What's wrong with my amp?!?

[Ron]

If it is a transistor breaking down in the op-amp, you will likely hear the crackling sound all of the time in the OD channel, even with no guitar plugged in.

I forgot to mention this symptom. If I had the amp plugged in, but no guitar plugged into the input, it crackled constantly.

Ding! ding! ding! Lonewolf called it.

[lonewolf]

But, like Jeff said, the actual cause isn't really the point. With the described symptoms, I would suspect the power supply to those op-amps in the OD preamp section. They may be running with true +/- voltage rails, and one of the rails is down or weak. If they are using a voltage divider to supply the op-amps, it could be a 1 cent resistor causing the whole issue.

I was thinking that too, until I saw the schematic.

All the op amps in both preamp circuits share the same +-15V power supply connected directly to the chips with no supply resistor. There is a 10uF cap across the voltage pins for each chip.

If there was a supply problem, it would most likely show up in the clean circuit too.

[lonewolf]

One final note:

Since the clean and OD channels share 1/2 of IC1 & IC7 op amps, although its possible, its probably not those.

The acoustic simulator channel shares IC6 with the OD channel, so if the acoustic simulator doesn't work right, its probably IC6.

If the AS channel does work OK, although its possible, its probably not IC6.

The only op amp that is exclusive to the OD channel is IC8--that's where I'd start.

[JackANSI]

She could have applied "power" to the system when the static electricity jumped from her finger, to the speakers connector, then through the circuit (power transistor might have been able to take the ESD no problem) to the opamp (would have a very hard time handling an ESD). Tada.

http://www.charleswater.co.uk/PPT/ESD%20Basics.ppt

Its not like the only way to damage a circuit is through connecting power or even connecting something wrong...

My main question is "Why did he replace the power IC's when the clean channel works?".

I also highly doubt if it was ESD. The charge would have had to somehow jump past ground to the tip connector inside the jack. I don't see how it could. The op-amp circuit would also have to be more robust than that. Consider how often the tip of a guitar cable is touched while it is being plugged into a guitar, while the other end is plugged into the amp. On the input side it is practically going straight to the op-amps.

For a static charge to fry a bipolar analog circuit like those op-amps, the charge would have to be really hefty. I've purposely tried to fry op-amps before with ESD... not with simple static charges, but a cattle prod that could spark across a 2 inch gap and make you scream like a little girl. After hours and hours of abuse, we could not get any of the op-amps to fail, and they were cheapo 1458s. A JFET op-amp is a different story.

But, like Jeff said, the actual cause isn't really the point. With the described symptoms, I would suspect the power supply to those op-amps in the OD preamp section. They may be running with true +/- voltage rails, and one of the rails is down or weak. If they are using a voltage divider to supply the op-amps, it could be a 1 cent resistor causing the whole issue.

If it is a transistor breaking down in the op-amp, you will likely hear the crackling sound all of the time in the OD channel, even with no guitar plugged in. When this happens in a preamp circuit... look out, since the noise can get worse quickly over time and easily push the amp to full output... really, really loud and potentially very harmful to those freshly replaced output chips.

It sounds like the tech needs a schematic, signal generator, o'scope and a troubleshooting primer.

The part number is a JFET opamp, just like the rest of the 072 family. And they REALLY don't like any kind of voltage above about 200V coming in on the output pins. And it has more to do with where the voltage is applied than how much, as you found out. The amount of current would effect the extent of the damage.

A resistor failing in a voltage divider would be much more gradual than "it works then it didn't" without some serious outside involvement with sustained current. Chances are the power supply would be shared across all opamps with only capacitor isolation (and possibly shared with the +/- rails of the power amp's power supply).. So the clean would be messed up as well, unless it goes straight to the tube, which I highly doubt. It sounds like they are using the tube as an EQ/Compressor in the configuration they have. And you'd be stupid not to put an input buffer first before the channel switching circuit.

And when a transistor is damaged by ESD, it can sometimes pass small amounts of current cleanly, with a much smaller saturation voltage due to the damage (which distorts the signal in a very unpleasant way) but the signal currents at this point should still be low enough that the opamp can handle it without total failure.


Edit: Oh and after some google searching I found a mention of this problem...

http://music-electronics-forum.com/t1322/

someone says something about a TSB because of OD channel failure, due to ESD on the input... So it might have happened when she went to plug in after hooking up the speaker again...

I could be of more help, but I can't find the TSB or the schematic through google with the time I have available right now. But I'm certain that Marshall hasn't taken the time to rewrite the book on amps with this model...

Totally agree that the tech working on this is an entry-level one and probably shouldn't be paid for his time, just the parts he used.. And shame on Marshall for not taking more time with their design..


Edit edit:

And U8 was the culprit on that thread. Once again, nice work lonewolf.

[JackANSI]

LOL, I don't know....I took it to Music Emporium. Is anyone familiar with who does the amp repairs there? I don't know the guy's name. I figured they probably have a lot of experience fixing amps but at this point I wish I had tried somewhere else instead.

They probably deal mostly with people who accidentally shorted their outputs and fried that stage, the fuse, and/or the power supply. Outside of that, they probably would rather have you look at a new amp so they don't really try.

Might or might not be the case, but I've seen that syndrome the most.

Its probably not the same amp guy I remember from Jarret Music. That guy knew his stuff. (and called the cops on me one night when I was trying to get into one of the Pittsburgh area 2 meter repeaters from the hill in front of Valley View park. He thought I was a vandal and I had no idea he lived up there)

[lonewolf]

And U8 was the culprit on that thread. Once again, nice work lonewolf.

Thanks. Ya gotta wonder why the Brits use those goofy reference designators. After years of working with Mil-Spec-202, that Marshall schematic seemed more like a multi-page connect the dots puzzle.