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Denniske1976
May 12th, 2011, 01:02 AM
Hi all,

I am in need of a little help. Here's the thing:

Last week I wanted to power up my IBM XT again, you know, something with "Commander Keen" and such ;-) Plus I picked up an old 5153 color screen somewhere and wanted to see if it still worked...

So, I switched the machine on and the only thing that happened was "poof!" :shock:

Opened up the case and it seemed like the mainboard had a bad case of "C56-blown-ness" (the tantalum cap near the keyboard connector). Searched the internet only to find that "actually" the machine should power up without C56 because it only filters the PSU power range for the mainboard (correct me if I misunderstood). Only thing is: when I power up the computer with that mainboard, only the PSU fan starts and nothing else happens (no screen, no floppy noise nuttin' else).

So, since I'm quite a-technical and also don't have any tantalum caps around so solder in, I replaced the mainboard with an old mainboard I had from a 5150 Portable (looks to be the same mainboard anyways). Installed all the cards, set the DIP switches, connected screen and keyboard. But when I turned the PC on, I get no screen :-( Eerything else seems to happen, because I see it reading the floppy drive and then switches to read a bit from the 10MB harddisk (probably loading DOS 3.10 I think that's what's on it).

So, connected the 5153 to my HeadStart LX-40 (that still won't read HD floppies) and got screen there... conclusion? The CGA card probably also blew up.

So now the bottom line of my question is: Does anyone have one of those old full length IBM CGA adapters? Or can I somehow figure out what's happened to this one? Looking at the card, I don't see anything blown or black or something. Card looks 100% OK. I did look on ebay and the cards are there, but $186 for some old CGA card (plus $45 shipping!!) just because it's IBM made? I mean if that's what those things are worth, I think I have about 233,000 bucks of other IBM cards in the basement :D

Anyhoo, anyone know what to do? Ever repaired one of those cards? I might just have one somewhere in the basement but I'm pretty sure I don't. The only other one I have is in my 5155 Portable and I would like to keep that in there (they were stock IIRC). I'd like to keep my XT original if possible, especially now that I have another 5153 screen to go with it that actually works :-)

So, thanx for reading and let me know your thoughts

modem7
May 12th, 2011, 02:19 AM
Opened up the case and it seemed like the mainboard had a bad case of "C56-blown-ness" (the tantalum cap near the keyboard connector). Searched the internet only to find that "actually" the machine should power up without C56 because it only filters the PSU power range for the mainboard (correct me if I misunderstood). Only thing is: when I power up the computer with that mainboard, only the PSU fan starts and nothing else happens (no screen, no floppy noise nuttin' else).
Are you sure that is C56? C56 is a good 4" away from the keyboard connector. Whichever tantalum it is, it may look look like it has blown open circuit, but instead may be short circuit (or partially short circuit). So remove the capacitor then see if the motherboard starts.


So, since I'm quite a-technical and also don't have any tantalum caps around so solder in, I replaced the mainboard with an old mainboard I had from a 5150 Portable (looks to be the same mainboard anyways). Installed all the cards, set the DIP switches, connected screen and keyboard. But when I turned the PC on, I get no screen :-( Eerything else seems to happen, because I see it reading the floppy drive and then switches to read a bit from the 10MB harddisk (probably loading DOS 3.10 I think that's what's on it).

So, connected the 5153 to my HeadStart LX-40 (that still won't read HD floppies) and got screen there... conclusion? The CGA card probably also blew up.
Confirm for us that switches 5 and 6 are either OFF/ON or ON/OFF.

If either, maybe one of the switches didn't switch properly - try setting switches 5 and 6 to OFF/OFF, then to ON/ON, then to ON/OFF.

Denniske1976
May 13th, 2011, 04:13 AM
Hi modem7,

Thanx for the reply... just checked and you're right: it's C56 alright but not so much near the keyboard connector. My bad, dunno how I came to think that. C56 is behind and in between the J4 and J5 ISA slots. Anyhoo, that's the first thing I tried: cutting off the 3-pin cap and see if it would work again because I read somewhere it should without C56. But it didn't :-( I then tried to wire-bridge the 3 holes with a wire soldered in and that also won't work.

But, I've just wnt home for lunch and checked the "new" installed 5155 mainboard and switches 5/6 were both ON and that's the "no monitor installed" setting according to the IBM Technical maintenance and reference - Personal Computer XT Part 2 (luckily I kept all the documentation). So now I've set them to 80x25 and I had screen!! Yay!

