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:
http://www.curto-circuito.com/esquem...0Sanyo%20Hd60%
20Mtk1379%20Mtk%201369%20Service%20Manual.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
Matsu****a-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,
--
Phil.
http://www.philpem.me.uk/
If mail bounces, replace "10" with the last two digits of the current year