Repairing Bestec TFX0250D5W

[everell]

Had a Bestec TFX0250D5W come my way. It had several bloated capacitors. So I thought I would try replacing them and see if that fixed it. It didn't fix it! Here are some pictures I took of another Bestec, same model, not as many bloated capacitors. The earlier Bestec that I fixed was a dead 5vsb with bloated C36 capacitor. This one had a handful of bloated capacitors.

[everell]

Here are some pictures of the silicon in this psu

[everell]

After replacing the bad caps, the power supply tester LEDs just blinked, the fan just gave a tick. So I measured the output diodes thinking that maybe one was bad. No, they all checked good. So I started tracing the output circuit to see what was in this power supply. To my surprise, there are NO load resistors on the output rails. So I set up a jig with a 12 ohm resistor for the 12 volt rail, a 5 ohm resistor for the 5 volt rail, and a 3.3 ohm resistor for the 3.3 volt rail. The power supply now came up working as expected.

The reason for writing this post..............make sure the power supply you are working on has load resistance on the output rails - or you may be chasing a ghost.

For anyone else working on this particular model Bestec, here are some drawings of the 5vsb circuit and the output circuit. Also a roadmap of the output capacitors. Perhaps this will be helpful to someone.

[momaka]

Nice work everell! Great photos as usual too.

With 17A available on the 12V rail, spec-wise this looks like a really nice 250W PSU.

[Micro System]

Thanks this images helped in repairing The TNY276PN was burnt and from picture i was able to identify the part no

[lewisp69]

Originally Posted by everell

Had a Bestec TFX0250D5W come my way. It had several bloated capacitors. So I thought I would try replacing them and see if that fixed it. It didn't fix it!...

I also have a Bestec TFX0250D5W and need some info on replacement MOV. I have two blown MOV and I can see some info on one of them, TRV 07241, but I'm not finding this available in the USA. Shipping from outside the USA takes too long right now so I would like to find an equivalent replacement.

The MOV are ZR1 & ZR2, they are connected to the wire coming from the voltage selection switch, and to + voltage and ground, after the bridge rectifiers. Pics attached.

[lewisp69]

Originally Posted by lewisp69

I also have a Bestec TFX0250D5W and need some info on replacement MOV. I have two blown MOV and I can see some info on one of them, TRV 07241, but I'm not finding this available in the USA. Shipping from outside the USA takes too long right now so I would like to find an equivalent replacement.

The MOV are ZR1 & ZR2, they are connected to the wire coming from the voltage selection switch, and to + voltage and ground, after the bridge rectifiers. Pics attached.

Not able to upload pics right now

[momaka]

Originally Posted by lewisp69

I also have a Bestec TFX0250D5W and need some info on replacement MOV. I have two blown MOV and I can see some info on one of them, TRV 07241, but I'm not finding this available in the USA. Shipping from outside the USA takes too long right now so I would like to find an equivalent replacement.

The MOV are ZR1 & ZR2, they are connected to the wire coming from the voltage selection switch, and to + voltage and ground, after the bridge rectifiers. Pics attached.

No worries about not being able to attach pictures (though if the forum is rejecting them, it may be because they are too big in terms of size or resolution - must be scaled to 3000x3000 max. or less.)

The placement of those MOVs is pretty standard for older PSUs like this: one in parallel to each input 200V mains capacitor. The MOVs protect these caps from over-voltage due to surge or whatever other external factor.

That being said, since both of your PSU's MOVs blew, you might want to check first if something else didn't go bad. Most likely the fuse on the input has gone bad, and possibly the NTC thermistor as well. Bridge rectifier can sometimes go bad from that as well. If you have replaced those parts already and nothing else appears to be bad, you can actually test the PSU without the MOVs, just to verify if the PSU is working at all after whatever caused these MOVs to blow. The MOVs are there only for surge protection, so they aren't actually necessary for the operation of the PSU. Of course, they are good to have in the even of over-voltage / surge to prevent more damage to the rest of the circuit. On the negative side, they do sometimes happen to fail by themselves from old age. That's because over many years, they probably have seen many "micro" surges. Every time one of these MOVs has had to operate (clamp) a surge voltage, a little part inside it gets destroyed and makes the MOV more likely to conduct next time. So overtime, that can make them fail by themselves, so to speak.

