Function
- MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2, 12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin 7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias voltage resulting in the phase inverter outputs being unbalanced.
- replace silicon rectifier with 1N4007 and 470 ohm resistor (check this, it's in the restoration video)
- change 1.8K series resistor to 5.1K, check bias for -45 or slightly higher (-46) when B+ is at 435V on the KT88. If you have a fully meterd variac it should draw 1.2A at 120V.
- While you are at it replace the bias filter, a 22uF 450V cap will stiffen it up and last forever. (? I think 12uF is stock?)
- The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5 of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470 before I measure pin 5.
- If a pair of resistors is marked 1% in the schematic this means to each other not the listed value.
- For all resistors in McIntosh, unless noted otherwise they are 10% tolerence.
Transformers
- Room = 75F
- Front = output transformer = 88F
- Middle = output transformer = 92F
- Rear = power transformer = 110F
Aesthetics
- use Wenol (http://www.wenol.com/) to clean the chrome. Auto wax works too.