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Help to repair a MFM 8-Bit ISA Card

ayandon

ayandon

Experienced Member
Joined
Aug 10, 2022
Messages
52
Location
Kolkata, India
Hello,

This is my MFM Hard-disk Controller 8-Bit ISA Card.
May be its CXT-101.

It was in running condition when used years ago. (This is not a scrap-card)
Now, when testing, I only noticed that there is a short on +5v line!

Can you pls help me to identify the Capacitors and those few ICs with no markings on it?
It will be a great help.

Also, pls share your pics if you have similar card.
 

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You could lift a leg of each capacitor one by one to try and find a short. Also let the card run powered in the machine for a while and see if one of the chips becomes very hot, if it does then you know that IC is probably shorted.
 
VeryVon said:
You could lift a leg of each capacitor one by one to try and find a short. Also let the card run powered in the machine for a while and see if one of the chips becomes very hot, then you know that IC is probably shorted.
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Actually since I have only one retro system, I am too afraid to put it in for testing 😥
 
Granted, I did not see this board before the "short", but U9 appears to be well-done and U5 appears to be medium-well done. Given your admitted and entirely understandable fear of testing in the system, buying another controller has to be considered, in my opinion.
 
syzygy said:
Granted, I did not see this board before the "short", but U9 appears to be well-done and U5 appears to be medium-well done. Given your admitted and entirely understandable fear of testing in the system, buying another controller has to be considered, in my opinion.
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Agree.
I am interested to replace U2, U5 and U9 if I can find the IC nos.....
 
ayandon said:
Agree.
I am interested to replace U2, U5 and U9 if I can find the IC nos.....
Click to expand...
So, as an onlooker, I see you up against a lot.

Are you comfortable with the skill level to replace components on the board, including ICs?
Identify the ICs at a minimum by a picture of a good board, but ideally with a schematic?
Find and install the ICs - NOS? ok, add that to the burden.
Find and install some possible number of caps/resistors that may either have been damaged in the short or caused the short.

THEN, you still have to test the board and you only have one system.

I am a great fan of perseverance, but I think you have to balance all that against getting a new/used working board - which may not be terribly expensive?

What HD(s) were attached?
Does the underside of the board show any heat burns anywhere?
 
VeryVon said:
You could lift a leg of each capacitor one by one to try and find a short. Also let the card run powered in the machine for a while and see if one of the chips becomes very hot, if it does then you know that IC is probably shorted.
Click to expand...
I found it perfectly effective to just measure the resistance on all tantalum caps on the shorted rail and cut the one that has the lowest reading. Did two repairs like this over the last weeks and in both cases it was the correct one on first try. :)
 
Timo W. said:
I found it perfectly effective to just measure the resistance on all tantalum caps on the shorted rail and cut the one that has the lowest reading. Did two repairs like this over the last weeks and in both cases it was the correct one on first try. :)
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Both Cap issues?
 
Here is a similar board it is NOT identical but given the similarities, it may be of some value. I think that you have your work cut out for you and I think that if you really want to do this, a first step is to identify the exact board that you have. If I could find a similar one in a few minutes, with enough searching, you may be able to find the exact one.

Also, you did not answer the questions as to your skill/comfort level - I know you very much want this restored, but the more info that you provide, the better chances of getting help.
 
Timo W. said:
Yes, both had a shorted tantalum cap. When you have a short on a voltage rail on something that worked and was just in storage, this really seems to be the most common cause for it.
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Can you pls let me know these Cap values? Are they all same?
Mine are very very hard to read.... may be as I used IPA and Toothbrush to clean the PCB....
 
Okay, call me benighted--I don't care. I don't see any tantalum capacitors on the board---MLCCs, sure, all of those yellow ones. I'd start by removing C9 and C10 (electrolytic) and see if they're any good.
 
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Chuck(G) said:
Okay, call me benighted--I don't care. I don't see any tantalum capacitors on the board---MLCCs, sure, all of those yellow ones.
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Not sure how you think you can tell that by the look of it. These can be either MLC or tantalum. Yes, tantalums also exitsted in that axial drop-shaped form. You can even see that C14, C15, and C21 are apparently not of the same type (might be a mix of MLC and tantalums). I don't see how that matters for finding the shorted cap, however.

Can you pls let me know these Cap values? Are they all same?
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You should be able to read the values once the cap is out or a pin lifted. Try to do what I suggested: find the cap with the lowest reading (resistance). Lift or cut one leg and measure the voltage rail again.
 
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Timo W. said:
Not sure how you think you can tell that by the look of it. These can be either MLC or tantalum. Yes, tantalums also exitsted in that form. You can even see that C14, C15, and C21 are apparently not of the same type (might be a mix of MLC and tantalums). I don't see how that matters for finding the shorted cap, however.
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Given that it's a Far East origin (Taiwan?) board with lots of missing components and 2 electrolytics where tantalums might be a better choice, I strongly suspect MLC just from the point of saving quatloos by the vendor. I'm willing to be proved wrong.
In particular, where's the BIOS PROM? I suspect that U6 is a 28 pin EPROM and U8 is a PAL used for address decoding. Clearly this was intended for an 8 bit system with hard disk support in the motherboard BIOS--not all that common. Didn't the M24 have such support built into the BIOS?

Just some random observations from an old coot.
 
Chuck(G) said:
Given that it's a Far East origin (Taiwan?) board with lots of missing components and 2 electrolytics where tantalums might be a better choice, I strongly suspect MLC just from the point of saving quatloos by the vendor. I'm willing to be proved wrong.
In particular, where's the BIOS PROM? I suspect that U6 is a 28 pin EPROM and U8 is a PAL used for address decoding. Clearly this was intended for an 8 bit system with hard disk support in the motherboard BIOS--not all that common. Didn't the M24 have such support built into the BIOS?

Just some random observations from an old coot.
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Friend, it was with my XT System (NEC V20 + Faraday Chipset)
 

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Then your board had to have ST506 BIOS support in its BIOS. Note that your card is depopulated with regard to BIOS extension and address decoding for said BIOS. Your card will not work with a standard XT system without special consideration for the lack of BIOS support on-board.
 
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