Bose 1800 Manual de servicio - Página 28

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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

Date 10/05/1979

Bulletin #17