840AV dead PSU

[grieverheart]

I had my quadra 840AV (which I recapped and had working) plugged in for some time without using it and randomly decided to turn it on today. Unfortunately it did not turn on. After checking the PSU, I found out I was not getting anything from 12V, only from the 5V rail. After a few minutes, I removed the PSU and plugged it back in the electrical outlet and it suddenly started making a weird sound and soon after smoke started coming out. I quickly unplugged it. I opened it up but on first instance I didn't see anything out of the ordinary really.

Tomorrow I'll clean it up and look closer. Unfortunately the PSU seems really difficult to work on. If you have any suggestions on what to check, I'd really appreciate it. Should I risk it and plug it in the electrical outlet again and try to spot where the smoke is coming from?

I'll post photos tomorrow with my progress.

 

[grieverheart]

I dusted the psu off this morning, and discovered what seems like a probable blown capacitor — the bottom black plastic is hanging, and on the other side of the board it looks darkened. There is also corrosion on some pins on the same daughter board, and I think at least one more capacitor is leaking (there looks to be some green stuff). Does anyone know what the function of the daughter board is?

Hopefully it’s just as simple as replacing these two caps. If it works then I can work on replacing the rest!

 

[grieverheart]

Bad news, I removed the daughter board successfully, and then went ahead and removed the capacitor with the hanging base. I then went ahead and plugged the psu without the board in, but started hearing burning sound again, I sprinkled some alcohol to try to locate the problem, but it resulted in fireworks, so I quickly turned off the power. I suspected one of the large caps, it was also warmer than the other one. I removed it, but it seemed to test fine.

With the large cap removed, I decided to turn the psu one more on. This time it took some time for something to happen, but when it did, it triggered the house’s breaker and started a small fire! One of the varistors, next to the large cap exploded. The weird thing is that the fuse on the board was still not blown.

Now, from what I understand, the varistor is implied to be the cause of the psu issues, but rather the symptom. I’m also afraid I should have never tried to turn the psu on with components missing, is this correct?

Do you have any suggestions how to proceed? Is it worth the risk of recapping? Or do I risk wasting time and burning new components?

 

[pl212]

Hm, I would avoid switching the PSU on when you have removed components, and I would definitely advise not spraying it with a highly-flammable liquid.

 

[grieverheart]

Indeed, doesn’t sound very intelligent of me when I read your post!

 

[cheesestraws]

Yes, I very much regret to inform you that if that wasn't toast before it's probably toast now.

 

[grieverheart]

Yes, I very much regret to inform you that if that wasn't toast before it's probably toast now.

You think so? Everything else looks fine. I’m now wondering if just the MOV short-circuited. I read that this can happen, and indeed catch fire. I can’t find any obvious shorts anywhere.

So I’m now wondering if I should risk it and replace the MOV and some of the caps and try again. BTW, I mistakenly threw the MOV away, do you have any idea what rating I should go for?

The alternative is to either buy an ATX psu (found one from Inter-tech with the correct rating for under 30€), but it means I have to modify it to work with the quadra, or I buy a secondhand 840av psu (saw one for 60€), and replace all the caps to prolong its life. What do you suggest?

 

[joshc]

Do an ATX conversion. Leave the old one alone. Be careful doing the ATX conversion - no removing components or spraying fuel on it while its powered up.

Converting it is easy, this has been documented across the forum in various existing threads.

 

[grieverheart]

The reason I asked if I should go with revealing an old one or converting an ATX one, is due to This article. Apparently, the newer cheap ATX psus are not as good as the old ones.

 

[demik]

That's not exactly true. back in early 2000 cheap ATXs were fire hazards. Now they are all dead
So while a modern bad ATX can be worse than an old good PSU, a good modern ATX will trump both, tolerances are tight

 

[grieverheart]

That's not exactly true. back in early 2000 cheap ATXs were fire hazards. Now they are all dead
So while a modern bad ATX can be worse than an old good PSU, a good modern ATX will trump both, tolerances are tight

Problem is, I cannot find any ATX power supply with 30A on the 5V rail from any reputable brands. I assume the Inter-tech I mentioned above is on the look low end.

 

[Byrd]

It's always a compromise choosing a modern/ATX PSU replacement, as you will never get the exact same specs but presuming the 840AV isn't loaded with multiple drives, cards whatever you can source that's close to the rated specs and of good quality you'll be fine. Beware of the outlandish amazing ratings on new, cheap ATX PSUs which are not worth the sticker they are printed on.

 

[tjjq44]

I had the same issue few years ago, my 840AV psu failed suddenly. I decided to swap out the internals from a good ATX psu I had on the shelf. I don't remember the +5V rating but it certainly wasn't 30A, more like 20 or 25 if I remember correctly (I threw away the ATX chassis of course).

