We recently had the privilege of performing a Restoration and Repair on this McIntosh C32 Preamplifier, shown below with a restored MC2105 power amplifier. The unit had a tape output that wasn’t operating, and other outputs were intermittent. After removing the covers it was also clear that the flat flex cables had delaminated and needed replacement. We identified some previous flat flex cable rework which needed improvement.
Repair and Restoration
We disassembled the unit, cleaned out the prior rework, and replaced the flat flex cables with a new set. Once that was complete we were able to reassemble and check out the unit. The fresh set of cables resolved the intermittent behavior. We isolated the lack of a signal at the tape output to a bad relay. In addition, as we evaluated performance over frequency. In doing so we identified an issue with the high filter. The filter would not disengage in one channel, resulting in loss of high frequencies. We isolated this to a bad connector. In this case the connector pin was not making reliable contact with the control signal in the flat flex cable. After removing and replacing the relay and connector the unit was fully operational.
The unit had been re-capped previously, primarily in the signal path. The main power supply filter capacitors had been replaced, but showed signs of leakage. We removed and replaced the filter capacitors, and rewired the loads in to the supply. We also removed and replaced power supply and signal path electrolytic capacitors on all boards where there had not been prior rework.
Original delaminated flat flex cables 
Previous flex cable rework – needs to go 
Prior rework removed, ready for new cables 
Restored chassis with new cables installed 
Output relay replaced 
Leaking power supply filter capacitors and prior rework 
Replaced filter capacitors with appropriate parts and re-cabled the supply and loads
Performance
Once complete and reassembled, a thorough functional and performance check identified an intermittent connection. The photo shows the broken solder joint where the input connector ties to the board, center. The wire was floating in the joint, sometimes making connection, other times not.
Reflowing the connection had it running solid and we verified excellent performance. We measured distortion under 0.05% in both the preamp and monitor amp outputs.
We spent some time listening to the C32 paired with a Bob the Tech Audio restored MC2105. It sounded great and made a nice photo. If you have an older McIntosh C32 preamplifier that is in need of restoration or repair, Bob the Tech Audio can help. Reach out to us via our service request form.
