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Compaq SLT 386s/20 power supply pinout

mikerofone

mikerofone

Experienced Member
Joined
Aug 9, 2020
Messages
69
Location
Switzerland
Hi all,

I recently picked up a Compaq SLT 386s/20 at a local flea market. It is missing the power supply, and it has a very nonstandard plug of which I cannot find the pinout.

Q: Does anyone have the pinout, or have its power supply and could measure the voltages?

Long winded post with details :)

This is said connector:
IMG_20210609_224923.jpg

I'm numbering the pins as follows for reference:
Code: 
   3  4
1
   5  6
2
   7  8

There also is an internal battery, which has four pins:
IMG_20210609_225126s.jpg
And I'm numbering these as
Code: 
1 2 3 4

Some pictures here: https://photos.app.goo.gl/QVJqG6U6zHF8YZfn8

I'll use P1-P8 for the PSU connector pins, and B1-B4 for the battery pins. Using my multimeter, I was able to determine:

* P1, P2, P5, P7 and P8 are ground pins, as is B4.
* P4 and B3 are directly connected (multimeter beeps).
* P3 isn't directly connected to any battery pin.
* P6 isn't directly connected to any battery pin.

There is an r/vintagecomputing thread that has some pictures of the PSU label, which just reads "18.3V DC, 3A, 33W". The service manual says "Input Voltage
10 - 18.2 VDC" so I guess it's not picky, but there are still too many pins for me to just go randomly poke apply power to them and see what works.

Well, actually, after also seeing in the manual that the battery is rated at 12V, I tried applying 12V+ to B1 and GND to B4. The laptop actually turned on for about 5 seconds and the screen illuminated, but then it shut off and wouldn't boot again. So there is of course a chance that I already caused some damage, though no pops or magic smoke where noticed.

Once I'm done with my current project, I might take it apart and hopefully learn more about the individual pins, but I'm hoping someone might already know the answers. :)

Cheers
mikerofone
 
Hi,

I have revisited this computer and, after realizing that the SLT 286 and 386 use the same PSU, was able to find some more information. Most importantly, it made me discover https://www.vcfed.org/forum/schenkzoola/1070484-compaq-slt-386-or-slt-286-power-supply-pinout (thanks, schenkzoola!) which confirms that my attempt of powering via the battery terminals, or via (what I denoted as) pins 3 and 7 in the power plug, was valid. https://www.vcfed.org/forum/forum/genres/pcs-and-clones/58214-compaq-slt-286-ram?p=706300#post706300 suggests that my machine still not working might be due to bad caps in the DC-to-DC converter board. Everything looked fine, but it's possible the looks are deceiving. ;) So I'll try that next, whenever that will be.

Cheers
mikerofone
 
I think I'm able to give a definitive answer to this, not only based on reverse engineering - but based on an special cable they used to make.

I managed to put my hands on this "unobtanium"-class cable which is a 12V DC power cable for Compaq SLTs, to connect them in a car.

chrome_stnhRsbjB7.jpg


There is little doubt this cable only takes 12V DC as input, and was able to power an SLT without battery (otherwise; the whole cable makes no sense to me )

Interestingly, and following the above pinout, only pins 3 & 7 are populated on my cable.

chrome_qvc7a7xkL6.png

This is as simple as it gets ; you can power an SLT with just 12V DC on pins 3,7 (the two up & down - closest to the back of the case), and let the rest in the air.

ApplicationFrameHost_Pfq3c80TW2.jpgchrome_BwUdbTt7re.png

I tried it for you, it perfectly works.
With a compact flash, my 386SLT draws around 1 Amp at 12V DC , up to 1,5 if the floppy seeks (boot time).

Any decent "real 2 amps" 12V DC power brick should do the trick.

chrome_US6yWB0uBy.jpg

It is unclear to me what the other pins do, they must be related to the battery charging.

If you don't care about the battery, I would recommend everyone creates their own "modern" power supply for your SLT, by cutting the original cable and wire it to a 12V brick on pins 3 & 7. One could argue you're not "restoring" here, but the stock power pack is very heavy, very hot, probably requires a recap, and even when recapped it has internal complexity that is useless if you don't charge your battery - and generally speaking I restore PC so that they work for the coming 20+ years.
My last argument for the "cut cable and get rid of the original PS" being that you can also very easily plug an external 12V battery on your home made 12V cable if you put a 5,5mm jack adapter on it (which I did). This makes your laptop portable again, without NiCDs. Anyway just my 2 cents, I leave it up to you.


The best is of course to find a second cable so that you can both keep your original cable for "preservation" AND have a modern 12V powersupply for actual use.
 
Last edited:
Hi CedsRepairs,

thanks for sharing this information! Great to have the pinout confirmed. I haven't yet made any progress on my machine, which I still believe to suffer from a failed DC-to-DC converter, even though all the caps I managed to check look and measure fine. I managed to fry my bench PSU while poking around^1 so for now, my experiments with my machine are on hold.

