1) welcome on AK !
2) what numbers are "stamped" on your ( both ) driver boards ?
3) by the fact that you found " rockwoods 5WL.15" inside your unit, did you see that they look like replaced resistors ? somebody worked on this unit using a bad reference resistors ?
Hi, the heatsinks correspond to what is pictured on the schematic as boards (1 to 4) 045630, 045950, 045951, 045634. As for input boards, these are marked 045928. The amp looks inside as totally stock, not a single intervention as far as I can say (unless everything was changed at once a long time ago. I would then think that the rockwoods are then appropriate to this model, or at least they have been mounted at the factory. Now, I still do not know if WL means inductive or, on the contrary non-inductive. In the discussion above, opionions diverged on this topic, or at least there seemes not to be a clear consensus...
One driver has e-b shorted, it is labelled 198 Malaysia 8526, I think this is MJ3237 since the other channel has the corresponding driver (not blown) marked as Motorola MJ3237.
A MC2500 with blown outputs.....that is a rarity. Did the whole channel fail in a cascade?
I am sure that factory parts can supply you with the correct schematic.
Hopefully your driver transistors are OK.
I now always replace the darlington bias transitors when ever I have a MC2255 family of amps near the bench with a hot iron nearby.
The issue with inductive/ non inductive resistors were with the early series of MC2205 family. They need to have the inductive resistors to insure they do not oscillate.
As always your local tech should have the facilities to properly test these amps under stress to assure you get what you are paying for and are repaired correctly.
McIntosh engineering did not discover that the resistors manufacturer had changed their build design until the amps were tested under stress and found to need that little bit of inductance to maintain stability.
It was after that mutual discovery that the amp circuit was modified to allow the use of non inductive resistors.
I believe that subsequent amp designs (after MC2205 family) could use either. The MC2505 amp family (includes MC2300) need .33 and .56 inductive wirewounds......
I do not know the history of the amp so no idea what happened. Some output transistors seem ok, but I shall change them all, no chances takes here... As I wrote to clinic-audio, I still don't know what about the WL meaning... The Rockwoods sure seem stock...
As I wrote, one driver is blown, looking for substitute. Thanks for the hint on the Darlington, I shall change them, they seem easily available. Concernig the WL issue, do I have to understand reading your post that in my case inductive or non-inductive are both ok ?
Working on the mac is part of my hobby, so I shall try to cope with the situation by myself, or at least going as far as possible without however making things worse. Hence all my questions.