On the other hand, since both MOVs blew at the same time, you may also want to check your house's wiring. In particular, check the AC voltage at the plug where the PSU was plugged in and monitor it for a while - especially when a high-power device comes On in your house. If living in a house with dual-phase 120V AC supply (most single homes and townhouses in the US), an open/bad Neutral can cause some plugs in your house to go much above 120V AC, while letting others sink way below that. If you turn on a high-power device in your house and some lights flicker or go dim while other seem to go lighter (well, I guess this may not apply so much anymore, now that people have switched to LEDs and CCFL light bulbs), that's definitely a sign of your house's Neutral connection opening or going bad... and if so, you ought to call your electric supply company right away, as this is actually an electrical fire hazard.

And finally, for replacement MOVs... you really just need ones that are rated for 200V DC. Looking at some other PSUs, these part numbers should work too:
TVR 10241
Z151
There are many other too, but these are the ones I can recall seeing recently in a PSU.

After replacing the MOVs, make sure to install heatshrink tubing on them for shrapnel control (in case they blow again.) Same if original fuse was blown and you replaced it with a glass type. Glass fuses should be heatshrinked to contain glass fragments if the fuse blows.

[lewisp69]

Thanks momaka, this is good info. The computer, and several household appliances and the air conditioner sufferd from a surge during a thunderstorm. Not my house, my brother-in-law's mothers house. If that wasn't bad enough, the power company was trying to restore power while her power meter box was ripped off the house by a tree limb, and exposed wires were on the ground shooting sparks. I'm trying to get the PC running again so she can play her games. Also, I know I could just replace the power supply, but this is a bit of hands on education for me. The fuse, bridge rectifier and two MOV appear to be the only components blown. I'm currently trying to find one source for the components, so I don't have to pay separate $8.00 shipping for parts that are less than a dollar each. Hopefully your replacement suggestions will help with this. Thanks so much!!!!!!!

[momaka]

You can check Digikey.com or Mouser.com for parts, if you haven't already. That way, you can get everything in one order (and shipping.) There are other parts places online too, but these are the main ones (at least here in the US.) Alternatively, if there are any computer shops left in your area, you can ask them if they have any junk PSUs they're throwing away to try and scavenge parts from them... or if you see a busted CRT TV on the side of the road with PCBs still in it.

Originally Posted by lewisp69

If that wasn't bad enough, the power company was trying to restore power while her power meter box was ripped off the house by a tree limb, and exposed wires were on the ground shooting sparks.

Oof, that must have been a not-so-fun light show to watch.

[lewisp69]

Finally, pics attached.

[momaka]

^ Doesn't look to bad. Just need to clean the board well from all the soot where these two burned. IPA should work OK for that.

Output electrolytic caps could probably also use replacing with (if you'd like the PSU to last as long as possible), even though they don't look bad. Should be just a few more cents per cap to a $1 max. for the larger ones. Note: no need to replace the big 200V caps, unless they are bad... which hopefully are OK and the MOVs protected them. Otherwise, they'd be the most expensive thing to replace @ $3-4 ea. usually.

*EDIT*
I think these might actually be drop-in replacements:
https://www.digikey.com/en/products/...7D241K/2407535
In case link above goes bad, these are Bourns Inc. MOV-07D241K (Digikey P/N: MOV-07D241K-ND)

Alternatively, there is also the LittleFuse V150LS2P (Digikey P/N: F5595TR-ND or F5595CT-ND):
https://www.digikey.com/en/products/...50LS2P/1009272
^ More or less the same thing as the first one above for more or less the same price.

[PeteS in CA]

You might want to look at the inrush limiter thermistor, the green disc next to the fuse. It looks like it is chipped, and it might be cracked.

BTW, that power supply is almost 11 years old, very decent product life.

[lewisp69]

PeteS, the inrush thermistor measured 3.4 ohm. There is no damage on it but some white adhesive is on it.

BTW, tried to do some testing while waiting for the MOVs to come in and blew the 5v stand-by control chip, U2 (TNY276PN). Ordered some of these too. I had replaced the bridge rectifiers diodes and fuse and the chip blew as soon as I plugged the PS into the outlet. I hope it was already damaged and I don't end up blowing up the new chip.
The hot side power was normal when I checked it after the chip blew.

[lewisp69]

I love J.R.R. Tolkien!!!!!!!!!!!

