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Thread: Upgrade CPU in Tandy 4825SX Computer

  1. #1

    Default Upgrade CPU in Tandy 4825SX Computer

    I have a Tandy 4825SX Computer and I want to upgrade the CPU. I have an Intel DX4/100 and I'm wondering if I can pop that in there and get 75MHz out of it? I also have an Intel DX40DPR100 overDrive processor and I'm wondering if that would work? I'm thinking it won't because I need the DX40DP75 instead of the DX4ODPR100, but I'm wondering if it would work but not at 100MHz but at 75MHz.

  2. #2
    Join Date
    Aug 2015
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    Kansas City area, US
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    I doubt the motherboard supports the voltages needed for the DX4/100, but the OverDrive cpu ought to work. Might be a jumper to switch from 25 to 33 Mhz to get 100 Mhz. There's a youtube video of someone with one-- https://youtu.be/qq0wn4iadZE

  3. #3

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    I saw his video this morning and that's what gave me the idea to try the overdrive processor I have. TJBChris' Tandy 4825 SX has an Intel DX40DP75 in it. I was reading on stason.org and the Tandy 4825 SX has dip switch CPU type selection for an 80486DX2, so I would assume that sets the bus speed to 33MHz, thus making my DX40DP100 work. I just want to make sure I don't fry anything on my Tandy or my overdrive processor by doing so.

  4. #4
    Join Date
    Aug 2015
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    Worth a try on the dip switch, but they did make 25x2 486dx2 50's. There's a four prong chip near the riser board that looks like it might be an oscilator. Is it marked 50.00 Mhz? Early 486 motherboards without adjustable clocks usually had an oscilator double the speed of the cpu. If it does, you could theoreticaly replace it with a 66Mhz clock to give a 33Mhz cpu speed.

    Just noticed you've got an ODP overdrive instead of an ODPR cpu. The ODP is meant to go into a special 'overdrive' socket and has an extra pin that might not fit into the 4825's socket. https://en.wikipedia.org/wiki/Intel_80486_OverDrive
    You could look for one of the voltage converter boards to run the DX4 - https://en.wikipedia.org/wiki/Intel_DX4 , but it would probably be as cheap and easy to find an evergreen 486 upgrade or an ODPR overdrive.

  5. Default

    I was going to reply, then I saw my video did it for me. When I first got the 4825 SX, I was digging around the archive of Tandy's Faxback documentation at http://www.oldskool.org/guides/tvdog/faxback.html, the SW 2-1 and 2-2 switches set the system to expect a DX/DX2 or ODPR processor to enable the math co-proresssor, but the core bus speed remains 25 MHz. If I remember correctly, it's up to the processor itself to double or triple the clock (my DX40DP75 accepts the 25 MHz input and triples it to 75 MHz). The 4850 EP is the same motherboard as the 4825 SX, but has a 50 MHz DX2 processor installed.

    My OverDrive processor was installed when I got the machine; I'm surprised it wasn't ODPR, since there's no OverDrive socket in the 4825 SX. So I'm not sure if Tandy's socket has the 169th pin or if the previous owner did their own surgery. I guess I could remove the CPU and find out...

    I highly recommend the upgrade if you can find a reasonably-priced DX40DP75. The machine is very capable with early-mid '90s games (Doom plays very well, Duke Nukem 3D is playable, and other games like StarGunner or Windows games like You Don't Know Jack run very well.

    -Chris
    Last edited by TJBChris; October 8th, 2019 at 08:07 AM. Reason: Added some ODP/ODPR background on my 4825 SX

  6. #6
    Join Date
    Aug 2015
    Location
    Kansas City area, US
    Posts
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    Wups! First post you say ODPR, third post ODP. ODPR should work on that motherboard (at 75Mhz). Pretty much the type of system the ODPR was designed for.

  7. #7

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    Quote Originally Posted by eswan View Post
    Wups! First post you say ODPR, third post ODP. ODPR should work on that motherboard (at 75Mhz). Pretty much the type of system the ODPR was designed for.
    Yes, you're correct. I have the ODPR, I missed the R on the third post. I'll give it a try and see if it works @ 75Mhz.

  8. #8

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    Quote Originally Posted by TJBChris View Post
    I was going to reply, then I saw my video did it for me. When I first got the 4825 SX, I was digging around the archive of Tandy's Faxback documentation at http://www.oldskool.org/guides/tvdog/faxback.html, the SW 2-1 and 2-2 switches set the system to expect a DX/DX2 or ODPR processor to enable the math co-proresssor, but the core bus speed remains 25 MHz. If I remember correctly, it's up to the processor itself to double or triple the clock (my DX40DP75 accepts the 25 MHz input and triples it to 75 MHz). The 4850 EP is the same motherboard as the 4825 SX, but has a 50 MHz DX2 processor installed.

    My OverDrive processor was installed when I got the machine; I'm surprised it wasn't ODPR, since there's no OverDrive socket in the 4825 SX. So I'm not sure if Tandy's socket has the 169th pin or if the previous owner did their own surgery. I guess I could remove the CPU and find out...

    I highly recommend the upgrade if you can find a reasonably-priced DX40DP75. The machine is very capable with early-mid '90s games (Doom plays very well, Duke Nukem 3D is playable, and other games like StarGunner or Windows games like You Don't Know Jack run very well.

    -Chris
    Tandy made a nice little computer. I like the fact that it looks very much like my Tandy 1000 TL/2 and TL/3. The only shortfall in the 4825 SX that I see is the lack of L2 cache. Seeing your video made me want to stick my AWE32 in there. The only thing that I will be missing is the cupholder. Sadly, mine didn't come with a cup holder.

  9. #9

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    Hmm, I had some kind of Tandy 486 some years ago. I think it was originally a 486 DX/2-66, but I can't remember for sure. Maybe a 50. Anyway, I dropped a DX/4-100 into it and used it for some time like that. Unfortunately it got left behind after a bad breakup and is probably in a dump somewhere now, and I can't remember the model number.
    -- Lee

    May Be Interested In Acquiring: Mac IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff, Peculiar Old Serial Terminals.
    Old Computer Fun! (Muh Projects Blahg) | Muh YouTube Channel that goes with it

  10. Default

    I decided to crack the 4825 SX open today to see if it indeed has the 169th pin (allowing an ODP processor as well as an ODPR). It turns out my DX4ODP75 is not modified, and has its 169th pin. Tandy's socket also includes the extra pin. So at least for my motherboard revision, you can use the ODP processors without modification. Hopefully that makes upgrading easier.

    Image_2 2019-10-09_10-19-22.jpg Image_3 2019-10-09_10-19-22.jpg

    Quote Originally Posted by MrRedHat View Post
    The only thing that I will be missing is the cupholder. Sadly, mine didn't come with a cup holder.
    The cupholder is definitely a nice feature. Sadly, I make heavy use of cringe-worthy jokes like that in my videos. The phrase "hello you rummy buzzard" appears in a large number of them.

    -Chris

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