Re: FG501 repair


um-gs@...
 

Hi,
just want to continue with my lessons learned on the same subject.
The device was operative, suplly voltages were adjustable to their nominal values, but no output.
1. No output
a) open R251 and inject signal voltage: Voltage appears at output, so amplifier is working
b) checking voltage at junction R32/R34 when turning the frequency dial: voltage varies
c) checking voltage at output of U15 pin 6 : does not vary - replace U15 which was defect - oscillator is working again, output signal is generated
2. Sine wave output distorted and adjustment R150 has no effect
a) replacing both R150 and R170 allows adjustment of distortion free sine wave output
3. Output signal not at symmetrical duty cycle
a) adjustment of R45 (step7) has no effect
b) output voltage at U40 pin6 does not vary when R45 is adjusted
c) swiper of R45 constantly at zero
d) opening up/replacing of C40 solved the problem (it was shorted) - output duty cycle can be adjusted

Regards, Gordian

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