Marshall 5210 lunching a resistor

[primofacprima]

Hi all, I was wondering if someone could give me some advice.

I have a Marshall 5210 that keeps burning R60 on this diagram - http://www.drtube.com/schematics/marshall/5210-iss3.gif - It's the one right at the bottom of the schematic on the D bus.

I have swapped out C47 & C27 but notice that there is no resistance across the mounting holes for C47 with the capacitor removed - would this be normal?

Having a quick poke with the multimeter (with a face full of smoke) showed 26v at the R60.

Any help would be fantastic, Thanks for reading.

Joe

Edit: I've just removed ZD3 and found no resistance across it. New one on order. What can do this?

Cheers, J.

[g1]

  The ZD3 zener may have just failed.  They usually short when they fail which explains the low resistance reading.
  To be sure there are no other faults on that -15V line, check resistance across the points where the zener used to be.
  A low resistance reading there will mean there are other faults.

[J M Fahey]

Hi all, I was wondering if someone could give me some advice.

I have a Marshall 5210 that keeps burning R60 on this diagram - http://www.drtube.com/schematics/marshall/5210-iss3.gif - It's the one right at the bottom of the schematic on the D bus.

I have swapped out C47 & C27 but notice that there is no resistance across the mounting holes for C47 with the capacitor removed - would this be normal?

Having a quick poke with the multimeter (with a face full of smoke) showed 26v at the R60.

Any help would be fantastic, Thanks for reading.

Joe

Edit: I've just removed ZD3 and found no resistance across it. New one on order. What can do this?

Cheers, J.

1) "no resistance" is not a reading.
I guess you mean either "open/infinite resistance" or "shorted/zero resistance" shown on the multimeter display.

2) from *other*  things you mention I guess it means a short; to be more perecise, a shorted zener diode.
Yes, that would place more voltage (= higher stress) across R60 .

What worries me a little is that you don´t mention a symptom that is evident to anybody trying to use that amp, without even opening it: that it does not work, because that´s what happens when you lose the entire -15V rail 

Reminds me of the old Doctor joke, where a patient phones in the middle of the night complaining about a headache.
The Doctor suggests "take an aspirin and come see me in the morning"
1 hour later he phones again, complaining "it didn´t help" .....
After 3 or 4 such calls, he sheepishly admits : "I forgot to mention, I was in a bar room brawl and the owner hit me in the head with a baseball bat" .....

As of the exact reason why it failed ... sh*t happens ... stuff does have an average service life and average means that some last more than that, some last less.

It´s a reality of Life, applies everywhere, that´s why there´s Techs, Mechanics, Doctors, and Servicing in general ... and Forums like MEF 

[primofacprima]

Thanks fellas.

g1 - Cheers, I will check that.

J M Fahey - Yeah, I did mean shorted.

I got the amp as a gift for a mate, and when I looked in the back some clown had used pick-up wire to connect to the speaker. It was making a loud 50hz sorta noise too so I stripped it down and re-flowed some suspect joints and reconnected a lead. That cured the noise, but on testing it I then got the smoking resistor. For the short time it was on it seemed to work fine, but I obviously switched it off as soon as it started smoking.

I apologise for being as clear as mud! I am on a steep learning curve, and didn't know if there are common things that could cause this kind of thing to happen.

Thanks once again!

[J M Fahey]

 :dbtu: ...

[primofacprima]

GGAaaaaaahhhhh!!!  :grr

I replaced ZD3 and repaired an open circuit to the transformer along the C rail.

Fired it up and wow! What a great clean tone from the clean channel... but the boost channel doesent work now. 

No noise from it at all with or without the footswitch. The LED does come on with the footswitch.

On testing I have:

1. Continuity from the jack skt to IC2b , so ok to there.

2. 9 odd volts at the 5.1v zener ZD1 (bottom of schematic, C rail, middle take off)

3. TR12 + 13 26v, TR14 +15 -27v.

I'm going to change ZD1, but should I also replace TR8 + 9? I don't know how to test these... I assume that they could be damaged by the extra voltage.

Are there any other components that are likely to have fried from the open circuit that caused my original problem?

Thanks guys.

[Enzo]

WAIT!!!

You say the clean channel sounds loud and clear?  That means the power amp is working, so stop fixing it.  You have a dead preamp channel, look THERE.  Besides, I see a 9.1v zener on my drawing.   Regardles of what is happening in the power amp, the power amp has no way to know which preamp channel is in use.  It cannot selectively go silent when one channel is selected.

You have a dead channel, so trace the signal through it.  IC2 is the lead channel, so make a quick check for DC on its two output pins, hopefully no DC there.   Put a signal into the input jack, is it present at pin7 of IC2?  Pin 1?   TR5 TR6 are mute transistors for that channel.  If they are turned on, it squashes the signal to ground.  So while the channel is on, and no sound happening, is ther a positive voltage on the base of those transistors?  While it is running, is the collector of each shorted to ground?  Or low resistance anyway?

