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Old 10-16-2021, 07:18 PM   #8
momaka
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Default Re: Repairing Bestec TFX0250D5W

Quote:
Originally Posted by lewisp69 View Post
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.

Last edited by momaka; 10-16-2021 at 07:21 PM..
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