How to: recapping the Mega STe power supply

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Pacman
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How to: recapping the Mega STe power supply

Post by Pacman » Mon Oct 16, 2017 11:41 pm

I recently decided to replace all the electrolytic capacitors (recap) in my Mega STe power supply (Phihong PSM-5341) as a safety measure, considering it's close to 30 years old a "ticking time bomb". Better to replace before everything goes wrong than after!
NOTE: the Atari TT also uses the same power supply, so TT owners pay aTTention as well! Sorry, couldn't resiST :lol:

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!


Sources/background info
Here's where I've found most of the info:
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.
MSTe PSU schematic (new Exxos capacitor values).png
MSTe PSU schematic (new Exxos capacitor values).png (199.38 KiB) Viewed 1226 times
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): And while you're at it, order a dummy load power supply resistor kit from Exxos if you want to check the power supply without connecting it to the computer. In case you've done the recapping wrong and the PSU goes "poof!" at least you won't risk damaging your Mega STe in the process.
If you want to order these resistors from elsewhere you need to buy:
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!
Mega STe power connector.png
Mega STe power connector.png (19.67 KiB) Viewed 1120 times
  • 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:

    +12V (yellow)
    -12V (blue)
    +5V (red)
    -5V (white)
    GND (black)
    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)
2) Drain the PSU capacitors for 24 hours
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! :lol:

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!
20171009-215859_CCF_09102017.jpg
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20171010-131529_IMG_8516.jpg
20171010-131529_IMG_8516.jpg (317.37 KiB) Viewed 1226 times
4) Desolder (remove) the old capacitors
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 effectlively 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
20170703-181040__MG_0749-Edit 2.jpg
20170703-181040__MG_0749-Edit 2.jpg (623.91 KiB) Viewed 1134 times
6) Check its voltage outputs independantly from the computer
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 :o
  • 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" :thumbup:
  • 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 :D
7) Reattach PSU and reassemble the computer
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! :bravo:


ADDITIONAL NOTES
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).
20171102-131127__MG_2263.jpg
20171102-131127__MG_2263.jpg (96.51 KiB) Viewed 1079 times

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.
Noctua fan connector.png
Noctua fan connector.png (10.28 KiB) Viewed 1191 times
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.

Malfunctioning opto-isolator
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).
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.
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).
20170703-181040__MG_0749-Edit-2b.jpg
20170703-181040__MG_0749-Edit-2b.jpg (155.18 KiB) Viewed 1133 times
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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exxos
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Re: How to: recapping the Mega STe power supply

Post by exxos » Thu Oct 19, 2017 2:10 pm

Pacman wrote:
Mon Oct 16, 2017 11:41 pm

Malfunctioning opto-isolator
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).
I had not seen or heard about that one before. I would have thought screen flickering would be more related to ageing capacitors than anything else.

Some optos do have better ratings and speed great than others, ultimately if it makes any difference or not.. it is not something I have tried with the mega power supply.

I did try it on some ST FM power supplies while ago but did not see any difference between two different spec parts.

I do have a lot of these power supplies to repair at some point, so I will investigate this when the time comes :)
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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Pacman
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Re: How to: recapping the Mega STe power supply

Post by Pacman » Thu Oct 19, 2017 2:16 pm

I read about it here.
Apparently recapping didn't make a difference in this respect, but replacing mentioned part did.
What does that opto-isolator even do?
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: How to: recapping the Mega STe power supply

Post by exxos » Thu Oct 19, 2017 2:27 pm

Pacman wrote:
Thu Oct 19, 2017 2:16 pm
I read about it here.
Apparently recapping didn't make a difference in this respect, but replacing mentioned part did.
What does that opto-isolator even do?
The problem is we do not know what capacitors he used to re-capping. There are many types and specs, and just using any old capacitor isn't going to do anything. So would need to know more information about this before concluding the capacitors did not change anything.

The optocoupler is part of the feedback loop for the voltage regulation. As an example, the Opto simply connected across the 5 V rail, and when the voltage gets to something like 5.1 V the Opto turns on, and turns off the main switching transistor to the transformer. So obviously no energies being dumped into the transformer core at this point, and the voltage will start to drop. The 5 V is now only powered by the capacitors alone.

