Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: Littlewyan on January 14, 2014, 06:20:59 PM

Title: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Enzo 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: J M Fahey 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: J M Fahey 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly 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;
(http://www.ozvalveamps.org/repairs/xform_test.gif)
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Enzo 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Enzo 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly 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).}
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly 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?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan 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.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly 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?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 17, 2014, 01:12:47 PM
HT Stays steady. Now shortly before this issue started occurring this amp was repaired as it wouldn't work at all. Issue was a bad solder joint on a heater connection apparently. Just checked the heater voltage for the valves, on the 12AX7s its 5.8V which is acceptable I think, however on the EL84s its 2.6V! So I unplugged the Heater Winding from the board and read 6.4V directly on the PT itself. Now there are two 100ohm resistors going to ground from either Heater Winding and with the Heater Winding unplugged they are effectively seperated from each other. Measured them and I get 50ohms, odd, measured the ground side and theres 5ohms between them and the chassis. Looks like I'm going to have to take the board out to investigate further.

EDIT: Looking at the schematic the EL84's heaters come straight off the PT Heater Winding and should be at 6.3V, the Pre Amp 12AX7s come off a Bridge Rectifier which is connected to the PT Heater Winding and should be at 12.6V DC. Now when I measured the 12AX7's Heater Pins I found about 3V DC but 5.8V AC. Possibly a rectifier issue?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: g1 on January 17, 2014, 01:33:52 PM
When you are measuring the heaters, measure across, not from either side to ground.

When the amp was cutting out, was it instantly or did it take a second or 2 to fade out?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: g1 on January 17, 2014, 01:48:17 PM
Attached is schematic, I don't think it has been posted yet.

You mentioned trying to measure the 100R resistors and said they should be separated if the heater winding is disconnected.  The tube heaters are still connected as is the bridge rectifier BR102.  So they are not separated and that is why they still measure 50ohms, which means they should be good.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 17, 2014, 02:43:47 PM
Looking less and less like the OPT.  Phew.   :cheesy:
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 17, 2014, 02:53:31 PM
It was instant and I took out the rectifier to find it was good. However when it is it tests bad. I did find two tracks had been shorted by some clumsy soldering, went across this with a screwdriver to seperate them but still the issue remains and I still get AC on the DC side of the Rectifier. Perhaps the rectifier is bad? Can they go bad under load? I just tested it with a multimeter.

EDIT: Just tested it again and found 12VAC on the other side of the rectifier, however its all running on a 6.3VAC Winding ......... Its not the valves as I got a higher value when the valves were out. DC was at 5V or so.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: g1 on January 17, 2014, 03:53:04 PM
 Yes the rectifier could be bad under load.  Or the filter caps C73 and C78 could be bad.
 But also, are you getting proper heater voltage at the power tubes, you said it was bad before, is it still bad?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 17, 2014, 03:57:33 PM
No I measured it properly and it was ok, about 6V. What doesn't make sense is when I removed the valves I turned the amp on, checked the heater voltage AFTER the rectifier and got 17VAC and 8.8VDC. When I turned the amp off the AC drained very slowly. What on earth could be storing AC and how is 6VAC turning into 17VAC?! Can't see any issues with the tracks now.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: g1 on January 17, 2014, 04:47:32 PM
  The schematic shows the preamp heaters being run in parallel.  So you should have 6 to 7VDC coming out of the bridge.
  Some meters have trouble reading AC where DC is present.  This may be the problem here.
  If the heaters were causing the cutting out, it would be a fade out, not an instantaneous full stop. 
  I think we are chasing a ghost here.  Turn off the lights.  Watch the heaters when the fault occurs.  If they are not dimming I think we can forget about them and move on.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 17, 2014, 05:46:14 PM
Guys I hate to say it but I've given up. Spoke to my Grandad about it and he agreed its one for Marshall to look at.

Thanks for all your help
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: J M Fahey on January 17, 2014, 06:30:33 PM
Cheap meter is reading 7VDC as 17VAC.
Cheap meters don't *really* measure AC but use the same DC multimeter circuit they already have, just add a diode in series with the input and multiply display reading by roughly 2.5X
They save a lot of money that way and it works ... sort of ... if you measure "pure" AC, such as the line voltage or a transformer secondary ... but if you measure DC mixed with AC (such as what you did, or trying to measure ripple) they actually measure DC (yes, on the AC scale, the d*mn b*st*rds) and multiply that by 2.5 .
That's why you have "17VAC" where really there is 7VDC.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 18, 2014, 04:08:31 AM
O right, in that case i'll just use my Avo in future.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 18, 2014, 10:13:58 AM
Quote from: Littlewyan
I turned the amp on, checked the heater voltage AFTER the rectifier and got 17VAC and 8.8VDC. When I turned the amp off the AC drained very slowly. What on earth could be storing AC and how is 6VAC turning into 17VAC?!

Caution: most meters will respond to DC on their AC ranges giving silly readings.  If you want to measure AC in the presence of DC you need a blocking cap in series with the meter.  I'm pretty sure your AVO will do the same.  Older meters had a socket marked "Output" which simply includes a cap in series with the AC volts input, but it's intended for audio frequencies and may read low on power line frequencies.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 19, 2014, 06:44:39 AM
Guys i've had an idea, i think its definitely oscillation like you say as I had the exact same issue yesterday whilst I was experimenting with my 18w (found a coupling cap that I added after a volume control was the cause, removed it and all was well). Now the way I see it this amp used to work, only thing that could have possibly changed were wires. So what about the output phasing? Sure it doesnt have NFB but wouldnt the output phase still make a difference?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 19, 2014, 10:49:02 AM
Quote from: Littlewyan
only thing that could have possibly changed were wires. So what about the output phasing? Sure it doesnt have NFB but wouldnt the output phase still make a difference?

