VOGONS


How to fix your dead on-board floppy controller

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Reply 80 of 86, by amigasith

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A little bit of thread necro here, but I hope you'll like it 😀

Put into a couple of sentences, I had to replace the Super I/O chip on my MB-8433UUD-A and it turned out to work great! Here are my personal 2 major conclusions from this adventure:

- Despite a couple of people reporting in this thread that replacing the Super I/O chip didn't solve their floppy disk drive issues, it definitely solved it for me.

- Although my board only had "UM8663AF" on its silkscreen, a UM8663BF works perfectly fine on my board.

Here's the long story in case you're interested:

Since a couple of weeks, I'm a proud owner of a Biostar MB-8433UUD-A mainboard. It's the original version aka Rev 1. Unfortunately, shortly after receiving this beauty, the floppy disk drive stopped working properly. While I still got the familiar grunting noise during POST when the floppy disk drive is being initialized, I only got I/O errors when the board tried to boot from floppy. On top of this, COM1 didn't work either, no matter which mouse I connected to it. COM2, on the other hand, still worked fine.

"Okay" I thought and started reading a lot about this problem, including this thread here, of course. Right from the beginning of this long journey, I suspected the Super I/O chip UM8663AF to be the culprit, but I wasn't really sure whether it really was the root cause of my floppy disk drive issues or not. Therefore, I made quite a number of experiments, in order to rule out any other possible issues:

- I checked all traces between the 34-pin floppy cable connector on the board and the UM8663AF, and they were all connected properly.

- Next, I double-checked whether all pins of the UM8663AF made good contact to other places on the board or not, aka "continuity testing". This step was the one that took me a lot of time to complete and I had to spread it over a couple of evenings. But it was worth it every minute 😉 Since some pins of the UM8663AF are connected to vias that are placed underneath of the IC, I had to look for some pictures with desoldered UM8663AF chips, in order to properly follow the traces. Our dear forum here turned out to be a great source for this 😀 See for example feipoa's post here.

- Since all pins of the UM8663AF turned out to be connected properly to other places on the board, I tried booting from floppy disk drive with a Multi I/O card and this worked like a charm.

- In addition, I checked all resistors, capacitors, and diodes for shorts / open circuits. I know that doing this in-circuit is not a 100% bullet-proof method, but still better than nothing. None of the parts were showing errors, however.

So in summary, all important connections / traces "along the floppy path" seemed to be intact: from the 34-pin floppy cable connector to the UM8663AF, from the UM8663AF to the ISA slot, and from the ISA slot to the southbridge. The proper connectivity from the ISA slot to the southbridge was proven by the Multi I/O card experiment. Realizing all this made me feel more comfortable that the root cause of my floppy disk drive issues really was the UM8663AF. And so I decided to give it a try and replace it.

Here is a picture of the area on the mainboard before the "surgery":

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My first action was to simply reflow all pins on both the UM8663AF and the UM8667, which controls the serial ports. This did not help, however. I still got the mentioned I/O errors when trying to boot from floppy.

Next, I desoldered the UM8663AF. This is how the silkscreen underneath of the UM8663AF looks like on my board:

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As you can see, it only says "UM8663AF", but not "BF" as it does on other pictures I have seen both on vogons.org and other places on the Internet.

Here is the desoldered UM8663AF - I used roses metal, a lot of no-clean flux, and hot air for desoldering it:

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Last but not least, here is a picture of my board after the "surgery":

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As you can see, I used a UM8663BF and not a UM8663AF that used to be on the board before. Why? Well, first of all, simply because I couldn't easily find a UM8663AF and UM8663BF's are still widely available, even on ebay, for example. And secondly, because somebody on vogons.org mentioned that the UM8663BF's are more reliable than the UM8663AF 's. And so I thought "why not give it a try".

Of course, I was quite nervous when I started the actual replacement of the UM8663AF, mostly because of 2 things: (1) Whether the ICs that I received were actually legit and not some relabeled dummies and (2) whether a BF would actually work as a replacement for an AF. But everything turned out to work just fine - my MB-8433UUD-A works again like a champ 👍 I can now finally boot from floppy disk drive again without any problems and also COM1 is now working again as it should. Mission accomplished!

If you've made it down here: Thanks for reading 👍 And sorry for the rather long post, but I'm so happy now that I had to tell the whole lengthy story 😊

Last edited by amigasith on 2021-05-20, 20:33. Edited 1 time in total.

Reply 81 of 86, by feipoa

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It is nice to hear another users solved their issue by replacing the Super I/O chip. The issue of a defective COM port or diskette controller is becoming more and more common on these older motherboards, and is not specific to the MB-8433UUD. I have quite a few 4DPS boards in need of similar work, and they use a different chip.

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Reply 84 of 86, by pentiumspeed

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I had to do same with my nearly new Asus FX chipset motherboard back in the day after ESD zapped through parallel port and replaced the multi-i/o chipset from junk board brought it back to living.

Cheers,

Great Northern aka Canada.

Reply 85 of 86, by feipoa

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Mine just died while in the case sitting in its usual spot after years of use, and one day I rebooted and tried to boot from the floppy and it was spitting out errors. Perhaps electro-migration killed it.

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Reply 86 of 86, by amigasith

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Thanks for your comments, guys 👍

I forgot to mention that I also checked all passives for shorts and open circuits. I now added this to my original post. Not sure if this is a common fault on old 486 boards, but it definitely happens on Amiga boards, especially when they're flexed too much when they're put in and out of their cases. This can lead to cracked components, for example. Not that it happens very often, but it happens.

EDIT: The UM8663AF Super I/O chip by the way was the first IC that actually died on me in an old computer "just like that". It would be really interesting to know why this happened, but I guess I will never solve this one... I mean I had faulty ICs before, simply because I did something stupid like plugging in wrong devices but never out of the blue.

Cheers!