Repairing a PVR power supply

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,

--
Phil.
usenet10@philpem.me.uk
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Philip Pemberton
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The TEA1522P incorporates the power FET within the IC. It also derives startup power from the drain input. Given that it doesn't drain the mains filter cap, I'd verify no output diodes are shorted, then replace the TEA1522P before wasting time with the opto isolator, shunt regulator, or other unlikely candidate. If you MUST fiddle, use a pair of 9 volt batteries in series to provide startup current via pin 1. (Unless you have an isolation transformer, DON'T use a bench supply).

Many SMPS controllers use a large value resistor to obtain startup power. Using an internal switching regulator to obtain startup power is common, and those ICs are prone to failure. This design has gone one step further, incorporating the switching FET in the package, this sacrificing reliability in the quest for high efficiency and low cost.

PlainBill

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PlainBill47

Output diodes are all A-OK. All around 0.5V forward drop, according to my DMM.

Right, fair enough. I don't like blaming ICs for failures without some proof-of-failure, but in this case I think you're right -- if the FET has blown open, it's not going to be doing much...

Actually, my plan was to test it with the AC supply disconnected, which would allow me to push a small current into the 5V rail and check that the shunt reg and opto were working.

One more note for the lab notebook!

It looks like the chip gets pretty hot too -- there's a lot of discolouration of the PCB laminate around the pins; the diode pads show similar discolouration. In hindsight, that was probably a Big Red Flag...

Thanks,

--
Phil.
usenet10@philpem.me.uk
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Philip Pemberton

Swapped the 1522. PSU still dead, no output or startup.

Suspects are now the opto, the shunt regulator, and the resistors which measured high.

--
Phil.
usenet10@philpem.me.uk
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Philip Pemberton

Philip Pemberton wrote in news:4c12566c$0$17407$ snipped-for-privacy@news.astraweb.com:

did you check the ESR of the electrolytics,particularly those around your IC? I've seen where high-ESR caps kept switchers from starting. They loaded the IC supply enough that it would not start. I've also seen where carbon film resistors that charged the "housekeeping" IC power supply cap had increased in value to the point they were open.

--
Jim Yanik
jyanik
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Jim Yanik

All checked good, but were replaced anyway (they were off-brand Chinese things that I've seen fail in the past).

Thus far I've replaced: TEA1522 Optocoupler Shunt regulator One shorted diode in the 5V loop. Looked good on the meter in-circuit, took it out of circuit and my DMM and the Atlas DCA reported it as dead- shorted.

I've checked the resistors around the shunt regulator and the TEA, they're fine. All the other stuff around the TEA looks good, but still noting... "Beam me up, Scotty, there's no life out here."

I'm getting sorely tempted to disconnect the secondary diodes for all but the 5V rail, then reconnect them one by one...

--
Phil.
usenet10@philpem.me.uk
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Philip Pemberton

OK, I disconnected all the rails and it still wouldn't come up. Replaced the TEA1522 and it came up OK with only the 5V rail active. Fair enough, reconnected each rail in turn, turns out the SR560 Schottky on the 12V rail had shorted *again*.

I'm now onto my last TEA1522 and getting more will be "entertaining" to say the least (and probably involve buying £10 worth of them). What gets me is that the shorted diode did NOT cause the PSU to go into shutdown: it blew the TEA instead...

The shorted-turn trigger trips when the voltage at SOURCE is 0.75V. Thus, the 1R resistor that's in there now will cut the power when the primary pulls 0.75A, and the chip is rated to a drain current of 1A. This seems a bit close to me -- so I'm thinking of adding about 10R to this (two 0.7W

22R resistors in parallel to share the load). Good idea, bad idea?

I'd try the Lightbulb Trick, but I'm willing to bet the contents of my wallet that it won't offer much protection against a secondary side short.

--
Phil.
usenet10@philpem.me.uk
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Reply to
Philip Pemberton

Huh? From what I've seen, high ESR caps will cause the switcher to shut down, but from over voltage sensing rather than over loading the outputs. Shorted caps will do what you say, but an ESR meter is not much good in those cases. A good low range ohm meter for tracking shorts is best in a situation like that. Better still if it has 4 wire inputs.

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JW

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