NOTE: the Atari TT also uses the same power supply, so TT owners pay aTTention as well! Sorry, couldn't resiST
Electrolytic capacitors have a certain lifespan so this is probably a wise investment if you care about keeping your computer healthy and fully working.
What follows below is not an explanation on how and why recap or how a capacitor works and so on, but a practical "how to" guide for those of you who just want to get it done, so here we go!
Here's where I've found most of the info:
- Recapping info for the PSM-5341 Mega STe power supply
- PSU why & how to replaced the dreaded capacitors in the Atari ST
- Atari ST PSU capacitor & upgrade information
- Mixing different PSU valued capacitors
- MegaSTe hardware problems -help locate it please
- Phihong PSM-5341 power supply schematic for the Atari Mega STe (and I believe also the TT)
- Discharging capacitors, PSU testing with dummy loads
Choosing and obtaining replacement parts
Exxos sells complete "recapping kits" for the various Atari ST power supplies (or "PSU" = Power Supply Unit).
These are high quality parts with the right values and physical sizes. I urge you to support him, being one of the few still active Atari ST hardware developers, while also taking out the hassle of figuring out the details of getting the correct parts yourself.
Alternatively, source the parts yourself (I did this because I was about to take out a large order of other electronic components from Farnell anyway).
Just take care to buy from a reputable dealer (stay away from eBay and the such because you'll run a high risk of receiving fake/inferior quality components which just isn't worth it). Also remember to get the correct values, physical sizes (diameter, height, pin-spacing) and of course a well known quality brand who stands behind their components. You hardly want to recap again just because you went for the cheaper alternatives.
Panasonic FR-series capacitors are the way to go (long life, high temperature rating and low ESR values (low internal resistance) and general high quality throughout in combination with competitive costs), which is what I used.
For the two 400V capacitors however I had to get "regular" (not low-ESR) capacitors as they weren't available within the FR-series, so I ordered well known brand name capacitors with similar hour and temperature ratings as the FR-capacitors.
Regarding capacitor values, Exxos has chosen slightly different values from the ones originally used, the reason being for improvement purposes. Here are the parts I ordered (Exxos' recommended replacement values where available) along with the part number (i.e. C15) on the Atari PSU circuit board, dimensions and quantity, linking to Farnell (NOTE: part numbers/models constantly change, so don't be surprised if the links at some stage no longer work. Exxos' recapping kits on the other hand are a no-brainer if you want to keep it simple):
- 1500 uF/16V 20mm height/10mm diameter/5mm pin spacing (4 pcs: C15, C16, C17, C18) Panasonic FR-series EEUFR1C152B
- 4700uF/16V 25mm height/16mm diameter/7.5mm pin spacing (2 pcs: C13, C14) Panasonic FR-series EEUFR1C472B
- 100uF/16V 11mm height/5mm diameter/5mm pin spacing (1 pcs: C19) Panasonic FR-series EEUFR1C101B
- 10uF/50V 11mm height/5mm diameter/2mm pin spacing (3 pcs: C20, C23, C26) Panasonic FR-series EEUFR1H100
- 220uF/50V 16mm height/10mm diameter/5mm pin spacing (2 pcs: C8, C24) Panasonic FR-series EEUFR1H221B
- 10uF/450V 20mm height/12.5mm diameter/5mm pin spacing (1 pcs: C7) Panasonic EE-series EEUEE2W100
- 120uF/400V 30mm height/18mm diameter/17.5mm pin spacing (1 pcs: C6) Rubycon BXW-series 400BXW120MEFR18X30
If you want to order these resistors from elsewhere you need to buy:
- 2.2 Ohms/17 W high power resistor (1 pcs) TE connectivity 2.2 Ohms/6.1V/17W/10% SBCHE152R2K
- 10 Ohms/17W high power resistor (1 pcs) TE connectivity 10 Ohms/13V/17W/5% SBCHE1510RJ
Replacing the capacitors
We're now ready to do the actual job which consists of the following:
- (optional, but recommended) Checking and taking notes of output voltages (independant of the Atari Mega STe)
- Draining the power supply capacitors for power
- Disassembly, taking notes of original component placement, values and polarity
- Desoldering (removing) the existing capacitors
- Soldering the new capacitors in place, reassembly
- (optional, but recommended) Checking and taking notes of output voltages (independant of the Atari Mega STe)
- Mounting and attaching the PSU to the Mega STe
1) Remove PSU from Mega STe and check independantly
We will now measure its voltages without the computer and take note of the results:
- Power off, disconnect and remove the PSU from the Mega STe. Pay close attention to the proximity of components close to the PSU as they can easily be knocked over and damaged in the process (yes, I'm talking from experience)
- Insert the 2.2 Ohm/17W power resistor between the +5V (red) and one of the GND (black) pins of the power supply output connector.
