New member with first project. Comtex 386 PC

[LorneinCanada]

I have been hording this old 386 for many years. Thought it was time to have a go at bringing it back to life. End goal ATM is maybe loading some form of DOS and using it for trying BBS. I'm into HAM radio so maybe if I can figure out how. Maybe BBS over HF. There is a further introduction, what I have tried and finally questions at the bottom. I welcome comments and suggestions on what to read up on, try etc.

I have pretty limited skills especially around software. I'm a mid 50's guy. Built a few PCs from P4 to LGA775. My favorite OS is XP. I dabbled a wee bit with Ubuntu. Its cool but mounting software was a bit much for me. On this project I plan (so far) to use FreeDOS.

So plugged it in and booted it up. I was going to get all fancy and test the PSU first, but that didn't happen. Then it was give it a go with the load limiter in series...... for reasons to many to go into here we ended up just winging it and did a "plug it in and see what happens" approach. No magic smoke. It sent me directly to the bios because the coin battery on the board was dead. I see a 1.2 meg drive A and no drive C. 386-BIOS (C) 1991 American Megatrends Inc. There is a VGA video card, Trident chip. 640k of system RAM. The HD is a Seagate ST3144A apparently it will be 124.6 Meg formatted. Found the HD manual online. I believe the mombo model is MB-1333/40PM-CH. There is no math co processor. I have been told I will want a "a 68-pin pga 80387, it needs to match the dx or sx variety of the cpu.
After a few fire ups I noticed something new I had not before. In the BIOS it detected a floppy.... I didn't look close enough. I assumed since the 3.5 was plugged in and bios said that it was seeing a 3.5. No it sees a 5". My main challenge so far revolves around getting the BIOS to show an enabled HD and Floppy.
Looking at the IO card there are a dozen or so jumpers. Someone helped me out with a link to a web page for it. A Biostar FI33290UIO1012.I played with jumper pin settings. Basically trying to enable the floppy and HD while turning other functions off. Nothing I did was successful.
I have asked this question in another forum. "As to swapping IO cards. I have others. But they are from AT cases, might be some more in the Tandy's or Commodore. Is an ISA IO card interchangeable on any of these?" The feedback I recieved was that most ISA I/O cards should swap on the 386. But maybe not "into" certain Tandy's. I think the question of mobo BIOS being an issue came up. And theres a question pertaining to this below.

So far I have:

Cleaned the ISA slots and video and storage controller card... Not sure of proper nomenclature. The ISA card that the floppy and HDD plug into.

I tested the HD supply power voltages.

Swapped another control card in.

Swapped another HD in from a AT? Pc. The orig was a seagate ST3144A. I thought it was spinning yesterday... But today not sure. Seems dead. I swapped in a Caviar 2120. It spins and the LED on the front lights up on power up. Still nothing in bios. I downloaded manuals for both HDD's to make sure I had the IDE cable plugged in correct, master jumper etc.

I tried prompting the BIOS to default settings.

I tried yet another HD. A Caviar 2540. First I put the HD on my newer pc to see if W10 could read it. It couldn't, but it spun happily for some time. {I don't know what it is about these old HD's and the onboard floppy. But anytime I go near them W10 goes full Cartman on me. (lets see how many figure out that plug lol)} I tried running said HD on the 386 and it did nothing. But eventually I smelled trouble. Not sure if it was just the HD going Chernobyl or the PSU or both..... Need to try the PSU ,outdoors, later and see what we see. HD is definitely dead now. Recently going through the old HD manuals I noted some HDs power in pins were wired in an opposite fashion on some HD's. I don't think that was the case here. I cooked it for sure when I tried spinning up the HD from a variable PSU. I tried putting 12v on the 5v pin by accident and that was fatal.

Perhaps adding fuses or relays between the psu and the components of a vintage pc might be a good idea???? Even if just to save them from me?

Questions:

Do I have to load DOS via the floppy on eacj start up or will it be loaded once and then live so to speak on the HD.

If I can't get the PC to recognize the HD and floppy... Will I have to reprogram the BIOS. Can I reprogram and old BIOS chip.


I would like to try mounting these old HD's to another PC to see if they function. I tried via a USB adapter and a PC on W10. Got nothing. W10 made that mounting sound on the Caviar. But nothing showed up on file manager. Or under the "This PC" tab. Can W10 even recognize the Partitions on old HD's?? I think/ assume we are talking FAT16 or something.

I threw in some screen shots of what I'm seeing in the Bios etc. Not well organized or labeled I'm afraid.

