PS/2 Model 30 (8086) Power Supply Question \ VOGONS

PS/2 Model 30 (8086) Power Supply Question

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First post, by Guld

Posted on 2024-12-26, 17:25

Guld Offline

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I received an IBM PS/2 model 30 a while back (originally posted here IBM PS/2 Model 30 Restoration (8086 version)). I've cleaned up the boards a bit but I have some concerns about the power supply before I try to plug it in and see if it works.

I'm not all that familiar with power supplies and what may or may not be normal, so any help would be greatly appreciated.

I measured resistance across the outputs on the P3/P4 connectors from the power supply and get the following values. This power supply is completely disconnected from the system, although the fan inside the power supply is still connected.

1Pwd Gd, GND       1030 Ohm
2Pwd Gd, +12 V     1050 Ohm
3Pwd Gd, -12 V          Open
4Pwd Gd, -5 V      1110 Ohm
5Pwd Gd, +5 V      1020 Ohm
6+12 V, Ground       41 Ohm
7-12 V, Ground     1080 Ohm
8-5 V, Ground        74 Ohm
9+5 V, Ground        15 Ohm
10+12 V, -12 V      1120 Ohm
11+12 V, -5 V        115 Ohm
12+12 V, +5 V         27 Ohm
13-12 V, -5 V       1150 Ohm
14-12 V, +5 V       1090 Ohm
15-5 V, +5 V          88 Ohm

So, what gives me pause is the resistance from +12 to ground, -5 to ground, +5 to ground, +12 to +5, and -5 to +5 seem a bit low. Is this normal?

The F1 fuse in the power supply is intact. I don't see any obvious signs of issues that I can see. But I'm sure it has the original electrolytic caps in it.

Any advice on how to proceed with this power supply?
Does it need a load to test or is the fan in the power supply case sufficient?
Anything else I should try to check before trying it?

Any suggestions would be appreciated.

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Reply 1 of 3, by mkarcher

Posted on 2024-12-26, 18:49

mkarcher Offline

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Don't worry about the low resistances. They are intentional. Look at the second photo you took: Below the smaller coil (the big one is the transformer), there are two big white boxes, which are power resistors. One of them is 15 ohms, the other one 27 ohms, which is exactly what you measure between +5 and GND and then +12 and +5. These resistors are there as dummy load, because the control circuit in the supply is unable to reduce the power transferred through the the transformer to zero, so some energy is burned in those resistors to get the device into a safe operating area. I don't know whether the supply needs any further load to start up, but I am confident it will not blow up if tested without an extra load, because that's the sole point of the dummy load resistors.

I expect it is obvious to you, but just to be safe I still explicitly warn you to not touch anything inside the power supply while it is connected to mains or shortly after. Keep any pets out that might walk into the supply as well. Better yet, as it doesn't seem you need to have the supply open for, close it before applying power.

Reply 2 of 3, by Guld

Posted on 2024-12-27, 02:01

Guld Offline

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Location: Virginia, USA

mkarcher wrote on 2024-12-26, 18:49:

Don't worry about the low resistances. They are intentional. Look at the second photo you took: Below the smaller coil (the big one is the transformer), there are two big white boxes, which are power resistors. One of them is 15 ohms, the other one 27 ohms, which is exactly what you measure between +5 and GND and then +12 and +5. These resistors are there as dummy load, because the control circuit in the supply is unable to reduce the power transferred through the the transformer to zero, so some energy is burned in those resistors to get the device into a safe operating area. I don't know whether the supply needs any further load to start up, but I am confident it will not blow up if tested without an extra load, because that's the sole point of the dummy load resistors.

I expect it is obvious to you, but just to be safe I still explicitly warn you to not touch anything inside the power supply while it is connected to mains or shortly after. Keep any pets out that might walk into the supply as well. Better yet, as it doesn't seem you need to have the supply open for, close it before applying power.

Ah, thanks for explaining. Does anyone have a link to a site where I can learn a little about how power supplies like these work? Maybe just an overview?

And yes, thanks for the reminder! I stay clear of big caps 😁. And I plan to close it before testing, I opened it to blow all the dust out.

Reply 3 of 3, by Guld

Posted on 2024-12-28, 01:21

Guld Offline

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Closed up the power supply and powered it on today and it worked! Fan turned on after a second or two, and voltages seemed okay. Haven't connected to the motherboard yet, but soon! One step closer!

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