[AlanInSitges]

Got something a little newer on the bench today, no tubes, no nicotine glaze! It's not a TV but this seems the most-likely forum to put it in.

I have a no-name Chinese DVB set-top box that I've been using for a few years. It's unique in that it has an analog composite video output, and can also be switched between PAL and NTSC outputs - this is important to me since I collect both European and American sets. For the last year or so I've left it tuned to the local oldies channel, feeding an NTSC signal to a HLLY 2W transmitter, which blankets my house in a clean signal.

Until last week when the STB just turned itself off. They don't make it anymore and there don't appear to be any other boxes with that odd but useful PAL/NTSC feature. Can't find any on eBay either.

Inside on the single PCB is a simple SMPS that's not working. It uses a Chipown PN8370 chip. I've checked voltages all the way through the primary side of the power supply and they are as expected.

Output voltage jumps all over the place, between 0 and 13v, mostly 0. If I put a scope across the secondary of the transformer, I get a sawtooth-ish waveform around 12V that appears for a fraction of a second and then goes flat for about 2S, then repeats. Looking at the documentation for this chip, and based on what I know about SMPS in general (not much) it seems it's starting up and then either shutting down due to an overload or because it's not getting the output it expects - this circuit appears to use a winding on the transformer to generate feedback for the chip vs. the more common optoisolator and I really have no idea how that part of the circuit works.

What I've done so far: checked and changed all the electrolytics (originals are all labeled "low ESR 105C"), including primary and secondary filters and the VDD startup cap, checked the bridge rectifier on the primary side, along with a couple of diodes on the primary side, checked what I assume is a Schottky diode on the secondary side (labeled PD300 on the board). Now I'm at a loss. Anyone able to give me a suggestion for where to go from here?

Attached are a photo of the board and the datasheet for the chip. Thanks.