Smoke from MegaSTe PSU (PSM-5341) !

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Pacman
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Smoke from MegaSTe PSU (PSM-5341) !

Post by Pacman »

Today, after not having used my Mega STe for a month or two I turned it on right away and smoke started came out of the power supply (Phihong PSM-5341). The computer didn't turn on and I immediately switched power off.

So I disconnected the PSU, opened it up and noticed a fried resistor (R 35). No other visible changes to the PCB.
20210201-114237_P2010007.jpg
20210201-114237_P2010007.jpg (306.6 KiB) Viewed 3824 times
20210201-114249_P2010008.jpg
20210201-114249_P2010008.jpg (364.53 KiB) Viewed 3824 times
20210201-114226_P2010006.jpg
20210201-114226_P2010006.jpg (283.86 KiB) Viewed 3824 times

R 35 appears, according to the schematic, to be connected to the fan and is apparently 10 Ohms (it now measures around 5 Ohms)
MSTe PSU schematic (new Exxos capacitor values).png
MSTe PSU schematic (new Exxos capacitor values).png (750.11 KiB) Viewed 3824 times

Any idea what has happened here, why and what to do about it?
Is it likely that I've fried my Mega STe? For some reason the PSU fuse didn't blow.
Come to think of it, before turning on the computer I might have quickly plugged in the fan connector if it was left unplugged (I've been doing some servicing on the Mega STe main board lately), and perhaps I didn't pay attention and plugged it in the wrong way (it's a 3-pin plug):
Noctua fan connector corrected_final_correction.png
Noctua fan connector corrected_final_correction.png (16.63 KiB) Viewed 3821 times


Could that have caused this to happen?

I recapped the PSU about 3 years ago using Exxos' own PSM-5341 power supply repair kit, and I even wrote a "how to" guide on recapping this particular power supply. It's worked fine ever since, until today.
STe | MonSTer with dual IDE-CF memory card adapter | NEC Multisync 1990SXi | ST_ESSC | RSVE | Link '97 | Sony HD floppy drive/AJAX controller | Exxos HD floppy module | Battery-backed Ricoh realtime clock module | Discovery cartridge | C-Lab Unitor-2 | C-Lab Export | C-Lab Combiner | C-Lab Steady Eye | C-Lab Human Touch | Unicorn USB

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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by exxos »

Check the resistance across C24.. Also check the resistance across the fan to make sure it is not short circuit...

EDIT:

My first guess it is probably just dumb luck that the resistor is just simply not high enough what is to charge the capacitor from totally discharged...
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Pacman
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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by Pacman »

So I would need to discharge C24 first, before measuring?
I haven't done that (not sure if simply shorting it is safe), but it measures around 120 Ohms (in-circuit that is) -did you mean I need to desolder it first?
The fan measures around 1 M Ohm between the + and - terminals.
But only around 0.7 Ohms between the GND and the yellow wire (whatever that is). If I plugged it in wrongly I would likely have done it like this:

red wire (fan +12V) to -12V (PSU)
yellow wire (fan something else) to GND (PSU)

... and with the low fan resistance (0.7 Ohms) I assume this means I basically short circuited the two fan terminals of the PSU.

I remember struggling with the polarity of the fan vs. the PSU fan-connector, but apparently this is the correct connection:
black wire (fan -12V) to -12V (PSU)
red wire (fan +12V) to GND (PSU)

So if this only affects the fan, can I safely desolder R35 and use the PSU without the fan until I get to replace R35 (and perhaps C24)?
STe | MonSTer with dual IDE-CF memory card adapter | NEC Multisync 1990SXi | ST_ESSC | RSVE | Link '97 | Sony HD floppy drive/AJAX controller | Exxos HD floppy module | Battery-backed Ricoh realtime clock module | Discovery cartridge | C-Lab Unitor-2 | C-Lab Export | C-Lab Combiner | C-Lab Steady Eye | C-Lab Human Touch | Unicorn USB

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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by exxos »

I would suggest just replace the resistor, and power up the power supply and just check the voltage is correct.. Then if you plug the fan in and it burns the resistor again, you know there is something wrong with the fan...
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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by exxos »

BTW is this is the fan you are using

https://noctua.at/en/products/fan/nf-a6x25-flx

0.12A 1.44W.. could be why it burned the resistor.. the resistor is probably only rated for 0.25w... So I would check it is not getting warm when the fan is connected...
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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by Pacman »

Ah! Good catch!
Yes, that's the correct fan and ratings. So the resistor watt-rating is likely too low for the replacement fan?
I'll see in my parts box, but I'm not sure I have such a small resistor as 10 Ohms, so I will have to order online.
Would a 0.5W resistor be a better choice?

Until I get a replacement resistor (I've desoldered the burnt one for now), is the PSU likely to safely work without frying the computer?
Heat is not a factor as I'm currently working with an open case while doing some minor repairs to the motherboard, but killing the MSTe is something I'd want to avoid! :shock:
STe | MonSTer with dual IDE-CF memory card adapter | NEC Multisync 1990SXi | ST_ESSC | RSVE | Link '97 | Sony HD floppy drive/AJAX controller | Exxos HD floppy module | Battery-backed Ricoh realtime clock module | Discovery cartridge | C-Lab Unitor-2 | C-Lab Export | C-Lab Combiner | C-Lab Steady Eye | C-Lab Human Touch | Unicorn USB

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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by exxos »

I would suggest you fit that resistor else your machine will be without -12 power.. I don't know if that will cause damage.. better not to risk it..

Is there not some other fan with a more sensible current rating you can use ?
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https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by Pacman »

Oh! I'd better not run it without -12V.
Unfortunately I don't have any other fan, but I can use the computer (for the time being, with the case open and for shorter periods of time) without the fan at all.

If I don't find a 10 Ohm resistor, are there other values I can use or is this critical?
STe | MonSTer with dual IDE-CF memory card adapter | NEC Multisync 1990SXi | ST_ESSC | RSVE | Link '97 | Sony HD floppy drive/AJAX controller | Exxos HD floppy module | Battery-backed Ricoh realtime clock module | Discovery cartridge | C-Lab Unitor-2 | C-Lab Export | C-Lab Combiner | C-Lab Steady Eye | C-Lab Human Touch | Unicorn USB

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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by exxos »

Pacman wrote: Mon Feb 01, 2021 1:05 pm If I don't find a 10 Ohm resistor, are there other values I can use or is this critical?
Could probably use something between 4.7R to 22R I guess... Without an in-depth analysis of the who circuit it is powering I cannot really say much else... They could just be using it as a fuse... or as a surge protection resistor..
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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Re: Smoke from MegaSTe PSU (PSM-5341) !

Post by Pacman »

Sounds good. I'll see what I can find.
Thanks for your help!
STe | MonSTer with dual IDE-CF memory card adapter | NEC Multisync 1990SXi | ST_ESSC | RSVE | Link '97 | Sony HD floppy drive/AJAX controller | Exxos HD floppy module | Battery-backed Ricoh realtime clock module | Discovery cartridge | C-Lab Unitor-2 | C-Lab Export | C-Lab Combiner | C-Lab Steady Eye | C-Lab Human Touch | Unicorn USB

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