Give me a clue mate, can't find a 33v zener...

Hi Stu,

5302 brings up a 12/20 watt keyboard amp. I think you mean 5203?..

http://www.drtube.com/schematics/marshall/5203.gif

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

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

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.

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!

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.

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!

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.