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Author Topic: Marshall DSL201 OT Possibly Blown  (Read 14935 times)

Littlewyan

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Marshall DSL201 OT Possibly Blown
« on: January 14, 2014, 06:20:59 PM »
Hi Guys

My mate has a Marshall DSL201 which I think may have a shorted Winding in the Output Transformer. Now the issue he gets is when he turns the volume up above a certain level the volume suddenly goes down real low, so low you can only just hear sound coming out of the speaker. Doesn't matter if you have the pre amp volume up and master down or vice versa.

So I've gone through the amp with my oscilloscope and found the volume stays constant all the way up to the EL84's Grid Stoppers, however on the Anodes of the EL84's is where the drop occurs. Soon as you go above a certain level the output from these valves drops significantly. So I believe it could be the Transformer as I would have thought a shorted winding would have the effect of pulling down the Output of the valves. Also when the amp is played at low volume with distortion the distortion doesn't sound as good as I think it should.

Now I've checked the voltages when the volume drop occurs and the B+, Anode and Bias all stay at the same level when the issue occurs. I've also changed the valves which didn't make any difference. My mate did say that the problem started occurring when he lent the amp to a local band who cranked it for a gig, although I don't think this should have killed it as Marshalls are designed to be run at full volume. I should also add that the board inside this amp is actually for a DSL401, looks like a tech changed it a while back, most likely to overcome the Bias Drift issue that these amps are known for (among many other issues!)

Sound like Output Transformer to you guys?

Enzo

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Re: Marshall DSL201 OT Possibly Blown
« Reply #1 on: January 14, 2014, 06:45:31 PM »
The only difference between the DSL401 and the DSL201 is the number of output tubes, and maybe the power transformer.  So they both use the same circuit board.  Just on the 201, they leave out certain parts.

J M Fahey

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Re: Marshall DSL201 OT Possibly Blown
« Reply #2 on: January 15, 2014, 03:14:25 AM »
The quick check is to grab *any* old but working  OT, even a 15W one, disconnect the old one (without removing it yet) and tack soldering the new one straight to the plates, +v , and to a lonely "flying" jack where you connect your cabinet.
Do not even worry at NFB.

Does it reasonably work now?
Even 15/20W àre quite loud, should be much more than what you can get with a shorted OT.

Post results.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #3 on: January 15, 2014, 03:36:25 AM »
I'll see if I can use the transformer from my 18watt amp.

I should also mention that when the volume drop occurs if you turn the standby switch off and on then sound comes back for a second before dropping again and will keep doing this until you turn the volume down.

J M Fahey

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Re: Marshall DSL201 OT Possibly Blown
« Reply #4 on: January 15, 2014, 07:42:34 AM »
Doesn't look like a shorted OT to me, they don't "self heal" , but try the spare transformer anyway just to discard it.

I much more distrust speaker / wiring / jacks / impedance switches / etc.

Also if you have the modern 1960 cabinet, which is mono/stereo and multi impedance, they do all that with lots of switches and jack leaf contacts in series, any of them can cause a similar symptom.

Measure *after* those plates,meaning straight at the transformer secondary taps and along the path: tracks/switch/jack/plugs/cable/speaker terminals.

Even the speaker might have crumbling tinsel wires.

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #5 on: January 15, 2014, 08:00:10 AM »
Quote from: Littlewyan
when he turns the volume up above a certain level the volume suddenly goes down real low, so low you can only just hear sound coming out of the speaker.

...

I should also mention that when the volume drop occurs if you turn the standby switch off and on then sound comes back for a second before dropping again and will keep doing this until you turn the volume down.

This is not what I would associate with a shorted turn in an OPT - normally it just soaks up the power.  This is also less likely in a combo than a head because the normal cause it the load coming disconnected under drive conditions.