[momaka]

Originally Posted by lewisp69

BTW, tried to do some testing while waiting for the MOVs to come in and blew the 5v stand-by control chip, U2 (TNY276PN). Ordered some of these too. I had replaced the bridge rectifiers diodes and fuse and the chip blew as soon as I plugged the PS into the outlet. I hope it was already damaged and I don't end up blowing up the new chip.

Yes, sounds like it was damaged already from before. Of course, before putting in new TNY276 chip, check its surrounding components for faults. Otherwise, this could repeat.

Also, to actually test the PSU in a safer way (or at least the 5VSB circuit and the main PSU without any load), you can put a incandescent (or halogen) light bulb (40-100 Watts typical) in series with the PSU's input, like this:
https://www.badcaps.net/forum/showpo...4&postcount=70
The above method should prevent the new stndby IC from blowing up, as well as the fuse and anything else on the primary side.

First, just hook the PSU with the series bulb and see if the 5VSB comes up correctly. If it does, you can try shorting the PS_ON (typically green wire) to ground to turn on the PSU - but do so without any load (or very light load, such as a PC fan or two or an old useless HDD... or 12V car bulbs.)

The incandescent (dim) bulb "trick" works with many PSUs (switching and linear)... though there are a few exceptions, like for example, switching PSUs with APFC circuit. In the case of the Bestec in this thread - it's an older design SMPS without APFC circuit, so dim bulb trick will be OK with it.

[lewisp69]

Momaka, Thanks for your help with this PSU. The lamp worked wonderfully, although turns out I didn't have any issues when the PSU came to life. I ended up replacing the two MOV, using parts you suggested, the fuse, bridge rectifier and TNY276PG. Overall I'd say the PSU took the lightning strike pretty well. Momma "O" will be tickled pink to get her computer back .

[frend2001]

Hi Everyone,

I also have this PSU with me. It turns on but the output voltages are slightly low on outputs.

11.15v at place of 12v

4.6v at place of 5v

3.1 at place of 3.3v

it turns on but can't boot the CPU it's powering up.

[frend2001]

Originally Posted by momaka

No worries about not being able to attach pictures (though if the forum is rejecting them, it may be because they are too big in terms of size or resolution - must be scaled to 3000x3000 max. or less.)

The placement of those MOVs is pretty standard for older PSUs like this: one in parallel to each input 200V mains capacitor. The MOVs protect these caps from over-voltage due to surge or whatever other external factor.

That being said, since both of your PSU's MOVs blew, you might want to check first if something else didn't go bad. Most likely the fuse on the input has gone bad, and possibly the NTC thermistor as well. Bridge rectifier can sometimes go bad from that as well. If you have replaced those parts already and nothing else appears to be bad, you can actually test the PSU without the MOVs, just to verify if the PSU is working at all after whatever caused these MOVs to blow. The MOVs are there only for surge protection, so they aren't actually necessary for the operation of the PSU. Of course, they are good to have in the even of over-voltage / surge to prevent more damage to the rest of the circuit. On the negative side, they do sometimes happen to fail by themselves from old age. That's because over many years, they probably have seen many "micro" surges. Every time one of these MOVs has had to operate (clamp) a surge voltage, a little part inside it gets destroyed and makes the MOV more likely to conduct next time. So overtime, that can make them fail by themselves, so to speak.

On the other hand, since both MOVs blew at the same time, you may also want to check your house's wiring. In particular, check the AC voltage at the plug where the PSU was plugged in and monitor it for a while - especially when a high-power device comes On in your house. If living in a house with dual-phase 120V AC supply (most single homes and townhouses in the US), an open/bad Neutral can cause some plugs in your house to go much above 120V AC, while letting others sink way below that. If you turn on a high-power device in your house and some lights flicker or go dim while other seem to go lighter (well, I guess this may not apply so much anymore, now that people have switched to LEDs and CCFL light bulbs), that's definitely a sign of your house's Neutral connection opening or going bad... and if so, you ought to call your electric supply company right away, as this is actually an electrical fire hazard.

And finally, for replacement MOVs... you really just need ones that are rated for 200V DC. Looking at some other PSUs, these part numbers should work too:
TVR 10241
Z151
There are many other too, but these are the ones I can recall seeing recently in a PSU.

After replacing the MOVs, make sure to install heatshrink tubing on them for shrapnel control (in case they blow again.) Same if original fuse was blown and you replaced it with a glass type. Glass fuses should be heatshrinked to contain glass fragments if the fuse blows.

Hi, your inputs are welcome to my problem with this PSU