Compaq Presario CDS 520

In this post I will write some hopefully useful observations from working with the Compaq Presario CDS 520, with the goal of turning it into a MS DOS gaming machine, similar to my Presario 433 I owned as a child. According to the wisdom of the Internet, Compaq stands for “Compability” and “Quality”. Compability refers to IBM PC compability.

(This post is updated on a regular basis).

Compaq Presario CDS 520 specifications

  • Year: 1994
  • CPU: Intel 486 SX2 66 MHz (upgraded to an Intel DX2 66MHz)
  • 8 mb ram (upgraded to = 20 mb (max = 64), through 2x8mb + 4mb on board)
  • Cirrus Logic 512 kb graphics adapter
  • ESS 688 onboard sound (upgraded to a Soundblaster 16, see below)
  • ~500 mb hdd (upgraded to a 2gb compact flash card, see below)
  • 3.5¨ 1.44 mb floppy
  • CD-rom
  • Internal modem
  • MS DOS 6 + Windows 3.1 default (now: MS DOS 6.22)

General remarks

  • BIOS: As explained by, these machines does not come with a vast range of configuration options in their BIOS. They autodetect your disks, and can boot from floppy and hdd. Larger drives than 540 mb can be used, although the limit is unclear (2.5 gb confirmed ok). To access BIOS, press F10 when the white rectangle is flashing to the right top of the screen during boot-up. If this fails, you do not have a setup partition on your computer. Then, you need to boot up from a floppy disk to access the bios. To prepare a floppy you need to download the SP1363.exe SoftPaq (see above, I will also provide links here shortly) and make a disk out of it. If you are making a clean install of DOS, you can also use this disk to make a setup partition on the hdd.
  • Integrated builds. One limitation with these all-in-one machines is of course that if something fails, the whole machine might be rendered useless. They also have limited room for additional cards and other upgrades. That is essentially the cost of the compact form factor. My Compaq Presario 433 did not have an integrated CD-rom for example. Due to the slow LPT-port, the performance of parallel port CD-roms were limited, so I used long IDE- and Y-power cables and simply positioned the CD-rom outside of the machine. The advantage with the CDS520 is that it comes with an integrated CD, which is really beneficial when installing games etc. I still keep a lot of CDr’s at home for my Dreamcast, and other consoles.

Installing a Soundblaster 16

My biggest problem with this machine is the ESS688 onboard sound. Although it is soundblaster compatible in games, the sound volume tends to be to loud, and the front volume control does not do a great job adjusting it (neither did the software mixer). So, what I wanted to do first, was to install a faithful soundblaster 16 ISA card while still being able to utilize the internal speakers, and being able to accurately control the volume.

I found a soundblaster 16 CT2960 card on Austrian eBay for a good price. Although reading negative remarks about the VIBRA16-series I am perfectly fine with its performance. To use the internal speakers with this card, I connected them to the 3.5 mm out jack of the soundblaster, with a volume control on the audio cable coming out from the back of the computer. It works very well! Only problem is that the right speaker also served as the internal PC speaker. I solved this by simply using a speaker from a broken Nintendo Gameboy as PC speaker. This speaker had a most pleasant tone to it than the original. So, now I can enjoy games at a pleasant and adjustable volume, with full soundblaster compability. The installation is fully reversable and no harm was done to the machine.

Compaq Presario CDS 520 Sound

Compaq Presario Sound

The VIBRA16 driver installation was done by using the Sound Blaster 16 Value PnP (Vibra16) CD from Since the card is Plug n Play, it installs a TSR program, followed by additional configurations, such as the set blaster environment. Although these take up some memory it was not that bad (616kb of conventional memory left at the moment), they can probably be optimized later (I removed the content the installation added to config.sys).

Update: PC speaker and harddrive

I experienced two problems with the Presario lately:

  • The harddrive started to act up (dying).
  • The PC speaker went almost silent

Now, the harddrive was the easy part: I simply replaced it with a Compact Flash to IDE adapter, which enables the use of CF-cards as solid state hard drives. I was expecting to have to perform this upgrade sooner or later. I initiated the card and installed MS DOS 6.22 + added a bunch of games and Compaq files on it via the instructions on this site. I can really recommend having a virtual machine or two using VirtualBox or similar software. Very handy! This CF card had a boot delay of about 1 minute for some reason, but I switching to another CF card fixed that problem.