 

[ajacocks]

Welcome to the forum @LorneinCanada !

So, lots of questions here. I’ll take a crack at a couple of them, some others may be able to help with others.

1) yes, DOS can live on the HDD. You won’t need a floppy for every boot
2) yes, you can program a BIOS chip, or ask someone else to help you to do that. BIOS chips can be programmed with EPROM programmers.
3) yes, W10 can indeed mount old HDDs. Those old drives (if they were used under DOS) are FAT (file allocation table) formatted, and use the MBR (master boot record) partitioning scheme. When bring up an old 3.5” hard drive, you usually need extra power, the USB adapter can’t supply enough amperage to get one running.

- Alex

 

[LorneinCanada]

Thanks Alex. Been making time to read some threads here at VCF. So much to learn. I was reading the other day a thread about a new ISA board kit that allows newer drives on the XT buss. Whatever that is. Its something I will be reading up on more. I was watching some vids the other day about a usb BIOS chip reprogramer. In one video the guy modified the voltage output on his to match older BIOS chips. All very interesting stuff. The 386 project is set aside for a wee bit while I study up more. I'll be back with a progress report soon.

 

[LorneinCanada]

Status report... or just more questions... A bit of both maybe.

Spent the last week working on sorting through power supplies. Had a couple boxs of AT era PSU's. I eventually found two or three that showed some signs of life. I took one and completely recapacitated it. I seem to have lost the lid though. And the lid kind of holds the box together. I'll take a pic later. Hard to explain. The one from the Comptex PSU got a bath and new rectifier caps. I pulled the transistors and diodes that are mounted to the heat sinks to test them. Well with a DMM. I wonder if there is a tester that would test them at their working voltages... Well I guess that would be the PSU..... Anyways watching vids on PSU trouble shooting and poking about trying to learn how to repair them.
I think I found some details for the MOBO. https://stason.org/TULARC/pc/mother...TECH-INTERNATIONAL-CORPORATION-386-MB-37.html

Question for anyone that happens by. How do I identify what capacity these older RAM chips/modules are. I tried googling the chip numbers. That was not much help... so far. Looking at the board layout documentation and then looking at my actual board. It seems like Bank 0 is fully populated. Bank 1 is not. I have four modules. I think I have read that these modules get installed in pairs. What about the Cache chips. What are the U32 and 33 chips?
My BIOS chip is physically labeled 1981 AMI. The BIOS onscreen read 1990 American Megatrends. Does that sound right or?
I'm reading as much as I can. But a lot of it is just oozing back out. I was having trouble with the HD not showing in the bios or boot screens. Am I remembering right that some BIOS's had a HD limitation of ??? what was it 500 MB's? I have not gotten as far as an OS yet but if I go say W95 that too has some limitations and maybe learning how to configure the settings for Cylinders and heads n stuff?? The Seagate it came with is 133 Mb's or 124 formatted I think. This seems like a fairly small drive to my mostly uneducated view. IRQ's... This is something else I have to learn more about. On the web page about the Mobo I don't see any mention of IRQ jumpers. I don't remember if there were IRQ configurable options in the BIOS. Any good resources for simple explanations of IRQ assignments for this era PC would be appreciated.

 

[daver2]

Don't you just select the floppy disk type that is connected within the BIOS and save it? You have replaced the battery, right?

It is so long ago that I played with these things...

Dave

 

[LorneinCanada]

Hi Dave. Put a new battery in. In the bios it came up no HD detected. There was a line to enter HD parameters (right word?) but it was greyed out and could not type in it. The HD I/o is a separate ISA board. Maybe next time I get it booted I will see if its possible to set an IRQ for the I/O board. Maybe thats where I need to look.

 

[LorneinCanada]

I get to give everyone a status update today! It's Alive!!!!!.....
I got the Comtex to boot with the HD that it came with. Windows 3.1 on board. I gave up (for today) on fixing the original PSU and used another that is unrehabilitated but supplying power. I figured out how to get into the part of the BIOS that allows you to set HD parameters. I guess thats what got the HD going but I also used a different I/O card. I noticed the original I/O card looked like one of the chips was getting really warm. The little label on it was melting. Found another in the parts box that had an install manual online. I wish I could locate my infrared thermometer gun thingy. I wanted to monitor temps in the PSU and elsewhere. But my finger test (unplugged) said the main filter caps were still cold after being on for ten mins.
I want to build some sort of breakout box (5.25" tray) that would allow me to monitor all the voltages and current draw would be nice. But that's a subject worthy of its own thread.
I'm pretty pumped now. Time to throw all the parts back in the case and start adding things. I should really break down and order a new PSU...... But the wife is on my case. Give things a week or two to cool down first.
So my cpu is an A80386DX-33 IV. Can someone refresh my memory on the co processor. A 387?? is it SX or whats the suffix. What is a Weiteck Processor. I saw that in the BIOS.

