CFG250 Frequency Generator Repair

-Dead Service Manual Link- (Removed)

The Tektronix CFG250 2MHz frequency generator was sold as non-working. When I tested it, there was no signal output from the generator for all the waveforms and frequency ranges.Inside there were a few toasted resistors in the output stage. Using the service manual, I checked the 30V, 15V, 5V, -15V, and -30V rails which all were in spec. I might have missed the 15V1 rail which is just an LC low-pass filtered voltage branching off the 15V rail.

After checking the voltages, I noticed one of the heat-sinks for the output transistors Q13 was discolored. Instead of being black like the PNP transistor, Q14, it was brown. I de-soldered the transistor and measured the base-collector and base-emitter junctions. The base-emitter junction had a 0.05 ohm resistive short across the junction. The transistor must have shorted out causing it to heat up and discolor the heatsink.

When the transistor was removed from the circuit, I powered on the device again for some reason, the inductor L1 started to get hot and melt the plastic encasement around it. This was puzzling because it has not done that when the transistor was still in circuit. I eventually narrowed down the problem to a faulty dipped tantalum capacitor C17 that had shorted out. I replaced the capacitor with a 33uf 16v surface mount tantalum capacitor.

I ordered replacement parts for Q13, R93, R94, R95, R96, R97, R100. After the parts were replaced, the device worked again. Overall it was a successful repair, all the signal types and frequency ranges work. Another piece of equipment to add to my lab. This will be useful when developing audio circuits.