McIntosh MR78 Cleaning

[jdandy]

Originally Posted by Vintage Pete

Okay Dan, installment two is up!

Pete.......Hey, what can I say. I'm a McIntosh tuner buff, and a big fan of your collection.

[jdandy]

[Vintage Pete]

Originally Posted by jdandy

Installment three is up Dan....

Pete

[jdandy]

Pete.......I am thoroughly enjoying this thread. It is great fun to watch you go through your beautiful MR78, making it sparkle once again. And, there are the RIMO filters, invented by Richard Modafferi, the designer of the MR78 tuner.

My MR74 is clean, but your thread has me thinking of going back in again. You are doing a great job.

[Vintage Pete]

Thanks Dan...after I grab a bite I may put the last installment up tonight.

Pete

[jdandy]

Pete.......There's more interest in this thread than you might think.

[Beans]

I sure wish that were my MR78. I've written to you about it before. It's still in the damn shop
The 1st time it was down there, they supposedly changed all the bulbs. But mine always has had very dim lettering around the knobs. Are those bulbs you're putting in brighter than the originals?

[jdandy]

Pete.......This lovely young woman said she could care less about an old MR78 whatever.......she was just looking for Pete.

[Vintage Pete]

The final installment is up!


Pete

[Vintage Pete]

Originally Posted by Beans

I sure wish that were my MR78. I've written to you about it before. It's still in the damn shop
The 1st time it was down there, they supposedly changed all the bulbs. But mine always has had very dim lettering around the knobs. Are those bulbs you're putting in brighter than the originals?

I can help you with this. Mine used to have that problem too. After consulting the schematic, I discovered that there is (was) for some reason a resistor in the circuit that powers the panel illumination rail. I have no idea why this resistor was placed in the circuit, these are 6.3V lamps, wired in parallel and fed by a 6.3V transformer secondary.

You need to remove (and jumper) the 2 Ohm, 5 Watt Series Resistor R508 from the Panel Lamp Filament Circuit to provide a full 6.3 Volts To the Filament Circuit. This will fix your panel lamp brightness problem, and optimize the panel lamp brightness. This will not affect the dial or meter lamps, only the lower panel lamps. I am sure this is what your problem is. I removed this resistor 7 years ago and have not had a bit of trouble as a result, only a correct looking panel...using the type 1866 lamps that the design calls for.

I will be opening the tuner up again shortly to replace the foam light seals (I discovered they were in there longer than I thought, once I checked my records).

I can show you exactly where this pesky resistor is located.


Pete