 

[LorneinCanada]

So I guess I need a 387DX Copro.
I'm still lost on the RAM situation. I found this info so far for the MOBO. At least I think this is the same mobo

MB-1333PM-CH/MB-1340PM-CH Motherboard Settings and Configuration

MB-1333PM-CH/MB-1340PM-CH motherboard and socket layout, connections, user configurable settings, DRAM configuration, Jumper configuration, floppy drive configuration

stason.org

If this is the correct mobo info. It looks like I can add on up to 32mb ram and 256k cache. When it boots I believe it tells me there is 640k of system ram. I'm not sure if that would include cache or any ram on the video card. So in the large DRAM modules, at least that's my guess at what they are. The chips are labeled MCM6264CNP25 three chips per module. 30 pin modules. I'm not getting much back from google. Except maybe 8kb x 8 bit. Does that mean I have 96k of RAM? I guess DRAM to be exact?

The chips located lower right are labeled HY514400J-70. I'm guessing these are Cache chips. There are 8 of them. In the same area there are two slightly smaller MCM6288CP20 chips.
Any help figuring out what I have would be appreciated.

 

[krebizfan]

I think the labels on the SIMM modules were misread. I would expect the three chip SIMMs to be 256 KB using two chips of one megabit (256 kbit x 4) and one chip of 256 kbit. Thus, the 4 SIMM bank would provide 1 megabyte giving the 640 kB shown.

64 K of cache would make sense for an entry level 386 but chip number leads me to 512 K Byte (Mbit x 4) so I am bit confused.

 

[LorneinCanada]

My sincere apologies. I totally messed that one up. The HY part are the Drams and the MCM626 are the Cache.

 

[krebizfan]

Okay, that makes more sense though I am thinking that the SIMMS then would be 1 MB each for a total of 4 MB and the BIOS should be displaying about 3 MB of extended memory in addition to the 640 K conventional. That fits with a system manages to boot into Windows 3.

Find the coprocessor socket. It should be close to the 80386 chip. If it is square, it should be a 80387 socket and would take a 387-33 chip. If the socket is 40 pin rectangular, the motherboard is older and needs a 80287 or the 287XL (which is a 387 that fits in a 287 socket). Note that few programs that will run at a good clip on a 386-33 will benefit from a match coprocessor. Games that need coprocessor generally expected a faster CPU.

 

[LorneinCanada]

Okay, that makes more sense though I am thinking that the SIMMS then would be 1 MB each for a total of 4 MB and the BIOS should be displaying about 3 MB of extended memory in addition to the 640 K conventional. That fits with a system manages to boot into Windows 3.

Hey thanks for your reply. I'm still pretty confused. But that's pretty much a normal state for me. Online it says MCM624 chips are 8k x 8 bit. There's 8 chips. So I don't know how the Bits fit into the equation. But lets say its 64k. I'm looking at all the jumpers for the Cache and if I have pin 1 figured out correctly. The jumpers are set for 64. Not that any of this is end of world stuff. Because the machine boots and runs. Ya it did say 640. Then it counted more ram. Maybe that was the extended ram count. I'm not too sure of my terminology yet. I can't fire it up till I get this Turbo switch figured out. But it counted up to 3400 or something like that.

Find the coprocessor socket. It should be close to the 80386 chip. If it is square, it should be a 80387 socket and would take a 387-33 chip. If the socket is 40 pin rectangular, the motherboard is older and needs a 80287 or the 287XL (which is a 387 that fits in a 287 socket). Note that few programs that will run at a good clip on a 386-33 will benefit from a match coprocessor. Games that need coprocessor generally expected a faster CPU.

Oh man I pretty much had myself talked into ordering a copro when I got home from work today.... I have not read to deply into the subject. I was wondering if there were any downsides to the extra chip. Do they have a high current draw? Do they gobble up some resource like an IRQ?

