Marshall Valvestate 8080

[kdawg]

I'm working on a buddy's Valvestate that blew up - the T64/65 MOSFETs fried and must have taken out some other parts with it.

I tried replacing the MOSFETs with TIP142/147 (will that work?), but they also fried soon after hearing distorted audio, so before trying another set I'm just trying to make sure the preamp has good signal coming out.

I'm kind of stuck now though, the Clean Channel LED will not light (boost will though), and I'm seeing square wave in to the tube, regardless of volume or channel selection. I cannot get clean sine wave.

Power seems OK, +/- 15V and tube heater voltage is OK. I've removed the tube until I can make sure the preamp is good.

The only place I see good audio is on the output of IC1 pin 7. I tried replacing IC2 also.

Anyone have some ideas??? I was going to replace TR2 but I don't have any BC184 right now.

Thanks

-kdawg

Schematics
Preamp:
http://www.drtube.com/schematics/marshall/pc0689.pdf

Power Amp:
http://www.drtube.com/schematics/marshall/pc0689p.pdf

[Nigel Goodwin]

Originally posted by kdawg
[B]I'm working on a buddy's Valvestate that blew up - the T64/65 MOSFETs fried and must have taken out some other parts with it.

I tried replacing the MOSFETs with TIP142/147 (will that work?), but they also fried soon after hearing distorted audio, so before trying another set I'm just trying to make sure the preamp has good signal coming out.

You can't replace MOSFET's with bipolar, but the diagram you posts doesn't show MOSFET's, it shows bipolar darlingtons?.

Generally it's bad practice to just change the output devices, often they have either damaged other parts as they failed, or their failure was caused by other parts.

[Albertb]

You are quite right to try to fix the preamp side before worrying about the power circuitry. That will come later.

Just an observation, but the outputs of both those channels are going through IC5. This is an M5201S with a control voltage to select its operation. This control voltage comes from your footswitch unit which you suspect may have dodgy circuitry. I would also check out the operation of the switch itself.

Check around the footswitch circuitry to find the following:

1. When it is in the normal position, does the footswitch pull the voltage at the junction of R9, R32, R67 down to 0v? (This will put out the Boost LED of course, does this happen?) You could try shorting this point down to ground by hand just as the switch does,

2. Assuming it does work, then the voltage at R31, R66 junction should go up to almost 15v.

3. The voltage at TR2, R33 junction should be a little below this, and the TR2 Vbe voltage, (top of R8 to bottom of R10), should be about 0.6v.

4. With the switch in the Normal position, the first voltage should be about 8v, and in turn the voltage at the junction of R31, R33 should come down to about 0.5v. The Vbe of TR1, R32/TR1 junction to ground, should be something about 1-1.5v in this situation.

Things which you may find wrong will include:

1. The switch is stuck in the Boost position - replace it.

2. The voltages at R9/R32/R67 are correct in both positions but the following voltages at R31/R66 and R31/R33 stay either both high or both low - TR1 is blown.

3. The voltages around TR1 are correct but the voltage following TR2 is permanently low. Or more subtly, TR1 is fine but TR2 Vbe is 0v and the voltage at the top of R8 follows the voltage at the bottom of R33 from 0v up to about 0.5v - TR2 is blown.

TR1 and TR2 are general purpose NPN transistors with nothing special or demanding about either of them. TR1 can be replaced by any small signal darlington, or even a pair of small signal transistors linked together correctly with collectors connected and the emitter of the first to the base of the second. TR2 can be replaced by almost anything NPN, it is not even specified as a B or C category of gain.

Any transistors of the correct polarity which have a greater than 15v VCE will do! There is ABSOLUTELY NO GOLDEN EARS situation here, we are talking about very low speed switching. Put any silicon transistors you have handy in there and it will work!

When you have had a chance to check things out give us an update and we will try to give you more.

[abmelo]

I was trying to fix my Valvestate 8080 too. Just bought it pretty used.

The reverb is always ON and the footswitch doesn't do anything. In fact this is a minor detail since I can turn the pot to 0 and just shut it down... Anyway, the channel footswitch button is working ok.

I gave up on the reverb problem when I found that lots of capacitors on schematics where actually resistors on the PCB! It doesn't seems to be a mod done by the previous owner... it seems to be industrial soldering.

Does any know if in any hardware version, capacitors C1, C2, C25, C30, C23, etc. where replaced by resistors?

I'm not happy with this amp sound and would be happy if this resistors where the problem. Can anyone check it for me?

Thanks a Lot!

Alexandre Melo
Brasil

[kdawg]

What color are the "capacitors"? Those are most likely capacitors, they look like resistors but just caps in a resistor type package. The color read similar to resistors.

I still haven't gotten around to working on my 8080... I got parts to try out soon though.

-kdawg

[abmelo]

Maybe caps in a resistor type package?

Take a look on a picture that I took from caps (?) C11, C13 and C14. Are they really caps? In this case, what's the name of the package?

--
Good look with your 8080. Getting the correct parts is the most difficult step! Once you get them, just be cool and everything will go right!

[abmelo]

Took one of the strange capacitors just to measure resistance. They really are capacitors! Damn... trick one!

What is the name of this capacitor package? Color coded, by the way!

see ya,
Alexandre

[unclejed613]

those amps use TIP142/147's from the factory as well as the european transistors. when powering up an amp after replacing power transistors, i use a 100W lamp in series with the power line. if the bias and everything are ok, the lamp will light for a scond or so while the power supply caps charge up, then get dim or go out. if there is a problem that causes the amp to draw too much current, the lamp will stay lit.
you probably have shorted driver transistors (tr7 or tr4) or an open bias transistor (tr9)..... tr5 and tr6 could also be shorted.

some earlier versions of this amp have been known to be like Acoustic 360 amps, i.e. once they go up in smoke, they're gone.....

[antant]

Hi unclejed613 I have also o problem with my 8080 and I sent you en email, I just registered here also, you have a fantastic forum , full of tech info !

Antonis , Greece

[Peckerwood]

Does it switch channels by itself if you unplug the footswitch?