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Struggling to build a working 486

J

JonnyGators

Experienced Member
Joined
Mar 9, 2020
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Attleboro, MA
Having working early DOS machines, a decent XP/98 dual boot machine, and of course plenty of current machines, I've found the gap in my collection of functional machines is something of the Windows 3.1 era. So I've decided to build a decent 486 machine.

But, buying a bunch of used parts, and putting them together, more often than not results in a useless doorstop being built.

How in the heck am I supposed to figure out how to get a working machine together with nothing but used parts, often sold as is/for parts repair, because sellers can't be bothered to test anything?

Standard troubleshooting has always told me to start with everything disconnected from the motherboard, and add things one by one. That seems to not be an effective method in this era, though.

I have a new power supply. It powers up individual components. I know that's good.

So I started with power supply and motherboard (and pc speaker). Nothing.

So I returned the motherboard, reported it as dead which was my findings based on what has been standard troubleshooting all of my professional life in computers, and found a listing for one that was tested.

Same results. Another dead board.

So I reached out to the seller. He asked if I put RAM in it, said RAM is required for it to turn on.

Tried that....still nothing.

Now I need to sort out, is it a bad motherboard, or bad RAM?

What even is the right RAM for this? The motherboard is dx6900 ver 1.4, found this link about it - https://stason.org/TULARC/pc/motherboards/A/AMPTRON-INTERNATIONAL-INC-486-DX-6900-VER-1-4.html

It looks like the better option for ram is the 72 pin slots. So I went looking for 72 pin RAM on ebay. Lots of options. Some say for MAC Apple. Some say for mac and pc. How do I figure out what is or isn't compatible with this board? The link I found is great for pin and jumper settings, but offers no help as far as RAM information.

Could someone point me in the right direction as far as what RAM to get, and how to obtain tested and known working RAM of this vintage? I understand troubleshooting and testing is necessary when dealing with older things, but once again, I run into nothing but roadblocks around here, and am getting rather sick of acquiring piles of garbage that are absolutely useless to me. It seems every time I try to pivot away from one roadblock to something that should be easier, it's just another roadblock, waste of money, and pile of garbage.
 
If your RAM is bad, you'll still usually hear a post-code over the PC speaker.

To even get that far, though, you'll need to have the CPU installed and jumpered correctly.

Unless you are running a DX4/100, you'll want to jumper for 5v. If there is no jumper for that, your mobo either only supports 5v, or autodetects CPU voltage. You can test for this by putting a jumper wire into a certain pin in the CPU socket and then measuring voltage on a certain other one, but let's just forget that for now unless you explicitly need a DX4 mobo.

Your front-side bus speed can be set to either 25MHz or 33MHz. This is combined with a multiplier setting to get the CPU speed set right. For example, a DX2/50 (50MHz) would have the FSB jumpered to 25MHz, and the multiplier jumpered to x2. A 33MHz CPU would be jumpered to 33MHz and x1. 66MHz CPU jumpered to 33MHz x2. DX/4-"100" would be jumpered to 33MHz x3. If in doubt, try running the machine at 25MHz x1. Most if not all CPUs ought to be able to run at a lower-than-rated speed, but jumpering it too high can cause instability or even immediate crash on boot.

Once you get that far, insert some RAM, a video card, and connect a speaker. If you are lucky, you'll get a BIOS POST screen. If you are unlucky, you should at least hear beep codes that you can look up in your mobo manual.

Be careful with 72 pin ram. Most of what you'll find for sale is 60ns EDO ram designed for Pentium systems, and most 486 boards were designed to work with 70ns DRAMs. Some 486 boards will run EDO RAM fine, some will behave erratically with it, and some won't work at all. I think it has something to do with the refresh rate, but I am not 100% sure on that.
 
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You do not want Mac RAM. It is non-parity and will not work with a motherboard needing parity RAM (indicated by the x9 or x36).

Testing would probably be easier with 30-pin SIMMs. Find a set of 4 SIMMs each having 9 chips running at 70ns. Those are the most common type of 1 MB 30 pin SIMM so should be easy to find some that work and are affordable. There were so many varieties of 72 pin SIMMs and that motherboard only works with a narrow range of them. Checking the label of each chip could let you determine if the 72 pin SIMM matches but that is a lot of work. Don't expect to find any SIMM with a larger capacity than 4 MB.

Guaranteed working RAM? I don't know of a good source for it.
 
What I bought is parity RAM, but I've not found a straightforward guide for what works with parity, what works with non-parity.

The seller of the motherboard says the board takes EDO RAM. Don't know what that is, or ...... I'm all confused and just want a clear guide as to what to buy that will work exactly.

The motherboard came with processor chip, was tested with that, jumpers should be set right for that processor.

When I put in one of my parity ram chips, connect power, and turn it on, power stops shortly after. I've connected the fan on the processor as well. With only the fan connected, turning on the power supply results in the fan spinning. When I connect the motherboard, turning it on results in the fan spinning briefly and immediately stopping. Turning it off and back on results in no fan movement, until I disconnect and reconnect either the ram or the power supply.


