Compaq PDP-121P - Baked MOV

[momaka]

Re: Compaq PDP-121P - Baked MOV

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.

[socketa]

Re: Compaq PDP-121P - Baked MOV

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

[PeteS in CA]

Re: Compaq PDP-121P - Baked MOV

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.

[stj]

Re: Compaq PDP-121P - Baked MOV

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.

[socketa]

Re: Compaq PDP-121P - Baked MOV

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?

[socketa]

Re: Compaq PDP-121P - Baked MOV

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)

[stj]

Re: Compaq PDP-121P - Baked MOV

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

[socketa]

Re: Compaq PDP-121P - Baked MOV

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?

[PeteS in CA]

Re: Compaq PDP-121P - Baked MOV

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]

Re: Compaq PDP-121P - Baked MOV

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?

[R_J]

Re: Compaq PDP-121P - Baked MOV

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]

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?