Compact Flash to IDE adapter

The PC speaker was more tricky. I tried to look for some sort of mixer or BIOS setting, but I have not heard about any such way of controlling the sound. I also tried with different speakers, without any improvement: the sound was just barely noticeable. Unacceptable on a DOS gaming machine! I figured the system board on the Compaq failed to deliver enough voltage to the speaker. So I went digging inside the chassi… There is a board that contains IDE, floppy and speaker connectors, which is firmly seated with four screws in the heart of the integrated build of the Presario. Perhaps replacing a few caps could help? After removing the board, I replaced three of them. Without any improvement. Speaker still silent 🙁

Inside Compaq Presario CDS 520

Compaq Presario CDS 520 system board

Compaq Presario CDS 520 system board that connects to the mainboard.

Compaq Presario CDS 520 work in progress

Finally, I found what was wrong with the sound: the TDA7053 amplifier chip! I order a new chip from Italian eBay, but it would not work since it was a TDA7053A (with volume control). However, I ordered a regular TDA7053 chip from China and that fixed the problem!


CPU upgrade

The 486 SX/SX2 series cpu is rather slow compared to later models, such as the 486 DX2 66 MHz (which seems to be a bit of a flagship for retrogaming, based on eBay prices). The original cpu in the Presario CDS 520 lacks a coprocessor for example. First I upgraded the CPU to an Intel Overdrive 486 DX4 75 MHz I found cheap on Swedish eBay, but later decided to go for a DX2 66 MHz with the 33 MHz bus. I also benchmarked both these CPU:s as shown in image below. The DX4’s performance in Quake is slightly higher, but it shows some odd readings in Norton System Information.

486 DX2 and DX4 benchmark

CMOS battery replacement

The CMOS battery ended up dying on me. It is just a button cell battery so it won’t leak, but it is quite firmly attached to the motherboard. To replace it, I snipped and desoldered the connectors, and added a battery holder as seen below. Next time I do some work on the computer I will probably replace it with a horizontal holder, but this fix will do for now. I also replaced two capacitors on the mainboard (100uF, 25V) just because.

Compaq CDS520 mainboard

Compaq CDS520 cmos battery2


Images: Compaq Presario CDS 520 motherboard and modem

Compaq Presario CDS 520 motherboard

Compaq Presario CDS 520 modem

Software for the Compaq Presario CDS 520

Drivers and tools at Vogons

Tabworks at

Restore CD for CDS524 at (Interestingly, they used the image from my site for to illustrate this download). One of the comment reads: “Looks like this same set can be used to restore a CDS 520 and a CDS 510 as well. If the restore disk asks for a system serial number, enter one of the following:”
– 6801HQS2G786
– B548HQS21848
– 6436HLM30596


  1. Joao Cunha

    Lovely post! This was my first computer back in the 90’s (1994, I believe). And I was able to buy one in mint condition last year for 200$.

    I already did some upgrades:

    – Intel DX40DPR100 Overdrive Processor
    – 2x32mb 60ns FPM (gives a total of 68mb ram but the system caps it at 64mb)
    – Atrend ATC-6631 (YMF715B-S) Yamaha OPL ISA sound card

    Some notes:
    – The DX4 overdrive gave some performance improvement but nothing spectacular, I just ordered an Evergreen AMD 5X86 133 MHz and hopefuly this one will bring a noticeable performance kick.
    – Regarding the sound card, I didn’t face your problem regarding the sound volume being too loud, I remember tackling that issue with the ESS sound volume config utility. And don’t forget… the onboard ESS688F actually has an authentic YAMAHA YMF262 (OPL3) chip, which is great for retro gaming, it justs sounds awesome. I only got the Atrend card (which also has real OPL3) just to compare if there is less noise and interference compared to the onboard card.

    BTW, have you considered extending your speakers hack to also use the volume up/down/mute buttons on the chassi?

    I will stay tuned on your next posts. Let me know if you need some help!

  2. Dreamcast (Post author)

    Thank you, Joao! Yes, optimally I would have used the volume buttons on the chassi for the perfect integration of the SB16 card, but that would require a bit more disassembly than the current modification: another day perhaps 🙂

    Regarding the overdrive cpu:s, back in the days they were described as mixed bags due to other limitations / bottlenecks in the systems, although I never tried one myself. How do the DX4 fare for games that would normally run rather slow on the CDS 520, such as Duke Nukem 3D and similar 3D-games?

