MC275 rebuild

[Brice]

Hi guys,

It has been a long time since I got a new McIntosh and working on it. My Christmas present came early this year and I now have a nice vintage MC275. It's not the prettiest in the bloc but it works fine for its age.

Despite a rough shipping with broken tubes, it's now here for some TCL.

I'll do the usual PS caps upgrade with KTL25, rectifier swaps with some fast SS diodes, change all the power resistors in the amp, all the electrolytic caps (I found 12uF 250V film capacitor which should fit) and various components upgrade.

Starting with the PS, I notice that the thermistor is on it's last journey before Valhalla. It literally holds by a thread of copper to the core. Quite funny.

I was wondering what you guys replace it with. The specs I believe specify > 25 ohm 5 amp device.
But I did not find a modern equivalent in the CL family. I have a bunch of CL60 which are 10 Ohm 5 amp as the closest replacement. Should I put 2 in line to match the vintage thermistor?

Thank you,

Brice.

 

[rhing]

Maybe this would be helpful:

http://www.digikey.com/en/product-highlight/a/amphenol-advanced-sensors/cl-series-current-limiters

Good call on using the Authenticap can caps. If you don't mind spending the extra money and working with larger resistors, I am very pleased with the results with using Ohmite Brown Devil vitreous enamel wirewound resistors in my MC240 power supply rebuild. Mouser carries them along with the Ohmite Carbon composition resistors.

 

[Brice]

Thanks Rich.

 

[Brice]

Hi guys,

Reviving that thread as I had an issue with my MC275. I rebuilt it 2 years ago and I am very happy with it.

Some time ago I was playing it, when I've heard a low volume, low frequency stump in the speakers, like a small bang.
Could not know if this was from 1 speaker or both. That could not be good.

Put it on my bench, no stump. Ok I played some music and I immediately heard a distortion in one channel only.
Scope + distortion analyzer and yep, one side was really bad. My ears are not that bad after all.

After some measurements I deducted that 1 resistor was not right, the 27K plate resistor of the upper 12AU7 stage.
I installed it and it was a metal film resistor Holco H4P 27K 1% 50ppm 1W resistor. I always measure components when installing them so I know that one was good at some point in the past.

It measured 639K... No wonder the splitter was unbalanced.

I replaced it and the amp now works perfectly. I now will swap the others just to be on the safe side.

So beware of these Holco H4P resistors, they are not reliable at all.

 

[x3workshop]

I know I have some of these in my amps. Will have to review which ones.