ATARI SR98 RECAPPED BUT ...

[atari_man]

Thank you EXXOS for all the recapping info etc ! So I bought branded caps Panasonic Nichigon etc, a bridge and the 2 resistors all the correct values you mention from a known electronics store, then I put them on and yes I get better power and about 40 mv max VPP noise, BUT I noticed high temperatures in the resistors r205 r103 r104 r105 : 47` Celsius at 1st minute these are the 2w big gray ones. The atari ste sometimes didn’t boot and sometimes crashed.
Also The psu stopped giving 5v but 0,4 v and the resistance output was about 1 ohm - connected due to a broken part I thought. I try to find what was the cause and then the output came back to normal after cleaning resoldering some parts and output resistance raised to 100 Ohm again from 1 that was. So it worked again.
As I told you I change all the electrolytic caps, and the AC-DC Bridge only. I think something is starting to fail the temp rises and I am afraid it may burn my precious STE. I am thinking to replace the whole psu with a meanwell for safety. My mistake was that I didn’t check the temps before the recapping so I don’t know if it was my recapping or not who made this problem to appear. Or is it normal these temps and there is another problem ?

Please give me some directions where to search or what to do to be secure and safe!

[exxos]

I've never measured the temperature of the resistors so cannot really comment on that.

As why you had a voltage drop to 0.4v and strange resistances.. if the capacitors were not fully discharged when you did to resistance measurement they could well read odd resistance or even a short circuit. You should discharge the caps then try resistance and the value should slowly rise.

When you say changed all the capacitors , what exactly did you change because one of the smaller ones is a feedback cap which I generally suggest not to change.

If you changed the resistors on the feedback circuit did you adjust the 5v with the trimmer pot ?

It would be otherwise be rather problematic to diagnose such problems. I was certainly use resistors or a light bulb to load the power supply to see if it is reliable before using it on your STE.

[olivier.jan]

Hi, I’ve recapped a few SR98 and I also noticed the 2 gray resistors get very hot, but it’s been working like that for more than 2 years now without issue. I would say this is normal and shouldn’t be a problem as long as it doesn’t touch anything (including your fingers).

[rubber_jonnie]

First off, if you're testing off load, then you may get odd results. I use the load resistors from exxos store whenever I test any ST PSU, or need to adjust the 5v output. I don't do any adjustments when connected to a machine, it's too risky.

Secondly, the large 2W resistors are that size because they are intended to dissipate power up to that value.

Atari probably wanted the PSUs as cheap as possible, so rather than opting for something that was way over specced, they used something that was only just in spec for the circuit, without much wiggle room, but did the job for a price they liked.

So they are going to get hot, but looking at generic specs for 2W resistors, you could potentially easily operate up to 200C with no problems depending on the resistor composition (So carbon, metal oxide or wire wound).

I don't think 47C is that hot in the grand scheme of things for a 2W resistor, and potentially when used for long periods, it will reach a working temperature that is probably hotter than that, but still within the operating range of the component and as @olivier.jan has mentioned, they can do so with no ill effects.

One of the reasons these parts have such long legs is to lift them above the PCB and prevent the heat damaging them, it's a very common thing in PSUs of this vintage.

I wouldn't particularly worry about it unless the PSU is having difficulty in delivering it's voltage/power levels.

[atari_man]

Thank you friends for your answers. Trying to explain the odd 0,4V ... Suddenly the STE power off and I checked the power output and was 0,4V as I said. There was connected some leads for measuring the output volts while working and maybe I made a shortcut or a debris did this and the transistors or some other ic was dying or puted in safe mode. I remove the psu and after cleaning it thoroughly and resoldering and reconnecting the tested diodes it get back to life. Strange but happened.

>As why you had a voltage drop to 0.4v and strange resistances.. if the capacitors were not fully discharged when you did to resistance measurement they could well read odd resistance or even a short circuit. You should discharge the caps then try resistance and >the value should slowly rise.
I tested the output resistances always with discharged caps.

>When you say changed all the capacitors , what exactly did you change because one of the smaller ones is a feedback cap which I generally suggest not to change.
I changed all the electrolytic caps and the feedback cap 1uf which had 2ohm esr with 1uf at 1,5ohm esr measured with my atlas esr meter.

