Marshall Valvestate 8080 - VS80 - overdrive channel not working

[callai]

Hello guys,

I am new to the forum, and last saturday I have posted on a old thread my doubts, got an opinion and a friendly reminder that here is not The Walking Dead world......so i´m starting a new thread and will try to copy the previous post to here. Hope you could help me.

Its a while since I used to work on a daly basis with eletronics, I have a solid foundation, however eletronics are like playing...without practice you got rusty...

So hands on:

I recently got my hands on a second hand (meybe third or more) VS80. It has a not working pre amp, which is in my opinion the soul of this amp. I got it without the 12AX7 tube. I took some readings and will post it in a while.

[callai]

this was my original POST:


"Hi guys, I know this thread is a little old, but as is the most similar to my problem i´ll give it a shot.

I got my hands on a Valvestate 8080 that my friend had lying around for some time. It had a problem on the overdrive chanel. Almost no sound. the clean is working fine. I plugued my guitar and confirmed it.

After that I open it and found out the 12AX7 missing. Thanks to the previous technician who worked on it.

I will put a working tube on it, but first i decided to take a look on the circuit. Found on leg of R113 loose. I re-solder it and took some mesurments. Here is what I found.

on D3 diode i found 335Vac and 474Vdc.
refering to the ground (chassis) the tube conections reading were the following:
1 - 0V
2 - 0V
3 - 0V
4 - 465 Vdc
5 - 36 Vac
6 - 36 Vac
7 - 0V
8 - 465 Vdc
9 - 470 Vdc

between pins 5-8 and 3-4 i found 456 Vdc on each pair.


I intend to conect the tube soon, but I am worried about the voltages, it seems to me that they are higher than usual. Anyone can confirm that? Some direction to proceed?

Remembering that I took all readings without the tube.

Thanks in advance"

[callai]

And here is the response I got from nickb (thanks man!):

"I think you got the pin numbering in reverse order If that is true then the voltages are OK but the HT is very high. When you put the tube in it will come down. The problem I see is that C53 and C59 are only rated for 350V so I hope they have survived. Poor design IMHO.

It's better to start a new thread than tack onto an old one."


Nickb got it right, I took the pi numbering in reverse...next time will use a mirror to help me....

Any way, a got the ECC83s from JJ, and plugged into the socket, it sound creamy and loud...beautifull....but, beyond the sound I have gotten some smoke...

It apears to come from R113 or R112.

As I told before it´s pretty old and I don´t know what the previous owner/technicians have done to it.

I´m plannig to change this whole part of the circuit. R112, R113, R114.


What do you think about it? someone have already dealt with something like that?

[52 Bill]

Any way, a got the ECC83s from JJ, and plugged into the socket, it sound creamy and loud...beautifull....but, beyond the sound I have gotten some smoke...

It apears to come from R113 or R112.

As I told before it´s pretty old and I don´t know what the previous owner/technicians have done to it.

I´m plannig to change this whole part of the circuit. R112, R113, R114.

Those resistors are part of the AC voltage power supply for the high tension and heater supplies. Resistors normally do not burn up by themselves, they burn up because something is causing them to get too hot. The three resistors are wire wound power resistors rated at 5 and 7 watts. They are supposed to get hot, but not hot enough to smoke.

You need to find out exactly what component is burning and then figure out why it is burning before you start to replace things.

[callai]

Those resistors are part of the AC voltage power supply for the high tension and heater supplies. Resistors normally do not burn up by themselves, they burn up because something is causing them to get too hot. The three resistors are wire wound power resistors rated at 5 and 7 watts. They are supposed to get hot, but not hot enough to smoke.

You need to find out exactly what component is burning and then figure out why it is burning before you start to replace things.

That´s the problem Bill, Acoording to the schematic I posted above, those Resistor are the only components on the heater path. So there´s nothing else to be changed...or maybe the pcb track could be restricted causing overheating???

[52 Bill]

That´s the problem Bill, Acoording to the schematic I posted above, those Resistor are the only components on the heater path. So there´s nothing else to be changed...or maybe the pcb track could be restricted causing overheating???

Are you sure that one of those resistors is in fact smoking? Are you sure that one or both is overheating? Measure the resistance of each resistor and then the ac voltage drop across each resistor and see what you find. Has one gone off value?

[callai]

I took me a very long time to resume to the maintenance of the VS 8080, but i did figure out the problem and got it working. So Im here to give the feed back as future references for those with similar problem.

I found out that the smoke i saw on the circuit was not the resistor, but the soldering on its legs!!!

I´m not sure if it´s a issue of the original design or a mod done by some of the previous maintenance the amp went through, but my perception is that the resistor got hot and the heat irradiation + the joule effect (conductor getting heated by the current) cause the solder on the board to melt, in the long term leading to lack of sound.

I solved taking both resistor off the board to allow a better dissipation of the heat. I conected them with a wire and place them near the chassis (taking care not to cause anny short circuit).

The VS8080 already accomplished a 3 hour gig without any trouble.

Thanks for the help.

PS: I had a minor upset when maintaining the amp. I broke the od1/od2 switch. Anyone knows this part anme/specs??? I wanna replace it but aint sure with part to buy.

thanks

[nickb]

Good to hear you got it working Anyplace a components runs hot is a likely source of failure.

You can order spare parts direct from Marshall +441908375411 . Give them the Amp model number, the component reference SW3 in this case ( but please check) and the function OD1/2. They will be able to find the right part and send it to you.