Uncreative Labs

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

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.

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

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.

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

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, it´s usually one of the TTLs and via versa Rolling Eyes

I am going to readout the 27256s tomorrow morning. Hopefully, they are still OK.. I won´t 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.

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

I haven´t done anything yet because I am not sure about the PSU.

However I´ve 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

Joined: 02 Oct 2004 Posts: 594 Location: Western NC

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...

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

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