L3/Repairs

Symptom: scope would not power on with 7L5 (+L3) installed. A slight smoke smell was present.

Checking resistance between power rails indicated a short between +15V and ground. Eventually I noticed a discolored resistor connected to the edge connector for the L3. I disassembled the L3. Even though I used the proper size pozidrive screwdriver, two screw heads stripped before they loosened, and another screw sheared off. I resorted to drilling these screws out with a 1/16" bit. I think they were over-torqued, and there appeared to be spalling on the countersink and threads of the mostly-intact screw.

I traced the +15V rail short to connection `M` on the A125 attenuator. Only two components connect to this, K130 and C142, so it was easy to determine which was faulty: C142, a 15 μF 20 V electrolytic cap. After replacing this and the overheated resistor in the 7L5, it functions.

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