Oh, and try this before anything else:  I bet you are not using a footswitch?  Look at the footswitch jack.  When you push a plug into it, the contacts rise, but when the jack is empty they sit on lower contacts.  The one closes to the panel is grounding.  Use a meter probe or small screwdriver and press down on that front contact.  Does the channel come back on?  That contact keeps TR5,6 turned off when no footswitch is used.  See in the schematic?  On top of the FS jack ground, the line goes over to the bases of the transistors.

[primofacprima]

Thank you Enzo,

Spot on, I can see the 5 is a 9 now! I am dyslexic too which doesn't help with these things... sometimes I can't see the wood for the trees, which is why I am desperately grateful for everyones help.

I did try with the footswitch and it did make the LED come on, but the channel was still dead. I will do exactly as you suggest, and see if I can't break something else!

[Roly]

I did try with the footswitch and it did make the LED come on, but the channel was still dead.

This looks good, the LED tells us that at least part of the switching control circuit is working, and that tends to make us presume all of it is working, and start looking at the actual signal chains for something like a dead op-amp.

Do we have confirmed +/-15V supply rails?

2. 9 odd volts at the 5.1v zener ZD1 (bottom of schematic, C rail, middle take off)

ZD1 is part of the power amp input circuit (not the power supply) and should have 5.1V across it.
Don't fix wot ain't broke.


My intuitive guess is that IC2b is boofed, exposed input 'n all.

Measure and post all the voltages (ref:ground) on IC2, pins 1 to 8.

[primofacprima]

Cheers Roly,

IC2-
1. -0.38v   8. 16.4v
2. -0.13v   7. 0v
3. -0.13v   6. 0v
4. -16.3v   5. 0v

Only had time to check voltages at TR5 + 6. near as dammit 0v on collector base and emmiter referenced to ground.

I also get an almighty crack and thump when turning it off... I know SS amps can tend to do this but this
is mega loud.

[Roly]

IC2-
1. -0.38v   8. 16.4v
2. -0.13v   7. 0v
3. -0.13v   6. 0v
4. -16.3v   5. 0v

Only had time to check voltages at TR5 + 6. near as dammit 0v on collector base and emmiter referenced to ground.

Given my hunch, and the absolute zero's on prime suspect IC2b compared to the small offsets on the other half, on the bench I'd now take a shot at fitting a socket and trying a new dual op-amp, just about anything will prove the point.

How are you on removing IC's?  Got a SoldaPulIt?  Wick? (bright lens-light)

I also get an almighty crack and thump when turning it off... I know SS amps can tend to do this but this is mega loud.

Common, and with these, but another issue unrelated.  There have been a few threads about de-thumping power switches, pretty simple.

Mains switch snubber;
http://www.ssguitar.com/index.php?topic=2439.msg19970#msg19970
{for afters, one problem at a time}

[primofacprima]

Will do Roly. dip 8 skt and 1458 on order now.

No problems with soldering/desoldering, got masses of experience with that. I'm with you on the snubber, it was just so loud I thought I'd mention it.

Joe

[Roly]

I have a very dim senile memory that we have touched on both of these here with the 5210 before; dead IC2a, and loud switch-off "crack" - and fixed both.  Seem to be generic with this model.

{'course Murphy dictates it will be something else, one of the switching transistors perhaps... (sigh) ...but it's worth a bit of a shot in the dark.}


Yeah, well the snubber ... X2-rated cap, and it's mains wiring, lotsa heatshrink.

[primofacprima]

Wasn't the op amp. Swapped it and no change.

I used to repair card readers and PBX machines that could dial out 400 lines, then in 2001 had a massive career change. Since then my electronics has been limited to guitars (rewires, tapping, phase switching etc). I hadn't realised that I had forgotten how to read schematics.

Thanks to all the previous posters I've pieced it all together again!..

Irritatingly I'm working on a 1985 amp using the later circuit diagram which has different component numbering. The earlier diagram is largely unreadable... http://www.drtube.com/schematics/marshall/5210-iss5.gif

So now with my refreshed memory I know where I should be looking. I really miss having an oscilloscope on the bench though!

Cheers Fellas

[Roly]

Wasn't the op amp. Swapped it and no change.

Bugger!  Sorry - thought it was worth the punt.  Still think it's a shorting IC (and now we know it isn't that one).

Back to methodology...

Post voltage on 33V zener and 24V rail.  {oops, Wrong amp}

So now with my refreshed memory I know where I should be looking. I really miss having an oscilloscope on the bench though!

Now you sound like a man with the bit between his teeth, so I'll just stand back here out of the way (and possible flying debris).  Don't forget to keep us up to speed on your progress (or lack of it).

No CRO!  You poor deprived lad.  Any old CRO is better than no CRO at all.