    1. João Cunha

      Regarding the DX4 there was a small improvement on Duke Nukem 3d, I would say 3/4 fps.
      I just installed the evergreen 5×86 and now Duke Nukem runs awesome! Even Quake became sort of playable 🙂

      One awesome thing that I notice on the compaq motherboard, is the missing pins for the jumper that would allow switching from write-through cache to write-back.

      See this picture:

      This would allow us to use some 486 DX2/4 variants and also the evergreen 5×86 that support internal write-back cache. I would imagine we could squizz an additional 10% performance improvement. I still have to test it whenever I get a soldering kit.

  3. Dreamcast (Post author)

    Ah, well I’ll see what upgrades I come across. Checking Swedish eBay on a regular basis. Old cpu:s seem to be high in demand nowadays 🙂 Interesting hidden gem on the motherboard! I wonder if they planned on supporting it later or why they did not put the jumper there to begin with..

    1. João Cunha

      It’s an interesting dilema… for sure those pins connect to something on the PCB, the traces are not visible on top of the motherboard, but whenever I can, I’ll check the back of it, I’m hopping they connect the cpu to the VLSI chipset. I did some research on the chipset and I saw some references to the write-back cache support for l1 and l2 cache ( missing on the CDS 5xx), so there is hope. But even if the chipset supports it, the WB cache may be disabled at the BIOS level… and as you may know, if we can’t configure much on it 🙁

      1. Dreamcast (Post author)

        Yeah, the bios settings are rather limited. Perhaps you can shorten the pins (“dots”) somehow before soldering, just to see if it works?

  4. Joao Pinheiro

    Hey all!

    I found this page through searching Google for the Compaq Presario 520 CDS, which was also my first computer!

    I’ve been searching for one and I actually might have a change of getting the exact same one I had back in the day! I still don’t know in which state it might be but I intend to restore it to factory conditions if hardware permits.

    Which brings me to the question: do you know if there is any place where I can get the original Windows 3.11 with Tabworks and Compaq Media Pilot, etc, etc…? I’ve been searching around a lot but I can’t seem to find anything…

    Any help you might give me, It would help me a lot…

    Thank you in advance!

    1. Dreamcast (Post author)

      Hi! I have not seen the original Windows disks anywhere, but I did run into this site, from which you might be able to re-create some of the Compaq applications:

    2. Al Woodring

      Still looking for a Compaq 520 CDS? I’ve got one in perfect working order. I upgraded to a 66mz overdrive processor and added RAM. it’s been so long I’ld have to open it up to see what chips I put in! I’ve got the Tabworks 2.0 CD in the original box it came in. I’ve got a lot of Windows 3.0 disks and a Windows 3.11 upgrade disk.
      I realize I’m answering an old post but I thought I would reach out to you.


      1. Cory Flammer

        You still selling your 520 CDS?

      2. stanley heyman

        Al- I have a Compaq 520 CDS that I have used sparingly for a couple of decades. I just had NONSYSTEM DISK OR DISK ERROR come up and have tried to access BIOS but have had no success using ESC, F10, etc. when I turn it on. Any suggestions to access BIOS? Thank you.

        1. Dreamcast (Post author)

          The software used to access the bios on these machines is stored on a partition on the hdd. If the partition is not available anymore (e.g., due to a faulty hdd), you need to make a floppy with the necessary files. See this link:

  5. Frank Clough

    My first computer was a CDS 520 and I had a lot of fun upgrading it. I replaced the 486SX 66Mhz CPU with an Evergreen AMD 5X86 133 MHz Overdrive which gave a big boost. I replaced the 450Mb Hard Drive with a 2Gb drive, but the bios wouldn’t recognize the full size of the 2Gb drive so I installed overlay software I downloaded from the drive manufacturer’s website after which I could use the full 2Gb. The biggest upgrade I did was on the display. Onboard video memory was 512Kb which would only allow 16 colors so I added a 1Mb ISA graphics card which improved the colors on the display greatly. To do this I had to open the monitor case and unplug the integrated graphics connector from the monitor and replace it with a cable I made which I plugged into the VGA output on the display card. Another adjustment I made on the monitor was getting rid of the 1/2 inch black border round the picture. There are a row of potentiometers next to the CRT, a couple of which can be used to adjust picture height and width. Adjusting these made the picture full screen. My CDS 520 was a pretty decent computer after all the mods and upgrades and ran Windows 95 really well, but after a while it was time to move on so I gave it to a family I knew with young kids who wanted a computer and I built myself an AMD K6-2 500Mhz computer. But I really liked my old CDS 520

    1. Dreamcast (Post author)

      Wow, those were some really nice upgrades. I was actually curious about whether the monitor could handle a standard VGA connection. Good to know for future reference!

