Marshall VS 8080 IC1 Blowing.

[Nelec]

I repaired a VS 8080 for a fellow awhile back. It was in fantastic condition, just repaired a few dry solders joints, and it worked well after that.

About 3 or so months later he called back saying it had stopped working.

I loaned him my 8080 test amp, and grabbed his back for a look. I traced it down to IC1, popped a socket in, and replaced the IC, and it was all good again. I gave it a thorough playing with a couple of guitars to test. I also replaced all the power section electrolytics for good measure.

In the meantime the loaner amp failed. This amp is a little modified, so I wondered if my mods had taken something out in the preamp, but when I got it back it had the same symptoms. I replaced IC1 and it was fine again.

Then his amp failed again with the same symptoms, within a short timeframe.

My gut feeling is that something being plugged into it is taking it out. Given that the exact same fault has occurred on two different 8080's, it seems unlikely that it is a pure coincidence.

He is a little concerned, which is understandable, as his small Peavy practice amp (I'll chase its details up) works with the same setup without failing. I'm thinking it has a more robust input section for whatever reason.

I asked him if anything was changed gear wise around the time of the first failure, and he had started using a loop pedal, and something else I can't recall. I'll chase these details up also.

Preamp - http://www.drtube.com/schematics/marshall/pc0689.pdf

Power Amp - http://www.drtube.com/schematics/marshall/pc0689p.pdf

My questions are as follows:

1/ Has anyone else come across this fault? Is it common? I haven't found it in any of the other 8080 posts that I've read.

2/ If this is a weakness, any suggestions as to how I can make this section more robust without affecting the signal? Some kind of buffer pre IC1 perhaps?

3/ I was going to take the loaner back, tell him to plug his pedals into the FX loop instead of the front end, and see how it stands up after that. Is it fair to say that the FX loop would be more robust as this is its intent, or will IC8 be the next candidate to start blowing (both IC1 & IC8 are TL072 or similar), or I barking up the wrong tree here?

Thanks.

[loudthud]

There is probably something in the owners rig that is applying excessive voltage the the input. It could be leakage current from an ungrounded power supply on the pedal board or an ungrounded outlet that the amp is being plugged into. The fix would be to add protection diodes to the input(s) and a small series resistor. Diodes should be small signal types 1N914, 1N4152 or the like. 50V 200mA should do the job.

[Nelec]

Thank you very much for your response. I had looked at a few different schematics and noticed that the marshall V100, 8280, and a peavy amp I found a schematic for all had some form of protection.

I was thinking of using a pair of 1N4148's to the rails, but wasn't sure if the resistor is needed.

What effect will it have on the signal, and will I need to modify the anything else to obtain a "transparent" addition of this protection?

Thanks again.


p.s. I copied your signature, a very important point to make.

[loudthud]

The way it's drawn there will be a slight signal attenuation and high frequency rolloff. They should be too slight as not to be noticed. The 10K could go on the other side of the 1 Meg, it all depends on where there is room on the PCB. Just make sure both inputs to IC1 are on the side of the 10K where the diodes are. I would solder the diodes on the solder side of the PCB right under the IC. Then find a place where a PCB track could be cut to bridge with the 10K, or just lift one end of C1 and solder the 10K in series.

[Nelec]

Great, thanks for that.

I've had a look at the pcb, and will lift one end of C1 as you mentione to add the series resistor. I looked at suspending a daughter board above the area to add these components, but I think I'll just solder to the underside, with a bit of heatshrink over each diode.

Thanks!