Cheers
mikerofone


^1 I accidentally crossed the leads, which blew the breaker and PSU fuse, and new fuses keep blowing. I thought it had some sort of overload protection, but apparently that wasn't cut out for the job.
 
Well greetings, I’m new here and just purchased a decent SLT/286 and upon first 15 min of usage, PSU started smoking! Yucky and of course wifey is pretty mad for stinking up the place. Oops
 
Greetings everyone,

Definitely late to this party, but I'm revisiting my Compaq SLT 286 that was gifted to me by a mentor in high school. I started poking around years ago but ran out of time to work on it then. I love this thing's look and want to get it running. I'm not quite sure where to begin, I've gotten as far as attempting to plug in the AC adapter and turn it on. As expected, no signs of life on the actual system. The AC adapter still lights up with the green LED, going to start poking around with my multimeter.

I'm very new at this, so any help from those who have navigated this before me would be appreciated. My unit looks to be in decent condition, the removable keyboard's cable is disintegrating a bit and as expected the case has yellowed considerably.
 
Went ahead and removed the battery after I verified the AC adapter was providing the correct values on each pin. I got a green light on the power indicator on the case after the initial flash of the keyboard lights. No output on the built-in screen even when adjusting the contrast and brightness. Thinking I'll hook it up over VGA just to check for output.
 
ABowlOfDesolateWaste said:
Went ahead and removed the battery after I verified the AC adapter was providing the correct values on each pin. I got a green light on the power indicator on the case after the initial flash of the keyboard lights. No output on the built-in screen even when adjusting the contrast and brightness. Thinking I'll hook it up over VGA just to check for output.
Click to expand...
No such luck on the VGA output either it seems, Following the original manual flowcharts seems to either point to a display issue(which seems less likely) or "replacing the system board" Seems like it's time to take things apart and check for evidence of magic smoke. Thanks for the help so far!
 
ABowlOfDesolateWaste said:
No such luck on the VGA output either it seems, Following the original manual flowcharts seems to either point to a display issue(which seems less likely) or "replacing the system board" Seems like it's time to take things apart and check for evidence of magic smoke. Thanks for the help so far!
Click to expand...
Update:
No magic smoke found yet, looks fairly pristine. Found a tiny bit of yellow-orange goo near the HDD, but nowhere else. Noticed that J1 was set to pins 1-2 vs 2-3 even though this seems to have the Intel N80C286-12. snapped a pic for reference.

20230315_214456.jpg
 
ABowlOfDesolateWaste said:
No such luck on the VGA output either it seems, Following the original manual flowcharts seems to either point to a display issue(which seems less likely) or "replacing the system board" Seems like it's time to take things apart and check for evidence of magic smoke. Thanks for the help so far!
Click to expand...
I just picked up another one on ebay for $50. I thought the screen didn't work at first but I think I was sliding too quickly or too soon for the old system to get to the DOS screen. Did you try it in a dimly lit room? The screens are not great in bright lights, takes a little while for my eyes to see it well, besides the sliders, well at least the new one, can easily slide past the sweet spot. My new one's display isn't as crisp as "CedsRepairs" display. Although I was outside when testing it.
 
Yes you have to switch the VGA output on, and as a side note ; the key doing that varies in your have an US or "international" unit.

On my european one I don't remember the keys right now, but I remember I had to specifically look up the doc for this particular unit to find out the keys switching VGA on.
MOst references on the internet are about the US units and this key combination doesn't work the the other half of the world - just a FYI.
 
I recently purchased one (slt386s/20) for $80.
It lacks a power supply connection cable and has a horrible LCD.
This weekend, I'll try to boot it.
微信图片_20230616095759.jpg
 
That's a great price for a 386 model.
Be sure to remove the battery when you plug in the power. The batteries are usually bad that the systems won't even boot off of the power supply if the battery is in place. I wished someone knew how to crack the battery case open so I could replace my NiCad cells (I believe). I tried but the lid would not budge off the battery case.
 
CloudInMyHead said:
That's a great price for a 386 model.
Be sure to remove the battery when you plug in the power. The batteries are usually bad that the systems won't even boot off of the power supply if the battery is in place. I wished someone knew how to crack the battery case open so I could replace my NiCad cells (I believe). I tried but the lid would not budge off the battery case.
Click to expand...
I just damaged the LCD :(
The internal cable of the LCD detached from the glass :(
Do you know how to fix it or can I replace it with a comparable one?
0230616180827.jpg
16181156.jpg
 
I would look for a 286 on sale without the power supply as those are pretty inexpensive. You can make the power cable.
This set in the link below has the end that plugs into the computer. You'll need to get a multistrand wire for it, but the plug will fit!
Dual Row Wire Housing
 
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