    2. José Luis

      I’m stuck with my old CDS520 because of the damn CRT, it doesn’t turn on anymore, I was searching any local tech to give it a try, but, no one is able, because it’s an old monitor and etc…I’m actually struggling against its components to see if I could repair it on any way, but, it seems it will be impossible.
      I found very interesting your “cable” idea, to plug on a db15 connector, I know about the db15 pinout but I have no clue about the J5 connector on the CDS secondary board, could you be so gentle to explain its pinout to me?, please, so, I could use an LCD 12’1 to substitute my dead CRT and keep enjoying this little treasure.
      Thank you so much in advance!

      José L.

      1. Luis Evangelista

        I recently bought a CDS520 and the monitor doesn´t work either, i found that was a small capacitor in the CRT neck board, after replace it, it is working again.
        There is a link from Vogons, that the OP had the same problem and mine was the same too.
        i leave you the link here:

        Knowm I install windows 3.11 for workgroups,and the windows refuses to run with the cpu that was in this machine a AMD DX2 66mhz, i replace it with the Intel DX2 66Mhz and know is runing windows just fine. but if i install a network card like 3C509 the windows refuses to run. Very strange.

  6. Frank Clough

    Looking at some old notes I made I remember there are three potentiometers to adjust in order to get full screen display. If you should decide to increase the display to full screen it will need to be adjusted in both SVGA and VGA modes in order to get full screen in both DOS and Windows.

    On one side of the CRT there are a row of potentiometers. There is one pot marked “vert size” one marked “SVGA hor” and one marked “VGA hor”. The “vert size” is common to DOS and Windows. After adjusting for full screen in Windows using “vert size” and “SVGA hor” pots, reboot into DOS mode and adjust the “VGA hor” pot to widen the screen area for DOS as well.

  7. Walter Wold

    Recently upgraded one of these machines. Compaq CDS 524. But its a really bad display hardware.
    Frank Clough: do you have a video or link for how you did your upgrade with the monitor?

    RAM 36MB
    CD ROM 36x
    AWE 64 GOLD + Roland MT32
    New CMOS
    Recovered BIOS configurations with softpaq
    Installed Windows 95

  8. Pedro Veiga

    I still have my 524CDS working, however the monitor lost the brightness (its too dark to see), however i dont know how to disassemble it and take it to any TV repair man. Joao Pinheiro and Joao Cunha, i believe you are from Portugal. Hail

  9. Diego Salvetti

    Hello, excellent post for the Presario CDS 524. A friend of mine gave me one of these last week. The PC was full of dust so before I check if it was working I cleaned up just a little.
    After that, I connect the power chord and it started but without image (CRT was off). The PC boot normally but no image was on the CRT. (after it boots I heard two long beeps and two short beeps)

    If some one can help me with some experience so I can start digging this problem and try to fix it.

    1. Dreamcast (Post author) Does not mention 2L2S beeps, but 1L2s means a video error. Try connecting an external monitor to the VGA out to see if video out is working at all.

  10. João Cunha

    I don’t know how I never noticed this, but yesterday I was cleaning my CDS mobo and I noticed an unsupported (?) jumper below the bus speed jumper, for sure it would allow the selection between Write-through and Write-back cache. I currently have an evergeen 5×86 133mhz that supports WB cache, so I will try to short the WB pins to see if in fact the support for WB cache was hidden during all this time 🙂

    1. Dansekua

      Did it work ?

  11. Walter Wold

    I’m having a hard time figuring out how to use a CF card. I’ve tried 4GB and 8GB you say this was the easy part of upgrades. Could these machines have max 2GB size? It’s like the BIOS don’t want to set correct sector, cylinders etc.

