SR98 PSU GETS MORE LOVE

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

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

psu-new.png
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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.

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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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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
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Re: SR98 PSU GETS MORE LOVE

Post by Pacman » Sat Oct 21, 2017 11:12 pm

Having completed recapping my Mega STe PSU (and documenting the process) I'm preparing to recap my STe PSU (SR98). I've bought Exxos' SR98 recapping kit for that and also his PSU dummy load resistor kit (which I've already used when testing my Mega STe PSU without attaching it to the computer, thus no risk of blowing it up in case I did something wrong). :shock:

I have a few questions before I proceed (in return for information to fill in the gaps I'll consider writing yet another "How to" similar to what I did with the Mega STe PSU), this time for the SR98. I've made a preliminary illustration which should help whoever can answer my questions:
20171021-200718__MG_2226-Edit.jpg
20171021-200718__MG_2226-Edit.jpg (667.76 KiB) Viewed 417 times

1) I noticed that the kit didn't include a replacement for C206 (a small 1uF/50V capacitor close to the power switch). There's also no mention of it in the replacement listing in the SR98 section of The Last Upgrade -PSU, although I see discussions of a 1uF capacitor which I assume is C206 (relevant text made bold by me):
SR98 PEAK CURRENT UPDATE November 3, 2015
I have found the SR98's peak current demand isn't as good as it could be. This is generally seen as a dim on the video when the floppy drive is being accessed. So I started to look at this issue.

At first I changed the opto coupler for a faster one (actually same one as used on the ASP PSU) thinking it would give a more rapid response time, but it changed nothing from what I could see. I later changed it back (after doing all the tests below). The faster one might be slightly better in regulation, but it is so slight its probably not worth changing.

I then started to follow the resistor networks on the PCB and found that there is a capacitor which is pretty much directly across the opto's emitter diode. What this would mean is there is a small delay when the voltage is applied to the diode as it has to charge up a 1uF capacitor. It would then mean the 1uF would delay in turning off also. It is difficult to exactly work out whats going on with the diagram but I did notice when the 5V was turned up to 5.5 - 5.7V things improved a lot.

For this test I used a STE. I always had some odd ghosting on it and wavy lines on the screen. When the voltage was turned up (way to high in fact for 5V) the video noise problems and ghosting pretty much went away. Though I did notice some odd juddering on the edges of objects afterwards. But the most interesting thing was when the floppy drive was turned on, the video did not appear to dim as it did before.

So I found 2 resistors R201, R202 which seemed to be a voltage divider which also goes via the preset pot. This divider then is feed to a small transistor which in turn charges the 1uF capacitor to the opto diode. R203 is in the mix aswell, seems to be another resistor in the 0V line to the transistor. In anycase, I added 10K resistor across the top of R201, R202 and now the video problems went away at 5.25V. Also the dimming problem is not there. Any lower voltage then all screwy video problems come back. So clearly there is a biasing problem. I have also replaced the 1uF with a ceramic to rule out aged capacitor issue, Though I am not totally sure it made a difference.

Overall how it looks is when the PSU gets under load rapidly (when floppy motor turns on) It takes to long to charge the 1uF capacitor and the voltage drops (actually only about 0.02V) but this is visible to a quick eye that the video dims. So the idea is to increase the current though the circuit to give a faster charge rate of the capacitor, which will turn the opto diode on faster, and this does seem to be drastically improving stability.

I have also tried lower resistor values down to 2k2, but this did not appear to change anything. I upped the 1uF to 2uF. Its hard to say but this might have improved some very slight video issues. This particular issue only seems visible on the "drag bar" on GEM windows where the horizontal lines have vertical noise issues about every 1cm. Though without this new PSU mods, those lines start flickering pretty bad. This STE seems to run happiest at 5.25V. Any lower and the video noise issues start coming back. Overall the peak power can be improved with just a couple resistors and a new capacitor :)

So the final round up.

R202 2.4 k red yellow red + 1.8k = 1.029k Ohms
R201 2.7k red purple red + 2.2k = 1.212k Ohms

Replace R202 with 1K
Replace R201 with 1.2K

The trimmer pot may need to be adjusted back to 5.00volts on the 5V rail.
I'm a little confused since it appears changing C206 from 1uF to 2uF makes a difference (is that a ceramic capacitor instead of an electrolytic? Why not just use an electrolytic?), but it's not mentioned in "the final round up" at the bottom of the quote, neither is it part of the recapping kit. Surely all electrolytic capacitors need replacing after so many years, so what should I do to finalize the PSU servicing?

2) I understand (from the above quote's "final round up") that replacing the following two resistors should make a vast improvement to the video output noise, and considered one of two "ultimate improvements" to the PSU apart from recapping/changing the rectifier):
replace R201 (original value 2.7K/0.25W) with 1.2K/0.25W (new improved value)
and
replace R202 (original value 2.4K/0.25W) with 1K/0.25W (new improved value)

then (using the small VR201 trimpot next to the PC101 opto-isolator I assume?), adjust the output voltage on the +5V line to 5.0V (of course with the dummy load resistors in place and not connected to the computer)? Or should that be adjusted to +5.25V? It's a little confusing.
Have I understood the details correctly?

3) There's mention in this thread's first posting as well as in in the SR98 section of The Last Upgrade -PSU that the output diode (originally an HRW34) should be replaced with a STPS10H100CFP for much better performance (the second of the two "optimal performance" modifications I think).
I assume this is D202 (a 3-pin TO-220AB package attached to the large silver heatsink next to the 47uF/400V capacitor (C104)?

