Re: FG502 Troubleshooting Help Requested

Roger Evans

The charging and discharging currents for the timing capacitor have to pass through the switching circuitry of S155 so bad contacts there will stop the triangle generator working and the other waveforms all depend on the triangle generator. Measure the voltage drops across each of R140 and R175, this would show that the constant current sources are OK. Try this on different ranges since the current varies with the frequency range. The two voltage drops should be very similar. Measure the voltage at the junction of CR140/CR170 and at CR145/CR175, this shows whether the timing capacitor should be charging or discharging. Check that when you change the frequency dial the voltages at the outputs of U140 and U175 vary smoothly and approximately mirror each other, that should rule out a problem with the op-amps.

It sounds like you have a scope so monitor the emitter of Q210 while changing the frequency multiplier knob back and fore, you might see the occasional burst of activity even with poor contacts on S155.

There could also be a problem with the circuitry for externally gating the triangle generator, if Q315 is socketed you could remove it since it should be non-conducting in normal non-gated operation.


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