Similarly, with the 10 Ohm/17W power resistor, insert it between the +12V (yellow) and one of the GND (black) pins. These two resistors will stay in place during the whole voltage check for all output voltages. You can't/shouldn't turn on a switch mode power supply (SMPS) without anything attached to its output!
- Get a (preferrably digital for best accuracy) multimeter ready, set to measure DC voltage and plug it into GND and the first voltage pin of the connector you want to measure (e.g. -5V (white) and GND (black) )
The voltage pins are marked as follows:
PG "Power good" (orange): should be around +5V
- Turn on the power supply (a switched power strip is very useful for this as you can then easily turn it on/off safely at a distance), let it settle for 2 seconds and take note of the output voltage the turn it off again (you should do this in less than 30 seconds as the power resistors will get extremely hot otherwise!)
- Power off, reconnect the multimeter to the next pins you want to check, power on and measure, then power off again (within 30 seconds). Repeat until you're done checking all voltage outputs ensuring you've taken notes of them (so you can compare voltages before/after recapping)
Some of the capacitors can hold a lethal dose of power in them (yes, even when it's turned off and disconnected), so for your safety (and equipment damage) you should unplug the PSU's electrical AC cable from it and leave it for 24 hours (to be on the safe side). This pretty much ensures that the capacitors are drained when you start working on it. Patience, patience!
3) Disassemble and take notes
- Disassemble the PSU by removing its metal housing screws, carefully removing its top metal housing without touching any of the components (just in case there's any charge left in the capacitors). Continue by carefully removing the PCB (Printed Circuit Board) screws and finally turning it over (carefully so as not to touch your fingers or any metal)
- Set the multimeter to DC voltage and check if there is any power left in the capacitors (especially the 400V capacitors). If there is, leave the PSU for yet another few hours and check again, if not you can continue. Check all the capacitors like this, one by one.
- Make a rough note on paper of the capacitor placement, value and polarity. Also mark each capacitor with its part number corresponding to what it says on the PCB (i.e. "C5", "C14" etc.). In case something goes wrong you'll know exactly which original component went where and which way. Remember: the old capacitor values, sizes, placement and polarity is your reference, so be sure everything is noted/drawn before you remove them!
Without any power in any of the capacitors, all of them marked and notes taken you can safely remove them by desoldering.
Everybody's method is different for this. Some people use solder wicks, some use manual solder pumps, other again just heat up the pins with a soldering iron and pull the capacitors out.
Personally, I got hold of this cheap but very effective 40W Duratool desoldering iron for this very purpose and can highly recommend it! It looks like a regular soldering iron but has a hollow tip and a mechanical suction mechanism, so it first heats up the solder and then you press the button which effectively sucks up the hot solder, removing it from the PCB. Mine came from Farnell (I figure it might at least be electrically approved with better quality control than the ones you get through eBay despite their identical appearances). I'm sure there are many different sources if you only look around a bit.
5) Insert the new capacitors
Now comes the fun part: putting the new capacitors in place!
- Start by putting them all in place, ensuring that their values and polarities are correct (if mounted the wrong way round they can and most likely will violently explode!) and bend their pins so they'll be held in place even with the PCB held upside-down
- When done, check all capacitors at least twice for correct value and polarity. Take your time and if possible have someone else double check for you as well
- When you're 100% sure you've got it right you can solder them in place and cut the remaining pins off. Again, take your time so you'll end up with clean, good soldering connections
- Double recheck everything and reassemble the PSU
This is the slightly nervous part where things can go bang!!! if you've done it wrong, so we'll want to check the newly recapped PSU away from the computer. At least getting a replacement power supply will be easier/cheaper than replacing the entire computer.
