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Author Topic: Marshall DSL201 OT Possibly Blown  (Read 14244 times)

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #15 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?
« Last Edit: January 17, 2014, 01:24:56 PM by Littlewyan »

g1

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Re: Marshall DSL201 OT Possibly Blown
« Reply #16 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?

g1

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Re: Marshall DSL201 OT Possibly Blown
« Reply #17 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.

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #18 on: January 17, 2014, 02:43:47 PM »
Looking less and less like the OPT.  Phew.   :cheesy:
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 #19 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.
« Last Edit: January 17, 2014, 03:25:02 PM by Littlewyan »

g1

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Re: Marshall DSL201 OT Possibly Blown
« Reply #20 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?

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #21 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.

g1

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Re: Marshall DSL201 OT Possibly Blown
« Reply #22 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.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #23 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

J M Fahey

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Re: Marshall DSL201 OT Possibly Blown
« Reply #24 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.

Littlewyan

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Re: Marshall DSL201 OT Possibly Blown
« Reply #25 on: January 18, 2014, 04:08:31 AM »
O right, in that case i'll just use my Avo in future.

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #26 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.
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 #27 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?

Roly

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Re: Marshall DSL201 OT Possibly Blown
« Reply #28 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.
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 #29 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.