psu.why & how to replace the dreaded capacitors in ATARI ST!

[alexh]

How many different PSU designs were there for all Atari ST(e)?

Do they all have the same number of capacitors? Of the same type & value?

Perhaps us Atari-Forum members could come up with a guide showing all the different types of Atari ST(e) PSU with a list of capacitors, their location, type and value? A sort of shopping list for before you start.

[Sudum Gumudu]

There is a lot to read regarding capacitors at http://www.badcaps.net , including a large forum. The general consensus there seems to be there are six reputable brands, namely Panasonic, Rubycon, Nichicon, United Chemicon, Sanyo and Samxon.

[1024MAK]

The types of capacitor that age the worst (and therefore degrade over time) are the Electrolytic capacitors. These are the large cylindrical ones. These are the ones that should be replaced to extend the life of a power supply unit (PSU).

11299701.jpg

77073801.jpg

11310001.jpg

Any replacement capacitor MUST have a voltage rating the same as, or greater than the component that it will replace. The amount of electrical charge that a capacitor can hold is measured in Farads. Nearly all capacitors (except for memory back-up) are smaller than 1 Farad, so a smaller unit is used. For normal power supply electrolytic capacitors, these come in ratings measured in µF (micro Farads). A replacement capacitor should have the same value. If you are unable to get the exact value, it normally does not cause problems if you fit a slightly larger value (if it will fit on the circuit board).

The types of power supply unit used in modern computers (including Atari machines) are the switch mode type. These operate by rapidly switching the power so that the average voltage produced at the output terminals is the correct value. In order for the power supply to work correctly the electrolytic capacitors have to act as temporary stores for the electricity. This "smooths" the voltage. As the switching action is fast (value depends on the type of PSU, but can be from 20kHz to 200kHz) the capacitors have to be able to cope. Because the electric current flows in high frequency bursts, to be fully effective only use the "low impedance" types in switch mode power supply units.
Power supply units normally get warm during operation. Heat is produced from the other components around the capacitors and from within the capacitors. Electrolytic capacitors age and degrade quicker as they get hotter. I recommend that you use the 105°C types.

To sum up, in the Atari ST power supplies use 105°C Low impedance electrolytic capacitors.

Sometimes Tantalum capacitors cause problems. In some power supply units I was repairing (these were not Atari units) one Tantalum had failed short circuit in each PSU resulting in the PSU shutting down as soon as it started up!
These look like this

11100001.jpg

Do note that normal electrolytic capacitors and all tantalum capacitors MUST be fitted the correct way round. If you fit one with the wrong polarity, when you switch on there may well be a BIG BANG as your nice new capacitor explodes.

Most of the other smaller capacitors do not cause too many problems, so do not need changing. However, if you find any that run warm, or live next to a component that runs warm, it would be a good idea to renew it.

Any good electronic supplier should be able to supply these components. Some of the big companies that supply the UK market are
Rapid Electronics Limited, Severalls Lane, Colchester, Essex C04 5JS United Kingdom http://www.rapidonline.com/
Premier Farnell UK Limited http://uk.farnell.com/ and http://www.farnell.com/
RS Components Ltd. Birchington Road, Corby, Northants, NN17 9RS, UK http://uk.rs-online.com/web/ and http://www.rs-components.com/index.html
I have used all three of the above companies. You do not have to be a business, they will sell to individuals (they take credit card orders over the phone).
There are also some smaller firms, but they have a more limited range. Some of them can however help with sourcing components.

Happy soldering...

[Nikolas]

I just whant to let you know that i put 1000 mfd caps close to big chips and it makes ST more stable

[spiny]

beware cheap components

free market capacitors.jpg

[pic not mine]

[Erazotropa]

In PSU's you just can not put in any type of capacitor.
For switching power supply's you realy need Low ESR type's. This because of the switching of the psu at high frequencies 10khz >.. when the psu load suddenly increases, the amount of ripple current also increases..
This behaviour is funest for normal types of capacitors. They get warm and will shorten in life span.
Therefore Low ESR types are designed..