When the voltage drops to something like 4.9 V, the Opto will turn off and turn on switching transistor dumping energy into the transformer core which then starts to raise the voltage again. Then when the voltage reaches 5.1 V the Opto will turn on again and turn off the main switching transistor.

How fast the Opto can turn off and on can relate to how good the regulation is. Not only that, the voltages I state were just a example, I do not know the actual voltage levels. But also some Optos can be very slow or very fast to turn off and on. This depends on the gain between the Opto itself and the phototransistor gain. Higher gain Optos will have tighter control over the regulation and ultimately switch faster. Of course some of those can switch faster than others it all depends on the specifications. And there are a lot of them!

Of course generally putting a higher gain and faster switching Opto should generally be better. But this is not always the case. Sometimes the main switching circuit cannot keep up with the Opto isolator and you can actually end up making things worse with the regulation.

As I was saying about the capacitors, if the capacitors are not good rating, or too low of a value, the power supply will ultimately have to switch faster to maintain the regulation. So using a fast Opto relating capacitors could work better. Though better rating capacitors will improve the regulation anyway, so fast Opto would not really do anything.

Really the Opto should have scope readings done on it before and after the change to properly document what was actually happening.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: How to: recapping the Mega STe power supply

Post by Pacman » Thu Oct 19, 2017 4:09 pm

Although a bit over my head I vaguely understand what you're saying: that components interact with each other.
I suggest you contact Czietz and ask about the details.
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: How to: recapping the Mega STe power supply

Post by exxos » Fri Oct 20, 2017 12:57 pm

Pacman wrote:
Thu Oct 19, 2017 4:09 pm
Although a bit over my head I vaguely understand what you're saying: that components interact with each other.
I suggest you contact Czietz and ask about the details.
I will investigate it when I get chance anyway. It looks like you will have to wait until I get one of the power supplies which I have worked on before.

EDIT:

And the postman just handed me such a power supply :lol: someone sent me one to have a look at due to a failed re-cap...

It has a PC111 in there anyway...
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: How to: recapping the Mega STe power supply

Post by Pacman » Sat Oct 21, 2017 1:44 pm

I wish I had a postman like that :lol:
Well, sometimes I do :D
Mega STe | MonSTer with dual IDE-CF memory card adapter | STe | SM-144 |NEC Multisync 1990SXi | IDE doubler | ST_ESSC | RSVE | ICD Link II | Link '97 | HD floppy drive/AJAX | HD floppy module | Minolta PCMCIA card-drive | Realtime Clock module | Discovery cartridge | Unitor-2 | Export | Combiner | Steady Eye | Human Touch | Unicorn USB

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Re: How to: recapping the Mega STe power supply

Post by Icky » Wed Nov 08, 2017 11:39 pm

Great guide. I have added one more thing to finish off the PSU recapping and refurb. I found that the floppy and HDD cables were all loose and sometimes got in the way when closing the case. My MegaSTE had black insulating tape around the cables at various places. I had seen that modern PSUs for PCs employ an expandable braiding around the cables. After sourcing 1m of 4mm braiding that goes from 2mm - 8mm It made a better job of the cable management.

Image

Using some heat shrink wrap finishes off the ends to stop the braiding coming unraveled

I picked up my braiding from http://www.hobbytronics.co.uk/braided-s ... d=sleeving for just over a £1

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Re: How to: recapping the Mega STe power supply

Post by IngoQ » Thu Nov 09, 2017 8:32 am

Nice! Thanks for the tip :)
Ingo :geek:

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupéry

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Re: How to: recapping the Mega STe power supply

Post by Bikerbob » Sat Nov 11, 2017 12:04 am

MSTE PSU REPAIR KIT PHIHONG PSM-5341 240V type £25 22 In Stock [0064]


So this is what I want I assume.. but I have a questions.. your line there says 240v type - but I am to understand this is a switching PSU.

SO.. am I buying this.. because its for the PSM-5341? OR have you changed something that only makes it a 240v kit?

James

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