This might help;

If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Marshall DSL201 OT Possibly Blown
« Reply #6 on: January 15, 2014, 10:11:23 AM »
My secret tech senses are suggesting to me that the amp is going into oscillation at RF when cranked.  The transformer wouldn't know how high the volume was turned.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #7 on: January 15, 2014, 12:44:08 PM »
J M: The speaker is fine as I have also tried my own speaker cabinet and get the same issue. I haven't properly checked the solder joints for the secondary winding I must admit as the volume loss occurs at the Anodes and like I said the volume almost cuts out so its like something starts to short when the output voltage goes so high. Wouldn't be a Primary Winding as the Anode Voltage stays constant so I thought it may be a Secondary winding dragging the AC down. Then again would I be right in saying that if the speaker joint was bad then the impedance could change so much and would affect the signal coming out of the Anodes?

Roly: I think I'll try and do that test if I need to as my 18w Output Transformer will be a pain to take out of my 18w amp. First though I need to build myself a current limiter to ensure I don't blow the PT.

Enzo: I checked the Anodes of the EL84s with my scope and when the volume dropped I couldn't see any oscillation. Then again I'm guessing the oscillation will be obvious like a spike in the waveform?

Enzo

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Re: Marshall DSL201 OT Possibly Blown
« Reply #8 on: January 15, 2014, 03:36:16 PM »
RF oscillation is at a frequency way higher than audio.  If anything it would look like a wide band of light across your scope screen.  It will not look like a glitch on the audio waveform.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #9 on: January 16, 2014, 01:50:29 AM »
Ok no didn't have that. What I will try is connecting a speaker directly onto the transformer to rule out the board tracks, if that doesnt work then i've got a new 50w transformer coming for another project so i'll try that on the amp and see if it makes any difference.

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #10 on: January 16, 2014, 05:03:19 AM »
I'm inclined to a similar feeling to Enzo, a sudden drop of output as the volume (or sometimes treble) is advanced will normally have us looking for supersonic/RF oscillations.

It is just possible it could be a voltage sensitive breakdown on the primary (never the secondary) but I think that would be unique in my experience; normally they work, or are shorted, no half measures.

A word of caution about probing valve output stage anodes under drive.  Even in the era this could embarrass bench instruments, but with the dominance of solid state most modern instruments can't cope with anything higher than about 500 volts, and if it's a 350V supply and being fully driven then there will be a peak of 2 * 350 = 700V on each anode.

The poor man's way to do the neon test is simply hook a battery across the primary then let it break connection while you are still holding the transformer leads - if it bites you, it's still alive.   8|

{what is going on here is "inductive kick back" a.k.a. "back EMF", very similar to what happens in a car ignition coil - you connect a voltage and let the current build up to max, then when you disconnect the battery the magnetic field rapidly collapses and induces a very high voltage in the winding, causing the neon (or you) to flash.  If the tranny has a shorted turn then the energy gets dissipated in that (instead of you).}
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #11 on: January 16, 2014, 07:17:58 AM »
Ok I might get my mate in to do that test as its his amp :). Its definitely something in the output stage though as when using the D.I. Output which is just before the Output Valves the amp is fine.

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #12 on: January 16, 2014, 08:58:13 AM »
Quote from: Littlewyan
Ok I might get my mate in to do that test as its his amp :).

Sounds reasonable.   :lmao:

Quote from: Littlewyan
Its definitely something in the output stage though as when using the D.I. Output which is just before the Output Valves the amp is fine.

Oh yeah, really?  Hmmmmmm...

Any idea how old the output valves are?
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #13 on: January 16, 2014, 09:08:45 AM »
No but they werent gd, checked the bias, one was at 19Ma and the other at 36Ma! So i swapped them for two that I knew were gd and biased at 20Ma and 24Ma, same issue.

Also I should mention that if you turn the volume back down then volume wont come back until you flip the standby off and on.
« Last Edit: January 16, 2014, 12:18:03 PM by Littlewyan »

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #14 on: January 17, 2014, 11:02:41 AM »
Also I should mention that if you turn the volume back down then volume wont come back until you flip the standby off and on.

Again, that is what I would associate with a supersonic oscillation, not a dead OPT.

Please try this; monitor the HT voltage at the OP transformer centre tap, apply a steady signal, and slowly advance the volume from zero until the sudden drop in output occurs.  The HT voltage should initially fall a bit as the output starts to come up, but the critical question is - what happens to the HT voltage when you get to the "sudden drop" point?  Does it remain steady, or does it rise a bit, or fall a bit?
If you say theory and practice don't agree you haven't applied enough theory.