All sorts of things could have happened, caps go leaky or open, resistors drift in value, semiconductors can fail in all sorts of ways.

No, the phasing of the output should make no difference, besides, even with NFB how could it change without some component misbehaving?

Something has changed within the amp, and if you haven't changed the lead dress before the fault appeared then it is most likely one of the caps I mentioned above.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 19, 2014, 11:00:08 AM
Right, well its definitely between the EL84s and the OT so that narrows it down a lot. Might take the amp back off him to have another look.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: g1 on January 19, 2014, 02:29:37 PM
Have you checked R71?
Have you resoldered the EL84 sockets?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 19, 2014, 03:02:03 PM
No I must admit I havent
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 20, 2014, 03:06:22 PM
There isn't much point now am afraid to say as my friend took the amp apart last night to check for cold solder joints and when he put it back together he mixed up a red and black lead. Anyway he said something popped and smoke came out the back, fried PT :(. I did tell him to wait for me as he didn't know how to discharge capacitors, i told him he was lucky there was no charge left in them!
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: joecool85 on January 21, 2014, 08:04:43 AM
There isn't much point now am afraid to say as my friend took the amp apart last night to check for cold solder joints and when he put it back together he mixed up a red and black lead. Anyway he said something popped and smoke came out the back, fried PT :(. I did tell him to wait for me as he didn't know how to discharge capacitors, i told him he was lucky there was no charge left in them!

Wow, that's not good.  Glad no one got hurt.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 21, 2014, 10:53:14 AM
Quote from: Littlewyan
fried PT

You sure about that?  It's generally not that easy.

Offer him $5 for it.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 21, 2014, 12:56:59 PM
He said the PT looked damaged so I'm guessing thats where the smoke came from, I'll double check for him when I see him. No thanks Roly, that thing is a can of worms!
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 22, 2014, 07:59:37 AM
Guessing is okay, but facts are better - you're guessing on his guess.  It may be the tranny, or it could be smoke from a burning resistor came out near there.  Does he even know what a "damaged" tranny looks like?  I remain dubious until proven.

Something a tech learns early on is that most people are truly lousy witnesses, particularly when smoke is involved.  I've even had somebody bring me their "valve" amp to fix that turned out to be fully solid-state.   ::)

"...are you sure?"
"Am I ... WHAT?"  :o   :trouble
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Enzo on January 22, 2014, 11:37:12 AM
Agree^^^


Usually, the thing stops working - smoke is even better - then they come up with the one name of an electronic component, and suggest, "It's probably just a (component name here)."


Transformers do fail now and then but they are the LAST thing on a list of potential failures.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 23, 2014, 04:55:26 AM
^^^Yup.  :tu:  OT's certainly do fail, particularly if a valve amp is driven without a speaker connected, but even then they hardly ever smoke (which makes them tricky to diagnose).  Power transformers are generally a lot harder to kill.

I'll make a small bet that if you go sniffing (literally) under the chassis you will quickly turn up a burnt resistor, and where that is will give us a strong clue as to what happened.

Can you be more specific about the black/red wires he misconnected?
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on January 31, 2014, 02:58:01 PM
Hi guys sorry its been so long, been very busy with a new project. My mate wants to look at the amp himself now so I've basically left him to it, I'm not sure what hes done but I don't think he can get sound out of it anymore, he does however now know how to discharge capacitors!
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on January 31, 2014, 11:28:32 PM
I don't think he can get sound out of it anymore, he does however now know how to discharge capacitors!

It's sulking, and it bit him as a dying gesture.  ::)

Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on April 12, 2014, 04:49:19 AM
Hello everyone! Been a while. I took the amp back off my mate so I could have another look, however I've got to fix an additional mess that he created. Pretty annoyed here. First off, he was getting sparking, this was caused by no solder on the joint of the mains fuse holder, second, hes replaced the rectifier for the heaters with one that doesn't mount to the board so I've got to mount that on the chassis (cant put the old one back as hes broken the tracks) and third he didnt put the nuts back on the pots when testing and this is the amp's only ground points.

A lot of nuts are also now missing, found big blobs of solder on cable cut offs loose underneath the board and the whole thing is covered in cat hair. One is far from impressed!
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: DrGonz78 on April 12, 2014, 05:10:33 AM
Good thing it is back your possession, poor amp! But that's not too bad of work done on the amp.... if the cat was doing the work  :lmao:. Good luck!!
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on April 12, 2014, 11:35:03 AM
Quote from: Littlewyan
I've got to fix an additional mess that he created. Pretty annoyed here.

We know just how you feel.    :trouble

Quote from: Littlewyan
the whole thing is covered in cat hair.

(http://www.sign-farm.com/images/116-0292.JPG)
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Littlewyan on April 22, 2014, 03:33:46 PM
Right, sorry its been a while. Got the Amp powered up, however I had a very low distorted output. Did the battery and neon test on the OT, dead. Checked the Amp with my scope and found the Output was fine until I got to the otherside of the OT. I think my mate fried it when he connected one of the primaries onto the heater supply. So thats the end of that.
Title: Re: Marshall DSL201 OT Possibly Blown
Post by: Roly on April 23, 2014, 10:44:58 AM
Some you win, some ...   :duh  (sigh)