exxos's Atari, Amiga & retro forum

I'm getting ready to recap a SR-98 power supply (belonging to an STe).
Fortunately it didn't explode when testing (disconnected from the computer, and using Exxos' dummy load resistor kit) with the voltages appearing OK (+12.395V and +5.005V), which of course is good news knowing that the STe never received any "lethal" voltages :lol: I bought an Exxos SR-98 recapping kit 7 years back which I still haven't gotten around to install, so I think it's about time (the recapping kit has been in normal room temperature storage for all that time -should that concern me, needing to buy a new recapping kit first?).
Around that same time I successfully recapped a Mega STe (using Exxos' Phihong PSM-5341 recapping kit), so I wrote a Mega STe PSU recapping step-by-step guide in case someone's interested (a lot of the guide is applicable to recapping other STs as well).
A useful side-effect to writing a guide like that is that I can go back to it myself when I need to do it again but forgot all the details ;)

But the STe power supply is a little different.
First I noticed the +12V output wire is BLUE (not yellow like with the Mega STe). The +5V output wire on the other hand is RED; the same as with the Mega STe. I thought there would be some sort of standard here, but apparently not -so checking with a multimeter is a necessity!

Secondly: there's a trimpot (VR201) on the power supply PCB which is (according to this posting) for fine-adjusting the +5V output. Is it safe to assume that (after having replaced the capacitors and completing the PSU recapping process) I should attach the two dummy resistors (instead of plugging it into the computer) and adjust it to +5.00V (or as close to that as possible)?

Finally, is the output connector a 6-pin Molex KK?
(at some stage I'm going to replace the PSU with a Meanwell RD-50 or similar, as suggested in this posting, so I need to replace its connector, or make an adapter cable).

Yeah it's blue for 12v.
Yes calibrate the 5v with your load resistance on, outside of the computer. I would recommend trimming to 5.10V to compensate for any voltage drop.

Thanks.
So there's no risk of frying the STe with the 5.1V output?
Is there a certain tolerance as to what the STe motherboard can take, so it doesn't have to be exactly 5.0V?

5.1V is well within the acceptable limits for TTL logic.

Also, as Steve hinted, the voltage drop through the cables etc. will bring it closer to 5V anyway.

stephen_usher wrote: 03 Nov 2024 22:22 5.1V is well within the acceptable limits for TTL logic.

Also, as Steve hinted, the voltage drop through the cables etc. will bring it closer to 5V anyway.

OK. I'll go for that.
So... recapping is done, nothing exploded this time either and I've checked the outputs (with the two dummy resistors attached) and got these values:

Blue: 11.43V
Red: 4.95V

I then adjusted the trimmer (VR201) to adjust the 5V line and got this:

Blue: 11.74V
Red: 5.10V

The 12V line appears to be a little on the low side. Is this acceptable, or have the differing values of the capacitors made an unwanted difference?
The SR98 recapping kit I got from Exxos had these components compared to the original ones (slightly different from ones in the Exxos PSU repair article where C104 is replaced by a 68 uF/400 capacitor but I received a 47 uF/400V capacitor which is the same as before. The other capacitors are the same as in the article while the rectifier diode in the article differs from mine, the one mentioned there being a KBU 1005, but I'm sure that won't make any difference in the voltage outputs):

- C104 (originally 47 uF/400V) ---- Exxos replacement: 47 uF/400V
- C201 (originally 2200 uF/10V) ---- Exxos replacement: 4700 uF/16V
- C202 (originally 2200 uF/16V) ---- Exxos replacement: 4700 uF/16V
- C203 (originally 2200 uF/16V) ---- Exxos replacement: 4700 uF/16V
- C204 (originally 1000 uF/16V) ---- Exxos replacement: 1500 uF/16V
- C205 (originally 1000 uF/10V) ---- Exxos replacement: 1500 uF/16V
- D101 (originally RB155) ---- Exxos replacement: GBU 806

That's fine for the 12v line.

Personally I try to get as close to 5v as possible, so I'd tweak it back a smidge, however it is within specs if you want to leave it.

rubber_jonnie wrote: 05 Nov 2024 12:07 That's fine for the 12v line.

Great! What are acceptable specs for this line?

Personally I try to get as close to 5v as possible, so I'd tweak it back a smidge, however it is within specs if you want to leave it.

OK. I've adjusted the trimmer again, so now I have this:

Blue: 11.53V
Red: 5.011V

Pacman wrote: 05 Nov 2024 12:33

rubber_jonnie wrote: 05 Nov 2024 12:07 That's fine for the 12v line.

Great! What are acceptable specs for this line?

Personally I try to get as close to 5v as possible, so I'd tweak it back a smidge, however it is within specs if you want to leave it.

OK. I've adjusted the trimmer again, so now I have this:

Blue: 11.53V
Red: 5.011V

5v is usually between 4.75v & 5.25v for logic levels (There are other conditions I won't go into, but I'd expect it to work in that range). According to the line driver spec, anything from +15v to -15v is needed so 11.53v will be fine.

Thanks. Good to know for later!

PS: what kind of 6-pin connector (female) is used for power supply and likewise on the STe motherboard (6 pin male).
I tried a 6-pin Molex KK type, but because of ridges on the sides it won't fit. The pin spacing on the other hand appears to be the same (2.54mm).
PS PS: I just found a thread entitled Source for ST 6 pin power supply connector, but with there didn't seem to be a general concensus on the exact model. Perhaps someone here knows?