IBM XT 5160 power source & drives troubleshooting \ VOGONS

IBM XT 5160 power source & drives troubleshooting

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First post, by vmr_

Posted on 2020-05-24, 17:40

vmr_ Offline

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I have an AT PSU (clone?) for IBM 5160, and getting some strange readings on voltages with drives connected.

PSU: Schrack Eletronics EG 071068-D P/N 1501438 - in good visual condition, opened and no leaking capacitors
HDD: ST-412 10MB drive full-size
FDD: 356k floppy disk drive full-size

test1. When PSU powered without anything connected, on molex connector I always get the 5V (between groud black wire and red wire - as expected), and sometimes 0L instead the expected 12V (between ground and green wire).

test2. After e few restarts of PSU with nothing connected to it, I start getting 11.25V between ground and green wire on molex connector

test3. when I manage to get 11.25V a few times in a row for test2, I connect the 10MB HDD and on the other molex I now get 11.91V instead expected 11.25V (as per test2)

test4. when I manage to get 11.25V a few times in a row for test2, I connect the 356k floppy drive on molex, and I get 0V instead expected 11.25V (as per test2)

Wondering if the PSU is acting, or the HDD and FDD are at fault here.

NOTE: after careful inspection of other adapters in the IBM 5160, the Hard Disk Controller card to which HDD was connected has an exploded tantalum capacitor - not sure what the previous owner did. Wondering if this could have impacted the PSU, or the HDD.

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Reply 1 of 3, by vmr_

Posted on 2020-05-25, 10:40

vmr_ Offline

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At least for test 3 and 4, my guess is there could be some shorts on the connected devices, so the PSU does what is designed for - namely it switches off.

Cannot explain behaviour for test 1 - will retest again. Possibly linked to old capacitors and need to replace it?

Reply 2 of 3, by dionb

Posted on 2020-05-25, 10:53

dionb Offline

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Test 2 is not valid - a switching power supply isn't designed to work without load, in fact it can damage the PSU. Disregard any measurements made in this way.
Test 3 is valid and shows the PSU working correctly - 11.91V is well within tolerances of 12V (and incidentally better than 11.25V...)
Test 4 is valid and shows something shorting out. Are you using exactly the same Molex connector as on the HDD? If so, it's the FDD at fault. If using a different connector it could be either connector or FDD. Try to use same connector on HDD and FDD to determine which.

Reply 3 of 3, by vmr_

Posted on 2020-05-25, 14:12

vmr_ Offline

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Joined: 2018-09-06, 08:14

Tahnks @dionb, this helps.

for test 2, I realised when I left voltmeter connected to the molex, and turned the PSU on, I always got the 11.25V reading - so PSU delivers when a consumer is connected. Guess I did not make the distinction there, thanks.

For test 3, although I saw the jump from 11.25V to 11.91V on the other mollex when HDD is connected, HDD did not make noises, had no red led on or spin at all - to suggest it actually uses the power. So something is draining power, with no visible outcome - other that slight increase on voltage for 12V wires. Will check the board for HDD as well.

For test 4 - decided to start looking for capacitor replacements on the FDD side, as some have small black dots (and some measurements suggest shorts) - so the PSU looks like does its job there and protect itself 😀

Once I change the tantalum capacitors on FDD board, will retest.

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