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Tuesday, 26 March 2019

McIntosh MC2125 and C28 conversion to 240V and replacement volume control.

Colin's mate Ian got in touch

"Got a McIntosh power amp and pre-amp, both need converting to 240V, and the pre-amp needs the volume control changing. Can you have a look?"

Yeah, why not....

The stuff duly arrived, delivered by a delivery driver who was somewhat out of breath. When I picked up the package with the power amp in, I knew why....


It weighs a lot... enough to have it's own gravity and bend light around it, which makes photography rather tricky ;) It's an odd design that has output transformers, despite being fully transistorised. Must have seemed like a good idea at the time! (or did they just miss the memo?!?!)

McIntosh had provided a nice wiring diagram of the transformer. The two primaries should be in parallel for 120V operation....














... off with the bottom, and indeed the primaries are wired as we thought. The wiring is on the tag strips in the bottom left. All that's needed is to disconnect the Black/Red lead, the Black/white lead and connect them together. The join is soldered and heat-shrinked.





The unit is gently powered up via the variac whilst monitoring the 60V rail just in case, and everything checks out ... good. On to the pre-amp...



The C28 pre-amplifier is also a bit of a beast, weighing as much as most normal power amplifiers! 

Nice block diagram on the top cover...


... and under the cover the power transformer once again has a nice wiring diagram...

So off with the bottom, locate the wiring ...  


... and modify as before :)

Once again it's powered up via the variac to check for any mistakes. It's fine.
Now, the volume control. Ian had supplied a new pot. It's also got a mains switch on it. 

It's the top right control on the front panel. Sure enough the left channel track is open about half way along.
Knobs off ... 

 Front panel removed and put somewhere safe (it's glass) 
... and finally the top panel eased up to allow (tight) access... There's a few cable clips to undo to get it this far...

After much cussing at the sharp metal edges, and a slightly different shaped pot supplied to the original one fitting, the new pot and switch are finally fitted... 

... and (thankfully) works well, giving an impressively low THD.

3 comments:

  1. Hi,
    I am thinking of doing the same with a 120V mcintosh 2125 (switch it to 230V with this method). It is in original condition. But do you think that the original capacitors in this amp will handle this new 230 voltage? Are they able to do so?

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  2. Doz, I'd like to ask you - I want to do the same with my mc2125 - wasn't there any problems with the capacitors? Can the original capacitors handle the new 230 Voltage?

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    Replies
    1. There's no change in secondary voltage, so the original capacitors will stand the original voltage just fine ;)

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