STFM Screen noise

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JonB
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STFM Screen noise

Postby JonB » Sat Jan 07, 2012 5:52 pm

Hi there

I have just got a second hand power supply from eBay and it's working fine, except I see a some mild noise on my monochrome monitor (it's a standard VGA monitor with an adapter). It's most apparent on the grey desktop (due to the 50% grey pattern I suppose).

I should explain that I'm running a 520 STFM motherboard with TOS 1.0, encased in the steel chassis but without a floppy drive - I bought it for the ram chips but found out it was all OK and decided to resurrect it with a 4MB memory upgrade (just bought some SIMMs for this) and a home made case. I have another STFM here that I previously upgraded to 1MB RAM and I borrowed the power supply from it to try with the bare motherboard. The picture was rock solid, so I am suspecting that the power supply I bought from eBay is not quite 100%.

It says its model number is ASP34-2 (on a stuck-on label) and the board has REV.3 silk-screen printed on it, as well as an Atari logo and "C100000-001 GX-211 VO" and some dire warnings about the internal fuse :D. Otherwise it looks like a bog standard STFM power supply.


So to the question (and please note, I have searched but - well, let's just say the forum search here isn't the best :wink: ):


What's the best way of cleaning up the output from this power supply? Capacitors, I imagine, but which ones?

I'm thinking C5 (the one with the big red label warning that it is connected to the mains) which looks to be 47uF @ 250v. There are other similar sized (I mean in terms of dimensions, not capacitance) electrolytics on the board as well. Which ones are the most likely culprits?

Regards

JonB
Too many retro computers here: 1040STfm, Mega4, Ultrasatanx2 , Amiga A500, A500+ and A1200, Acorn Atom x2, BBC Micro, Electron, Spectrum 48k x2, +48, +128k, QL, Oric Atmos, ZX81, Memotech MTX512, Amstrad 6128 x2..

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Athlord
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Re: STFM Screen noise

Postby Athlord » Sat Jan 07, 2012 7:10 pm

He,
look for the elctrolytic capacitors.
They over 20 years old and in the capacitor the electrolyt is drying out.
Cheers
Jürgen
You better run - Motörhead

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Zogging Hell
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Re: STFM Screen noise

Postby Zogging Hell » Sat Jan 07, 2012 7:36 pm

Could anyone versed in this thing put together an order list for a reputable electronics firm (or even Maplins :wink: ) with a list of each capacitor, quantity and the order code for whichever company? I would love to change my capacitors having read the thread on the subject, but I'm bamboozled with all the different types. A straight up list with each part, quantity, and a part number with the electronics firm, which people like me who don't have half a clue about electronics, but can solder ok (ish, been practising ready for a blitter installation), can just grab the parts and then swap them over. I know the other thread lists some of them, but I got confused and don't know where to start! :oops: Would much appreciate someone doing this :D

Edit: grammar
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JonB
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Re: STFM Screen noise

Postby JonB » Sat Jan 07, 2012 9:43 pm

Zogging Hell wrote:...been practising ready for a blitter installation...


I did this when I was at college. Sadly the chip holder I got had the wrong pin pattern so I had to solder it to the motherboard with ribbon cable. That was an epic wiring job (worked first time, too, although I couldn't see much difference in performance). Not a job I'd do again - if you want to do this yourself get the right footprint on the IC socket and it'll be like shelling peas.. ;)

Back to the power supply - I think Maplin sell them, they are pretty bog standard electrolytics. Replace like for like and you can't go wrong. Of course, the only problem with Maplin is they are very expensive, so check Farnell, Radio Spares (dear as well) and the usual suspects. My question is which ones go first? I would rather not have to get every one replaced if all I really need is one or two.
Too many retro computers here: 1040STfm, Mega4, Ultrasatanx2 , Amiga A500, A500+ and A1200, Acorn Atom x2, BBC Micro, Electron, Spectrum 48k x2, +48, +128k, QL, Oric Atmos, ZX81, Memotech MTX512, Amstrad 6128 x2..

MRAtari
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Re: STFM Screen noise

Postby MRAtari » Sat Jan 07, 2012 10:44 pm

on that particular Atari PSU (made up until late 1980's) replace ALL of the caps as they are well over 20 years old

the 2 main caps and the others on the primary (main) side of the PSU will go first / the 2 large caps are always dry

any noise you hear from this PSU is usually the transformer as i've had a few "noisy" ones

ebay is the best place to get new caps cheap

alternatively i have a few refurbished PSU's with all new caps for £15 each plus shipping


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