I see from the datasheet that there are two variants to choose from: the TO-220AB or the TO-220FPAB insulated package. Which one should I go for? Or will either do?
STPS10H100FCP.png
STPS10H100FCP.png (113.53 KiB) Viewed 417 times
Here's the schematic by the way (I think I originally downloaded it from Exxos' schematics/manuals section)
sr98 schematic.png
sr98 schematic.png (448.84 KiB) Viewed 417 times
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Re: SR98 PSU GETS MORE LOVE

Post by exxos » Sun Oct 22, 2017 11:10 am

Pacman wrote:
Sat Oct 21, 2017 11:12 pm
I'm a little confused since it appears changing C206 from 1uF to 2uF makes a difference (is that a ceramic capacitor instead of an electrolytic? Why not just use an electrolytic?), but it's not mentioned in "the final round up" at the bottom of the quote, neither is it part of the recapping kit. Surely all electrolytic capacitors need replacing after so many years, so what should I do to finalize the PSU servicing?
Basically leave the capacitor alone, I will write more as to "why" later. aka when woke up ;)

Pacman wrote:
Sat Oct 21, 2017 11:12 pm
then (using the small VR201 trimpot next to the PC101 opto-isolator I assume?), adjust the output voltage on the +5V line to 5.0V (of course with the dummy load resistors in place and not connected to the computer)? Or should that be adjusted to +5.25V? It's a little confusing.
Have I understood the details correctly?
When you change the resistors, it alters the reference voltage, so you end up with 5.25V or 5.4V etc, so you need to adjust the pot back to 5.00V.

Pacman wrote:
Sat Oct 21, 2017 11:12 pm
I see from the datasheet that there are two variants to choose from: the TO-220AB or the TO-220FPAB insulated package. Which one should I go for? Or will either do?
You should use the TO-220FPAB insulated package to isolate it from the heatsink.
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Re: SR98 PSU GETS MORE LOVE

Post by exxos » Sun Oct 22, 2017 1:57 pm

OK, so C206 is basically a time delay capacitor,which slows down the voltage feedback loop.

take this falcon wave form as a example.
falc5a-min.png
falc5a-min.png (2.12 KiB) Viewed 397 times

Assume the high spike is repeated multiple times across the image.Where the voltage is likely going to be something like 7 volts. Now if that capacitor was not in the circuit,the feedback loop would be extremely fast.So the feedback loop will see the 7 volt spikes.So the power supply will then think that the 5 volt output is actually 7 volts,so the power supply will start lowering the voltage down to about 3 volts,which is of course not correct. So in order to combat this problem,we use a capacitor to slow down the feedback loop.So basically what happens is the 7 volt spike is ignored.

So if we change the capacitor,for example a higher capacitance,the feedback loop will be slower and it would ignore more of the spikes.Initially this seems desirable,but on the reverse side of this,because the feedback loop is slower,the power supply will not respond as fast to transient spikes such as when the floppy drive turns on.So obviously there is a compromise between the two.

So the question is do we change this capacitor? basically no,because if we put a better quality or specification capacitor in the circuit,then the feedback loop will be slower and the transient response will suffer.

Changing the two resistors also speeds up the transient response with the feedback loop speed.Also the low ESR capacitors on the output will reduce those 7 volts spikes down to basically nothing.So that feedback capacitor actually does not do that much in this case.

Technically the power supply could function without the capacitor at all,but it is not good to let the power supply frequency run wild.So having a small capacitor there will help keep the feedback loop stable.

It is probable that something around 0.5uF or even less would be fine,but I have not tested this a whole lot.So while the capacitance may drop due to age etc,it really does not matter that much as long as there is some small capacitance there.Of course if people use poor quality output capacitors,then the voltage spikes will be higher and the feedback capacitor will need to be a higher value,or basically the original value.Because a lot of people recap themselves with various capacitors,a "one size fits all" capacitor is almost impossible to do.

Technically I could recommend a 100nF capacitor for example,when using my own recap kits and the transience response and regulation would be better in this case.But if someone used poor quality capacitors then they would likely have to go for the 1uF value,or there about.So i cannot really recommend a value for this capacitor as there are too many unknowns. I think 0.5uF to 1uF is fine over all. Perfectly good enough for the aged capacitor anyway. So basically just leave the capacitor alone :)
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Re: SR98 PSU GETS MORE LOVE

Post by Pacman » Mon Oct 23, 2017 11:42 pm

Hope you got yourself some sleep ;)
Thanks for clearing things up. C206 seems like a complicated issue which I'm not going to get into -simply because I'm not qualified :lol:
Suffice to say I understand the conclusion is not to replace it with a different type (i.e. not a low-ESR type such as Panasonic's FR-series), but surely we should replace this almost 30 year old capacitor as well, only with the same value/type (1uF)?
(what is the actual voltage across this cap anyway? I see there's a 400V capacitor nearby in the schematic so it's hard to take a quick guess, but it's rated 50V originally -does this suffice even after the slightly different new capacitor values and low-ESR types?).

So how about I get a 1uF capacitor of a quality brand, but this time a "normal/general purpose" capacitor? Something like Panasonic's EB, ED or EE series? They're all long life (5000-10000 hours) and 105C rated. The latter two are also rated "high ripple" but that should only affect its robustness I think.
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Re: SR98 PSU GETS MORE LOVE

Post by exxos » Mon Oct 23, 2017 11:59 pm

Well, replacing a crappy capacitor with another new crappy capacitor still doesn't seem good :P

If anything a 1uF ceramic would be better. I didn't experiment much noting the regulation IIRC.. you could compare a 1uF and 500nF and check the ripple on a scope and see which value works better.

Lower values give better regulation , but also make the PSU work harder & faster. I don't know the "safe minimum" value..maybe 100nF...

But also, as I noted, you can cause video interference as well, that mostly what I was noting, not the regulation during those tests..

I do need to do more SR98s for my store at some point, so may look into it again when I do those..
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