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PS/2 Model 70 Power Issue

UPDATE: I pulled most of the capacitors from the power supply main board and I found three tested bad on the ESR meter: C28, C29, and C39. C28 and C39 are the two 220uf capacitors wahka recommended I replace. I am looking to source a complete set of capacitors for the ones I removed (essentially I did not remove the two large 680uf capacitors or the four on the two riser boards.

I also ordered some dual lead tantalum capacitors to replace the two motherboard caps.
 
oldpcguy said:
Just to confirm the tantalums I need to replace are 10uF, 25 volt?
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i don't remember what they are, but anyway you can't go wrong with that.

for the PSU, replace also the 2 2200µ. they are of the same series than the 220, and were dead on mine.
as you probably understand, thoses 220 and 2200 are the output capacitors, change them. maybe the 2200 are still good for now, but they'll fail for sure.
they are low esr.

you can replace them with modern very low or simply low esr without any issue.
as NeXT explained, some older PSU design don't like very low esr, specially for the regulation feedback, but no issue with this one.
 
whaka said:
i don't remember what they are, but anyway you can't go wrong with that.

for the PSU, replace also the 2 2200µ. they are of the same series than the 220, and were dead on mine.
as you probably understand, thoses 220 and 2200 are the output capacitors, change them. maybe the 2200 are still good for now, but they'll fail for sure.
they are low esr.

you can replace them with modern very low or simply low esr without any issue.
as NeXT explained, some older PSU design don't like very low esr, specially for the regulation feedback, but no issue with this one.
Click to expand...
I intend to replace all of them except for the two large 680uf caps as well as the four located on the two riser boards. I am having my cap guy put kit together. He mentioned it's getting more difficult to get parts so hopefully he can supply all of them. I know he has the 220uf caps in stock. I'm also going to purchase a second set to recap the working power supply.

EDIT: The supplier was able to put together a list of all the caps I require. Should be here mid next week so hoping to have this repaired next weekend.
 
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NeXT said:
You can try the automotive lightbulb method to put an amp or two on the +5 and +12 and that's usually enough to stabilize the rails.
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Since I am not a power supply kind of guy what is an appropriate load to put on a power supply for testing?
 
The tantalum capacitors arrived today and they're now installed. Powered it on and no smoke so I got the polarity correct. I suspect the power supply caps will arrive tomorrow so I should have the whole thing repaired shortly.
 
oldpcguy said:
This is great information. I am hoping, given the symptoms and the ESR meter testing, that the problem is limited to the bad capacitors. In fact I might even know today as they're supposed to be delivered this afternoon.
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But it needs to be expanded further - along with the other PS/2 PSUs.
 
The capacitors arrived this afternoon, they're installed, and the system has been put back together. It passed the smoke test so I plugged everything in and...success! Here it is completely assembled and almost ready to go. I just need to replace the CMOS battery (on order) and clean the outer top cover (only thing that I didn't wash when I was performing the original cleaning). Very pleased with the result.

One benefit this system has is the external drive cable. It looks to be a straight pass through so I can probably use an external drive to perform the system configuration (though the internal drive works fine).

IBM PS2 Model 70 Powered On.jpg
 
oldpcguy said:
One benefit this system has is the external drive cable. It looks to be a straight pass through so I can probably use an external drive to perform the system configuration (though the internal drive works fine).
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The IBM passive bay and card adapter or from a SYSGEN "Bridge" replacement riser? A Reference Diskette will only run from the A: drive. It looks great!
 
IBMMuseum said:
The IBM passive bay and card adapter or from a SYSGEN "Bridge" replacement riser? A Reference Diskette will only run from the A: drive. It looks great!
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How do I tell the difference? It appears it could plug into either connector (though it was / is currently installed in the B: drive connector). I'll take a look tomorrow as I'd like to get the case cleaned. $50 for the system and $9.00 worth of capacitors and it's all good.

What to do about the broken SIMM slot. Can I skip it and install memory in the third slot (the center one is broken)? I doubt I'll need a lot of RAM but I've got a stick sitting on top, waiting to be used (though I think it's only 256KB)
 
oldpcguy said:
How do I tell the difference? It appears it could plug into either connector (though it was / is currently installed in the B: drive connector). I'll take a look tomorrow as I'd like to get the case cleaned. $50 for the system and $9.00 worth of capacitors and it's all good.

What to do about the broken SIMM slot. Can I skip it and install memory in the third slot (the center one is broken)? I doubt I'll need a lot of RAM but I've got a stick sitting on top, waiting to be used (though I think it's only 256KB)
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At the bottom of this page, it shows the passive (4869-001) adapters: https://www.ardent-tool.com/floppy/5_25_Floppy_Adapter.html
 
oldpcguy said:
Since I am not a power supply kind of guy what is an appropriate load to put on a power supply for testing?
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a bit late, but you don't need to on this one.
you've maybe seen the big 4.7 ohm 7W ceramic resistor, and that's exactly what it does. it load the PSU when not connected to motherboard.
the counterpart of this way is it produce a lot of heat all time...

i think an interesting mod to do is to move this resistor elsewhere, as it contribute to heat capacitors in this area too.
and i think we can say the cooling of the 70 is weird. hot air have no real path to go out.
 

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oldpcguy said:
What I have is a simple cable where one end plugs into the floppy connector and the other is a female connector which mounts to the back of the system. It appears that a Type 4869 External Floppy drive would plug into the female connector on the rear of the system.
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Very similar to a couple that I have around - after-market, but not the SYSGEN "Bridge" adapter by what I can see. There were a few manufacturers putting out unbranded cable and drive assemblies for the PS/2s - One I received even had the power plug mixed up!
 
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