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.

(This post is updated on a regular basis).

Compaq Presario CDS 520 specifications

  • Year: 1994
  • CPU: 486 SX2 66 MHz
  • 8 mb ram (upgraded to = 20 mb (max = 64), through 2x8mb + 4mb on board)
  • Cirrus Logic 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! The CF card has a boot delay of about 1 minute for some reason, but I think switching to different card will fix 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

The solution I came up with was neither elegant, or any remedy for whatever is the cause for the problem. I simply used a small chinese audio amplifier (driven by 5V), and connected the + cable from the motherboard speaker between this amplifier and the speaker. It worked! It was even too loud, but I fixed that with a resistor. And now, the speaker works again!

5V Audio Amplifier

Images: Compaq Presario CDS 520 motherboard and modem

Compaq Presario CDS 520 motherboard

Compaq Presario CDS 520 modem


  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:


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