 

[LorneinCanada]

Turbo switch problem. My turbo switch has three wires. The board looks like its set up for a two pin line. Are Biostar turbo headers usually like open for turbo and closed for no turbo. Or vis versa? I was running it with no switch present and it seemed to run fine. So from left to right the header seems to read turbo led, cc (of maybe VCC?) , Gnd and two pins for Turbo sw. The case switch however seems to have the center pin as a common and the switch connects to one outer pin or the other. I'm worried I'll ground something out. Maybe I'll leave that off for now. How will I know if I'm running in Turbo or not. What difference does that make? I think I heard non turbo drops you way down to like 6 mhz or something? The Lock power led was a little confusing as well. But I don't see a majour threat of grounding there.

 

[krebizfan]

8K by 8 bits means each chip stores 8K bytes. The bit size means that the chip acts as if it was 8 chips each one bit wide. If you have 8 of them plus some additional chips for parity, that would provide 64K. Note that if you plan on increasing the cache, you may also need to change the tag chips. Vogons might have comparisons of gaming performance relative to cache size. I don't remember gains in gaming performance with larger caches but I could be wrong.

The 80387-33 seems to use little power. The TDP is listed as less than 2W for Intel's model. Installing one wouldn't have any negative impact on the system other than the increased power draw. So if you can find one cheap, install it. However, don't expect any boost to performance with most software. Sure, spreadsheets will be 10 times faster but a modern system would handle those same spreadsheets a thousand times faster.

Increasing the memory to 8MB will handle Windows 3 and Office 4 and most of the games that can run on this system. If you can find 4MB SIMMs that work with it, those might be just as cheap as 1 MB SIMMs and with 20 MB or 32 MB, the system could handle Win95 and Office 95. Excess memory shouldn't be a problem. Unfortunately, there were so many variations on SIMMs that I can't be certain which SIMM would work in which motherboard.

I don't know where to find an answer to the turbo question. I suggest trying benchmarking software with the turbo button pressed and not pressed. I can't imagine any system that has problems if the turbo button was installed backwards.

 

[modem7]

My turbo switch has three wires. The board looks like its set up for a two pin line.

The motherboard photo shows that there is space to the right of the 2-pin turbo header.
Therefore, four of the options shown at [here] are available to you.
However, that will be reduced to two, which two depending on what happens when the two pins are shorted by the switch.

The case switch however seems to have the center pin as a common and the switch connects to one outer pin or the other. I'm worried I'll ground something out.

'Common' is not the same as 'ground'.
At the switch itself, the common wire is not connected to the case (something that is grounded). If it was, then you would need to be concerned.

 

[LorneinCanada]

Another installment of the never ending saga. The other day I got the serial mouse working. Todays battle was floppy drives. So fidling with cables, jumpers and BIOS. All at the same time of course. Win 3.11 sees the 3.5 as D: A. And I installed my Darkwolf demo. That seems all fine and dandy. Even though I'm pretty sure I told the BIOS the 5.25 should be A. The jumper on the 5.25 is at D1. The cable is plugged in straight section to the 5.25 and twist to the 3.5. Latter has no jumper select. I'm ok with this so far as it works. But I still can't get D:B the 5.25 to read a disk. Admittedly the blank could be unformated. The light comes on and the disk spins the whole time the disk is in there. In fact it was still spinning when I pulled the disk out. I should clarify. The motor is spinning. Not sure if the disk is engaged. No spinny sound come whenth. Is that a word? I tried formatting disk B to no avail. The drive is a TEAC 5.25 FD-55GFR 142-U. The floppy's are double density.
Onwards and upwards. I guess. Spurred on by my partial floppy drive success. Time to install a sound card in the 386 project. I have two cards that are for sure ISA sound cards. The Gravis is too valuable to mess with till I get better acquainted with this old stuff. So I guess it's the Adlib. I see Adlib drivers in Win 3.11. I'm curious about IRQ assignments. How do I deal with that??? Plug it in and see what happens? My I/O board has IRQ jumpers. I don't think the mobo has a specific jumper for sound.
How will I know what IRQ it ends up on? I don't remember seeing anything about IRQ's in the BIOS and I don't think Win 3.11 has Device Manager. Could be wrong...
Are IRQ's in any way physically related to the individual ISA slots?
The Adlib output. Is it stereo on Mono?

 

[LorneinCanada]

I found a few pages like this
https://duropc.com/irq-settings-conflicts
And printed them out. I'll go through the BIOS tomorrow. The answer must lie there.

 

[krebizfan]

Windows 3 and MS-DOS 6 have a program (MSD.EXE) which can tell the currently used IRQs. Added cards need to be on unused IRQ. Plug and Play doesn't always grab a free value though I don't know if the specific Adlib card supports plug and play.