Bose 1801 Service Manual page 28

Subject: Thermal Improvement
Remarks:
I'm pleased to inform you that we have developed a method for improving the thermal stability
of the 1800 amplifier, which may have been a problem to you during the last several months.
This problem was caused by a change we were forced to make when RCA discontinued the
manufacture of the transistor we had been using as Q3 on the 1800 PCB's.
The following change should be made on 1800 PCB,s which have the new Q3 ( MMPS U10 ).
1. Change R13, the bias pot (P/N 102832), to a new one which has not been sealed with
glyptol. This will allow you to adjust the bias current later.
2. Change R11 (P/N 102977-1501) from 1.5kΩ to 1.8kΩ (P/N 107170-182).
3. Apply a piece of Mylar insulating tape over the small (3/4" x 1/4") vertical surface of the heat
coupler bracket that is immediately above and parallel to C12.
4. Apply a small blob of thermal grease to the Mylar tape.
5. Obtain a 10kΩ thermistor (P/N 111153) and place insulating tubing over the leads, leaving
them about 1 inch long.
6. Solder the thermistor across (in parallel) the new R11.
7. Place the body of the thermistor against the blob of thermal grease applied in step 4.
8. Obtain a thin piece of polypropylene or other springy plastic and cut it to about 3/8" x 3/4".
Punch a small hole in one end of it and fasten it to the heat bracket using the screw that
mounts Q3 to the bracket. The tension of the bent piece of plastic will keep the thermistor
pressed into the thermal grease.
9. Adjust the bias current to 90ma according to the service manual.
NOTE: This change will be made on all future production units of the 1800 amplifier.
SERVICE BULLETIN
27
Date 10/05/1979
Bulletin #17