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Troubleshooting an XT

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dongfeng

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Nov 16, 2003
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After a long wait, I finally received my XT today! Looks really nice, but she's going to need a bit of TLC to get running again...

It's fitted with a whole host of cards, including a "Tiny Turbo 286" board and VGA adaptor. Anyway...

When I turn the computer on... nothing. The power supply whirs into life, but that seems to be it. After I removed the unncessary cards, the hard drive span into action, but still a blank screen.

So... I'm thinking something must be up, and removed all the cards except the 286 card (as it has the processor) and the VGA card. Still the same! Also tried it removing both the drives.

Any ideas? I've also removed the 8088 from the 286 card and put it back where it should be, still no luck :(
 
Last edited:
How long did you wait for it to boot up?

Did you carefully examine the machine to make sure that nothing is being grounded by accident? How about the capacitors - are they all intact? Other physical damage?
 
Thank you for your replies :)

I've had it running for about 5-10 minutes and nothing appears on the display. I've checked the board and nothing seems to be out of place - but I'll post a photo of it in a moment just in case one of you can spot something I've missed :D

The 8088 was on the TinyTurbo board, so I have removed that and put it back in it's correct place. The 286 is missing the cable that connects the 8088 socket to the 286 board... which makes be think the seller was lying to be when he said that it worked when taken out of use 15 years ago . . .

I have a feeling that it might be something wrong with the display - I've tried two different VGA displays with no luck. I also put the VGA card from my Epson AX2e in it (set to 8-bit) and get swirly blue lines on the screen. I'll dig out a suitable computer to check the cards...
 
The mainboard:
xt_mainboard%20(Medium).JPG


Mainboard in very high resolution

Cards 01
Cards 02
Cards 03
 
VGA card tested in another machine and it's fine...

When an XT boots up... how long, and what sounds does it make? any beeps and such?
 
At XT booting up will count memory, so it will be obvious. An older 5150 PC might not count memory, but you will at least get a blinking cursor while it goes through POST (Power On Self Test).

Make sure the CPU is in the correct socket. :) A technical reference manual would be helpful here.

A bare XT (no slots except for video) should boot into BASIC. You might want to ditch the VGA card and use a known workingCGA card (and monitor) just to be safe. VGA should work, but the machine was designed back when Monochome and CGA walkied the Earth.
 
One more note .. it looks like you have a Monochrome/Parallel adapter. So if you have a true monochrome monitor, you can flip a switch on the motherboard and have it use that display adapter. (You can probably find the XT switch settings online .. if not, I can give you a sane set of settings when I get home.)

Which brings up another point .. you need to verify the DIP switch settings on the motherboard.
 
Also, removing the 'extra' 256K memory board may require another change in switch settings:

Code: 
Switch #          Function          Settings

1                    Test                On = loops on POST, Off = normal operation

2                    Coprocessor      On = yes, Off = no

3 & 4              System board RAM   On On = 64K
                                                 Off On = 128K
                                                 On Off = 192K
                                                 Off Off = 256K

5 & 6               Display adapter       On On = no adaptor
                                                  Off On = CGA 40 columns
                                                  On Off = CGA 80 columns
                                                  Off Off = MDA or more than one adapter

7 & 8               Floppy drives           On On = 1
                                                   Off On = 2
                                                   On Off = 3
                                                   Off Off = 4
...or mebbe not.

--T
 
Unfortunately I only have VGA monitors available to me, no monochrome or CGA :( But the VGA card came in the XT and apparently was working fine when it was last in use...

Tried the switch settings, but no luck.

Even if there was no monitor attached, it would still try and access the floppy when it boots up, yes? My XT doesn't even try to do that. I have a nasty feeling it's a dud board...

The hard drive is very noisy when it spins, is that normal?
 
yup... tried booting without it, and even replacing it with a different 8-bit VGA card...

I know they both work... I've tested them in another machine...
 
dongfeng said:
The hard drive is very noisy when it spins, is that normal?
Click to expand...
I don't think it affects how the motherboard works, unless there is a problem with the power supply such as it delivers bad voltages that would cause the hard disk to make noises and has fried the motherboard. :confused:
 
Ahh it doesn't sound good. I'll try plugging the hard drive to another power supply to see what it sounds like.

Another thing I have noticed is that the floppy drive was connected to the same card as the hard drive, and not the floppy controller card... correct?

It seems the previous owner really liked to modify their XT!

Any way to test the floppy drive in another (more modern) computer? And the HDD?

Any way to test the power supply for correct voltages? Theres a few clone XT boards on eBay which may do the trick, providing the PSU is ok...

Thanks for your help so far :)
 
Try the most basic configuration: Motherboard with just the power supply and the speaker connected (no cards at all). With the motherboard switches set for Monochrome (5=off, 6=off), you should hear beeps as soon as the power supply is turned on.

If no beeps, then consider:

1. Dead motherboard.

2. Are the two connectors from the power supply in their correct sockets (ie. P8 into socket closest to keyboard connector). I can't remember what the symptoms are if the plugs are reversed (and I'm not willing to try this on my XT).

3. Bad PSU.

For a quick check of the PSU (assuming you don't have a POST card):
On power up, when the XT PSU believes the voltages to be correct/stable, it puts 5 volts onto the 'power good' line (and that takes the motherboard out of its reset state). Measure that line with a multimeter. It's the orange wire. It will take less than a second for the line to switch from 0 volts to 5 volts.
 
Thank you, I have tried that but still there is no beeping from the speaker :(

In testing the PSU, it does not appear to have an orange wire, and there are several red ones. Which ones should I test?

xt_psuwires.JPG
 
The standard colour coding is shown at http://www.epanorama.net/documents/pc/pcpsu_connector.html

The 'power good' line in your case is the white wire.

If your 'power good' line isn't reading 5 volts, the power supply might be good but is being loaded down by a short (or partial short / heavy load, etc.) on the motherboard. To test for that condition:
1. Disconnect the motherboard from the PSU
2. Disconnect all other devices from the PSU except for one (i suggest the floppy drive). You need to leave one device connected because the XT PSU won't power up unless at has a load of some sort.
3. Turn on the PSU and see if the 'power good' line is now 5 volts.
 
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