Bob the Tech Audio began an evaluation of this McIntosh MC275-II Gordon Gow edition, recently in for service for a burnt up channel. It will need some work. I couldn’t resist posting a few pictures of the carnage as a start. The unit’s been repaired at this point, see the update below.
It looks like there’s been catastrophic failure of a resistor in the V7 driver bias. This has caused a small fire and burnt/melted the nearby parts. There’s a failed electrolytic capacitor here as well. There’s also been some prior work in the power supply. Somebody cut many of the supply wires in an apparent attempt to isolate an issue between the rectifier, multi-caps, series choke, and loads. The customer indicated that there is a short circuit in one of the output transformers. At least two of the 6550 power tubes have also failed. The normally black/gray getter has turned white indicating the tubes have lost their vacuum.
Update after Repair
Since the original post we’ve disassembled and reworked this amplifier and I’m happy to say it’s working great.
Power Supply
We started by checking the power supply and the complaint about the output transformer short. These parts all had wires clipped off so we needed to check things out before reconnecting. The main rectifier checked out OK. The series choke was loose on its mounts so we tightened that in and it looked good and was not shorted to the case.
Output Transformers
We measured and compared the two output transformers and they measured consistently. There were no short circuits between any pairs of windings, or between windings and case. To ensure that the output transformer was not arcing under high voltage (it sees 450VDC for bias and probably more AC when driving full output power), we placed 400VDC between the various pairs of windings with a bench supply and verified there were no shorts or arcs induced. The output transformer is OK.
With the output transformer verified, we reconnected all of the supply and output transformer wiring, removed the incorrect 220 Ohm screen grid resistors and replaced with the proper cement style, and verified the plate inductors were OK. The reworked supply and output stage are shown below.
Amplifier Board
With the supply ready to go, we moved on to rework the V7-related issues. We started by removing the damaged parts from the interstage coupling capacitors forward. A shot of the cleaned up board is below. The burnt circuit board trace looks like the cause of the problem, but I think it was a secondary effect. A shorted electrolytic capacitor likely shorted the -150V supply to the V11 cathode through R71/72. Those transistors burned up and took the trace and the interstage capacitor with them.
We reassembled the board with some fresh parts, wiring around the burnt trace and a few damaged plated through holes. The reworked board is shown below.
We’ve been listening to the MC275-II for a few hours now and it sounds great. Hopefully yours doesn’t have a burnt channel like this one, but Bob the Tech Audio is ready to take on your McIntosh MC275-II service, restoration, and repair. Contact Bob the Tech for your service needs here.
