Half-Saint wrote:

So best way is to just check tantalums for connectivity?

That's what I would do. I've repaired a few XT's by replacing (if needed) the caps around the power and keyboard connectors.

Okay, so I checked these caps for "anormal" resistance values.. I guess 0 and 1000000 is bad ?
If so, the good ones have between 395 (Ohms) and ~600 (Ohms) !? 😀

Edit: I forgot to mentions.. These are the values of the blue Cs I tested - In ugly machine readable form, of course.
(Oh, I miss the 50s/60s/70s era components with meaningful description!)

104
502
201
101
47
331
471

Since we're at it, what kind of "good" Cs should I use as replacement for the "bite the dust" tantalums ? Styroflex, Glimmer etc ?

Update. I did a full re-cap and replaced the aging caps with ceramic and electrolytic types.

Unfortunately, no change. No VGA signal, no keyboard reaction.. 🙁

Do I see right on your photos that there is no display at all on the POST-card? It should at least display "--".

You still have some problems with the power-supply. Are you sure the POST-card is OK and its not causing the shortage?

Did you already test the voltages between a black wire (GND) on the power connector and the corresponding Pins on the ISA-bus? Maybe with and without the POST-card installed?

If your PSU (while switched on and connected to the motherboard ) shows all values are good when measured with a multimeter, then try the minimum diagnostic check.
However, if you get no -12V reading, then you have a short on your -12V rail and you need to address that first.

If you have an MDA or CGA card and access to an EEPROM burner, then you can try the Supersoft/Landmark diagnostic ROM.

Refer here for more info (use the 5160 section and at the ROM section): http://www.minuszerodegrees.net

Thanks for the replies and tips!
The behavior of the mainboard is the same as posted earlier.

I checked with another PSU, too.
(Even replaced all the RAM caps)

Except for +12v (?), all other voltages are missing on the POST card.
I'm not 100% sure what this means, however. If checking the voltages on ISA, it seems they are there.

However, I'm afraid that doesn't necessarily have to mean anything,
because it doesn't say anything how the voltages behave under load (or how much current flows;
that's where the LEDs on the POST card would come in handy under normal circumstances). 🙁

PS: I've attached a current photo of the motherboard..

Are you sure the POST-card is OK...?

Supersoft-ROM is a good idea, but without fixing the power problems it doesnt make sense to start with this...

Predator99 wrote:

Are you sure the POST-card is OK...?

It seems to be. Tested in a Pentium 133.. I'll do soon in a 386, too.

Predator99 wrote:

Supersoft-ROM is a good idea, but without fixing the power problems it doesnt make sense to start with this...

Indeed. Programmed the EPROM, but no reaction.

Have you tried to turn on the computer withou the POST card after the full recap just in case ? I know that some of my boards don't play well with POST cards for some reasons. (they can output codes, but they won't POST with the card inserted. Weird isn't it ?)

As far as I know, those cheap Chinese test cards are meant for AT computers (286->) only and don't work with XT at all.

I also tried without any expansions cards and with a Hercules clone (should work as MDA in Supersoft), but neither the PC speaker, nor the keyboard show any signs of life. 🙁

sf78 wrote:

As far as I know, those cheap Chinese test cards are meant for AT computers (286->) only and don't work with XT at all.

Yes, I suppose that's true. The POST codes on PC/XT were completely different (as was the keyboard protocol).
I also have got a vintage POST card made out of discrete components (TTL chips).. - I'll use that, too, if I find it.

Btw, does the XT platform also support POST codes vis Parallel Port ?
Some PC/AT style computers, such as laptops, did support this.

If so, I could try to make and attach such a DIY POST card dongle to the Parallel Port of a Hercules clone.
The port address would be 3BC there, which is supported by even the oldest software.

Jo22 wrote:

I also tried without any expansions cards and with a Hercules clone (should work as MDA in Supersoft), but neither the PC speake […]
Show full quote

I also tried without any expansions cards and with a Hercules clone (should work as MDA in Supersoft), but neither the PC speaker, nor the keyboard show any signs of life. 🙁
[...]
Btw, does the XT platform also support POST codes vis Parallel Port ?
Some PC/AT style computers, such as laptops, did support this.

If so, I could try to make and attach such a DIY POST card dongle to the Parallel Port of a Hercules clone.
The port address would be 3BC there, which is supported by even the oldest software.

I rather think they will use I/O 378, as indicated by the POST code disassembling seen here.

Such a parallel port watcher would be a real good thing! I guess I'll make one too, while waiting for the ISA/PCI post card to arrive from China.

Aw, still no sign of life (with the diagnostic rom in place). 🙁

I'm slowly beginning to understand why people got rid of their XTs so quickly.

retardware wrote:

I rather think they will use I/O 378, as indicated by the POST code disassembling seen here.

Such a parallel port watcher would be a real good thing! I guess I'll make one too, while waiting for the ISA/PCI post card to arrive from China.

Thanks for the link!

Look in this thread: XT clone motherboard repair advice/help needed (KT 10 M/B)

We resurrected my very broken XT motherboard (which sat in a damp garage for years and it was probably broken to start with) and you will find some information there on what to check. Your board is very similar. My motherboard was showing very erratic behaviour - one time it was looking like it's totally dead, but sometimes it would try to communicate something over pc speaker. I'm pretty sure you can repeat the steps described by Deunan in this thread and it will shed some lite on your motherboard.

Personally I would start with removing all RAM and putting good (checked) memory instead - as little as possible (256kB?). One broken dram chip in the bank 0 and motherboard will appear dead.

Also Ruud's diagnostic rom or ever better Landmark diagnostic rom is a must but you already know that.
Look for ICs (TTL chips especially) getting warmer than rest of them. Pull the EPROM and check if it's working fine in some programmer (TL866-II is perfectly suited for our hobby and I recommend it to every retro computer fan).

Early XT motherboard (chipset-less ones) are super easy and fun to work on when you know what you are doing - very similar to 8-bit computers.