SR98 PSU GETS MORE LOVE

Various tweaks and fixes from video ghosting to audio fixes etc.
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exxos
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SR98 PSU GETS MORE LOVE

Post by exxos » Tue Aug 22, 2017 10:50 pm

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These SR98 PSU's are the most common that I have seen in the UK, the "trusty" SR98. Well these PSU's from my observations have been failing for over 10 years now and I have a stack of about 50 of them which fail to power a STE for very long before they "die" totally.

The electrolytics used were really cheap-ass ones and probably were not very good from new either. Problem for one is the heatsink on the right side being reaching 60c and above. Those electrolytics are being cooked by it reducing their life. People probably remember the "pea rectifier" being bad for overheating and bulging and basically exploding. Its was well known back in the day, ST format might have even mentioned it in a guide, can't remember now.

Of course its unfair to slate Mitsumi for their cheap work. I mean the Atari ST was "built to a price" and "cheap as possible" to get as many machines into the publics hands as possible. So thumbs up from that point. There is only so much you can do "to a price" and they have lasted around 20 years, so can't really complain. Don't get me wrong, I really like the SR98. It was likely pretty awesome when it was new! I don't think anyone could have seen these machines being used 30+ years later and these PSU's are mostly still chugging away out there somewhere.

But it doesn't stop there. While regulation can be made a lot better by using quality capacitors, I have also found changing the output diode also helps with regulation and also efficiency slightly! I chose the lowest voltage drop diodes I could find under a 2amp load to try and keep the "heat" down on the heatsink. As the heat warms up the capacitors a lot and one of the reasons the capacitors fail on these PSU's than maybe other brands of PSU.

I also went for a fast diode in terms of efficiency and regulation. I did not expect such a huge increase in regulation, though p-p was 0.5volts and dropped to 0.2volts after the new diode was fitted. So this cheap-ass PSU can be restored better than new and with the mods can be made into a pretty awesome bit of kit!

The SR98 can be brought back to better than new with one of my SR98 RECAP KITS. I Also sell complete refurbished SR98's (and sometimes DVE PSU's) in my store which will include the new diode in later batches.

There are other mods on my website to improve regulation and general service tips. For one, most of the PSU switches have horrid cracking, arcing, sparking sounds after a lot of use. I would recommend changing the switch if thats the case.

STORE PAGE
https://www.exxoshost.co.uk/atari/last/storenew/#PSU

SPARE PSU SWITCHES
https://www.exxoshost.co.uk/atari/last/storenew/#0032

PSU RESEARCH WEBPAGE
http://www.exxoshost.co.uk/atari/last/psu/index.htm

..wrap..
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Pacman
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Re: SR98 PSU GETS MORE LOVE

Post by Pacman » Sun Oct 08, 2017 6:32 pm

Your clear and right-to-the-point explanations in plain English are very informative and just what I need now that I'm about to recap just that PSU type for an STe. It's been working fine ever since I bought the computer (back in 89 or thereabouts I think) but I bought your SR98 recapping kit and dummy resistors just to be on the safe side. Better that than wait till it's too late :o

On the same subject I can report that I've just successfully recapped a Mega STe power supply. Time permitting I'll post a "how I did it" in case there are others who have never done it before and would like some guidance.
Last edited by Pacman on Mon Oct 09, 2017 8:40 pm, edited 1 time in total.
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rubber_jonnie
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Re: SR98 PSU GETS MORE LOVE

Post by rubber_jonnie » Sun Oct 08, 2017 11:42 pm

I can confirm that I used one of the re-cap kits from exxos (https://www.exxoshost.co.uk/atari/last/storenew/#0027), and mine now happily powers my 4160 STE with CosmosEx fitted internally.

The only issues I would say to watch out for are:
1. Be careful desoldering, the boards are very old now, and easily damaged by excessive heat.
2. When drilling out the holes for the new rectifier, go very careful. Those pads are not well attached, and just as easily damaged by a drill as overheating.

To add to what exxos said, the rubber 'bungs' in these PSUs dry out and shrink/perish due to overheating, crappy original parts and old age. Once the seal is gone, the electrolyte drys up, and it's goodnight capacitor.

That doesn't mean chucking the PSU away though, as an hour with a soldering iron can bring them back to life nicely.

My original how-to is here:http://atariage.com/forums/topic/250837 ... -be/page-2
Collector of old Atari things:
800XL + Ape Warp mod, 1010 cassette, 1050 + Happy mod, 65XE (128k) & XC12, 2600jr, 7800 and Lynx II
Atari 520ST + Gotek, 1040STF, 4160STE with 32Mhz booster, ROM switcher and CosmosEx
Plus the rest..
C64 + 1541 and cassette, 2x Sinclair QL, Mac Classic, Mac Performa 6200, Amiga 1200, ICL One Per Desk, Epson HX20, Dreamcast, original Xbox, SNES and a bunch of other bits n bobs I can't remember right now. :D
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