Vintage 130W IBM5160 Schrack Elektronik EG07168-E PSU Repair

[fmashockie]

I've had feedback circuit issues related to the zener diode on SMPSs where the zener still appears to test fine (when the circuit is not live such as with a DMM on diode test function), but it ended up being the issue.  Especially if thermal stress appears to make the problem worse.  I would be suspecting that blue zener diode as well.

[hozone]

THANK YOU! To all the people who help me during this repair, expecially @xavier60.
The guilty is the 1N823, thermally compensated zener diode. (https://4donline.ihs.com/images/VipMasterIC/IC/AMPD/AMPDS00002/AMPDS00002-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0)

I've put a 1N4735A (6.2V zener), I get now 12.35V and 5.3V almost with the actual load. (A pretty old pata 40Gb HDD).

Find attached a few picture. (I may have to replace the blue cap - should be 1n5K400 from another picture I have - that I've accidentally melt a bit).

Now I have to find a seller with decent shipping price. This diode is 4Eur almost and 20Eur shipping :-(

I must admit that the 1N4735 that I've mounted works well, even if I heat up with the heat gun it goes up to 5.35V and 12.45V, which I think could be in the IBM 5160 motherboard limits.
Now I'm going to reassembly that PSU and check it connected with the PC.

What's your opinion about leaving the 1n4735 there?

[CJay]

Ooof, I *think* that's the PSU from an IBM XT-286.

We used to repaiur lots of them and found that they fried themselves quite nicely and comprehensively.

Our fix was a rewound driver transformer (it gets destroyed) and a pile of other replacement parts as a kit but the details of that are *long* lost to time which is entirely unhelpful.

[xavier60]

Leaving the 1N4735A sounds reasonable to me.
If you want to alter the voltage, you could try fitting other 1N4735A's from your stock which could give you slightly different voltages.

[hozone]

Thank you,
today I'm going to reassembly the PSU, and tomorrow I hope I've time to mount this on the IBM and let you know.

[hozone]

Aseembled and working again!
THANKS!

[fmashockie]

Awesome! Glad you were able to get it running again!