As mentioned earlier, a switched power strip/extension cord is very useful for this. If the PSU short circuits, starts burning or whatever you can quickly turn it off without needing to touch the PSU or yank its power cable
- Go back to step 1 (remember to attach the two dummy load power resistors and not let the PSU run for more than 30 seconds at a time!), taking notes of all output voltages (this time after the new capacitors have been installed). Depending on the condition of the PSU to begin with you may or may not notice any difference in voltage outputs when comparing with the ones measured before recapping, but it's now likely more stable, with less ripple/noise and will probably last for years, giving your Mega STe much healthier "juice"
- Personally I measured almost identical voltages as before, but noticed that the display output was better than before! Set to hires mono mode (640x400 pixels) the picture appeared slightly sharper, the white was stronger white (as opposed to slightly gray) and some slight "noise" in the gray desktop background was gone. Even if you don't notice any difference I assure you that your work hasn't been a waste
Provided all the voltage measurements appear OK (and no smoke or strange sound/smell has appeared) you can put the PSU back into the computer (again, carefully so as not to knock over any nearby components in the process), reattach the internal power connector and give it a quick check before finally reattaching the top cover as well.
Congrats! You're done. Now give yourself a good pat on the back!
While the PSU is already opened you might as well do a few other things (this section might be expanded as I receive new tips/get new ideas)...
Replacing the (noisy) cooling fan
No doubt about it -the factory installed fan is quite noisy! Besides, cooling fans don't last forever -they accumulate dust and dirt, they wear out etc. so I recommend you replace it with something better.
This page describes in detail how to replace it with a low-noise Noctua A6x25. There's no need to modify the power supply or Mega STe case (although some people cut away the ventilation slits for a better airflow. Some people like to do this while others would never cut or modify the appearance of the computer in any way).
The Noctua fan has a 3-pin female connector attached but there is no need to cut it off and replace it with the original 2-pin connector. Just make sure only the two leftmost pins (black and red wires) attaches to the Mega STe power supply. The yellow wire isn't used.
The fan also comes with two "noise reduction cables" which effectively slows down the fan and thereby lowers the sound even more. Whether this is a smart move or not (will it overheat the PSU and/or computer?) is guesswork, but on the other hand, if your Mega STe internal SCSI hard drive is disconnected (or not installed in the first place) and a MonSTer board for instance is used as a super-fast solid state (and completely silent) storage solution (using an IDE to Compact Flash memory card adapter) it might be a consideration as the SCSI hard drive would surely draw more power and generate more heat.
According to Michael Ruge's Chips'n chips (German) documentation, some Mega STe computers have a flickering desktop background which was caused by a malfunctioning/poor spec'd CNY17-3 opto-isolator (a 6-pin DIL packaged IC marked "PC" on the PSU circuit board).
Replace this with a PC111 opto-isolator and the flickering should be gone! The PC111 might be hard to get hold of as it's discontinued, but one place to find it is at LittleDiode in the UK (or through their eBay store which appears cheaper).Wenn die 1.44MB Floppy nicht zuverlässig durchformatiert, kann es
am Netzteil des Computers liegen. Die Partnummer muss mit -002
enden. Wenn es ein -001 Netzteil ist, muß die mittlere Platinen-
Schraube entfernt werden. Außerdem muss der Stehbolzen, in den die
Schraube gedreht war, gegen die Platine isoliert werden. Klingt
zwar alles merkwürdig, ist aber tatsächlich so. Bei der Gelegen-
heit kannst du auch gleich nachsehen, ob der "richtige" Optokopp-
ler SHARP PC111 eingebaut ist. Wenn es ein CNY17 ist, kann der
Bildschirm ein wenig flimmern.
exxos EDIT 11/05/2020 - updated image, and additional info my Mikro in this post viewtopic.php?f=9&t=300&p=38661#p38655