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Compaq SLT/286 Not POSTing

CamiTheWitch

CamiTheWitch

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Joined
Jul 11, 2022
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17
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Denton, TX
Hi! A while back I picked up a full set of parts (with some spares) for a Compaq SLT/286, knowing I'd be in for a bit of a puzzle. I finally started working on it a few days ago. The original owner had removed the DS1287 from both motherboards I picked up, so I tested both with the DC-DC boards, and found neither board POSTed, and only one DC-DC board "works", meaning it'll illuminate the green power light but doesn't do anything else. I replaced the DS1287 in the board that was in better condition, but it didn't change anything. I suspect the DC-DC converter is the issue, but if anyone has any suggestions on what to check, I'd appreciate it! Thanks!
 
Poking around some, I see the board is only getting 3.5V on what I'd expect to be the 5V line, I'm presuming that settles it as the DC-DC converter?

edit: Pulling the DC-DC converter and testing it in and out of circuit shows 3.5V on pin 20 and 8.6 on pin 6 when connected to the motherboard. Outside of the board it's 5V and 12V respectively, is it too low on amperage or is it actually a motherboard issue? Given neither board worked I'd be a bit shocked if it wasn't the DC-DC converter, but I'm not sure now.
 
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The SLT is one of my favorite vintage laptops of all time and I have owned many (probably almost two-dozen or so at this point, I've lost count!). I had this same issue fairly recently.

Recap the DC-DC converter before doing any further troubleshooting! I had an SLT last year that gave a "power light but nothing else" and when I swapped in a spare DC-DC converter it worked/POSTed fine. Closer inspection of the original converter revealed a nasty fishy smell from the caps and once I replaced them, the original DC-DC converter was working fine once again.

But, if the original owner removed the DS1287s check the traces around it carefully. I had a person reach out to me awhile back that had an SLT/286 that wasn't posting, and it turns out the previous owner ruined many traces when removing the Dallas chip, so definitely inspect your boards.

The motherboards in these laptops are extremely reliable, so it's usually bad caps in the DC-DC converter or damage from a previous owner that causes them not to work.

Hope this helps!
 
That helped immensely, thank you! So, I replaced the three large capacitors (I misread the two smaller ones as 1000uF and this ordered the wrong part), and all the voltage rails are okay now, I get 12.08, 4.96, and around -12.8 (this is probably a bit high but I’m not positive about that). The screen shows a bit of activity, but otherwise there’s still no response from the machine. I checked the DS1287 pins before the repair, and they all seemed to be functioning, with the exception of the pins that didn’t connect to anything in the first place.

I do have two 100uF 25V capacitors but they’re both Sprague’s like the originals, and are actually older by about two years. Should I swap them in and try it or is there something I should check first? Thanks so much for your help so far!

EDIT: I presumed the motherboard had RAM onboard, I didn’t have the “Triumph Piggy Memory Board” installed, plugging it in made it POST successfully, with a 162 error, which I know how to fix :) Thanks so much again for your help!
 
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Okay so I’m not done yet it seems. I have a 602 error, which is Diskette Boot Record Error. I have two drives. One is completely seized, the other appears to work perfectly fine mechanically, but gives the boot error. I tried swapping the floppy head and motor controllers, I’ve tried writing several disks, and I’ve tried two different floppy cables, all either not fixing anything or making it worse in the case of the seized drive. I see at least some people have had this issue before, but it seems they either gave up or they fixed it by loading the setup disk, which I can’t do because it isn’t loading correctly. Any suggestions?
 
Glad you got it POST-ing! I figured the board was probably fine.

For the floppy drive, I haven't had that issue before actually. Your "good" drive may have misaligned heads potentially from the sound of it, that would be my first guess, because the "diskette boot record error" seems to suggest the drive is at least attempting to read it. If it was completely failing you would get a "non system disk or disk error."

I assume you are making sure to write the diag disk to a true 720k floppy? You can't write it to a 1.44MB disk or it won't work properly.
 
They’re definitely 720K disks. When attempting to read the head will move to the middle of the disk and give up
almost immediately. It tests the head motor fine, so I’m guessing you’re probably right and it’s the heads being misaligned. Either that or my desktop isn’t writing them correctly. I’ll see if I can convince one of my newer laptops to write the disk either before I go to sleep or sometime tomorrow just to confirm if it’s my desktop or the Compaq. I’ll add that I tried to boot from a 1.44M disk as further troubleshooting and it does respond with “non-system disk”, so I’m hoping it’s not the heads. If it is, is there a way I can fix it or do I need to look for a new drive?
 
CamiTheWitch said:
They’re definitely 720K disks. When attempting to read the head will move to the middle of the disk and give up
almost immediately. It tests the head motor fine, so I’m guessing you’re probably right and it’s the heads being misaligned. Either that or my desktop isn’t writing them correctly. I’ll see if I can convince one of my newer laptops to write the disk either before I go to sleep or sometime tomorrow just to confirm if it’s my desktop or the Compaq. I’ll add that I tried to boot from a 1.44M disk as further troubleshooting and it does respond with “non-system disk”, so I’m hoping it’s not the heads. If it is, is there a way I can fix it or do I need to look for a new drive?
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Ok, that's good to know.

Yes, it could be an issue with how the disk is being written too, so trying on a different machine can't hurt. When these machines haven't been run through setup after replacing the Dallas battery, they are really flaky about reading 1.44MB disks, because by default the drive is actually configured as a 360KB 5.25. Once you get it to the point of booting the setup disk, it will configure itself properly as a 1.44, but until then I wouldn't be too alarmed that a 1.44MB disk won't read.

It's very unlikely the heads themselves are actually bad, that rarely happens. Misalignment is far more likely. It's not hard to realign heads with ImageDisk (IMD), but since that's a DOS program you can't really run that until you get the machine booted, so that will be tough without a donor drive or laptop.
 
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