Marshall Valvestate 8080 Muddy, Scratchy and Noisy

[Pigyboy]

Hi,
I have a marshall valvestate 8080 that makes some sound but it is very muddy, and scratchy. I have replaced the preamp tube and the two 2200uf caps but this did not help. Is there anyone who has any idea where to check next?

[drewl]

Yes, the pots and jacks often go bad on these.
Clean all pots and jacks with a good cleaner like DeOxit, try running a patch cord between the fx send and return jacks to test them, and there's a good chance the pots and jacks may need resoldering to the pc board.

I'd start with this simple stuff before troublesooting the electronics.
But there may be a bad or failing opamp.

[Pigyboy]

Thanks for the input Drewl. I have gone over the board and the connections several times and cleaned the pots and jacks. The problem seems to not change. I will give it all another good look and let you know.
Thanks
pigyboy

[Enzo]

Then I would be looking for a noisy failing op amp. One quick check is look at the output pins of each op amp. Any that have a DC offset are suspect.

[Pigyboy]

Dc offset...

HI,Thanks for the input Enzo. Can you please give me a quick run down on how to check the dc offset?
Thanks,
Pigyboy

[Enzo]

Oh, it is nothing cosmic. Op amps run on a split power suply, which means a positive and a negative of the same value - typically 12 or 15 volts. The inputs and outputs should then sit in the middle at zero volts DC. So ground your meter and probe the output pins of each op amp. ANy that are not at zero DC are suspect. I am not woried about a few millivolts, I just don't want to see a volt or two or more. Well actually, I'd be concerned over 250mv come to think of it.

DC offset is not a good sign, but a chip can get noisy without offsetting its output pin too. So it is not a complete test.

[Pigyboy]

Marshall, Now the Mackie

Thanks Enzo. I truly appericiate the help from all in the forum. I am learning about these things as I go. I will give the Marshall a look now. I also have a Mackie 1642 VLZ powered mixer that has all of the overload lights lit. I am thinking the power transistors are suspect. There was a 6800uf cap with the top popped so I replaced that but it made no difference. How do I go about checking to see if the tranny´s are good or not?

[Alex R]

There was a 6800uf cap with the top popped so I replaced that but it made no difference.

Top of my head, probably it took some other parts with it. The overload is likely to be mains hum. Any power resistors in line with that 6800? Any looking burned? Any other filters along the line looking swollen? Is the one you swapped paired with another 6800? Might be best to replace that too; at least check its solder joints. Some prime suspects for a big mains hum are filter caps and rectifiers.

What equipment do you have to test it?

If the power transistors are bad they are very likely shorted, and if so they will blow fuses, and test short. So the mere fact that the lights are lit makes me suspect the preamp or the power supply instead. Look for diode effect between the terminals of the transistors, ie different resistances when you swap the meter leads around on them. This you should see in at least two places on each transistor. Does the amp have no output?

[Enzo]

Do you mean like all the input channel peak lights are lit? When I see a mixer with ALL hte lights lit, it is almost always a missing power rail. Whatever the op amps tun on - +/-12, +/-15, whatever - see if one is missing.

If it is just the power amp indicators, then it still could be a bad power rail, or blown channels.

[Pigyboy]

Sorry for the confusion...

HI guys,
I think I confused you Alex with yesterdays post. Enzo is having me check the dc offset on the opamps in the Marshall. The Marshall still has an output just very noisy. I am doing all of this with a standard voltage meter. I have been busy so will try to check the Marshall today.
I then added in my post a question about a Mackie mixer with all the overload lights lit and no output. It was on this I replaced the 6800uf cap. I will try to check the power rails on the Mackie today according to Enzo´s instructions. Thank you both very much for the help and bearing with my learning curve on these subjects.
Pigyboy

[Alex R]

HI guys,
I think I confused you Alex with yesterdays post

Don't need anyone to confuse me, I can do it all on my own.

[Enzo]

My wife is responsible for confusing me. I don'r need to do it on my own.

[Pigyboy]

Hey guys,
I am totally confused all the time that is why I moved to Spain where I can not even speak the language. Yes, I found IC2 to have a DC offset of almost one volt on the Marshall and in the process upon closer inspection I found the channel switch to be coming apart. I am ordering the switch and as a precaution I am waiting for it to arrive before I test anything out. This seems to me the proper order of taking care to not damage anything else. About the Mackie mixer. I am getting 16v +/- out of the power supply which seems right but where do I check the power rail at?
pigyboy

[Enzo]

That +/-16v has to actually get to the ICs that want it, it is not enough that the power supply makes it, so look at an op amp or two on the board and see if it has both rails at its corners. "Rail" is just another word for power supply.

[Pigyboy]

Closer Inspection proves Important...

Hey Guys,
First off thanks for all of the help. To close the Valvestate story I found that IC´s 1 & 2 both had a DC offset of almost 1 volt. I changed those using chip sockets in case they prove problem again in the future. When I was closing the amp back up I noticed a problem with the channel switch so I waited til the new switch arrived to check the amp. Once back together the problem was better but still not right. One more look inside revealed hairline cracks in the solder joints of one of the power transistors. Yes, I think this was the main problem from the start but without the aid of a magnifying glass I was unable to see the cracks in the solder. This is about the 5th or 6th Valvestate I have had that had this similar problem but all of the others the problem was always cracks in the solder joints of the large output caps. Noisy Valvestate? Check ALL the solder connections very carefully. OK, now for the Mackie mixer. Once I got it it had been opened already and yes it was apparent there was a blown cap in the power section. I replaced it and put it all back together but still all of the overload light were on. Closer inspection again I found it is able to reverse the cable that comes from the power supply section. Flipped the cable over and guess what? Yes it all worked perfectly. I learned several things here. First I understand DC offset now, next the term 'power rail´was added to my meager vocabulary and in the end it turned out to be common sense and really looking closely at what is going on that solved these problems.
Thanks everyone and Merry Christmas,
Pigyboy