The only other problem I have now is that at boot I first get a "3C 301" error, before it starts counting memory. I had a co-processor installed so took that out and set switch 2 to ON again. The error still shows up. What it does too is that the keys of the keyboard seem to "hang", because from DOS boot there's keys appearing and repeating in the screen while I don't touch the keyboard. Pressing a key won't make it stop and they're random keys (so a sequence of 2's and then ZZZZZZ and then bbbbbbb and 00000000 for example). So my guess is either the keyboard is gone or it's the keyboard controller chip that's kaputt on the mainboard :-(

But at least my 186 USD worth CGA adapter isn't LOL ;-)

per
May 13th, 2011, 04:26 AM
The only other problem I have now is that at boot I first get a "3C 301" error, before it starts counting memory. I had a co-processor installed so took that out and set switch 2 to ON again. The error still shows up. What it does too is that the keys of the keyboard seem to "hang", because from DOS boot there's keys appearing and repeating in the screen while I don't touch the keyboard. Pressing a key won't make it stop and they're random keys (so a sequence of 2's and then ZZZZZZ and then bbbbbbb and 00000000 for example). So my guess is either the keyboard is gone or it's the keyboard controller chip that's kaputt on the mainboard :-(

"xx 301" errors indicate that the keyboard has one or more "stuck keys". You're sure that it's a 83-key keyboar, or a PC/XT compatible keyboard? Modern AT keyboards won't work.

Denniske1976
May 13th, 2011, 05:39 AM
Yup, the keyboard is one of those original PC and XT IBM clicky keyboards (83-key with the F1-F10 on the left side). AFAIK it was 100% OK, it's been in storage for a few years though... in a dry area too.

NeXT
May 13th, 2011, 07:43 AM
have you removed the blown cap? I have had blown tants in the past remain shorted after they went boom.

MikeS
May 13th, 2011, 10:46 AM
...the first thing I tried: cutting off the 3-pin cap and see if it would work again because I read somewhere it should without C56. But it didn't :-( I then tried to wire-bridge the 3 holes with a wire soldered in and that also won't work.Say what? bridge the cap with a wire???

Druid6900
May 13th, 2011, 07:14 PM
Say what? bridge the cap with a wire???

Come on, MikeS, every motherboard needs a good dead short to ground of one of the supply voltages. It's traditional, ya know.

modem7
May 13th, 2011, 09:04 PM
Come on, MikeS, every motherboard needs a good dead short to ground of one of the supply voltages. It's traditional, ya know.
Modern motherboards have a LED which when on, indicates that power is applied to the motherboard.

The wire-to-ground technique on vintage motherboards achieves that same functionality. When the wire glows red hot, it means that power is applied to the motherboard :)

MikeS
May 13th, 2011, 09:24 PM
Modern motherboards have a LED which when on, indicates that power is applied to the motherboard.

The wire-to-ground technique on vintage motherboards achieves that same functionality. When the wire glows red hot, it means that power is applied to the motherboard :)Ah, I didn't realize that; thank you, modem7. But I would think that like in Edison's many experiments it would have to be special wire, no?

modem7
May 13th, 2011, 09:44 PM
Ah, I didn't realize that; thank you, modem7.
My pleasure. We learn something every day.
I don't know as much as Chuck(G), but I'm full of pearls of wisdom (and other stuff).


But I would think that like in Edison's many experiments
Edison who?


it would have to be special wire, no?
No. There's special circuitry in the PSU that can sense when the wire is about to blow open (irrespective of wire type) and regulates wire current accordingly so that the wire doesn't blow.

That special circuitry costs good money, which is why computer makers later switched to LEDs - it's cheaper - no special circuitry in PSU required ;)

JohnElliott
May 14th, 2011, 02:45 AM
Modern motherboards have a LED which when on, indicates that power is applied to the motherboard.

The wire-to-ground technique on vintage motherboards achieves that same functionality. When the wire glows red hot, it means that power is applied to the motherboard :)

Heh. I remember powering up a 386, and it failed to boot. So I opened up the case, and there was a flashing blue light on the motherboard. It took me a few seconds to realise that it was not in fact an LED, but an arcing capacitor.

Denniske1976
May 17th, 2011, 05:36 AM
OK, here I am again...

From the looks of it, it seems that wire-bridging a cap isn't really the smart way to go (well, I did mention I was quite a-technical... being able to solder doesn't mean being an electronics expert as well LOL):blush:

Anyhoo, I left the 5155 mainboard in since it's a 64-256KB XT mainboard anyways... installed the co-processor again, closed up the case and hit every key on the keyboard a couple o' timez.

And it's all good now :-) 640KB memory (256KB on-board with 2 added IBM memory cards), 10MB harddisk (ST-412 is 10MB right?), DOS 2.10, working 83-key IBM keyboard and a co-processor :cool:

Thanx for all the help guys, I'm very happy now... and learned not to bridge caps ;-)

Dennis