McIntosh MC2100

[xtian]

That’s confusing, the device symbols on the schematic, and the operating conditions, indicate those should be regular bipolar junction transistors.
What was the original device type part reference, and what did you use to replace them?

A 40W bulb may be excessive, reduce the mains voltage a bit?

Original was Motorola 32-070. New is MJ15001G.

Operation (and failure modes) is the same using a 40w bulb or 100w bulb. In both cases, amp draws about 20 watts when stable and passing audio, and much higher (up to 75w with the 100w bulb) when running away.

[Helmholtz]

Acc. to schematic the amp doesn't use any FETs.
The MJ15001G isn't a FET either.

How do you measure amp power draw on a bulb limiter?

[xtian]

Acc. to schematic the amp doesn't use any FETs.
The MJ15001G isn't a FET either.

How do you measure amp power draw on a bulb limiter?

Pardon my terminology. Power transistors. I have a Kill-a-Watt device in line with wall voltage to monitor wattage.

[Helmholtz]

I have a Kill-a-Watt device in line with wall voltage to monitor wattage.

That will show the sum of bulb and amp power draw.

[xtian]

That will show the sum of bulb and amp power draw.

Understood.

[Helmholtz]

It is essential to have zero DCV at the power amp outputs with the output transformers disconnected and without load.

[g1]

It's not uncommon for a solid-state amp to have issues with being powered up on bulb with load connected. The output can stick to one of the rails and you will get bright bulb. It may be every power cycle, or at random like this one.

I had thought you were also having the issue with amp unloaded, which would mean some other problem. But Helmholtz comment just made me realize the output is always loaded with the OT. And it will be a low impedance primary OT as this is not a tube amp.
In addition to checking for DC with the OT disconnected (post #21), see if the fault mode will even occur with OT disconnected.

[xtian]

When you say "with the OT disconnected", I think you mean no load connected. With amp running and no load connected, I see 24mV DC on one channel, and 14mV on the other. Is that close enough to zero DC, or does this indicate a problem?

Since this morning, I have been turning on the amp each time I pass my bench (100w LBL in line, 8-ohm speakers connected). My tally is six successful startups, and two fails. Not encouraging. If I gave it back to client, I'm sure the fuse would blow sooner or later.

[Helmholtz]

But Helmholtz comment just made me realize the output is always loaded with the OT. And it will be a low impedance primary OT as this is not a tube amp.

Yes, an OT is a DC short to the output. Impedance doesn't matter with DC (only DCR, most likely below 1R here).
No wonder you don't see any significant DCV with the OT connected even if the amp is faulty.

[Helmholtz]

When you say "with the OT disconnected", I think you mean no load connected. With amp running and no load connected, I see 24mV DC on one channel, and 14mV on the other. Is that close enough to zero DC, or does this indicate a problem?

OT disconnected means the OT disconnected from the amp.

[xtian]

I'm repeating you, so I'm absolutely clear on the idea: Are you suggesting I should disconnect the OT at the red X, then measure DC from power amp output to chassis ground?

[xtian]

I'm off to play a gig, so I'll catch up tonight or tomorrow. Thanks, friends!

[Helmholtz]

I'm repeating you, so I'm absolutely clear on the idea: Are you suggesting I should disconnect the OT at the red X, then measure DC from power amp output to chassis ground?

Yes, there should be no DC path to ground.

[xtian]

I ran test as described. With OTs disconnected, DC at each channel's output reads 24mV and 30mV. Same figures I get when OTs are connected normally, with or without load.

[g1]

Can you get the fault to appear with OT disconnected?
If not, I think the random issue is just test-rig induced by the bulb, and it's time to ditch the bulb.