AT Power supply experience

[ckrtech]

Hey guys -

AT is certainly too short a term to plug into a search!

I was wondering about some of you guys that work on AT power supplies and if your approach, concerns, etc. are different than that of an ATX power supply.

For details behind the reason for my question, see below:

I just rehabbed a 486 motherboard that had a leaky battery. It took quite some time to get the board ready to test, and now I am testing it inside the original case. The damage from leakage wasn't bad. It ended up only needing three jumper wires in the keyboard area, and the -12V trace had to be partially rebuilt. Speedsys (a benchmark program) runs fine. Unfortunately, I am encountering some freezes, graphical glitches, and reboots after less than a minute with the game Wolfenstein 3-D. It could be bad RAM, bad video card, bad BIOS timings - who knows.

Yeah. It's old. 486SX-33, 8MB RAM, 250 MB HD, VLB 1MB video, and I/O card. No fan or heatsink (as it has run for years).

1: Power ready gets up pretty quick. I see it cross +5V, and it (along with the +5V wires) settles into 4.94 or somewhere near there.
2: No electrolytics on mobo or video card, btw. Video card has tantalums only.
3: AT Power supply is a DVE DSP-1454P. I may adjust my space on the workbench a bit so I can remove it from the case and take a peak, however I don't have any reason to suspect it - do you? It just recently turned about 24 years old.

I have a lot of diagnostic swap/replacement/testing to do. Thought I would clue you guys in on my endeavor. Meanwhile, I gotta start digging up extra parts.

[Behemot]

Re: AT Power supply experience

Yeah I remember few years ago these machines were everwhere, now it's expensive collector item Also got about ten 486 boards where the batteries leaked and ate parts of the board… in my case the damage was usually excessive though some of them may still work, only some slots won't I guess.

Won't ever use that Ni-MH crap again, if something than I'll replace it with lithium batteries.

[ckrtech]

Re: AT Power supply experience

Update - I have been running this board with a Dell HP-P2507F3P power supply (along with AT adapter).

I still have some stability issues benchmarking Doom, and I noticed while probing the 12+ pin of the ISA slot that hard drive activity is represented inside the +12V waveform on my scope.

The voltage doesn't dip - you just see the access inside the +12V wave on the scope. Do you guys have any thoughts?

[stj]

Re: AT Power supply experience

not enough smoothing,
could be the caps on the psu or the motherboard.

it's pretty common, i have systems where you can actually hear the HD and even cpu activity through the speakers!!

[ckrtech]

Re: AT Power supply experience

The board itself has one tantalum cap on the +12V line - a cap I replaced due to corrosion/oxidation. I pulled it off another motherboard (a 386).

It is interesting that there appear to be two additional spots for caps that were not placed on the motherboard (cost cutting, I suppose?) for several of the power rails - including +12V. I marked them in red on the bottom photo (ISA slot 2 and 6). This photo is during the restore.

The blue arrow on the top photo shows the intentionally missing cap for slot 2 as well as the only cap (the one I replaced) on the +12V line (at least across the ISA bus).

I suppose I could use a different cap near the +12V line and even add others down the line - but I (at this point) am not sure that it is honestly where my problem lies.

[Behemot]

Re: AT Power supply experience

You measured that one? After 20 years I would not expect any of those caps to be OK.

[ckrtech]

Re: AT Power supply experience

I didn't measure it. And you wouldn't expect any of the caps to be OK? Why is that? I ask since they are tantalum capacitors for which age should be irrelevant.

That being said, I am uncertain about heat and moisture history as it wasn't in my possession for a number of years.

[Behemot]

Re: AT Power supply experience

Should=!is Besides, as suggested, such small capacity may not help much with filtering, especially if caps in the PSU are bad.

[ckrtech]

Re: AT Power supply experience

Hmm. So if I recapped the board <....sigh>, what should I sub in for the old tantalums? (mostly 16v 10uf). I've done a few PSU boards in monitors and tend to favor Panasonic series of several types (would have to check). Any recommendations for caps on the power rails on a motherboard and perhaps for IC power filtering as well?

(This project has gone on for a long time. Ha!)

[stj]

Re: AT Power supply experience

dont bother,
if your going to replace tants, you shouldnt use electrolytics.
if it's small, use MLCC's (multi-layer ceramic caps)
otherwise, use poly's

but 10uf caps wont make much difference either way - look into your psu i think.
(and take foto's)

[Behemot]

Re: AT Power supply experience

If the caps are on the power rails (+12, +5 or +3.3 V), than you can use lytics, I don't see a reason why not. In similar size I would go most likely for 100/16.

[ckrtech]

Re: AT Power supply experience

The bench/test power supply I am currently using is a Dell HP-P2507F3P.

Looks like majority of output stage are "Asiacon." They look OK - but I know that means absolutely nothing.

I'll try to add some photos of it.

And at some point we'll get some photos of this AT power supply that I have had since 1993/1994. (Maybe I can replace the holy-cow-loud-fan in it as well)

I am still skeptical that it is simply only the power supply at this point - unless this motherboard was just not designed to handle a +12V that isn't short of perfection.

Would be nice if I could scope this thing with a load that isn't a mobo.

[ckrtech]

Re: AT Power supply experience

Photos attached of Dell ATX. So difficult to get good photos of this tank without a teardown.

Interestingly, I built a second 486 with some other parts I had around - different board, processor, case, PSU, etc. That power supply tends to have the same reaction to hard drive reads in the scope when viewing +12V, but the AT clock signal is rock solid regardless of drive access, Doom running, etc.

I suspect my motherboard used in this main project of mine is letting noise into the signals, and that is causing instability.

Nevertheless, again - Photos of Dell ATX used for bench testing have been attached.