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SpidersWeb
April 9th, 2014, 12:53 PM
History
I got given the machine, it'd been outside for a while but was in surprisingly good shape. After 2-3 attempts it finally powered on. I used it reliably for the next few weeks (setting it up etc).
Then it sat for a month or so. Last night I was reading PC World 1988 with the PS/2 Model 50Z review and thought I should power it up!

Problem
Turn power switch on, keyboard flashes and machine clicks (when PC Speaker was in) but no LEDs / power after that.
PSU's safety function is on, you'll only get another flash/click if you wait 10 seconds before applying power again (as per tech ref).

Stuff I did
- removed the entire top tray (no HDD, FDD,battery, PC speaker etc)
- disconnected all attached devices
- tested resistance of 3 pin caps on motherboard - some show 589 ohms (fine) - others showed 8 ohms
- I removed the 8 ohm connected caps just in case they were the issue (I'll need to put them back later) but no change in resistance - I'm also not sure if 8 ohms is enough to trip the safety but I figured it was worth a shot.
- PSU is still connected, so now I think the lower resistance and lack of power is from inside the PSU. (I know I should have disconnected that beforehand and saved my capacitors but too late now - I have caps/soldering tools to replace if needed)

What I'll be doing tonight is disconnecting / opening the PSU.

But what I'm wondering is, are their common faults with these power supplies that I should be aware of?
Would the 8 ohm (not sure what line yet) definitely trip the over-current protection? or is it more likely over-voltage that's tripping it?

Just after some insight, usually I don't repair power supplies.

SpidersWeb
April 9th, 2014, 05:39 PM
I did find this thread on here - I'm hoping it's not the planar though :(
http://www.vintage-computer.com/vcforum/archive/index.php/t-10709.html

SpidersWeb
April 9th, 2014, 11:25 PM
PSU on it's own - 8 ohms on +5V, other lines normal
Without the motherboard connected the LED flash lasts longer. So I'm hoping it's over-current and a case of a bad output cap.

Just going to open it up when I find my security bit set. Out of interest it appears that at some stage the filter caps have blown - there is a slight black mark and a very very sad looking cable tie! But it appears someone had them replaced.

Edit: 8 ohms (actually 8.4) seems to be normal - there is two 5W resistors crossing the 5V line - dummy load I guess. Found some caps that are due for replacement - couple of fatties and one did have a little bit of fluff on top - nothing screams out broken though.

ryanita
April 11th, 2014, 06:51 AM
The 50Z's main board is much smaller than the planar of the original 50: almost half of the case is free. IBM used higher-integrated chips and put parts of the electronics (VGA memory, floppy clock generator and PLL) onto small riser boards. The non-standard 512Kx9 30-pin-SIMMs were replaced with a single PS/2 SIMM. Other sizes than 1 and 2 MB however do not work, so you have to victimise a slot for a memory board :-( http://dailydigitaldeals.info/wp-content/uploads/2012/01/11/33/buy.gif

SpidersWeb
April 11th, 2014, 04:03 PM
I'm not too worried about that stuff. Most of the PS/2 range is awkward like that.

For those that don't know, the 50Z was a follow up to the 50 - it came with larger hard drives (30 or 60Mb) and zero wait state memory (hence the Z) which gave it a solid performance boost. I have a copy of PC World 1988 where they review it alongside Compaq's, Model 70 etc epic edition to just stumble across.

I just need my power supply pumping again. I'm off today to start with replacing the electrolytic caps. Wish I knew more, or even had an electronics repair guy nearby.

Edit: New caps are in, in the low voltage section, only one I missed was a 120uF because all I could find was 100uF and a 4.7uF because I forgot to buy one (those two caps look OK though and not shorted). Replaced the 1000uF 220uF and 47uF electrolytics. No change. On power up the amber light flashes once - then I have to wait 10 seconds - as per the tech ref on short (which I can't find) or over-voltage. System board isn't connected, from what I've seen these PSU's power up on their own and both LEDs stay on. Sadly I don't have a decent digital scope to see what's happening.

SpidersWeb
May 15th, 2014, 09:28 PM
Does anyone have an experience with these supplies when they fail?
Circuit diagram?

I took it to a repair agent (I rang around until I found someone who was keen) - he's still looking in to it but his current status is "there is nothing obvious wrong". So if he fails to find the fault, I'd love to be able to go back to him with some more information or be able to look at something myself.

modem7
May 16th, 2014, 02:16 AM
Does anyone have an experience with these supplies when they fail?
I will presume that you mean switch-mode power supplies in general, not just the one in the 8550.
In my experience, faulty capacitors and faulty semiconductors (transistors, diodes, ICs, etc,) is common. But of course, you can get all the other kinds of electronics failures as well.


Circuit diagram?
Maybe a reader has the SAMS Computerfacts for the 8550, and maybe that contains a PSU circuit diagram.

SpidersWeb
May 16th, 2014, 05:30 PM
Cheers for the reply


I will presume that you mean switch-mode power supplies in general, not just the one in the 8550.
In my experience, faulty capacitors and faulty semiconductors (transistors, diodes, ICs, etc,) is common. But of course, you can get all the other kinds of electronics failures as well.

Actually more so the Astec models used in PS/2's. I was hoping someone would say "oh we used to get these in all the time, it's usually the xxxx or yyyy".
But if it's just a case of them suffering the same faults as other brands, then damn :(



Maybe a reader has the SAMS Computerfacts for the 8550, and maybe that contains a PSU circuit diagram.

I was hoping for something like that. I've seen that book on ebay but haven't grabbed it yet because the price of that + what I've spent + repair agent charges - it's going to end up well in to the hundreds of dollars for a simple PSU :/
With a lack of confidence in a repair, I did sneak out to ebay and purchase a replacement that apparently lights up, just as a backup (and because it was quite cheap). Should be here in a month-ish.