Marshall VS100 Combo Cutting out after a few minutes

[fleshtwister]

Can anyone help me? I have a Marshall VS100 combo. I found this thread and it seems people here might have an idea of what's wrong.

I replaced all of the transistors that are along the heat-sinks with the ones mentioned earlier in another thread but it did not change the problem.

The problem is, when I use the amp, it has a lot of crackle in it and the volume drops up and down but never as loud as it should be. If I pull up on the PCB near the transformer (see attached photo) the sound gets very loud like it should be. As soon as I let it go the volume goes back down to almost nothing. After a few minutes, the sound cuts out all together and nothing will bring it back except turning it off and back on. When I do turn it off I get a VERY loud pop through the speaker.

I have replaced the tube and that had no effect at all.


Thanks in advance for any help or suggestions.
BTW, I do have the schematics but I'm not very verse on all the electronic terms so you may have to dumb it down a little.

[Enzo]

If you have eliminated the phones jack, and if the phones are also cutting out, the jack seems innocent, then check the other things I mentioned elsewhere, in particular that resistor.

Isolate the problem, plug a cord into the effects send jack and connect it to the input of another amp so we can listen to what comes out over there. If that fades away too, then the preamp has the problem, but if that stays strong, then the power amp has it.

Did I suggest connecting a spare cord from effects send to return? Try that.

[fleshtwister]

Enzo, I just wanted to give you an update. I just got done playing my guitar through the amp and it seems to have cleared up. I'm not 100% sure but I think you were right about the headphone jack. It was doing its usual crackling and I put a 1/4" headphone plug in just a tiny bit and put a little pressure to one side and all the crackle cleared up and the amp stayed on level. Dirty or clean it worked perfect.

My new switches should be here Monday or so. Once I replace those, I'll give ya an update. Thanks again for all of your help!!

You Rawk my man!!

[fleshtwister]

The switches haven't arrived yet but I actually am to the point that I don't think they are the problem.

I was playing the amp today and it had the crackle again. This time nothing around the switches cleared it up. But when I jiggled the cable plugged into CON3 the crackle stopped and I could play fine. After about 20 minutes or so, the amp just cut out with a soft "pop". I couldn't get the sound back again without turning the amp off (loud pop) and back on. When the sound was out, I jiggled CON3 again and I could get some crackle sounds through the speaker but nothing else.

Also, I thought I would note, that when the sound cuts out the tube is still glowing but the two little LED lights near it (LED2 & LED3) no longer light up when I hit a note on the guitar. Also, I noticed that when the sound cuts out, the power light dims. When I turn the amp off the light dims more then shots back to real bright (loud pop) and then the light goes off. Of course, this is over a stretch of 1 second or so.

So, I'm starting to think it's not the rear switch plugs after all...

[fleshtwister]

Well, I replaced my voltage regulators. I'm thinking they must have been the wrong one since I burned out a resistor at R18 on board. It says 100R on the schematics for that resistor. Does anyone know which one I should get? The closest I could find on Mouser was here http://www.mouser.com/Search/Product...OlVnRtJw%3d%3d

Anyway, I think I may be getting closer but this trial and error isn't going to well LOL.

Thanks again,
fleShTwIsTeR

[Enzo]

ANy plain old 100 ohm will do there, it is not in the signal path.

I doubt the 7812s had anything to do with R18 burning up, R18 is in the power amp, where the 12v rails don't go. I fear your power transistors are shorted. Otherwise ther is little reason for that part to burn. Check the four power transistors - power off of course - for shorts between the legs - T9-T12. Also check the power resistor for each one for open condition - R19,21, 24,25 they are 0.33 ohms.

A shorted T11,12 and an open R19,24 would burn R18.

[fleshtwister]

The only reason I thought it might have been the regulators because I noticed one is 1.5A and the other is 1.0A so I thought that might be the culprit.

I will see what I can figure out on the ones you just mentioned.

Thanks again!
Kevin

[fleshtwister]

I was testing various pieces with my multimeter and I noticed the readout does not change when I check C17. That is a capacitor with a value of 2U2 63V. The trouble is, I can't fine those online anywhere.

Does anyone have an idea where I can find them? I couldn't find them on Mouser. It's the type that lays on the board with a wire coming out of each end.

Thanks,
Kevin

[Enzo]

The stand up caps with both wires at one end are called radial. The caps with a wire out each end - similar toa resistor - are called axial.

This ain't rocket science. If you need 2.2uf at 63v and can't find it, try looking for just 2 instead of 2.2. Or even 3uf. Can't find 63v? Look for 100v or sometimes 75v.

Did you remove the cap to test it? It might have a resistor across it on the board and that might confuse your meter.

[fleshtwister]

No I didn't remove it. I just touched the leads with the meter. The one next to it gave a reading (don't remember what it was) and it had the same values.

I'm honestly about ready to put this up on eBay as a parts amp since I have almost replaced everything and I can't find the parts that I haven't replaced.

I figured there isn't too much left to replace that's a major component.

Thanks for your help, I really appreciate it. I just don't know anything but to replace with exact parts. Sorry for my ignorance.

[Enzo]

It need not be a major component though. Some dinky little resistor open is all it takes to send the entire amp off into the blue.

[fleshtwister]

Yeah, I was thinking that right after I sent the message.

It's getting so frustrating because I have little knowledge of this stuff and I seem to be going backwards. The only thing I get out of the amp is a hum sound.

I tried running a patch cable from the FX Send to an input on another amp and the other amp gives the same hum.

I was blowing fuses instantly but I replaced the caps at C11 and C12 and that fixed that problem.

I just ordered a new Carvin amp so this will just be sitting if I can't get it fixed soon...

[fleshtwister]

Well I replaced the little Cap at C17 and now instead of just getting a hum out of the amp I get sound. But the sound is so faint that I have to turn the volume and gain all the way up to be able to barely hear it. It is like that on all 3 channels. The sound is so quiet that my electric guitar almost matches it in volume out of the amp.

It is an improvement from no sound but it's still not usable.

I tried running a cable from the FX Loop Send on the back into my new amp and I get good levels. There is a bad hum in the signal but it does come through. I tried running a patch cable from the send to the in (FX Loop) and it made no difference at all.

Any suggestions?

[Amp Kat]

What are the voltages on the power transistors attached to the heat sinks. From chassis ground you should get around +50 volts on one and on the other side@ -50 volts. The regulators on the preamp power board are around +/-15 volts and you can check those voltages on the op-amps also from ground to pins 4(-15v) and pin 8 (+15v). I think those are TLO-72's maybe. Inspect the tube retainer clip where it goes into the board it will sometimes dig into the trace to it's ground point killing the supply voltage. Remove it and fix the trace if it did this.

[fleshtwister]

Thanks for the info KB. Is it possible to explain how to do the tests you mentioned. I'm a bit lost on the settings for the multimeter and what exactly I should put it on.

I'm assuming that I would test the amp while it is off and not plugged in, is that correct?