Noisy valvestate 8080, and help finding a debuging page?

[Satamax]

Hi everybody!

I've had this 8080 on my bench (well gathering dust at the end of teh workshop for may be two years now. It's crackly or hissy. I'm pretty sure it comes from around IC1B but can't seem to find where's teh problem, well, up to my level of skill. I start to know how to debug a valve amp. I might lack methoid thought. But i don't know how to work my way around a solid state amp. Any advice as to what to check?

And if anybody knows a page like tube amp debug for solid state, please let me know.

Best regards.

Max.

[Enzo]

Noise in a SS amp is almost always a semiconductor. But regardless of tube or SS, if it seems to be IC1, did you replace it? If you thought V1 was noisy in a tube amp, wouldn't you replace it? WNy would it be any different now?

The essence of troubleshooting is to isolate the problem. If it is noise, scope the noise at the power amp, and work back stage by stage until you come to the point where the noise no longer exists. Isn't that how we troubleshoot tubes?

As with any sort of amp, if there is noise, turn the controls up and down. Any that affect the noise ar after its source. If the control has no effect, it is before uts source - or in a different channel or something.

[Satamax]

Thanks a lot Enzo. Well, i guess, i should "troubleshoot" the way you described. But i'm more of a debuger, going by the tube amp debuging page checklist and another one i have in a french book i have. And a few tricks i've learnt along the way. I tend to get the sig gen and scope only when i can't do another way. The problem is that my scopeing skills might not be up to par. Anyway, since VR1 affects the noise, teh problem should lie at the first stage. The funny part is that when crunch is engaged it doesn't have the problem anymore. May be it's noise cancelation due to the second stage using inverting input?

Here's teh schemo, i thought i had it linked in my previous post.

http://www.schematicheaven.com/marsh..._8100_8412.pdf

Thanks again.

Max.

[Enzo]

If you don't use the scope, you will never get good at it.

Let me restate: any control that affects the noise is after its source OR AT ITS SOURCE. In other words, IC1 might be noisy, or IC2, which VR1 controls.

Look at the preamp schematic. The top row of stuff is the clean channel. The bottom row of stuff is the dirt channel. Now look at the input jack. The input from the jack goes through C1 right into IC1-B pin 5. But look closer and you will see right after C1, a line drawn from right over R2 snakes down the left side of the drawing to go into the lower row of stuff at IC1-A pin 3. In other words the signal splits right at the jack for the two channels. No wonder the dirt channel is not affected, it is a completely separate circuit path.

Oh wait, you said crunch, not lead, I guess I was talking about the OD channels, sorry. When the crunch is on, there is a cap in parallel with the LEDs that are added in as a clipping element. That tends to mute higher frequuencies, noise included. Also in crunch SW1 ups the gain of IC1B some.

My vote is simple, replace IC1 and see what happens. Just for efficiency, I'd just replace IC2 while I was in there. Over here those ICs cost about 29 cents each, and replacing both is a lot cheaper than my labor for finding which one was the noisy one. ANd if it turns out some other odd thing is the problem, I am only out 58 cents for the confidence those ICs are not the problem. Most of the time, parts are cheaper than labor. Note the ICs are not the same. IC1 is a TL072, or most any general purpose op amp like the 4558, 4560, 2068, whatever. IC2 is a TL071, could also use a TL081, but 71 preferred. A 5534 would likely work well there also.

[Satamax]

Ok Enzo, thanks a lot for your reply.

Well, i never saw it under this light, parts are cheaper than labor!!! I'll see what i can do with thoses IC's.

Thanks again.

Max.