McIntosh C20 - can and should I bring it back from the dead?

[neevo]

It’s testing my brain a bit trying to go back and remember but it’s the components that come off the M6 and M7 switch that drives the RIAA curve (there’s also a right side too).

I couldn’t tell you exactly which ones are exclusively in the RIAA section but I believe it’s these ones. But also looks like a few extra parts down the signal chain too that might need changing too.

Depends what your approach is. If just repair essential items maybe others here can confirm exactly which parts (for example I would assume C19, C18 and R27 are in the single path of the treble RIAA curve).

I just overhauled everything as I wanted it as perfect as possible and had no regard for originality. Your approach might be very different.

Also triple check what’s in your preamp. There are many schematics and variations.



C15 and C19 are on the switch wafers themselves and doing them is a major pain in the arse in the chassis. I resorted to removing the entire switches from the chassis and did the capacitor replacement on the bench which was much easier to get a nice neat job.







That wasn’t the case for me with the treble and bass controls which can be done in situ.

 

[John Shriver]

To get accurate phono equalization, every capacitor and resistor on M6 and M7 should be accurate. The polystyrene caps neevo used are a great choice.

Do the values McIntosh used give perfect RIAA playback EQ? Maybe, maybe not. Such issues can get very contentious!

 

[Bill Ferris]

It’s testing my brain a bit trying to go back and remember but it’s the components that come off the M6 and M7 switch that drives the RIAA curve (there’s also a right side too).

I couldn’t tell you exactly which ones are exclusively in the RIAA section but I believe it’s these ones. But also looks like a few extra parts down the signal chain too that might need changing too.

Depends what your approach is. If just repair essential items maybe others here can confirm exactly which parts (for example I would assume C19, C18 and R27 are in the single path of the treble RIAA curve).

I just overhauled everything as I wanted it as perfect as possible and had no regard for originality. Your approach might be very different.

Also triple check what’s in your preamp. There are many schematics and variations.

View attachment 3230327

C15 and C19 are on the switch wafers themselves and doing them is a major pain in the arse in the chassis. I resorted to removing the entire switches from the chassis and did the capacitor replacement on the bench which was much easier to get a nice neat job.

View attachment 3230330

View attachment 3230359

View attachment 3230362

That wasn’t the case for me with the treble and bass controls which can be done in situ.

I hope that you remembered to heat sink clip each polystyrene cap`s lead before soldering it, as their very, very heat sensitive, and can be unknowingly internally damaged, if you were not aware, neevo !