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So after about 2 hours of me working to remove and replace that failed 8255, it finally works correctly. It correctly shows all RAM cards and chips and the dip switches do what they're supposed to do. Still having issues with now both my monochrome cards. (The ebay one doesn't work either...

So I got the replacement 8255 in the mail a D8255AC-2, the original was a P8255A-5 just want to confirm it is a proper replacement before I start trying to put it in (the data sheet seems to say it's the same just faster). And it turned on 4 or 5 times correctly on the old chip that is still...

I ordered a replacement desoldering gun (a Hakko FR301-03/P clone that doesn't use a bouncing solenoid), if you think those TMS2114-20NL 200ns replacements will work I'm will to give it a try. Should I be replacing U7 (74LS139) U16, U17, U18 (74LS157s) as well as U13, U15? I can't really read...

Was going to buy a few of these TMS2114-20NL 200ns replacements and try to fix the RAM, but found a replacement (untested so I may be repairing one of them anyway) for way too cheap on ebay (less than I could buy the chips and get them shipped). I just don't trust my ability to desolder that...

Well, it's got all new caps anyway now. :) I guess I'll have to pull the RDR to test it, thanks for keeping RDR updated BTW. Knowing that, I pulled the RDR and put the BIOS back in, and got this from the MDA and monitor. When I put the CGA back in I got an onboard RAM error (likely from the bad...

I tried using the MDA/parallel card instead of the CGA card and composite, the MDA doesn't trip the power supply protection (5 and 12 show up as expected) but it won't even beep on RDR with it installed. I purchased 5151 monitor but can't test it without the card, can the 9 pin on the CGA card...

Did the tests: All switches off = 00000 Grounded PC0 = 10001 Grounded PC1 = 01000 Grounded PC2 = 00000 Grounded PC3 = 00010 Every pin read 5v when the switches are off. Sounds like it's the 8255A. Now to try and find one. I'm not sure I have the ability to remove that 40 pin chip, I have a...

Upon closer inspection it's booting into RDR, but my CGA card seems to be scrolling just really fast and skewed (on the composite out, I don't have a CGA monitor yet). Edit: It seems that is what you get if you try to use a CGA card outside of the case, when I mounted it all back in the case...

I did initially when it had no power to the board and was just using the multimeter, but then used the right pins for all the tests after. Plugged it all in (motherboard, CGA card with composite to a tv, and power supply, and RDR), and I get nothing. Didn't even get to try the grounding of the...

Tried to test again, now it doesn't do anything. No video output. Not sure if it's because it's not in the case, I'll try mounting it in the case again and see if it gets better.

The RN2 diagram was invaluable, I had the pins completely wrong during my tests, and the probe color note was spot on, after switching the polarity around (and testing the correct pins) I was getting proper results. So after doing tests with the power unplugged, the switches all seem to work and...

I've been trying to take measurements across the various elements, but I'm not used to doing this level of repair. I can find schematics for the B255A so I can tell which pins are which, but I'm having to make assumptions for the pinout of the RN2 and when I test (what I think is pin 9 showing...

So I replaced all the failed RAM in the cards and onboard and I ran Rudd's Diagnostic, and it looks like there is something really wrong with either the dip switches for SW2 or whatever chip is being programmed by them. After a bunch of tests, here is the complete output of all Dip switch...

With the help from here and all your reference documents on www.minuszerodegrees.net I've gotten it back up (while waiting for the Rudd's to arrive in the mail). I have the ST225 and WD12 both working (someone had set the primary ST225 to something way off). I had to set the controller and...

So it works again, this time it was bank 1 chip 1 on the motherboard, after replacing it I added in one of the expansion cards and set the motherboard DIPs to 320KB but it looks like more ram is bad on the IBM 256KB card. I've been reading your RAM listings, and I think it looks like I can...

Well now I'm even more confused, should I undo my "Fix"?

I repaired it, I think it was pin 7 from the back on J4, which I think is IRQ4 for slot 4. So if right I guess unless I plugged a card in set to IRQ 4, I'd likely never have noticed.

It does look like it came like that out of the factory though, right? I'm not crazy?

Cleaned and de-oxed the sockets, replaced all the tantalum caps. Before and after. But I noticed a weird spot on the board, it looks like the trace is broken, but it looks like it was done before the solder mask was put on at the factory, is this normal? Should I try to connect this path or was...

I punked out and just requested an MCM68766 EPROM programmed with Rudd's 4.7 from member bzotto at https://eprom-hut.com/