Another MARSHALL Valvestate 2000 AVT150 Blowing fuses......

[g1]

On second reading it appears your problem is in the upper winding on the schematic, as you said when you disconnected the secondary the fans came on, and the fans run on the lower winding.
So if that is the case, you don't need to concern yourself with the regulators or anything powered by the lower winding right now.
So is it W103 and W104 that make the bulb go bright when they are connected?
If so, there are 3 main items running off the upper winding. The power amp IC's which run off PSU+VE and PSU-VE. You said disconnecting them made no difference so we will ignore them for now. That leaves the high voltage for the preamp tube.
Try lifting one end of R211 and see what happens.

[g1]

And if lifting one end of R211 makes no difference, then try lifting one end of R212.

[blearyeyes]

Light still bright after lifting R211 then R212

[g1]

Light still bright after lifting R211 then R212

So is it correct that the light goes dim when W103 and W104 are disconnected?
Have you checked C141 & C171? (you can measure resistance across R181, then R128)

[blearyeyes]

So is it correct that the light goes dim when W103 and W104 are disconnected?
Have you checked C141 & C171? (you can measure resistance across R181, then R128)

To be precise I pulled W102 W103 and W104. So I pulled the ground. I will try It with only W103 and W104 and then check the caps you suggest. What should I be looking for across the resistors?

[blearyeyes]

Yes W103 and W104.

I did check C171 and C141 directly from underneath the board and they were not shorted or showing Mega ohm readings.

[blearyeyes]

Should I have replaced R211 before lifting R212?

[g1]

No, it was not necessary to reconnect R211 for that test.
Here's a summary, please check and say if I'm wrong about anything.
1) Bulb lights bright with W103 & W104 connected.
2) Disconnecting all connectors from power amp boards, bulb is still bright.
3) With R212 disconnected, bulb is still bright.
4) No obvious shorts at L100 bridge rectifier or it's filter caps.
5) Disconnecting W103 & W104, light goes dim.

If all the above are correct, it's still possible the bridge or filter caps are failing but only when voltage is applied.
Or there could be a short on the board itself or a connector, but if this were the case I would have expected a low resistance reading at R181 or R128, maybe double check that. Again, it's possible a board or connector short is only showing up when voltage is applied.

Edit: I can't see PSU+VE or PSU-VE going anywhere else than the power amp modules. If anyone sees them going anywhere else, please speak up.

[blearyeyes]

After re checking R128 I'm getting a little rise then it drops to 0.000 ohms

P.S. everything in your list is spot on.

[g1]

That's good, some progress . It would appear the problem is something in parallel with R128.
For starters, I would remove C141, then recheck. If the low ohms reading is no longer there (across R128), replace the cap.
If you still get the low resistance reading with the cap removed, something else connected across those board tracks is responsible.
Because your reading rose a bit, then fell to zero, I'll bet on the cap being the problem.

[blearyeyes]

Removed C141. Measurement across R128 shows resistance. Cap out of circuit shows its shorted.

g-one, I have learned a lot about trouble shooting circuits. You are most excellent.

Is there any other component I should replace as a precautionary measure?

[blearyeyes]

Also I am assuming that when I tested the cap in circuit I was seeing the resistor or other components. I am getting a small glimmer of what this all means.

Lets see if replacing the cap fixes the amp.

[blearyeyes]

After replacing C141 bulb is now dark with W103 and W104 connected.
tube is glowing, fans are running, front panel LEDs and channel switching are all working bulb is dark.


Connected the power IC boards, powered up and pop, pop. ICs fried.
C141 got pretty hot. Bulb light up half way.

Pulled the power to the IC boards and tube is glowing, fans are running, front panel LEDs and channel switching are all working bulb is dark.

If the power ICs were already blown and took out C141. Could they make a pop sound again with fresh power applied?

[Enzo]

If C141 got hot, I'd wager it was installed backwards. C141 is a filter cap for a negative supply, that means the POSITIVE end goes to the ground trace. DO NOT install all the filter caps with the negative side to ground. Alternatively, if the W102,103,104 wires are mixed up, it is possible twice the normal voltage was applied to it, and that would also destroy the power ICs.

I'd verify with C141 removed that the AC voltages at W103 and W104 were the same, and with it installed that the voltages across it were appriopriate. Then BEFORE I connect the power modules, I'd verify the DC voltages they would face were in bounds.

I cannot think of any way the power IC modules could harm C141.

[blearyeyes]

Ok thanks. I'll go back and do what you suggest.
I have new ICs. I'll pick up another cap.

The original cap had a grey stripe with a negative sign. The new one had a grey stripe with arrows pointing down towards the bottom of the cap. I assume that grey is negative?

Well I'm learning from my mistakes.