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When I first saw this machine my first thought was that it was a later Tandy 1000 PC but on closer inspection I discovered it was an IBM PC clone machine. It seems every company made IBM clones back in the day and I shouldn’t of been surprised Tandy did either but I had never seen a straight clone from them, besides the later Sensation! machines which I think all used the Pentium and had some kind of backwards compatibility with Tandy sound. The price was right so I present the Tandy 425 SX.

t425sx1

The closest PC I have that I can directly compare the 425 SX to would be my Packard Bell Legend 115. I say that because they roughly use the same small desktop form factor and feature somewhat similar motherboards and the same base Intel 486 SX CPU. The 425 SX is a little wider but the expandability you get is worth the extra size. The machine I have here is bare stock and as far as I could tell was not upgraded in any way from the factory configuration. It almost seemed like NOS. There’s a power button on the far right and a reset button on the far left along with a key lock for locking the case shut I think which was kind of a thing back in the day. There is two 5 1/4 bays and two 3 1/2 bays one being fully internal and meant for a hard drive. Mine came with a 170MB IDE hard drive installed as well as the stock 1.44mb floppy in mine that had “1.44” printed on the eject button which I think is kinda cool. I like the two 5 1/4 bays because this gives you the option of a CD-ROM drive and something like a 5 1/4 floppy drive or a HDD caddy unlike my PB Legend 115 which only has one 5 1/4 bay which goes to the CD drive.

t425sx2

Here we have the back of the unit. The PSU that came with this machine has the monitor pass through which is always convenient. As you can tell right away from the expansion slot placement this machine uses a riser card but It has ISA slots on both sides of the card allowing for five cards which is really nice though the two on the left have to be short in length and you need to pull the power supply to get them in but at least you have the option. As far as ports from left to right we have Keyboard and mouse PS/2 ports, two serial ports a printer port and the VGA port for the built in video.

One thing to note about the VGA port is that it uses an older pin configuration and so one of the pin holes is blocked. This was an issue for me with my flat screen Gateway CRT monitor I use as it had this pin on the connector. One option is buying a VGA extension cable and snapping off the pin on the PC connection side or you can do what I did and drill out the hole so newer monitors will connect. this won’t hurt anything.

t425sx3

Opening this PC is very easy and just involved the two screws on the back and lifting the top away. So here we have a look at the internals. jumper sheet here.

t425sx4

1) CPU – The CPU for the 425 SX is, surprise, an Intel 25mhz 486SX. The SX designates that this CPU has either no math copro built in or has the math copro disabled. The CPU on this PC’s motherboard is soldered onto the board so it can’t be removed but luckily the machine had an extra socket to allow for upgrades. The 25mhz 486 SX is a lower end CPU and although does the job fine for older games it’s really suboptimal for later DOS stuff and an upgrade is highly recommended.

t425sx5

2) CPU upgrade slot – This is the CPU upgrade socket. This is a cheaper LIF socket so insertion and especially removal of the CPU is a little more difficult then a ZIF socket with a handle. This is where you would install either a 487 math coprocessor or the better option, a DX CPU. There are jumper setting to set the FSB to 25mhz and 33mhz so upgrading to a 66mhz DX2 or even an AMD 133mhz 5×86 should work fine. Once a CPU is inserted in the socket and the jumpers set the soldered on 25mhz 486 becomes disabled.

3) RAM – The RAM on this PC is a little strange. There are eight slots for 30 pin RAM as well as one 70 pin RAM slot but this machine seems very picky about what RAM it will accept. I couldn’t make it post with any of the 30 pin RAM I had and only one 8mb 70 pin RAM SIMM as well as the 4mb SIMM that it came with would allow it to post without error. All the other 70 pin RAM I installed allowed the machine to POST and was detected but they tripped a parity error. This board will supposedly support up to 40MB of RAM by using all available slots ( 1 x 8mb 70 pin and 8 x 4mb 30 pin ) but I suspect the right kind of 32MB 70 pin SIMM will also work, but as I said the 32MB stick I tried tripped errors on POST.

4) L2 cache – these are the sockets for adding L2 cache ram. This model did not come with any L2 cache installed which isn’t surprising as it was pretty pricy back when this machine came out. The 425 SX supports 64, 128 or 256kb of L2 cache. even though this isn’t as much as the Legend 115 which supports 512kb of L2 (but NOT 256kb) the chips needed for 512kb on that machine are very hard to come by and after 256kb you start getting diminishing results anyways. The downside is the number of chips and the size of the tag RAM chip needed is not the same as the rest. All cache configurations need either a 8k x 8 or for 256kb a 32k x 8 11 pin TAG RAM module which seem to be a little uncommon. All configurations also require an extra ninth 64k x 1 14 pin module. I don’t know why this ninth module is required, possibly for parity checking. It’s a little bit of a hassle considering that in many other boards all the cache modules you need are the same physical size or like the PB 115 where you just installed five modules of the same size for 128kb.

