Any McIntosh Audio GURU's? Need help fixing MC2500

[Puunda]

Hi All,

I'm having some problems trying to fix an MC2500. Well I'm not fixing it, a
tech is, and he's having some problems with it. The right chan 'Limit' light
comes on, although the amp is working fine. The tech has tried everything,
but is unable to find the fault.

Here is the email he sent to McIntosh

I have a MC2500 (#CS5591) in my shop for repair.

The unit works perfectly, except...the rt ch power guard limit light
illuminates at turn on. This happens with no i/p signal. Offset voltage is
2mV at 8ohm terminals. LDR1 remains off. I concluded that a faulty triac
driver, or triac could be the culprit, so replaced the lot. No change.

Next, checked all res on amp board & replaced all electros. Test: no
change. Replaced Q16 in case of leaks...no change. The voltage at Q16
collector is 9.5 v (should be 15.5v) The base of Q16 os 0V as reqd.

The reply from McIntosh was no help.
Here was another email from an experienced McIntosh repairer

Is there 15.9V on Q7 darlington that drives the Q8 triac., if no go to the
driver board and make sure Q23 the power guard driver is good, if it is good
(Q23) the darlington may be bad. The LDR uses the same drive that the PG
Driver does, if the driver fails the LDR should still work.

The the's reply was "Naturally I have already changed the suggested
components...to no avail"

So once again, the light is still coming on, and it looks like it's not
going to get fixed anytime soon.
Does anyone have any suggestions as to what might be causing this, or how to
fix it?

Any help would be appreciated.

Thanks in Advanced.

Yung

[Bret Ludwig]

Puunda wrote:
> Hi All,
>
> I'm having some problems trying to fix an MC2500. Well I'm not fixing it, a
> tech is, and he's having some problems with it. The right chan 'Limit' light
> comes on, although the amp is working fine. The tech has tried everything,
> but is unable to find the fault.
>

I'm not sure. But honestly if you are paying someone else to fix your
amp it is quite unfair to ask that he be tutored for free! I am sure
that McIntosh can fix your amp (probably by board swap so the whole
chassis need not be sent in) or if not that there are talented EEs he,
or you, could hire for a consulting fee. There are many out of work you
know.

If this were a DIY hobby project and I knew, I'd tell you. My guess is
that unless you can fully backwards engineer the circuit from the
schematic you are going to have to very painstakingly shotgun it (no,
not blast it with a 12-gauge, replace parts and redo connections) or
compare a lot of voltages and resistances to a working unit if one can
be borrowed.