Valvestate 8080 issues

[theguitarwookie]

Hey guys, first post here. I have a VS 8080, and I have the following issues:
-no clean channel. nothing.
-No distortion on drive channel
-Effect loop knob very scratchy and noisy.
-very faint tube glow (replacement on it's way JIC)
I have, thus far, replaced the input jack with a fresh one. Before, I got ZERO from the amp, even at full volume. Now I have meh...
I know little about electronics, and figured this would be a decent starting point for learning. I have read through the first 100 or so pages and found not much about this issue. I will be happy to try anything suggested, and hope to build my knowledge with a little help. Thanks in advance!

[chipprogr]

you will find the new tube also is a faint glow- thats the way they are -its a heater filament, not a light bulb. You state that the effects loop knob is scratchy, that tells me that the main amplifier IS working, you have a problem in getting signal from the preamp to the main amp. Try taking a 1/4" cable and go out of the efx send and right back into the efx return. Then hook up your guitar or signal source as normal and see if you get sound with that patch cable still in the loop.. If things work normal, you need to replace the efx return jack. If no go, grab something that makes sound and plug it into the efx return jack- got sound?

Let us know -Eric

[theguitarwookie]

Well, I tried that and still have the same overall effect as before. Zero signal from clean channel. Drive channel makes sound, but sounds terrible and flabby, with practically no distortion, with all knobs cranked to the max! That is the main reason I have surmised a bad tube. I am not sure if changing the tube will cure my ills, but this amp was used hard and put away wet for a decade straight. There is very little change in the sound when adjusting knobs (all on drive channel, clean is just beat) I get the most change with the bass and presence knobs. Like I mentioned, I have changed the main input jack with a new one just to get anything. All PCB solder joints look good, and those that were dull I re-soldered nice and sexy. I just noticed that someone has replaced the master volume pot with a 100K pot, where the schematic calls for a 10k pot. Will that affect things much? Again, assume I am a total moron with electronics!

[Jazz P Bass]

Marshall VS8080

Did you try plugging your guitar directly into the Effects Return, as asked for by ChipProgr?
Quote:"Well, I tried that and still have the same overall effect as before"
This an important piece of info.
Understand the signal will not be loud or distorted. In fact all the preamp controls will be bypassed.
Hense the test.
We are trying to ascertain whether it is a preamp or power amp problem.
I do think the problem lies in the preamp.
This test will prove it.
Do you own a Volt meter?
Do you know how to use it?
Cause thats next.

[theguitarwookie]

As I mentioned before, I AM a moron. After correctly interpreting the test, I have discovered that when plugged into the EFF Return, I have a super-clean sound, with level control via both the master volume and effects level knobs. Does that help any? I appreciate all the help guys!

[theguitarwookie]

Also, as I mentioned before, the clean channel does absolutely nothing in any respect.

[Jazz P Bass]

VS8080

Going in the Effects Return indicates that the power amp works.
So it is a preamp problem.
Can you gain access to a volt meter?

[theguitarwookie]

I sure do have a multimeter. Thanks for the excellent schematic, btw. I have previously been using a far less legible one from some obscure website. When I pulled the board out to replace the input jack, I did not notice any visibly poor solder joints or anything jumping out at me as bad. I know that means little, but I did reflow a few joints just to be sure. Do you think replacing the tube would be prudent just due to age, regardless of whether that is the primary failed component? I have a Sovtek 12AX7WA on order from Sam Ash, which should be here either today or tomorrow, so at least that will be a known good component. At any rate, I will recheck all connections on the board in the meantime. Thanks again!

