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 IBM XT 286 - PSU caused the "Super-Gau" View next topic
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xt286



Joined: 01 Apr 2007
Posts: 5
Location: Aachen / Germany

PostPosted: Sun Apr 01, 2007 10:13 pm Reply with quoteBack to top

Hi to all!

Currently, I am working on a IBM XT 286. I need it to run my programmer, to program a few MCUs and to play digger at least Smile Although I could bring it back to life at the beginning the power supply had a defect more or less soon. I built another AT PSU in the original XT PSU case and now I get all voltages except I have just 2,4V at the power good line. Now I think, my mainboard is toast, because the AT supply seems to be OK without mainboard. Well, I could buy a spare board, but this is not how I want to solve this problem. With my DMM I found out (switched to Diode - testing) that I have just 15R between GND and the +5V line on the TTLs f.e. and the DMM is beeping. This is not what I have expected. Does anybody know common mainboard defects in combination with a broken PSU? Is there any schematic online which could help me to find the defects with my osciloscope? Moreover I found two 27256 Eproms on the board. Where can I find the image in case they stopped working? And last but not least I saw several MMI bproms on the module... What is, if they are damaged?

Thanks for any help, Hanno / Aachen, Germany.
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Diky



Joined: 02 Oct 2004
Posts: 22
Location: near myself

PostPosted: Sun Apr 01, 2007 11:25 pm Reply with quoteBack to top

I think it should be a capacitor in short-circuit. I had a similar problem on an old XT motherboard but when i swap PSU the bad capacitor blows up. Motherboard still doesn't work but +5 is now +5.

BIOS eprom's could be easily dumped from a working xt286 (Who doesn't own a xt286... Smile ) i remember that on Simtel there was a program to dump BIOS rom for burning new BIOS eprom for solve some HD table problems, you can use that.

Good luck.
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xt286



Joined: 01 Apr 2007
Posts: 5
Location: Aachen / Germany

PostPosted: Mon Apr 02, 2007 12:06 am Reply with quoteBack to top

Thanks for the fast reply.

Usually I am repairing old pinball games with 6502 or 6800 MPUs. From my experience the defect part depends on the piece I desolder first: For example, if I am beginning with the capacitors, itīs usually one of the TTLs and via versa Rolling Eyes Rolling Eyes Rolling Eyes Rolling Eyes

I am going to readout the 27256s tomorrow morning. Hopefully, they are still OK.. I wonīt think about the Monolithic Memories yet Mad


I would like to know a few things before I proceed and desolder the TTLs:

Can anyone messure the resistance between +5V and GND on any TTL with power connection plugged off? Is there any difference between a standard AT PSU and a IBM XT-286 PSU?

Thanks, Hanno.
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xt286



Joined: 01 Apr 2007
Posts: 5
Location: Aachen / Germany

PostPosted: Mon Apr 02, 2007 8:46 am Reply with quoteBack to top

I havenīt done anything yet because I am not sure about the PSU.

However Iīve done a small service for XT286 owners: The bins are now online. You can download them here:
http://www.strikesandspares.de/RomsInet/xt286.zip

Moreover I found out that the ICs to repair the board a more expensive than a complete new XT286. Confused
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T-R-A



Joined: 02 Oct 2004
Posts: 579
Location: Western NC

PostPosted: Mon Apr 02, 2007 3:59 pm Reply with quoteBack to top

Quote:

Moreover I found out that the ICs to repair the board a more expensive than a complete new XT286.


That's the problem with obsolescence. You start hitting the other end of the "bathtub curve" when technology advances. More manufacturers are now making surface-mount components, and fewer are making DIPs, thus the price of the DIPs goes up because they're in short supply...
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xt286



Joined: 01 Apr 2007
Posts: 5
Location: Aachen / Germany

PostPosted: Tue Apr 10, 2007 11:14 am Reply with quoteBack to top

Good news. Today I could fix the motherboard. A TTL had gone bad. Iīve spent 2 evenings with messuring. Moreover I was the fool at work because Iīve told somebody, that I am trying to fix a 286 mobo. Well, they have just no idea!! Wink
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