First post, by HanJammer
- Rank
- Oldbie
So In the "stuff I bought" thread I let you all know about this XT clone machine I found in neighbourgh's garage…
First thing I noticed is that this machine's PSU is shot.
So I dismantled it and started testing it out part by part. I'm most interested in the motherboard and ISA cards…
Unfortunatelly motherboard (designed KT 10 M/B, Made in Japan) won't start.
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Things we know:
- BIOS EPROM is fine - I dumped the bin file: https://mega.nz/#F!JZcy1A7T!SxCYxYLulEkQZM7dIohowQ
- There are two BIOS sockets - 27256 (empty) and 2764 (with EPROM).
- Dip switches on this board are set correctly (I think) - according to specs on TULARC (I found two very similar XT board there: mobo 1, mobo 2). There are no additional jumpers and LED/buton connectors are in places shown in these specs.
- CPU is gettin quite warm (almost hot) after longer period of activity.
- UMC and Siemens chips in the middle of the board are getting slightly warm.
- On the U53 resistor block one of the resistors had too long pin on the back side of the PCB and it was bent and touching the pin of the resistor next to it - I removed the pin, but it didn't changed anything.
- I visually inspected the board and I didn't see any physical dammage to the traces or missing components. Traces I suspected may be bad (ie. I've seen some black spots on them) tested good.
- I removed all of the chips (but DRAM) from their sockets and inspected them for corrosion and/or bent pins - none found.
- I observed that IRDY (I/O ready) LED on the diag card tried to come on up (at first it was very faint, then it was blinking with low frequency, then it was lit constantly and brightly) - it did it at first, but it doesn't do it now.
- PC Beeper tried to beep several times but it was like the voice from the grave with a lot of screeching/crackling noise and so on...
- Most of the time nothing shows up on the diagnostic board, but from time to time it will throw some POST codes. Also I noticed it goes through these codes at extreme rates unlike my 286 motherboards. Looks like this (codes shown around 2:00):
https://youtu.be/pJtouhw7gmI
- I've tested the orange tantalum caps with multimeter and some of them show around 4,7 uF (I guess it's close to their rating) and rest behaves erratic (my cat agrees) but I have no idea how believable these reading are when the cap is mounted in the circuit (for spare caps I have they seem to be pretty fine). Looks like this:
https://youtu.be/RXXFbYE1Pnw
- I've tested some of the ceramic and tantalum caps on the rest of the board and got mixed responses (ie the cap over the 8087 socket show the same erratic behaviour as some of the orange ones).
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Do you guys have any suggestions what to do next (I would like to exhaust all possible diagnostic options before I start replacing parts). Also I don't have 5uF tantalum caps at the moment - can I safely replace them with 10uF caps?
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