Thursday, 15 June 2017

SR98 PSU gets more love

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.




1 comment:

  1. "ST format might have even mentioned it in a guide, can't remember now. "

    Magazines said a lot of things that still get mentioned today, I may have read something about DMA chips around 1991/2 or so have you heard of this problem? :p

    Amazes me these things have lasted as long as they have I wonder if its because they usually get left out of use for a few years when people got new systems? I wonder what the actual estimated usage/life hours figure was they came up with (prob just long enough to get out of warranty), of course this is going to vary due to load and use?

    How long would it last if left on 24hrs a day?

    " For one, most of the PSU switches have horrid cracking, arcing, sparking sounds after a lot of use."

    My Amstrad did that ALOT even when new...