Compaq PDP-121P - Baked MOV

[socketa]

Rescued a Compaq D51C computer
It has a deeper sized PSU than usual
The PSU fuse, and a MOV that was across a primary cap, were blown
The MOV is a zinc oxide varistor Z151 04U2, and i found another MOV on another parts PSU (that was also across a primary cap) that is a TVR 10241.
The clamping voltage is the same, but the peak current for the Z151 04U2 is 3500amps/2500amps, and for the TVR 10241 it is 2500amps/1300amps (whatever that means), even though it's a bit bigger in size - is that because it's not a zinc varistor?
Is it perfectly acceptable to replace the original MOV with this, despite the difference in "peak current"?
And does a clamping voltage of 395V mean that the voltage across one primary cap went higher than 395V? - that seems very excessive, since the primary caps are rated at 200V.
What could have caused the voltage to go that high?

[R_J]

The two caps are in series, if the cap that has the mov across went totally open, there would be at least 300vdc across it when the other cap was conducting. The TVR 10241 is rated maximum operating voltage 200vdc. The Z151 is also rated for 200VDC. It likely saw a spike that exceeded the max clamping voltage for the 8/20µs rating

[socketa]

Thanks.
I tested the primarys - they are Mashusihta CE 560uf, and they both measure 485uf and 0.2 ohm ESR (possibly haven't being used for a while).
When you say "spike", do you mean a spike from the grid?, or a spike because one of the primarys may have gone totally open (which i don't think has happened)?
What's the purpose of a MOV clamping voltage of 395V, when it's maximum rated operating voltage is 200V?
Anyway, is the TVR 10241 an acceptable replacement?

[PeteS in CA]

The input fuse probably was blown by the MOV, though it's worth checking the inverter transistors or MOSFETs. Since Compaq was acquired by HP in 2002, the computer could be 20 years old or more. The pin-out of the output connector, if of the Molex Minifit Jr. type (the type used for 20- and 24-pin ATX main connectors), may not be standard ATX.

More to the point, being around 20 years old, it's possible the MOVs clamped a good number of spikes (line surges due to lightning or inductive spikes caused by motors). MOVs are fast (unlike gas discharge tubes), can absorb a lot of energy (unlike TVS diodes), and are inexpensive. But every time an MOV absorbs a spike it is damaged a little, and the firing voltage is decreased a little. Eventually the firing voltage gets low enough that the MOV conducts under normal line voltage and soon fires, destroying itself and blowing an input fuse. If the inverter switch devices seem OK, the MOVs may have aged and self-destructed.

Another common way MOVs get blown on power supplies with input voltage selector switches is the power supply being set for 115VAC but plugged into 220VAC. If 220VAC or 240VAC is the normal line voltage in your country, check to see if the input voltage selector switch is set to 115VAC.

[socketa]

Voltage selector was in correct position
I'll first test the PSU with a light bulb, and then investigate further if it doesn't start.
So, as i asked in the first and third posts, is it OK replace the Z151 04U2 MOV with a TVR 10241? Would that be an improvement, or would it be a downgrade?

[stj]

it would be both.
your replacing a 150v part with a 240v part.
it will last longer - but be less sensitive to spikes.

[socketa]

your replacing a 150v part with a 240v part.

looking at the specs:

Max allow volt AC / DC:
Z151 04U2 150 / 200
TVR 10241 150 / 200

Varistor Volt:
Z151 04U2 212-259
TVR 10241 240

So, unless i'm not seeing correctly, they are about the same
The only noticeable difference, as previously mentioned, is in the spec, "Surge current"
So i'm assuming that the lower surge current means that it's a downgrade? (despite being a wider diameter)

[socketa]

So it looks like the lower peak current means that it's a downgrade, despite being a wider diameter (i can remember reading on this forum that a wider diameter is better; but that doesn't necessarily appear to be the case)

How about if i use two TVR 10241 MOVs in parallel, as a replacement for the cooked one?

[stj]

when you fit a mov - put it in heatshrink sleeving btw.
that way if it blows again it wont burn the board or spread soot everywhere.

[PeteS in CA]

Originally Posted by stj

when you fit a mov - put it in heatshrink sleeving btw.
that way if it blows again it wont burn the board or spread soot everywhere.

Shrapnel containment. Advisable with fuses as well.

