David Farber wrote:

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As I powered it up slowly with the Variac,
there was no gradual change in the duty cycle. It was either flat line
or full on.


** Very bad.

Check the 22uF on pin 8 of the PWM IC.

This cap enables the Soft Start circuitry.

https://www.ti.com/lit/ds/symlink/uc...ct%252FUC3525A

Using a Variac slowly may be an issue, but nothing will happen until the +/- 15V rails appear, then it should ramp up the PWM in about 1 second.


..... Phil