More information check: http://www.illinoiscapacitor.com/upload ... 09455A.pdf

[pterodactyl]

I just replaced the caps in my STE PSU with new low ESR 105 degrees C Pansonic types and... My STE display looks better! I use a Veiwsonic LCD monitor connectd with a ST to VGA adapter and there always has been vertical lines in the desktop display since I did this. They are now GONE! I can only imagine that the poor power condition contributed to some crashing issues I occasionaly experience. I think I'll replace them on the main board now.

I wonder if the FDD should not get its power from the PSU directly instead of from the motherboard.

[KLund1]

I saw a thread about the IC with a battery imbedded. I'd rather just replace the IC with a new one then try to pry out the old battery and replace. Where to get?

About replacing the caps. Just to keep it simple, could we pull the old caps off the PSU, take them down to Radio Shack and get exact replacements? Or better yet, spend a bit more and get a fresh PSU from BEST ELECTROINCS?

[pterodactyl]

About replacing the caps. Just to keep it simple, could we pull the old caps off the PSU, take them down to Radio Shack and get exact replacements? Or better yet, spend a bit more and get a fresh PSU from BEST ELECTROINCS?[/quote]



NO MAN! Ya gotta use new state of the art low ESR (low Impedance) capacitors, It will make a difference! A fresh PSU from Best may use these parts though - I could not say. But the avaerage replacement capacitor types are less effective even if they read the correct capaciance value. I am a believer.

[8bitbubsy]

What are common symptoms of bad Atari ST PSU caps?
And how should I measure the voltage lines? There are so many wires from the PSU to the mainboard...

[1024MAK]

There have been some comments about the size of replacement capacitors.
Modern electrolytic capacitors are normally physically smaller than older capacitors but have the same capacity (capacitance, measured in uF) and voltage ratings.
So don't worry or indeed expect that the replacements are the same size.
Just make sure that:
* the replacement has an equal or greater voltage rating,
* the capacitance is equal or greater,
* that it is a low ESR type,
* that it physically fits (the case may be taller or wider),
* and for the PSU capacitors that the replacement are 105C temperature rated type.

Most of the suppliers I listed in my earlier post (scroll back to near the top) will supply to most of Europe.

Happy soldering

[BeardSwitcher]

Hi all. I googled around and found some very informative posts on atari-forum regarding Atari PSUs.

I am not unexperienced with electronics (old electronics repariman and engineer) but i always want to investigate and read alot before i start tinkering with things

I have got hold on a old Atari 520 STfm that does'nt start at all. I figured out that it was the 2A Fast Fuse in the PSU that was blown. It has to have blown for a reason but i gamled and simply replaced it with a identical one and the result was a nice blue flash when the fuse immediately blew when the power was turned on again.

Any tips or suggestions of where to start? The PSU itself looks very cheap and have a yellowish colour, according to what i have read is this one of the units built with poor quality from start.

I will spend some time with it and a magnifying glass too look for poor soldering or burnmarks. but is suggest some shortcircuit somewhere or a powersurge in the capacitors.

I appreciate all help i can get

Br
Emil Billing Sweden

[BeardSwitcher]

Here is a closeup of the PSU.

atari520stfm-psu-browsfuses.jpg

Can not see any obvious leaks or burnmarks sofar...

[MRAtari]

you also need to change the opto-coupler otherwise it will highly likely blow again with all your new parts and trash them! believe me i learn't this the hard way!

whenever a fuse blows violently it always stresses if not blows the opto-coupler i've found

you will also need to change Q1 (3 legged BU508 chip on heat sink) and Q2 (3 pinned transistor) remove Q2 and look to see if has blown? (broken bits missing/cracks) if this is so then Q1/Q2/CNY-65 and a couple of resistors (R8 and possibly R4) will ALL be dead

the opto-coupler on this PSU is marked CNY-65 and is the 4 pin black square package to the rear of the PSU (in front of the large white square capacitor

good luck...

[TheNameOfTheGame]

Can anyone recommend a good capacitance meter that is reasonable in cost?

[Idek Tramielski]

Hi,

since my TT an MSTE psus seem to be at the limit of their life expectancy, I would like to fix them.
I opened the PHIHONG PSM-5341REV-A1 PSU and started identifying the capacitors.
Here is the list, that I was able to create with my limited knowledge:

list.jpg

In the first column, I color-marked the capacitors according to their type.
I need help verifying the orange areas in the fourth and fifth columns, since I couldn't find more details on them.
I also didn't find C21 on my pcb?!
C9, C27 C28 and C29 I cannot identify without taking them out. Will try this later......
Furthermore, I don't know what type capacitors the green (C10,C11) and yellow (C25, C27, C28) are?
Here is a picture including C10, C12 and C25:

caps.jpg

Maybe somebody can help completing the list and adding available replacement types?