As Byrd said, unless you have 3 NuBus cards + 2 or 3 hungry old SCSI drives, 30A is not necessary at all. In normal use it should not draw more than 10 or 15A.

 

[grieverheart]

I had the same issue few years ago, my 840AV psu failed suddenly. I decided to swap out the internals from a good ATX psu I had on the shelf. I don't remember the +5V rating but it certainly wasn't 30A, more like 20 or 25 if I remember correctly (I threw away the ATX chassis of course).

As Byrd said, unless you have 3 NuBus cards + 2 or 3 hungry old SCSI drives, 30A is not necessary at all. In normal use it should not draw more than 10 or 15A.

I’ve got nothing crazy and I don’t intend to. Also have bluescsi instead of a hard drive. It’s good to know that I can also probably use a better quality PSU with perhaps 20A on the 5V rail.

 

[tjjq44]

griverheart sent me a PM, I post it here so that everyone benefits

Het! You mentioned on the thread I started about the Quadra 840av, that you rebuild the PSU a few years ago using an ATX one. Could you help me out a bit? Can I test an ATX PSU without a soft power board? I guess I have two jump two pins, right? Do you have any pictures of how you rebuild yours? Also, something that’s not clear from reading the forums, but the 840av PSU also has an AC output. How can we keep this feature using the ATX?

Sorry but I don't have any pictures and no time to disassemble my quadra for now... For the modifications I gathered in this file what I could find at the time. This should be enough to understand how to modify "soft power on". Another issue is to fit the whole ATX circuits inside the Mac PSU chassis, tight but doable! For the switched AC out, I simply left it disconnected but a simple 12 or even 5v relay should do the trick.

 

[demik]

IIRC the +5V budget per NuBus slot is 2A. So if no cards, that's 6A off. Worst case, you can always use DCDC 12 to 5V converters for the drives, fans and stuff outside of the logic board

 

[grieverheart]

griverheart sent me a PM, I post it here so that everyone benefits

Sorry but I don't have any pictures and no time to disassemble my quadra for now... For the modifications I gathered in this file what I could find at the time. This should be enough to understand how to modify "soft power on". Another issue is to fit the whole ATX circuits inside the Mac PSU chassis, tight but doable! For the switched AC out, I simply left it disconnected but a simple 12 or even 5v relay should do the trick.

Thanks for sharing! This will certainly help. Did you go for splicing or did you use the staple method to transfer the pins to the Mac’s connector?

I will order the ATX PSU, and see if I can at least get signs of life from the 840AV. Hopefully it was just the power supply!

 

[tjjq44]

As far as I remember I kept a small piece of the original psu circuit board tied to the chassis, the one on which the DC out connector is soldered. I then soldered some wires to the ATX circuit outputs. In the end, the mod is invisible externally, you just have to connect the original cable between the PSU and the logic board.

 

[grieverheart]

Unfortunately, the PSU I ordered, didn’t have the advertised amperage, so that one is going back. I immediately ordered an alternative one, but soon after I found out that my PC also has a PSU with a quite high 5V rail of 28A, so I can always use that one

 

[grieverheart]

Just an update, but I decided to take two approaches. Try to repair the PSU and use an ATX PSU.

Fortunately, I swapped connectors with the apple and ATX PSU and was able to get the Quadra 840AV running again! This was a first test and after I confirmed, I quickly ordered PCBs (for the first time, so it was a bit scary) based on https://68kmla.org/bb/index.php?thr...d-other-things-with-a-22-pin-connector.46249/ .

In the mean time, I ordered some capacitors and other components for the PSU. I have no experience with PSU repairs whatsoever so I was stupid enough to just replace the varistor that blew up and a couple of suspect capacitors and turn it on! Sparks again, a bang, and the mains tripped. The second varistor blew up as well as the fuse. I later learned that to work more safely with PSUs, I should have made a series bulb tester/current limiter. I then started learning more about PSU repairs and did some more troubleshooting on the PSU. I found at least one TRIAC that had shorted, and after removing both main caps so that I could get access to the blown varistor, I found out one of them, the one close to the blown varistor, was showing capacitance in the nF instead of the rated 1200uF. Unfortunately, I cheaped-out and didn't order main cap replacements, so now I have to order those too! In the end, I will have invested a lot into keeping this PSU alive, but hopefully I will succeed!

By the way, while trying to understand how the PSU is working, I found out that it uses a 12V relay to switch the AC output on/off. I also think it's passing out the filtered AC instead of passing directly the mains AC. How easy would it be to use the same approach on a ATX mod? Is it just a matter of wiring a relay to the AC output and power it from the 12V rail?