McIntosh MC20 - 1960's - No Left Channel

Total noob here.

I sort of discovered some McIntosh equipment that has been in this house for 40 years in an old handed down stereo cabinet. Finally decided to see if I could get it to work. It's made a great little side table for the past few decades.

In any case, it's an original McIntosh 275 and C20 Preamp (and reel-to-reel/turn table/ and tuner)

I'm trying to get the Pre-amp/Amp working.

Inputting from a computer 3.5mm to RCA on Preamp for testing.

Left channel is not woking.

Tried balance of course and mode selector.

Confirmed that both speakers work by switching inputs in the 275.

I've done a continuity test on all the tubes (they were all the same, so I'm guessing the test was good. The same three pins show continuity with each other on all7 tubes)

I switched the input cables to confirm they were both working.

Changed the input to connect with Aux, Tape, tuner… all no left channel.

Tried High Level and Low level inputs... still no left channel.

Swapped V2 - V1 tubes (12AX7's)

Swapped V4-V3  (12AU7's)

Swapped V5-V6 (Also 12AX7's)

Then swapped V2 and V1 with V5 and V6 (as they're all 12AX7's)

No obvious issues to inputs internally when I open the back.

Next steps?



  Check all fuses.

Possibly the cable from your laptop to pre is NG?

Cable from pre to power NG?


Checked the one fuse I saw on the 275 and the one fuse I saw no the C20. I got continuity on both, and the C20 is working, I do not know of any other fuses. (On the 275 I replaced the fuse, was blown when I started this endeavor.)

Checked the cable from laptop (Switched RCA's - both would play through right channel.) So it's outputting left and right from the laptop.

The pre is on and working for the other channel. So power is "functional." Don't know if it's 100%.

2 wonderful McIntosh Tube pieces, well worth having refurbed even if working well.

I sent my working mx110z tube tuner/preamp to Audio Classics, they did a great job, including replacing all it's slightly undersized and corroded rca jacks with new gold plated ones.

I highly recommend having them refurb both the amp and preamp, especially because of the other vintage pieces with it.

I highly recommend Audio Classics, they have several excellent techs and their work is impeccable!