    1. Dreamcast (Post author)

      Hi Walter. Some CF cards simply wont work with the IDE adapter. It varies with brands, models etc. I am not sure about the maximum drive size. So my first recommendation is to try different cards. Another way is to prepare the card in your regular computer by using a virtual machine:

  12. Walter Wold

    Thanks for the tip! I’ll give it a try following the link and use this method. I also ordered two new CF cards – 2Gb and 4Gb.

  13. gius

    amazing article!
    Is there a way to update the video-card too? It would be such an impressive machine if it could bare at least 65536 colours.

    1. Dreamcast (Post author)

      Thank you! I suppose you could throw in an ISA graphics card, and do some custom wiring to the monitor, but it would require a lot of fiddling.

  14. Walter Wold

    That’s the only missing upgrade for my machine. Video card is lame! I’ve also used all ISA slots for other upgrades. If I could only solder of the internal video card on the motherboard…. Or something ☺️

    1. Dreamcast (Post author)

      Well, you can always add more ISA slots:

  15. GUO

    Well done. very very cool.
    I got my CDS520 in 1994. The 512KB Video RAM is so disappointed, it can be updated just by change the IC on board?
    And can you show what kind of RAM ranks that be added? they seems different from the RAM ICs onboard.

    1. Dreamcast (Post author)

      Hi! I have not seen anyone upgrade the video ram by replacing the original IC. Perhaps you could ask around on to see if anyone there have experience from such a procedure.

  16. Eva Milde-Hébert

    I loved my Presario CDS 520 and used it for the last few years as a cd player. When we changed to summer time, I was asked to press F1 or F2 and for some reason it did not boot properly anymore. The diskette and the CD drives work.
    I have the original bootable diskette, the QuickRestore Compact Disc Software restauration utility and SP 1333. As well a WIN 95 Upgrade. Following the instructions to use the software disc with the boot diskette, I simply cannot get it to work. Any suggestions or help would be greatly appreciated!

    1. Dreamcast (Post author)

      Perhaps the hard drive has died? Can you run some disk tools or similar from the floppy and see if it identifies the hdd?

  17. Goodbuster

    I’m having bios issues with this pc. I acquired it for free from a relative and it appears the drive has been completely wiped clean, including the bios setup. Which is proving to problematic as the floppy drive doesn’t seem to power on at all

    1. Dreamcast (Post author)

      Best bet would be to replace the floppy, unless you go with some sort of floppy emulator.

  18. Andy

    Excellent article, thanks, found one on the side of a road still working, this will be very useful 🙂

  19. Gaijin

    Any thoughts on how to recover a CDS520? The SP1363.exe SoftPaq doesn’t seem to recognize the HDD. I also tried a CF card with the same results.

    1. Dreamcast (Post author)

      What was the size of the CF card?

  20. Mica

    Hallo, ich hatte 1995 meinen ersten compag Presario mit Windows 3.11
    Ich habe noch die kompletten Disketten zum wieder aufspielen

    Einfach kurz Bescheid geben

    1. Dreamcast (Post author)

      Hello. It depends on which Presario you owned. For the CDS models, check out this link: For the 4xx models you can find the floppies here:

  21. Ruben

    Did you change any bios settings when you installed the Sound Blaster? Like disable the build-in audio or change irq settings in the bios before installing? Or clear the CMOS after installing?

    I’m asking because I have two known working Sound Blaster AWE64 Value cards and I tried installing one of them in my Presario 5220, but the system doesn’t even post with them. Just a black screen. I tried:
    – removing all other cards
    – installing in every slot
    – doing this for both of the Sound Blasters, in case one of them is in fact bad

    No luck. When I remove the Sound Blaster, the system works fine.

    So I’m thinking it might be a bios thing. But getting access to bios setup is still on my todo list, since I bought this pc from someone who did a clean reinstall of windows (no bios setup partition) and didn’t have recovery disks.

    Your post has been super helpful for figuring out and restoring my Presario. Thank you very much!

    1. Dreamcast (Post author)

      Hello Ruben. I am happy to hear that my post has been helpful. Regarding the bios and Sound Blaster: I have disabled the built in ESS audio card, but I am not sure if I did this before adding the SB16. If I recall correctly, I have booted the Presario both with the ESS activated and deactivated. To double check, give me a few days to find the BIOS setup floppy and see what happens if I activate the ESS with the SB16 installed. Do you have the opportunity to try any other ISA sound cards?


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