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Subject: Resistor Failure

Remarks:

We have found that the resistor R3 on the 1801/1800 amplifier PCB's have shown a higher

failure rate than normal. Studies reveal that the resistor normally runs at about 150-170°F. The

etch layout at R3 is not sufficient to provide adequate heat sink for this component, and the

resistor eventually changes value due to the heat.

This changing value causes the resistor, capacitor C2, and the zener diode CR15, to fail. The

symmetrical circuit, R2, C2, and CR6 all have sufficient heat sinking capability on the etch

layout and does not cause failures.

To improve the reliability of the boards, R3 should be changed from a carbon resistor to a

ceramic resistor of the same value. The ceramic resistor should be elevated slightly above the

PCB surface to provide adequate air ventilation for self cooling purposes. The new ceramic

resistor P/N is 108676-302.

In addition, R2 should also be changed to a new value. The resistor will be changed from 3kΩ

to 8.2kΩ, P/N 104064-822. This resistor is carbon, and due to the etch layout, need not be

elevated.

This modification must be preformed on all PCB's brought in for repair, regardless of the actual

failure.

SERVICE BULLETIN

Date: 09/19/1977

Bulletin #13A