Marshall Valvestate 80v 8080 troubleshooting noises

Hi all-
I picked up a Marshall valvestate 80v 8080, and I'm trying to troubleshoot a couple problems.

The clean channel is not working right, and makes this very weird sounding distortion;


With the volume turned down low, it makes a really egregious amount of hiss and low signal, with this distorted sound punching through, and then higher, it's all distorted. I did try cleaning the hell out of the pots and they are giving me normal resistance readings.

I checked the voltages on IC1 (a TL072) and it's showing -15v on pin 4, 15v on pin 8, and nothing on any of the other pins, millivolt readings. Is this indicative of a fault in the opamp? I have seen in other threads that there are a lot of cases of that opamp blowing in this amp for some reason. The opamp in IC2, on the other hand, is a different story, I think it has normal voltages - could someone tell me what to expect?

The other problem is a buzz that sounds like a ground loop, when using the boost channel, which sounds pretty good. It would be nice if it was related to problem 1, but I doubt it. I know this has been covered in other threads, but it would be great to get realtime feedback on what to try.

I have tried:
1. replaced the filter caps.
2. Tried re-flowing the solder on those big resistors that get warm R119, R120
3. Tried this grounding workaround noted on other threads, but I'm not sure I'm doing it right (remove a ground somewhere else?)
8080 ground loop b.jpg

4. Tried different 12ax7 tubes

The transformer is one of those guys that hums/vibrates audibly/mechanically with the amp turned on. I notice that this buzz is there at low volume with the amp turned on, everything turned down; turning up master volume does not affect it. But with master up, the volume on the boost channel (the working channel) increases the buzz. I'm not sure I understand this metal plate that the pots are all mounted to - it's like a ground rail with the pots 'isolated' from the chassis, but I get continuity there all around.

If anyone has insights on this well-known amp, would be glad to hear them. Schemo attached below

Marshall-Valvestate80-80W-8080-8100-8412-Schematic.pdf - Adobe Acrobat Reader (64-bit) 5_25_20...png

Marshall-Valvestate80-80W-8080-8100-8412-Schematic.pdf - Adobe Acrobat Reader (64-bit) 5_25_20...png
 

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Thanks, good suggestion. I think the input jack does not ground properly on the tip with the cord unplugged, but it separates ungrounded right with the plug in - so it needs the jack but I don't think that's the problem (?)

I tried a signal tracer and I don't hear the buzz on IC1, IC2, IC3, but it's there on the input of IC4, so I'm closing in on it.

Do those large capacitors C53 and C59 (pictured to the left of the tube) filter whatever hum or buzz may be coming from that autotransformer - would that be the failure point?

I can actually hear the buzz through the signal tracer by waving it through the air near this autotransformer pictured here. So I guess it is the source of the buzz,


Marshall_auto_xf.jpg
 
If you've checked the caps, then the next step is checking the resistor values.

I have to say I hate it when people design kit that places capacitors close to heat generating components like tubes that radiate the heat - especially when the cap is trying to radiate/cool itself due to the duty cycle at full tilt.
 
Actually I haven't replaced those caps. I replaced the main filter caps, but not C53 C59, which are the current suspects.

Yep it's a very poorly put together amp, fragile and wonky. I read somewhere that this amp can run without the tube, but then the HT voltage gets up to 450v, which is above the rating of those caps. That doesn't seem good.
 
I know the JCM uses a 345-0-345 transformer (ie 487Vdc unloaded) so it depends on the load but yes.. quite a few amps shouldn't be run with the tubes out due to the voltage rise and the power supply isn't regulated for your 12ax7.

Two 7W resistors on the board, and right next to it, tube the other side.. that would be a prime target for replacing with a 125degC cap.

Could also check the regulators on the IC power supply, they may be shorted/open and that could cause additional noise into the channels even if at low volume too.

It uses bridge rectification so it's going to have 100/120Hz noise if it's after the rectifier. If it's 50/120Hz then I'd say it's the transformer/shielding/ground/first filter cap rather than the other components.

There's no sign of anyone doing any mods to the amp?
 
No signs of modifications on the amp, it's just very poorly built - I think the 90s was the low point of quality.

The buzz definitely enters the signal at the preamp tube - it is not there earlier in the signal chain. It's not the tube itself or the socket. That's why I suspect c53 and c59.

What are the regulators on the IC power supply? Those diodes?
 
Checking all cap and resistor values is a long, tortuous and incomplete way, besides you mess with PCB soldering and pulling/replacing parts.

Amp is a system and you test functionality first, then you measure voltages around the suspect area and then replace parts which look suspicious or match weird measurements.

Inject 100mV 1kHz at amp input and scope it along the way, you will find somewhere a chopped-incomplete-broken-missing wave, measure voltages in that stage searching for abnormal values.

Out of 1000 failures, maybe 600 are caused by "bad parts" and 400 by poor soldering - bad connections - broken wires - supply voltages - biasing - etc. , my point being that even if you replace all parts on a PCB with fresh guaranteed good ones you are still NOT guaranteed it will work, by any means.

Not such a daunting task, by your description your problem is located on the clean channel between C1 and C16.
Might be a bad part or a bad connection, including bad solder joints and broken tracks.

As of:
I have tried:
1
2
3
4 ...
sadly all those are outside the suspect area.

Thanks for posting the schematic inside the thread itself.
 
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Thanks for the reply @JMFahey, that's helpful. For right now I am putting aside the failed clean channel, and might circle back to it. If I can get rid of the buzz in the boost channel I will just take the amp in that state.

That buzz seems to be first picked up in the signal tracer around R95 / R71. It's there with the tube pulled also. It's not there on R69. The buzz stops if I put a jumper from one side of R71 to the other side of R71, but of course the signal also goes to ground.

R95 and C54 are right next to the autotransformer seen in the picture in #7. I can hear the buzz from that transformer just holding the signal tracer in the air a couple inches away from it, if that makes sense.

That's where I am with it at the moment.
 
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Well, doing the divide and conquer @JMFahey I found the fault in the clean channel; it was the IC2 opamp, even though the voltages were right. Could sort of tell from the beginning that it couldn't be anything but an opamp by the sound of it. So let's clock that a win; clean channel restored with parts in my bin.

Still puzzled by the buzz. It's present at a low (but too much) level even using the clean channel, but it doesn't get amplified more with increased gain like it does on the boost channel.

I find it "first" with the signal tracer at around R95/R71 (also R109 ?), I think, but I'm not clear about how the signal flow even works, and whether it could be there because it's coming 'backwards' from a later point in the flow. Can someone help me understand that?
 
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Back to this amp;

I notice that when I pull IC8 (it put it in a socket), there is no buzz whatsoever. Dead quiet.
Okay, so that means it is in the preamp section for sure, right? Pulling IC1 or 2 (both in sockets) does not make the buzz disappear.

But curiously, when I short C18 to ground, or short C16 (end of channel one) or C17 to ground, the buzz does not go away.
Also, the signal from the preamp (I'm using a test tone) gets lower but does not entirely go away. I expected it to kill the signal.

I tried shorting R61 and R58 to ground to see if the buzz was coming 'back' in to the non-inverted input. Both make the buzz increase a little.

What does that mean?