Any help appreciated,

Wolfgang

[dml]

The green ones are polypropylene caps - mid voltage range (typical 100-300v) mid frequency, nonpolarised. The yellow ones probably Tantalum, lowish frequency, low voltage (typical 10-30v), polarised.

Tantalum nearly always have a tiny + marked, which helps. They also like to explode violently if you reverse polarity.

Note: A cap marked '222' usually means 2.2nF (or 0.0022uF). The first 2 digits specify the value (22), the last digit sets the decimal position - so you can work out other values when you see them. It's a common system for ceramics, polyprop and some others (but not the only one, unfortunately).

[TheNameOfTheGame]

The green ones are polypropylene caps - mid voltage range (typical 100-300v) mid frequency, nonpolarised. The yellow ones probably Tantalum, lowish frequency, low voltage (typical 10-30v), polarised.

Tantalum nearly always have a tiny + marked, which helps. They also like to explode violently if you reverse polarity.

And the green and yellow don't really need to be replaced correct? Isn't it just the electrolytics that age so poorly?

[dml]

And the green and yellow don't really need to be replaced correct? Isn't it just the electrolytics that age so poorly?

Polyprops are quite robust. They are only a weak point if over voltage and long term heating is a problem - unlikely here.

Tantalums are prone to failure if abused but they tend to detonate vs silently stop working

Ceramics are not a weak point generally in low voltage or mains circuits. Moisture or heat can get them over time and they go crumbly but you can usually tell (squash them with a fingernail but not too hard).

Electrolytics have a liquid electrolyte with a finite shelf life - they dry out. They are a common weak point.


Having said that, any circuit that goes violently pop can damage any other components invisibly - it's just that the probability for some types is pretty low so you should be ok. In this case the semiconductors and electrolytics are the main weak points. Replace all electrolytics and replace any semis which don't test positive (simple transistor, mosfet junction, diode tests usually enough - plenty of info can be found on this).

Simbo says a few of the other components can go bad in this PSU. That's from experience and it's circuit-specific so if in doubt that seems like a good idea too. Better replace a few extra bits and save a mainboard.

[krupkaj]

BTW is there schematics for ATARI TT/MEGA STE power supply (Phihong)?

[Idek Tramielski]

BTW is there schematics for ATARI TT/MEGA STE power supply (Phihong)?

I have not run into it, yet. Would be nice to have.

Anyhow, I replaced all electrolytic capacitors, the optocoupler and a couple of filters of the psu. All problems that I observed (noise on screen, wavering screen, ...) are gone! I have not replaced the diodes yet, since I have not found a small enough replacement with the same rating. Next time I will add some additional information on parts in the list above.

Update: I am still looking for a good replacement for the 1N5395 (400V/1,5A/DO-15) of the psu. I tried the UF5404, but they are DO-201 and don't fit mechanically.

[charles]

how did all this develop ,, was it successful?
any measures for a 1040stf power supply

that was for a 520stfm correct?

mine is 2amp 125v

north america
charlie

[krupkaj]

I have just found there is a schematics of TT/MEGA STE power supply on dev-docs server now.

[KLund1]

This is a very good thread. Very informative. But I'm a nubie when it come to reading circuits, bar codes on resisters, which are the positive leg(s) of a transistor, orientation of caps, etc.
I have several PSU's I'd like to repair. I would great if someone with much more knowledge could put together a baggy with all the parts needed, and a 1 to 1 replacement guide. (this new one goes where this old one is kind of thing. maybe a few pictures are always helpful)
I have no idea where to buy, what to ask for, etc. I'm ok with a solder iron and could do the work, just need the parts and a bit of guidance. The same for any parts on the motherboards.
I'm pretty sure there are quite a few of us in this boat. I'm sure we'd all pay a fair and reasonable price.
Just a thought.

[exxos]

I posted the fixes for the STFM PSU already if that helps.

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