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Compaq Deskpro 8086 motherboard troubleshooting

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    Compaq Deskpro 8086 motherboard troubleshooting

    After having no luck with the two motherboards I had, I bought a third Compaq Deskpro 8086 motherboard from TexElec. It is new-old stock, never used.

    At first it didn't show anything, but once I tried it with switches 5/6 off and an IBM MDA card, it showed something on a screen! It was some memory checking error.

    The board has bank 0/1 on sockets and 2/3 soldered in. Board came with 200ns DRAM. And as I understand, Deskpro can run at 8MHz. How is this possible? Does it insert a wait state?

    Anyway, I think one or more of the 6665 chips in sockets had oxidized contact, because after I swapped all of them for generic Chinese 41256 chips, the board stopped giving memory check error. Now it gives me 301/401/601, which is great to be honest.

    I even changed switches 5/6 to on/on and got it to show stuff using VGA card.

    So now I wonder if I should change soldered DRAM chips in bank 2/3 with socketed 4164 150ns chips. What do you folks think?

    Please answer if you can:

    1) Is it ok to use 200ns memory in Compaq Deskpro 8086? It runs at 8MHz. It came with 200ns chips, but never worked stable for me.

    2) System is unstable: it usually starts ok if it was powered off for a while, but gives continuous beep or nothing at all if you turn it off and then on. Any advise?

    3) With 6665 chips (same as 4164 but with 7-bit refresh), it starts, but always shows memory error, even though each chip was tested in DRAM tester and is good. Error is 2104 201.

    4) Is there something like Lexmark/Supersoft diagnostic ROM that would fit into 8KB ROM to work on Deskpro 8086?

    5) What does it mean when front led is green or red?

    6) Is there anyone in Greater Toronto Area with experience fixing XT class PCs and is willing to look at it? Please reach out, we can discuss compensation.
    Last edited by dhau; March 15, 2021, 08:16 AM.


      5 - is the cpu speed 4.77 or 8, I think.

      Last edited by jafir; March 15, 2021, 09:29 AM.


        Oh wow, you have a user manual! Please add it to your pile of things to scan Unless it's already scanned. I didn't find anything anywhere.


          Originally posted by jafir View Post
          5 - is the cpu speed 4.77 or 8, I think.

          You are correct.


            POST error 201 is usually a memory error. 2104 should be the address in hex.

            Do you turn the system off then turn it on right away? Or do you wait a 10 count? Some older machines don't react well to a quick off/on cycle.

            Can't speak for the Deskpro offhand, but one continuous beep is frequently no memory detected.

            I bought a NOS Deskpro motherboard from them as well, but have been too tired to install it. Still sealed. Let me check ... Yep, mine has 150ns memory. That's more normal for up to 8Mz XT class systems. I remember using 120ns on the fast 286 I had many years ago.

            Have you considered buying one of those diagnostic cards with an LED code readout? Some of them have one edge for PCI and one for ISA.


              Thanks Casey. I'll probably swap all memory from 200ns to 150ns or faster. It is confusing why it came with 200ns soldered in. I'll also recap all those 10uF 16V capacitors just in case. The ones that blew up were really spectacular. The soot was kind of nasty.

              I didn't know about 10 seconds rule, frist time I had problems like this. I'll check if 10 seconds is enough.

              I also think that the problem could be related to me using a power supply from Deskpro 386. It is rated at 5A @ 120V, and original was rated at 3A @ 120V. So I guess it has bigger capacitors or something. I'll be happy if I manage to fix the original PSU, but it just blows the fuses right now.

              The diagnostic card probably wouldn't help much, I think it only works on AT clones and up.

              What BIOS chip number did you get?



                Hmmm. I wonder if that was my problem the first time around. I picked up a Deskpro cheap because the power supply was bad. I figured no problem, just swap out the p/s from my spare Deskpro 286. The first time I turned that on, a capacitor blew. Someone here said it wasn't an important one, just clip it out and go from there. Did that, and all it does now is ... nothing. The LEDs on the enhanced keyboard blink on & off, but that's it.

                Given my recent energy levels, I shelved it until I saw the Tex Elec boards for sale. Like I said, I haven't tried plugging it in. Now I'm afraid to.



                  recently I bought Compaq Deskpro 8086 board, but all socketed chips are missing, also and the BIOS chip. I want to make it work.

                  I am very thankful to dhau for the ROM chip.

                  But need to know what is the ROM IC, that I need to find for there,

                  also is there something else needed for this board, like the missing socketed RAM chips and what they should be?



                    For some reasons I can't get the larger versions of your images. The image hoster isn't loading up for me. Ping me via direct message with your email, and I'll send you high resolution photos of my boards.


                      Originally posted by dhau View Post
                      For some reasons I can't get the larger versions of your images. The image hoster isn't loading up for me. Ping me via direct message with your email, and I'll send you high resolution photos of my boards.
                      try "open image in new tab" or mid-button click...


                        Originally posted by Milen View Post

                        try "open image in new tab" or mid-button click...
                        Nope. Not any larger that way either.


                          Hey mate, I think you're in good shape. The big missing chip on the left from the CPU is probably BIOS (27c64) and the one above is FPU, you probably don't want it anyway. You have some DRAM sockets there, use 41256 chips to populate for 640KB.

                          Your board looks a lot newer then what I got: