This McIntosh C29 preamplifier powered up fine, but there was no audio output. Both channels were out due to a faulty 2-channel Light Dependent Resistor in the signal path. In this unit the LDR is used in the signal path to mute the outputs while the unit comes up. Once up and stable, the LDR lamp is energized and the photoresistor resistance is reduced, passing signals. With the LDR not working, no signals passed.
The original parts use an incandescent lamp and are long since obsolete. I installed a pair of LED photoresistors as replacements. The original LDR is at left in the photo, with the replacements at right. I added dropping resistors to limit the current and match the original drive circuit, so no other modifications are required. The photo shows the original and the rework. I noticed this caution label on the top cover as well – so I fixed the unit, it sounds good, and I got a laugh out of it too.
If your vintage McIntosh C29 preamplifier has no output, or you have any other vintage or modern fine audio equipment. Reach out to us at the service page for your service needs.