5) Video – On board Video uses a Western Digital WD90c31A-LR chip with 512MB of video RAM. This is a decent chip and unlike the Headland chip in the PB 115 does not suffer any timing errors. I believe this chip is running on the ISA bus rather then a local bus. The chip can be expanded to a full 1MB of video ram by means of sockets next to the chip. The RAM should be easy to find especially if you harvest it from an old  generic or Trident VGA ISA cards.

t425sx6

6) Riser card supporting five ISA expansion cards (three full length).

7) One on board floppy connector and one on board IDE connector.

8) AT power connector.

9) external CMOS battery.

10) PC piezo beeper speaker for PC speaker sounds.

I like the Tandy 425 SX. Its a small machine but not to small so you have room for expandability. The only downside is the picky RAM on this motherboard as well as the need for extra L2 cache chips. I originally had plans for the PB Legend 115 for a special project setup but due to the ability to cache 256kb as well as the added expandability I think I may use this PC instead.

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7 Comments

  1. I love your articles. In a number of them, you say to harvest video ram from trident cards, but if you said why, I missed it. Why are trident cards bad, to the point they are only good for ram scavenging?

    • Trident cards are generally pretty low end products aimed at budget users. They work and the average user may be just fine with one but compared to something like a ATI or Tseng labs card they are slower, have poorer image quality and may be less compatible. for instance I’ve had people tell me about Trident cards incompatibility when using a MPEG decoader card where a Tseng Labs ET4000 works fine. They also tend to be cheaper and easier to find then the high end cards so it’s usually more economical to just scavenge their RAM. Not all Trident cards are terrible. the 8900D based ones are pretty fast and the 8900 cards also work in 8 or 16 bit slots making them pretty versatile but in general Trident is a budget offering. If a Trident is all you have It will get the job done but Id take a Tseng Labs, ATI or S3 card over them any day. They seemed to get even cheaper and worse as the PCI era came about as well.

  2. I toughly enjoyed your 425 SX piece. One question – say I used a standard 168 pin DX4 100 in the 425 SX as an upgrade – would it be able to run at full speed or be stuck at a maximum 66 MHz? I’m assuming I’d be better off with an overdrive type chip that could hopefully work around (as in double or triple) a lower motherboard multiplier and run faster than 66?

    • I don’t think an Intel DX4 would work because it’s a 3v CPU and the board only supports 5v CPU’s so you’d probably fry it. The safest bet would be a Intel overdrive ODP chip but even that’s not a sure thing. CPU compatibility with upgrade sockets can be hit or miss sometimes. I only experimented with a 486SX-33 and a 486DX-33 in the upgrade socket both of which worked fine.

        • Mike K
        • Posted December 31, 2017 at 06:51
        • Permalink

        Thanks for the heads up on the voltage, I completely forgot that the original 486 upgrade socket was 5V only. I’ll most likely go with the still readily available Evergreen / AMD upgrade as it has voltage step down circuitry built in and report my findings.

      • Please do, I’m curious as well.

  3. Hey there all,
    I came upon this webpage when I was searching online for any information about the Tandy 425 SX computer. The very first computer I owned was a Tandy 425 SX computer. It looked almost exactly like the one in the pictures on this webpage except that the 3.5″ disk drive and the empty bays are switched, I believe. I get a sentimental feeling when I think about my first computer. The computer that I had, had a harddrive and a 3.5″ floppy drive. It used DOS 5.0 and Windows Ver. 3.1 for the operating system and operating environment respectively and it also had software for America On Line (though I don’t know what version other than it was for windows), a program for setting up the installation disks for Windows 3.1 (which had two icons, one for an informational file and one to run the program). I had the computer for many years and I got a laptop as well. Eventually, I ended up putting the computer in the trash because it took up a lot of space and was old and out of date. Over a year ago, I found on the internet a program called DOSBox which allows old games and programs to be used on newer computers. With DOSBox I am able to run Windows 3.1 on my Windows 10 laptop. I would love to bring back the look of the desk top of my first computer. Unfortunatley I never copied the files for Edit.com which comes with DOS 5.0. I also didn’t make a backup of America On Line or copied the files and folders for making the installation disks for Windows 3.1. I wish that I had either taken a snapshot of the desktop or created an image of the desktop. I wish I also had the properties’ information of the icons for AOL, 2 icons for the program for creating the installation disks and the icon for edit.com. I also would have liked to make a copy of the autoexec.com file because I am interested in batch files. I am curious to know if you have tried to turn on the computer and if so, did it start up? I was wondering if you have made any changes to the computer? If so, what have you changed? I know that this webpage has been up for a while, so my response is late but I would love to know more! Thanks. Lorin


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