Re: Still need help with this VM..

Postby TM1 » 23 Oct 2017, 19:48

It appears that the TN2404KL's have been discontinued. I sent an email to Marshall here as Trower wants me setup for Marshall service as it is. hopefully the guys in Indiana can help me out. It use to be no problem when Korg was distributing as Mitch Colby had me setup with parts, etc. for the last 15 years that Koeg was importing them..

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Re: Still need help with this VM..

Postby TM1 » 11 Nov 2017, 16:24

Got an FET from my friend HUGH Gilmartin @ Marshall!!!! It works perfect!
Thanks Steve and Anitoli for all your help!!

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Re: Still need help with this VM..

Postby Anitoli » 12 Nov 2017, 00:51

Those pesky Fet's. Good you got it going.

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