[Jazz P Bass]

Clean Channel

Before doing anything, check IC1 pin 8 for +15VDdc.
Pin 4 for -15Vdc.
You can change out the 12AX7 if you want to.
It is only used on the Crunch channel.
As to No Clean Channel.
If you look at the schematic, Clean & Crunch both come off of C1 over at the input.
Clean goes into the first IC, IC1B. It is a dual opamp.
Pin 5 & 6 are input. Pin 7 is output.
Set your meter to Vac, black lead to chassis ground. Amp on Clean Channel.
Play your guitar and look at the point between C2 & R3 with the meter red lead.
Guitar signal at input should max at 100 mv.
I would expect some gain at the test point.
If not, or no signal at all, IC1 is bad.
If o/k, continue on to IC2. Pin 6.
If o/k, go to IC5, pin 5.
It is failing somewhere in that chain.
Also LED 4 & 5 on the circuit board should illuminate when playing in the Crunch mode.
They should not function on Clean.
Check this.

[theguitarwookie]

I measured 15.3V+ at pin 8 and -15.6V at pin 4.
Let me clarify an above statement. This amp only has signal on the Boost channel, and the clean/crunch (normal channel) do nothing. That may have mislead you.
Now here is where my electronics ignorance will come to light.
I do not see IC1B anywhere on the board. Is that to indicate that IC1B is one side of IC1? On the schematic I notice also IC1A in the overdrive section. I may have just answered my own question, but I notice that the pinout for IC1B uses pins 5,6, and 7, and IC1A uses 1,2, and 3. And where I took the V measurements uses pins 4 and 8. Have I just had an epiphany?
At any rate, I also noticed something that may be of significance. SW1 has a broken case and the retainer clip is rusty. I have removed it and replaced it (temporarily) with another similar switch from a parts amp. I have also noticed that none of the PCB LEDs illuminate at any time or in any switched configuration. Might that in itself be indicative of failure? Under what conditions should they illuminate? I have attempted all switched patterns and none (6 or 7 either) light up. Might it be a good idea to refresh all ICs in the chain with new parts? The fact that I am finding mechanical failure in switches and incorrect pots used leads me to believe that this unit has seen VERY hard use, much harder than I had been lead to believe. As I said before, I am using this as a learning experience and am thus not afraid o spend time soldering. Thanks again!

[theguitarwookie]

I have been away from the amp for a few days now, and have been giving thought to bypassing the preamp section altogether and using the output only, as running through the effects loop actually sounds very good and is quite dynamic. Dunno, but Marshall did a nice job with that. Any thoughts on modifying the circuit board to permanently bypass the preamp and maybe use a different case for the unit? Not worth it? I could then pull the tube and socket out in addition to any other usable parts to set aside for a tube pedal project?

[Pottumaa]

Before doing anything, check IC1 pin 8 for +15VDdc.
Pin 4 for -15Vdc.
You can change out the 12AX7 if you want to.
It is only used on the Crunch channel.
As to No Clean Channel.
If you look at the schematic, Clean & Crunch both come off of C1 over at the input.
Clean goes into the first IC, IC1B. It is a dual opamp.
Pin 5 & 6 are input. Pin 7 is output.
Set your meter to Vac, black lead to chassis ground. Amp on Clean Channel.
Play your guitar and look at the point between C2 & R3 with the meter red lead.
Guitar signal at input should max at 100 mv.
I would expect some gain at the test point.
If not, or no signal at all, IC1 is bad.
If o/k, continue on to IC2. Pin 6.
If o/k, go to IC5, pin 5.
It is failing somewhere in that chain.
Also LED 4 & 5 on the circuit board should illuminate when playing in the Crunch mode.
They should not function on Clean.
Check this.

I have this problem with my marshall 8080.

I got sound from return jack and then found that IC1 was bad. So i replaced IC1 with IC6 and still i don't got no sound.

IC2 seems fine, but when i test IC3 and IC5 they don't give nothing. I'm missing normal channel treble knob and boost channel contour&treble knobs, so it might be couse of them as IC2 doesn't give any output if all normal channel knobs are at 0.

Also led 4 & 5 ain't functioning even when crunch turned on.

All help would be appreciated. I could play through return jack, but it gives this annoying squeeling sound when playing louder.