Hi guys, Here's a new one for me... switchmode power supply repair.
I've got a "Nichimen" PVR sitting on my workbench; apparently it "jus' plain stopped workin'". It appears to have been made by some Chinese OEM or other, possibly "Cosmic Digital Technology Co. Ltd."
The power supply board carries the markings "CO-DPA118 Ver2.0
2002-12-13", and appears to have been used in a DVD player as well. Schematics are on the second-to-last page of this PDF:Here's what I've found out thus far: - The SMPSU I have is missing the components for the -12V rail, and the "1085" 3.6V regulator. This appears to be "by design" and an act of cost- cutting on the part of the manufacturer (why include power rails that aren't used by the product?)
- Power filtering and rectification is fine. I've got 330V over C9 (47uF
400V). It also seems to be storing its charge over power-down.. I forgot to discharge said capacitor and got a nice electrical burn for my trouble.- Powered up on the bench, U1 (TEA1522P) appears to be totally inactive, and its Vcc (between pins 1 and 2) seems to be dead too. Given that it depends on the transformer running before it gets Vcc (it has a separate winding all to itself), that isn't surprising.
- Drain (TEA1522 pin 8) is at 330V. Source is at 0V. The internal transistor is thus switched off (there's a resistor between Drain and Vss, there should be *some* voltage there).
- A few of the resistors are reading differently in-circuit -- specifically, R1 (next to the TEA1522), R7 (next to the transformer), R12 (near the TL431) and R13 (same place). I think my meter's (Fluke 25
3digit) getting confused by surrounding circuitry...Question is, where do I go from here? Obviously I'm going to desolder one leg of each of the "suspicious" resistors, but I'm left with three other possible suspects: TL431 shunt regulator PC817 optocoupler TEA1522 SMPSU controller
I've already shotgunned all the electrolytics (except the 47uF main filter) on the grounds that they were made by several known bad-cap manufacturers (Capxon, JP and one or two others). It's now loaded with Matsushita-Panasonic parts. So we can call the capacitors "known good" :)
My plan (after checking the resistors) is to solder some wires across the
5V output, then bring it up to 5V with an external current-limited PSU. That should allow me to test the TL431 (by increasing and decreasing the PSU voltage). Testing the optocoupler should be possible too, though I think I might need a second PSU or a 9V battery for that.Can anyone suggest some other things I could check? I'm calling this a startup issue at the moment, seeing as that's what's happening -- the SMPSU is failing to start up.
Thanks,