I just want a clear instruction as to what parts to buy to build a working machine so I can stop acquiring garbage piles and stop throwing my money away on more and more projects that turn into garbage piles. I am beyond frustrated at this point. I just want to build a working 486, and I am willing to work at building it and troubleshooting, I don't just want to buy a working machine, building it is part of it, but I want more than just non working piles of garbage.

Can I please for the love of god get some guidance in the right direction to buy a working set of parts to build a working machine?
 
I'm told here I want parity RAM. The parity RAM I have doesn't work.

The seller tells me I want EDO RAM. All 72 pin EDO RAM I search for on ebay is non-parity.


Can ANYONE just give me a straight answer.....what in the heck RAM works with the motherboard which model and web link info I've posted?

Does anyone know how to determine what RAM to buy?

Why is it so hard to get a clear answer about this? I've given the exact motherboard model, posted the link I found.....but is the information about what type of RAM this requires lost to time forever? Why is it so impossible to get a clear answer to just about anything around here?
 
If the AT power supply is kicking off after you switch it on that would indicate a short circuit. Is the motherboard installed in a case? Sometimes mounting plates or standoffs can contact a solder joint or a trace running next to a mounting hole and cause a short. Something to look for.

Other things to think about: Maybe the board was jumpered correctly at one time but jumpers fell off during shipment? Double check that the PC speaker is connected on the right pins? Connect a power LED and see if that comes on?

edit: I wouldn't assume the RAM is bad just yet. But it's tough to know whether parity or FPM/EDO even matters without knowing what chipset the motherboard uses
 
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The only source I can find mentioning the motherboard model claims it uses FPM memory not EDO. I expect that it should work with FPM 30 pin SIMMs. https://www.memoryx.com/generic-memory-30-pin-simm.html choose the 1MB 9 chip 70ns design. Those were the most common form of memory in the early 90s and useful in early 486, most 386, and even some 286 systems and memory expansion cards. Double check the status of the listed supplier. I think I ordered from them many years ago. They may not be as reliable now.

I know you want the 72 pin but it is difficult to find memory that matches and even harder since a lot of memory is incorrectly labeled.

Note: If you have the fake cache version of that motherboard, throw it out. It is going to be slow. It was also not very reliable.

http://www.elhvb.com/webhq/specs.htm
https://www.memoryx.com/fpm.html
 
Most PC RAM iirc is non-parity. IIRC it's (some of) the Macs that need parity ram, various old Unix workstations, and some server boards. If the mobo is supposed to use EDO RAM, you'll be fine with EDO RAM then I guess. The deal with EDO RAM is that it has a burst mode that the older 72 pin RAMs didn't, but EDO is *supposed* to be backwards compatible.

Surely a board that is new enough to have VLB will have 72 pin SIMM sockets, not 30 pin? If you can see any pictures of the board, zoom in on the SIMM sockets. If it takes 72 pin SIMMs you'll see a divider thing in the middle of each socket. If 30-pin, there won't be that divider (and the contacts will be spaced further apart and fewer).

Edit: Went back to the page you linked. Looks like that board will take both 30 pin and 72 pin RAM. Interesting. I bet you can't install both at the same time though, or if you do I bet the 72 pin ram slows down to 30 pin speeds.
 
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Make sure you have the AT power connectors in the right positions on the motherboard. Black wires should be in the center and red/orange wires should be on the outside.
 
From talking to the seller of the motherboard, I'm not too confident in whatever he said. He recommended checking the pin configurations, because he doesn't remember what processor he tested it with. Seems like when he listed it as tested, he means, at some point in time I tested all these parts, and they worked some time ago, but now I"m gonna mix and match parts and list them as tested without so much as details that they work together or have been tested recently or in this configuration. So....so much for relying on listings that claim they are tested, and receiving things ready to use after testing.

Meanwhile, the seller of the RAM suggested another listing of his RAM that should work with the board, so I'm working an exchange there.

"fake cache version" ????? How do I determine this?
 
JonnyGators said:
"fake cache version" ????? How do I determine this?
Click to expand...

Typically, the fake cache motherboards lack sockets for cache and instead the fake plastic objects are soldered in place. The other method of noticing is that the performance is anemic and the same whether the cache is turned on in the BIOS or off. http://redhill.net.au/b/b-bad.html covers the saga of the related motherboards produced under many names including PC Chips and Amptron like you have.
 
To my knowledge, 486 board with 72-pin memory universally use 72-pin FPM modules, not 72-pin EDO modules. Physically, they are the same (and Pentium systems take either), but FPM modules are hard to find, especially for larger memories. Therefore, always start with 30-pin SIMMs, they should be universally compatible. You need 4 modules of the same size (doesn't matter if 256 KB or 1 MB, or even 4 MB if you can find those).

The most basic test is to have only mainboard, CPU and speaker connected. If memory is missing (or faulty), you should hear a single long beep. Anything else requires memory, and a matching jumper setting. You should be able to get some beep codes, then. The next step would be to add a video card. If things do not behave as expected, a POST card is a useful investment to see where things fail.

Regarding the "fake cache": On some boards, the cache sockets are actually populated, but the chips are just plastic. Replacing them with actual cache memory fixes the performance, then. But that's not really important until the system actually does successfully POST....
 
