Refurbishing a TI PEB power supply

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.

About Chris Kobayashi

I'm a security systems engineer, specializing in UNIX, network, and physical security. I'm in Tokyo, and I'm mostly retired now. I'm well-versed in both electrical and software engineering, with a particular interest in old computers and game consoles. You can contact me here.
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