DSL15H mod failure and schematics

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Mart1234
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DSL15H mod failure and schematics

Postby Mart1234 » 24 Aug 2014, 02:54

Hi there,

I did som tweaking to my DSL15h last Night regarding the lead channel: Clip C75, Clip C76, Clip C34, add R82 150k, that's what others successfully did. Additionally I tried to:

- make those changes switchable (mini switches on the rear side of the amp)

- Install a variable resistor/poti for R40, as different values are reported to sound best. I wanted to find my best R40 value stagelessly.

Bad result: lead channel has lots of high feedback when gain/volume knobs are turned to certain levels (e.g. From both at 9 o'clock on) The amp is just squeaking without any guitar plugged in. The Feedback disapears when either gain or Volume are reduced. Feedback also disappears when the R40 poti is turned to higher values, let's say above 4.5 kOhm (I used an open 5k linear poti for testing). The clean channel works as before and doesn't Show feedback or Noise. Preamp tubes all have been exchanged to ensure that it's not a microphonic preamp tube - no improvement.

So is it possible that I have to raise R40 (which limits V1B current and hence gain on lead channel) to a value that no one reported before, e.g. 5kOhm or more? Why?

What else could have gone wrong with the V1B circuit (tried Not to Touch anything Else)

Additionally, to understand the amp circuit better and to find any future errors, would anyone be so kind and helpfully send me the schematics to the DSL 15h?

Thanks a lot in advance, I Hope I didn't confuse to much of the technical details and of the english words.

Bye,
Mart

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Re: DSL15H mod failure and schematics

Postby Anitoli » 24 Aug 2014, 05:09

Do you have a link to a schematic? I cant find one.

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Re: DSL15H mod failure and schematics

Postby Micky » 24 Aug 2014, 08:33

Put C34 back in and see if the feedback goes away.
In all reality, you should never clip anything out, maybe just adjust it's value a bit.
I would go with a 100p cap in C75+76.

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Re: DSL15H mod failure and schematics

Postby surfnorthwest » 24 Aug 2014, 12:32

Anitoli wrote:Do you have a link to a schematic? I cant find one.



Marshall has requested that schematics of current production amps not be published here, you can find discontinued amp schematics here.
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Re: DSL15H mod failure and schematics

Postby Anitoli » 24 Aug 2014, 12:41

Ok. Since I cant see what the circuit is, I would say put it back the way it was and leave it alone. :bgrin

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Re: DSL15H mod failure and schematics

Postby Mart1234 » 25 Aug 2014, 01:22

Well I soldered in some cords, so that I can interrupt the connection from outside the amp by means of switches. The squeaking feedback appears also when switches are in such position that everything should look like in original condition.

So either I made a mistake in wiring (I have 5 times 2 cords for R82, C76, C75, C34 and adjustable R40 poti) or I damaged some part of the hardware (circuit board oder V1 socket). To remove R40 I had to use the soldering iron rather long and also had to use some mechanical force to get R40 out. One pin was bent over at the bottom side of the board very much so it didn't slip out easily when trying to remove it.

What really perplexes me is that raising the value of my 5k poti (substitute for R40) will attenuate the feedback. That means that there's no short-circuit over R40/C34 connections (which I formerly thought), which could result in a too high cathode current and therefore cause the squeaking feedback of the V1 tube. Should I just give it a try with a much higher R40 value, like 8kOhm, just to see if feedback disappears?

Any other ideas?

Thanks a lot,
Martin

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Re: DSL15H mod failure and schematics

Postby Mart1234 » 29 Aug 2014, 12:15

Hey there,

I just wanted to post that I found out what caused the feedback, although I don't understand it thoroughly.

It's the cords that cause the feedback. When I establish contact between one end of the cord and a soldering point that is grounded, e.g. at R82 position, my amp starts to squeal. Same at C34, R40 or C75 position. The other end of the cord doesn't have any electrical contact to nothing.

Maybe the capacity of the cords I used is too big (I read about some hundreds of pF per meter) so that too much AC current could flow and the preamp tube got into feedback.

But then: How can switches ever be installed retroactively without disturbing the whole circuit due to cord capacity?

What cables should one use to install switches at the rear side of the amp? (Length per cable about 15cm)

Bye,
Martin

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Re: DSL15H mod failure and schematics

Postby Micky » 29 Aug 2014, 14:36

You should use small shielded coaxial cables to eliminate any induced feedback,
Something like Belden RG-174 or possibly a Teflon equivalent.

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