Svenska said:
To my knowledge, 486 board with 72-pin memory universally use 72-pin FPM modules, not 72-pin EDO modules. Physically, they are the same (and Pentium systems take either), but FPM modules are hard to find, especially for larger memories. Therefore, always start with 30-pin SIMMs, they should be universally compatible. You need 4 modules of the same size (doesn't matter if 256 KB or 1 MB, or even 4 MB if you can find those).
Click to expand...

Mid and late 486 boards started to be able to use EDO memory. Some had different memory capacities based on whether FPM or EDO was installed and late boards usually didn't care. I have an AST Advantage! 486SX/33 which does the former. With EDO, the maximum amount of RAM is 32 MB, but FPM bumps it up to 64 MB. My later ZEOS motherboard that was made in 1994 will use either type of memory and doesn't impose any penalties for using EDO.
 
You may find one of those ISA POST code cards helpful. You know, the ones that show diagnostic codes as the system is going through power-on self test. If your CPU is running but the RAM is failing you will definitely be able to tell.

I have one of the cheap ISA/PCI cards from ebay and it works fine.

Agree that FPM (fast page mode) RAM worked with the 486 systems I have had, and that EDO (extended data-out) RAM did not work on any 486 I ever tried it on.
 
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I can't make any sense of this stupid motherboard. I think the seller is a lying idiot. He posted a picture of a post screen, but sold me a board that won't even turn on, and can't even tell me if it's the same equipment pairing or configuration that he tested, or when that was. I think testing a pile of equipment, forgetting about it for a period of time, mixing it up, and posting a listing as tested and working with equipment that you can't even say when or how it worked, is getting into the territory of dishonest.

So he's all like, just google it, and check the pin settings. Uh.....you tested it, buddy, shouldn't you have already configured it to work with the processor you sold with it?

But, whatever.'

So, here's a page with the jumper information - https://stason.org/TULARC/pc/motherboards/A/AMPTRON-INTERNATIONAL-INC-486-DX-6900-VER-1-4.html

Problem is, as I go down the list, I get stuck early on. First of all, whoever laid out this diagram should be shot! It says JP31 should be closed. And shows on the diagram a section of 9 pins in a square, with the top right labeled JP31, middle left labeled JP33, and bottom center labeled JP32. Two HUGE problems with this!

1. Do they go across, or down? by selecting a different pin in each direction for labeling, it makes is COMPLETELY UNCLEAR whether they go across or down! What idiot labeled the chart this way? This is beyond stupid!

2. There are 3 pins, and the ONLY instruction it gives for these 3 pins is for them to be closed. HOW DO YOU CLOSE 3 PINS? Does it want 2 of them closed? Which 2?

From looking at what is actually on my motherboard, it looks like they go downward. And 32 and 33 are actually 4 pins each, so the diagram is not only confusing, but inaccurate. Which was confusing as hell, because the diagram looks like it's suggesting they go across, but, ok, they go down, the 3 rightmost pins are JP31, and they should be....uh.....closed?

WHICH ONES SHOULD BE CLOSED?!?!?!?!

And both motherboards I bought of this have no jumpers on any of those pins, so I have nothing to go on.

Who in the heck writes instructions that 2 out of 3 pins need to be jumpered, and leave out the only important detail? Ridiculous.


Anyone have any ideas what to do with this? Or do I just need to give up on this stupid manufacturer and return the boards, and find something else? All I wanted was a working 486, and all I get, even when buying "tested working" parts, is bullshit!
 
Also, another minor detail........WHICH WAY DOES THE PROCESSOR GO IN?

From looking at the 2 motherboards I have, it looks like the processors are facing different ways. I had assumed the seller would have put things in correctly, but......no......idiots on ebay. I used to feel bad for how sellers are treated on ebay. But now when I see that we're dealing with people flipping garbage that they know nothing about, and just want to take the money and be left alone when someone ends up with broken crap that doesn't work, I have no sympathy for that anymore. Screw them all!

The document I have seems to offer nothing in the way of information as to how to put in the processor, and it will fit in multiple ways.


How do I figure any of this out when the most basic of instructions are omitted from the instructions?

I've been building PCs since the XP era, and never faced problems such as this......why were the instructions so lacking pre XP?
 
Ok, I'm just done with this motherboard, I've sent a piece of my mind to the seller, and am processing a return for full refund. Back to the drawing board......can someone recommend a 486 motherboard and processor that doesn't suck?
 
The holes in the socket only match with a chip being oriented correctly. If the chip resists insertion, it is probably oriented wrong. Intel has a mark or indent on the corner of the chip with pin 1 which needs to match with pin 1 on the socket.

If you haven't done so, download some of the earlier releases of Upgrading and Repairing PCs. The publisher decided to give away PDF copies of those so it makes sense to take advantage of it. Unfortunately, the ones I have lack detailed pictures of chip and socket alignment.
 
It's interesting--the moment I saw the "photo" of the board, I said "That's a PCChips/Amptron board", even though I never had one of them. I do have the P1 version of the board, which looks amazingly similar, however. It works well, even though it's not a barn-burner.
 
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