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I have a sick IBM 5161. Help needed

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Exceter

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I have an IBM 5161 that I got from computer reset a while back but, up until now, I was unable to test it because I didn't have an extender card. I have since soldered together a replica extender card and I'm trying to get everything working. When I first connected everything, I got an 1801 error on screen which, according to -0 could be caused by a lot of different issues. I did, however, notice that my pair of Seagate ST-251-1 drives which I'd mounted in the 5161 didn't spin up and make their unique startup noises. I tried two different cables and 2 different receiver cards but still got an 1801 error on screen. I then decided to swap out the backplane because I happened to have a spare. After replacing the backplane, when I switched the 5161 on, the ST-251-1 drives spun to life and made their unique noises. However, a couple seconds after that capacitor C4 gave me the magic smoke. Not having a spare capacitor and reasoning that there was something wrong with the original backplane, I transplanted the C4 capacitor from the original backplane to the replacement backplane. When I switched the 5161 back on, the ST-251-1 drives spun to life just as before but then, exactly like last time, the C4 capacitor blew all over again. At this point, I don't know if there's something wrong that's blowing that capacitor or if I was just unlucky and transplanted a dead cap. If I'm just unlucky, I could steal C2 or C3 from the original backplane as well because they have the same value(106 +16k - There are horizontal lines both above and below the "K") but I don't want to do that if there's something else wrong which could blow them too.

Any advise, insights, or diagnostic procedures would be greatly appreciated.
 
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Exceter said:
Any advise, insights, or diagnostic procedures would be greatly appreciated.
Click to expand...
Regarding the filter capacitors on the backplane. The situation is pretty much the same as the 5160 motherboard; there are 10uF/16V tantalum capacitors and .047 uF capacitors on the various voltage rails. The backplane uses the following 10uF/16V caps:
10uF/16V on +5V rail: C7 and C8
10uF/16V on -5V rail: C5 and C6
10uF/16V on +12V rail: C2 and C3
10uF/16V on -12V rail: C4

We know from IBM 51xx experience of 10uF/16V tantalum caps, that the ones on the 12V rails are by far, more likely to fail than the ones on the 5V rails.

And from my web site: "My own experience is that the failure rate is relatively high if the tantalum capacitor has been unpowered for many years (e.g. I buy an 'unknown working status' motherboard from eBay), but the failure rate is very low for a tantalum capacitor that is periodically powered (e.g. I use my IBM 51xx computers at least every few months.)"

Might have both the 'Computer Reset' backplane and the 'spare' backplane been in the 'unpowered for many years' situation?
If yes, then C4 failure for both backplanes may have been a coincidence.
(And if yes, do not be surprised if C2 or C3 fail.)

For now, you could leave the C4 position empty. It provides partial filtering (not all of the filtering) for the -12V. The receiver card uses +5V only and so is unaffected. With the power supply now no longer overloaded by a short-circuited capacitor, there will be +5V to the receiver card, and with all else working, the 1801 error should go.
 
Might have both the 'Computer Reset' backplane and the 'spare' backplane been in the 'unpowered for many years' situation?
Click to expand...
I think that's a pretty safe bet. Both are from Computer Reset so it's unlikely that either have been powered on in a very long time.

Am I correct in thinking that these would be suitable replacements?
https://www.ebay.com/itm/16126828054...sAAOxyaTxTPzKy

For now, you could leave the C4 position empty. It provides partial filtering (not all of the filtering) for the -12V. The receiver card uses +5V only and so is unaffected.
Click to expand...
That worked!!! The 1801 error went away and my mouse works when plugged into a serial port that's in the 5161! Thank you!!
 
Update: I transplanted C4, C3, and C2 from the original board into C4 on the replacement. C4 and C2 blew within seconds. C3 is still running after 5-10 minutes. I've successfully booted from my XT-IDE with it installed in the expansion unit!
 
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