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Refurbishing a TI PEB power supply

TI-99/4A I've seen a number of people recommending replacing the older forty-ish watt TI PEB power supply with AT(X) power supplies.

As with most stopgaps/workarounds that the TI community has come up with, I'm not sure that's a good idea. The original PSU emitted +16VDC, +8VDC, -16VDC, and +5/12VDC for the stuff in the drive bay, and the PEB cards are expected to have on-board regulators to produce the desired voltage.

An AT(X) PSU, on the other hand, emits +12/+5/-12VDC. The folks that have done the conversion recommend that the original +8 and +16 be wired into the +12 rail, and the -16 wired into the -12 rail. That will work if and only if the voltage regulator on each card will go into bypass/shunt mode when the input voltage is below the regulator's input voltage floor (typically about two and a half volts over the specified output voltage).

If the card regulator goes into bypass mode, it might as well not be there at all. The entire point of the per-card regulator scheme has been negated -- you've lost the protection afforded by the onboard regulator, and are at the mercy of the PSU. The only case where this isn't an issue is on a +5VDC-only card, and if you haven't replaced the 7805s on the cards with switching equivalents, you'll see a dramatic increase in heat generated by the regulators.

Therefore, if one has an ailing PSU in a PEB, I reckon it's better to refurbish the original PSU rather than replace it with under-specced voltages.

The PEB PSU is pretty simple. It uses a gigantic multi-tapped transformer to step down the voltages to somewhere around nominal, simple full-wave bridge rectifiers and a few large electrolytic capacitors to smooth things into DC, and a couple of 78xx regulators to deliver the +5 and +12VDC. The schematic is here.

First, replace the 7805 (in a TO-220 form factor) with a switching equivalent. I used CUI Inc's V7805-1000, which handles one amp without breaking a sweat. It's probably better to get the -1000R, which has longer leads for mounting flush against the PCB, but the non-R leads are long enough to fit as well.

Second, replace the 7812 (in a TO-3 form factor) with a switching equivalent. These are harder to find ... I used the EzSBC PSU6, which is also good for one amp. Do not forget to put the screws back into the mounting holes; if you don't, the regulator won't be grounded and will go into bypass mode. That'll put an unregulated +16VDC onto the +12VDC rail, with bad consequences.

Third, replace all of the electrolytic capacitors. All are axial-mount 35V polarized units; you'll need two 4700uF, two 3300uF, two 1000uF, and two 47uF. The replacement 35V caps are probably thinner than the originals (mine were), but if you go up one more size to 50V, the physical dimensions should match pretty closely.

Fourth, replace D1 through D6 (i.e., all of the diodes) with 1N5401s. D3 and D4 are already 1N5401, but they're at least thirty years old and are near or past their service lifetime. The other diodes were 1N4002s, which are the less-beefy cousin of the 5401 (the 4002 can handle one amp, the 5401 is good for three amps). The 5401s are a bit bigger than the 4002s, but they'll fit into the mounting holes with a bit of elbow grease.

While you're in there, you might consider replacing the original fan (running on 110VAC) with a normal PC fan (running on +12VDC). The size is standard. Wire that into the regulated +12VDC rail wherever convenient. Instructions can be found here.

That's pretty much it. Plug the P1 cable from the transformer back into the regulator board, turn the PEB on, and verify the voltages at the P2 and P3 connectors against the schematic referenced above. If everything matches (the unregulated voltages will be high, because there's no load), then you're done.

This should return your PEB PSU back to original strength.

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Tim on :

Great information and well-written.

Back in the 90s when people were modding cases with the AT(X) power supplies, a few of the EE folks warned about the heat but never brought up concerns related to the 12v regulator dropout. I suspect that because it "worked" no one considered the issue you bring to light. Back then, the prevailing attitude was to minimize blowing out cards, because people were jumpering their regulators or removing the regulators completely, then unwittingly sticking them into a standard PBOX. Even worse, selling those cards without a warning because they either forgot about the mod, or were benefactors of someone else doing it for them.

Chris on :

Hey Tim,

Good to hear from you.

The AT(X) modification, even with Gazoo's recommendation, never really sat well with me because of the regulator dropout issue. Like most things in life, it works until it fails catastrophically killing everyone within a 5km radius ...

... as long as the filter caps are refreshed, and the diodes that make up the FWBRs are good to 3A, I don't see any reason why the stock PSU would need to be replaced with an AT(X) to handle a fully-populated PEB.

That having been said, I can see the need for a beefier +12VDC can regulator to handle a three- or four-ancient-floppy-drive unit ... but, honestly, the right thing to do with a PEB is to have a HxC and a single half-height drive to read legacy floppies into the HxC (which the 1A +12VDC regulator will handle fine). Can't fight entropy, and the magnetic media I've dealt with can handle one pass through a drive before dying-via-flakes.

I noticed that you posted a link to this post to the AtariAge '99 forum. Thanks for that. I sort of doubt that anyone will take heed of the warning, instead opting for (redacted AA user)'s patented cargo-cult "it's worked so far for me!" nonsense, but it's good to get actual engineering-based information out there. Maybe it'll save someone the heartache of having an ATX PSU take out their CorComp cards.

Cheers,

Chris

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