Weird fault in Marshall 100V model 8100. Advice appreciated!

[allante666]

Ok, soo Marshall Valvestate 100V 8100 head in just for a service, no faults reported other than a couple of dirty pots. Pots cleaned up ok and sorted a couple of potential dry joints other than that all ok, but, heres the thing.
If I leave it with no speaker connected and turn it on- all ok.
If I turn it on with no speaker connected and then plug one in- all ok.
If however I leave it and turn it on with the speaker connected, my current limiter lights up like a xmas tree! But, if I pull the speaker out its fine. Then, if I plug the speaker straight back in again, its still fine!
If I turn it on with the speaker plugged in soon afterwards its still fine but after being off for a minute or so it does it again.

I dont get it, its like its seeing a short when it starts with the speaker in but removing and then reconnecting the speaker removes the problem.

Even while the limiter bulb is lit the various led's in the amp light up as per normal.not that i've left it on for long.

Anyone any ideas where to start?

[52 Bill]

When the amp came in there was no problem with it blowing fuses? When you turn it on without the limiter and no speaker, what happens?

If the amp powers up fine with no limiter and no speaker, and no voltage on the speaker buss, then power it up normally without the limiter with the speaker and see what happens.

[allante666]

Hi Bill. I plug them in through the limiter by default so I haven't tried it without it. No there was no blowing fuses mentioned and the fuse in it looks like a pretty old one rather than one that's recently been replaced. I can try this tomorrow but whats your thinking behind it?

[Jazz P Bass]

Limiters have there use.

They can also be abused.

My take is you do not need the limiter on a working amp.

All you will end up doing is messing with the output section.

In your case, one rail is turning on when it should not.

[allante666]

One what rail? supply rail?

[The Dude]

Most likely, yes. If the rails do not come up evenly, the amp will output DC momentarily until the rails stabilize. That is likely why the blast of current on power up. With no speaker connected, there is no current path, so it doesn't matter. When the speaker is connected, well.......

Power it up with no speaker and monitor the DC on the output with a meter. That will give you a better idea of what's going on.

[allante666]

Its not a momentary blast it stays like it until I pull the speaker out.

[The Dude]

What "stays"?

[allante666]

the limiter bulb stays lit

[The Dude]

As mentioned above, ditch the limiter. Fire up the amp with no load (speaker), monitor the speaker output with a meter set to read DC volts.

[allante666]

ok Ill try that tomorrow and report back.

[g1]

If it fires up good on the limiter with no speaker, and attaching the speaker while amp still on does not light the bulb, then it is good.
As the others mentioned, it doesn't like starting up on the bulb when load is attached.

[allante666]

OK, so, I can now say that it is nothing to do with the limiter.
Running this directly from the mains and monitoring for DC on the output I get the following:
Switch on without a speaker- a spike up to about 28 volts for probably a quarter of a second and then sits at 0.02 volts
Switch on with a speaker- 28 volts and stays there until I unplug the speaker or turn the power off. If I unplug the speaker it drops back down to nothing (well 0.02) and if I then plug the speaker straight back in it stays at 0.02

[Enzo]

OK, the power amp is unstable at start up.

When you say a 100v 8100, do you mean an 8100 designed for the Japanese market where the mains power is 100vAC? And if so, are you operating it on 120v US power? If so, correct that.

If I am confused and it is meant for 120v, then proceed to the power amp. My first suspects are the feedback and the op amp. I'd probably swap out the TL072 because I have a drawer full of them, and it is a suspect.

TR11,12 are your limiters, lifting D4,5 disables them. I might do that during test, but I also would verify the resistors associated with them: R116,117,118 and R115,97,96. The two 0.33 ohm power resistors are likely either open or OK, not likely off value.

R61 is the feedback resistor, it says 33k, but it also has stuff in parallel, so in circuit it ought to measure something less than 2k.

The power amp relies on good +/-15v supplies, so verify both of those are OK. And are they good right at the power pins of that IC.

Does you amp have the BDV64,65 types for outputs or have they been upgraded to TIP142/147?

[allante666]

Hi Enzo. No its called a Valvestate 100V, thats the name not the voltage. its also named as a model 8100. I'm in the UK so its on 230 volts (is it not called a 100V over there?)

It has the t64 and 65 still in it although I do notice that one has been replaced as one is an ST and the other is a Marshall branded one. TR9 has also been replaced with a TIP31C although I think this is just a more sturdy version of the original TIP 29c
Yes 15 volt rails are both good all the way through. .33 resistors are both ok, Ill check the others in a minute