>If you changed the resistors on the feedback circuit did you adjust the 5v with the trimmer pot ?
YES at 5,20 without load and was giving 5,00V max 2A and 11,88V connected at the STE.
I didn’t have a branded 1,2K and I put a Chinese 1K+200Ω but I had VERY GOOD noise results as mentioned! 30mv peak to peak at 5v I think. Then I put the originals resulting bigger noise.I was afraid that I was pushing things so I put the old resistors and I leave it that way.

>It would be otherwise be rather problematic to diagnose such problems. I was certainly use resistors or a light bulb to load the power supply to see if it is reliable before using it on your STE.
YES I do that always before risking things and I also I tested it with dummy load to see the results.

Maybe the better in esr and bigger in capacity caps deliver more power and the transistors are in difficulty and some ic’s are shutting down ?

Ok about the big temps in the 2w resistors, but I always want my electronics to work in a cool and quite environment. Things are better then. I took photos to share with you about the temperatures.

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I think that these psu’s are some dangerous for our machines. I am giving second thoughts about the Meanwell solution to be peaceful and safe.
Did anyone moded such an open (new technology with OVP OCP etc) psu board on a atari?

[rubber_jonnie]

I honestly think you're overreacting to the temps, you have to remember that these PSU's were designed and built nearly 40 years ago, and there wasn't a great deal of care taken in having things run cool, but they do run in spec.

My TRS-80 model 1 has power supply components that run much hotter, it doesn't mean it's wrong, it just means it was the way they were designed back in the day. In fact a lot of my stuff, the MSX is another good example having an internal PSU itself, and that too get's very warm during prolonged use.

They will always run warm and there isn't a lot that can be done about it. I have STs with SR 98's and they run just fine for long periods, but yes, they get warm.

Ultimately, you are dealing with old designs that will get hot and no amount of tweaking is going to make it better, it's just the way it was designed.

If you don't like the older PSU's getting warm, then a Meanwell RD-50A is the perfect replacement, this one is in my Mega ST: Meanwell RD-50A

I personally haven't tried to fit one in an ST, just the Mega, so you may need to do some fettling to make it fit, but they are very cheap and provide more than enough power for your needs.

[Icky]

Also you could consider an Exxos PSU from his store that is drop in replacement and runs cooler.

https://www.exxosforum.co.uk/atari/store2/#0030

[atari_man]

I will try to upgrade the HRW34 as a last attempt with another Schottky Diode other than the difficult to find for me STPS10H100C,
I will fit again the two feedback resistors and then I will check the outputs and the stability of STE.
Finally because I am a very scholastic type, I will put the Meanwell RD-50A or the open frame RPD-60A for the future..
Thank you exxos rubber_jonnie and Icky for your answers.

[rubber_jonnie]

I will try to upgrade the HRW34 as a last attempt with another Schottky Diode other than the difficult to find for me STPS10H100C,
I will fit again the two feedback resistors and then I will check the outputs and the stability of STE.
Finally because I am a very scholastic type, I will put the Meanwell RD-50A or the open frame RPD-60A for the future..
Thank you exxos rubber_jonnie and Icky for your answers.

No worries.

[atari_man]

After many many years I had an Atari ST experience finaly! But not complete without running a game or a nice app. I made the scart cable with a fixed din13 to din13 male good quality cable cutting the one lead and putting in its place a scart plug, following the standart schematics WITHOUT the resistors and I think now I have the perfect picture in my tv-monitor. Not need for HDMI or any other converters etc..

My MEGA ST childhood atari has gone and I dont have anything (disks,etc..) from that era, and I dont have any software to play with.

So next in line is making a hard disk - or better a sd disk maybe with wifi something like octadrive or any similar open free econmomic project. Please tell me if there is something good out here to try.

About the PSU my STE is working fine, I let the temperatures aside out of my mind and while testing it no strange things happen at all!
Everything went as expected!

Some photos...

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Because I am new to the new era Atari users , what versions are my TOS'es ?

THANK YOU all and have endless retro fun!