Phonic MAX 860 problems...

[The Dude]

Hi,
No, it's definitely not a heating problem. As I wrote, when I once have got the amplifier to work, it work 100% every time I start it, even if it has been powered off for several days. It's just when it has been unused (powered off) for, say a month or so this problem/phenomena occurs.

Oops. Sorry. I must have misread the symptoms. In that case, I agree. It sounds like a bad cap. Do you have an ESR meter?

[Chrille]

Can you access the protect circuit to monitor what is happening?

I would go to the junction of R406 & C408.

Did you mean R426?

[Chrille]

Oops. Sorry. I must have misread the symptoms. In that case, I agree. It sounds like a bad cap. Do you have an ESR meter?

I can measure caps on my multimeter but only up to 20 uF. Otherwise I can measure discharge times on bigger caps on my scope. I'll check it out.

[Jazz P Bass]

Did you mean R426?

Yes.
The junction of R426 & C408.

[Chrille]

Yes.
The junction of R426 & C408.

I have measured it now and I see nothing remarkable here. Looks fine!
As mention earlier, the protection circuit is not (as far as I can see) detecting any errors because the RA1/RA2 relay is keeping holding the output connected, and the protection Led (LED405/LED505) is not lighting.

[g1]

So when the fault occurs, the peak light comes on but the protect light does not?

[Chrille]

So when the fault occurs, the peak light comes on but the protect light does not?

Yes, that's correct!

[g1]

Then let's not use the word 'protect' anymore, ok?

[g1]

Do you still have the problem if the limiter switch is turned 'off' ?
It sounds like the limiter is sometimes getting stuck on full limiting.

[Chrille]

Then let's not use the word 'protect' anymore, ok?

Yes, you're right, it's probably not the right word, but I didn't find anything better to describe it.

[Chrille]

Do you still have the problem if the limiter switch is turned 'off' ?
It sounds like the limiter is sometimes getting stuck on full limiting.

Yes, I've tried all button and all combinations but nothing helps.

[nickb]

Yes, I've tried all button and all combinations but nothing helps.

Just to be very clear, are you quite certain that with the limit switch set to 'off' you still lose the output on both channels, the protection LED is off and the limit LED is on?

[Chrille]

Just to be very clear, are you quite certain that with the limit switch set to 'off' you still lose the output on both channels, the protection LED is off and the limit LED is on?

Yes, almost 100% correct. It might happen on just one channel. (There's no dependency between the two channels.)
On the amplifier I work with at the moment, it is Led 101 so it's left channel. Right channel works fine.
On another amplifier, it's the opposite.
Worth mentioning is that when there's no signal in, the Led is not lighting but if I just have a tiny signal in, the Led is lighting.
I also noticed that when this state occurs, the voltage on (in this case) OP_OUT/L goes low (about minus 2-3 volt).
Then after a while it sudden jumps up to zero and it works fine!

[Jazz P Bass]

The only thing that I can think of that would put a steady minus voltage on OP_OUT L is either the opamp IC201A is acting up or Q204 (2N5401) is leaking and allowing B- voltage through.

Q204 can be monitored at it's Emitter leg.

max_860__Q204.pdf

[nickb]

Yes, almost 100% correct. It might happen on just one channel. (There's no dependency between the two channels.)
On the amplifier I work with at the moment, it is Led 101 so it's left channel. Right channel works fine.
On another amplifier, it's the opposite.
Worth mentioning is that when there's no signal in, the Led is not lighting but if I just have a tiny signal in, the Led is lighting.
I also noticed that when this state occurs, the voltage on (in this case) OP_OUT/L goes low (about minus 2-3 volt).
Then after a while it sudden jumps up to zero and it works fine!

Well this is a puzzle. I think the only way to solve it is to gather data. I see a difficulty in that it only happens with a signal which means DCV measurements may not tell us very much. Do you have a scope? Failing that, get it in the failing state and measure the DCV around the power stage i.e on the CBE of each transistor with no signal - we might learn something. First try the freezer spray mentioned earlier as that could get you a quick answer.