[socketa]

Yep, figured that out - the localized vicinity was well coated with blackness
And it only took me an hour or two to find out that "varistor voltage" is the voltage where the MOV starts to conduct - IMO, they should call it something like "initial conduction voltage"
240V is above the cap's rating, but possibly OK for the cap if it's a quick spike

[momaka]

Originally Posted by socketa

looking at the specs:

Max allow volt AC / DC:
Z151 04U2 150 / 200
TVR 10241 150 / 200

Varistor Volt:
Z151 04U2 212-259
TVR 10241 240

So, unless i'm not seeing correctly, they are about the same
The only noticeable difference, as previously mentioned, is in the spec, "Surge current"
So i'm assuming that the lower surge current means that it's a downgrade? (despite being a wider diameter)

Well, the biggest downgrade would be to not have any MOVs, obviously. And many cheaper PSUs, in fact, often lack MOVs.
That said, as long as you do have any MOV(s) in there, PSU primary caps should be protected from spikes. I think the current rating only matters in instances where you might have a voltage spike that is so high and so powerful that the MOV is unable to clamp it fully and some over-voltage does make it through to the other components. But in this case, I think both MOVs are rated too similarly for there to be any difference in how they "perform" (note: MOVs spend most of their service life waiting and doing "nothing" until a voltage spike comes along.) Therefore, I think you'll be OK with the replacements.

Other than that, I also second the notion with the MOVs getting heat-shrinked. Fuse too, if it's glass and isn't already covered. Ceramic - OK to leave as-is.

[PeteS in CA]

I've seen ceramic cased fuses that blew apart, though the fuses were SMD types. They blew apart due to the current briefly continuing to conduct by arcing. A longer ceramic fuse might not have arced quite as long, and with less dramatic results.

A higher rated energy MOV will handle higher energy single hits without catastrophic failure, though the same kind deterioration will still occur.

[socketa]

Put the primarys back in (a small bit of the sleeve came off when removing the tough white glue, so i just put plastic masking tape over it), and the replaced the MOVs
60W lightbulb 'fuse' briefly lights, then goes out
5VSB auxiliary rail (green wire) is working good
But it doesn't jump start it with a paper clip between the black and white ATX wires (no voltages on the other three rails)
There is a 5H0265RC Fairchild power switch (for the 5VSB?) and a 2SK2648 N-Channel MOSFET (for the main PSU?). There is no shorts on the fairchild power switch, and i get 13ohms across the MOSFET Gate/Source, so that's not a short either
Tried connecting a hard drive, but it still wont jump-start
There is 158V and 155V across the primarys and 313V across the bridge rectifier DC legs
This was all done with the lightbulb in place of the fuse
The main controller is the 8 pin UC3845B IC
I removed it, and there is 3 ohms between pins 5 ground and 7 Vcc
I luckily found a UC3845BN and checked the same pins and there is no contact between them
Should i replace it with that one, since i can't perceive any difference on the datasheet between the two ICs?
Anything else that i should check before replacing it, so that i don't damage the replacement controller?

[R_J]

The ic will be the same, the only difference is the N suffex which refers to the package (DIP). I suspect there will be a small electrolytic on the vcc line, I would replace it as well.

[socketa]

Thanks for the confirmation.
That Vcc cap tests good
So i desoldered the main mosfet and get 14 ohms across gate and source - so looks like that's baked as well
This mosfet is rated at 800V 1.5Ω 8A -Is it OK to replace it with a STW8NC80Z N-CHANNEL 800V 1.3 Ω 6.7A Zener-Protected PowerMESH™III MOSFET?

[stj]

if it's shorted to the gate then you need to check the gate-drive circuit too.

[socketa]

Originally Posted by stj

if it's shorted to the gate then you need to check the gate-drive circuit too.

That's only a couple of resistors, 20 and 200K - they test good.
I replaced the MOSFET with the other one that i mentioned
Good to to power it up now with a bulb?

[socketa]

I powered it up but have no power to Vcc on the UC3845
Vcc goes to an optocoupler via a BJT transistor.
The optocoupler (U3) has 16V on one pin of the primary, and nothing on the other, and 2.5V on both pins that are on the secondary side.
The transistor (222A NPN) is just above the UC3845 chip in the photo.

[R_J]

If there is 2.5 volts on each pin of the optocoupler on the secondary side, the difference in voltage is 0 so the internal diode is not lit and the primary side will act open circuit so no vcc
The secondary side should be supplied by the standby circuit, 2.5v seems low so check if the optocoupler is shorted, the sec should check like a diode. The secondary of the optocoupler will be controlled by the power supply ON/OFF signal