jtps28m smps can't figure out how the vcc to the tl-494 gets generated

[wcurlew@juno.com]

Working on a smps from jetstream. Looks like the power for the tl-494 chip comes AFTER the switch transformers. Can't figure out how the supply starts up. Getting a sawtooth on the VCC pin of the tl-494. Schematic attached.

[shakalnokturn]

To me it seems that it should self oscillate due to T2 xfmr (T2D winding), then when there's enough voltage on output TL494 kicks-in and regulates by controlling pulse width.

My first suspects would be C22, C19, C18, C7. (Then any electrolytic stuffed in a hot spot...)

[wcurlew@juno.com]

Thanks shakalnokturn, I was wondering what that extra winding was for. I did check the ouput diode pair and it looked good, I'll sniff around the caps and maybe scope the input to the T1A transformer to see if the oscillation is there.  Bill

[Bud]

Check if 5V appears on pins 13,14,15 as the regulator is trying to start, and check or replace C15 capacitor .

[amyk]

This is a common topology in older PC PSUs - it starts as a self-oscillating half-bridge and once the secondary gets enough voltage, the TL494 takes over. I've written about it before here:

https://www.eevblog.com/forum/beginners/old-at-computerr-ps-issue/

C15 is suspect, but I would check all the electrolytics and replace them anyway.