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Turbo XT woes

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First post, by FAMICOMASTER

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Now that my AT clone is working... Enough, I feel it's time to move on to my other project:
My Turbo XT clone.

It's a DTK Turbo 640 motherboard
Manual: http://minuszerodegrees.net/manuals/Turbo-XT% … ion%20Guide.pdf
TH99: https://th99.infania.net/m/C-D/30501.htm

It was originally in a case labelled "Turbo Beltron," but I swapped it into an other case, which says "PC Craft." Both cases have almost identical metalwork, the same power supply, the same diskette drives, and both cases had identical motherboards. The board that was originally in the PC Craft case had a clock battery explode on top of it and ruined the motherboard, it shows no signs of life at all. The only difference between the two cases is purely cosmetic, the PC Craft has no rust and the front plastic is designed more to look like an XT than an AT like the Beltron case. Otherwise they are identical.

When I bought the Beltron initially, the machine *seemed* to "work"
That is to say, I plugged it in, flipped the switch, the power light came on, and it made a POST beep. A single beep and it went silent except for the fan. Nothing out of the ordinary. I didn't have any sort of MDA/RGBI monitors to use with it, and I didn't have an 8-bit friendly VGA card either, so it sat under my desk for about a month.

I pulled it out recently since I finally got a working MDA monitor (A lovely little 12" Samsung green screen monitor I got for a very good price). The Beltron case looked awful, so while I was waiting for the monitor to arrive in the mail, I pulled the board out and stuck it into the other case. I also took the better of the two power supplies with it.

A while back, I'd pulled the first two banks of RAM (18x41256 DRAMs) to install in my AST Advantage for the AT clone. I pulled the bank back out of the AST Advantage and installed it back in the motherboard in the same two banks. I didn't touch the second two banks of RAM (18x4164 DRAMs). During this, I reseated the BIOS (the socket looked a little worse for wear) and installed an 8087-2 coprocessor. Used the manual to set the switches and put the board in the case, installed the generic I/O card it came with (https://th99.infania.net/i/C-D/52639.htm), which I set up to the best of my abilities... It is missing the two large jumper blocks across the bottom though, which are also unlabelled. FCC ID says it was made by Yangtech in Taiwan, but I have nothing else on it. Put a Video7 VEGA card in with it (https://th99.infania.net/v/U-Z/52394.htm), setting the switches for a monochrome monitor.

Flip the switch and... Keyboard lights flash and then one long beep. The beep never ends. Ever. It is at a constant tone and starts immediately after the keyboard lights flash, and nothing happens. Nothing on screen, no beep codes, just one tone until you hit reset or shut the machine off.
I've reseated all of the RAM chips (They all tested good in my AST, but just to be sure I swapped bank 0 and 1 with each other and still got the same results), I reseated the BIOS EPROM, and I completely removed the diskette drive controller and the 8087-2.

It's now an NEC V20, 640K, and a video card. There is nothing else installed.

It seemed to be working fine not long ago, what could have happened? How can I fix this? If I can just get it to put something on the screen I'll be content with it for now, even if it won't boot or something.

P.S. In this picture, it is the machine on the right. The machine on the left with no cover is my Turbo AT. The monitor is sitting on top of this Turbo XT.
MVIMG_20191010_204942.jpg

The current setup on the bench:

IMG_20191011_181354.jpg
IMG_20191011_181403.jpg

Reply 1 of 33, by Caluser2000

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Check the turbo switch cable routing. The one on my XT turbo ran under the mobo and caused a short of some kind when turbo was selected. Took a while to figure out what the issue was but in the end I disconnected the connector on the mobo and all was right again. Couldn't be bothered pulling everything out to reroute the cable and I can select the 3 various speeds by a couple of key presses anyway.

Last edited by Caluser2000 on 2019-10-12, 00:13. Edited 1 time in total.

There's a glitch in the matrix.
A founding member of the 286 appreciation society.
Apparently 32-bit is dead and nobody likes P4s.
Of course, as always, I'm open to correction...😉

Reply 2 of 33, by HanJammer

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Very nice. I have the same case and the same (or very similar, but those TurboXTs all look the same to me) board. I'm saving the parts for long winter nights having a plan to build some cool XT (this will be my third XT, but first built by me).

It's nice to see this Samsung monitor is working! 😀

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Reply 3 of 33, by FAMICOMASTER

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Caluser2000 wrote:

Check the turbo switch cable routing. The one on my XT turbo ran under the mobo and caused a short of some kind when turbo was selected. Took a while to figure out what the issue was but in the end I disconnected the connector on the mobo and all was right again. Couldn't be bothered pulling everything out to reroute the cable and I can select the 3 various speeds by a couple of key presses anyway.

There is no turbo switch connected at all. The old case had a reset button, but neither case had a turbo switch.

There's a jumper on the board to switch between a mechanical switch and a keyboard switch "Control + Alt + -" I believe. It's in the keyboard position right now. I think you could probably set both, but neither are.

I'll be pulling the board from the case entirely to check for shorts.

Reply 4 of 33, by FAMICOMASTER

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HanJammer wrote:

Very nice. I have the same case and the same (or very similar, but those TurboXTs all look the same to me) board. I'm saving the parts for long winter nights having a plan to build some cool XT (this will be my third XT, but first built by me).

It's nice to see this Samsung monitor is working! 😀

This is my first XT ever, I just got real lucky and ended up with a lot of parts for it.

Samsung is pretty good, it produces and EXTREMELY bright picture. Needs some adjustment and cleaned pots / switches, but it's great!
I've got a nice color monitor coming in the mail as well.

Reply 5 of 33, by Caluser2000

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It's great if you get one with a few spares. My Redstone XT Turbo flip top had a TVM EGA monitor with it but the hdd was dead. Just put it storage for a bit then decided to fit an XT-IDE hdd card in it. I got mine from MonoTech PC here in New Zealand. He was local and no shipping costs. Fitted a 2gig BigFoot IDE drive I had laying around doing nothing. Works wonderfully and certainly recommended. Saves hunting around for RLL/MFM matching hdd/controllers. I was lucky enough to get the mobo manual with it along with the complete system guide. If the monitor ever dies I've got a couple of 8-bit vga cards for it.

The attachment RedstoneXT.JPG is no longer available

They're certainly big beasts. I hope you get plenty of enjoyment from yours.

There's a glitch in the matrix.
A founding member of the 286 appreciation society.
Apparently 32-bit is dead and nobody likes P4s.
Of course, as always, I'm open to correction...😉

Reply 6 of 33, by FAMICOMASTER

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I won't be getting any enjoyment if it's not working at all - Like I said, it just makes one tone on power up. Flip the switch, keyboard lights flash, and it makes a beep. Nothing else.

Does anyone have a list of beep codes or any troubleshooting advice for this DTK board? Maybe a replacement I could buy for cheap if it's pooched?

Reply 7 of 33, by FAMICOMASTER

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UPDATE: I took it out of the case and put the original MDA video card back in. Also plugged the original power supply in (Even though they are identical)

The machine now boots!

I get this message on boot:

Phoenix ROM BIOS Ver 2.27
Copyright (c) 1984,1985,1986 Phoenix Technologies Ltd
All Rights Reserved


Keyboard Bad

8087 NMI at F000:E35E.
Type (S)hut off NMI, (R)eboot, other keys to continue_

I shut it off, reseated the 8087-2, and lo and behold I get a RAM count!
It still says "Keyboard Bad" (I'm using an IBM Model M - I thought they auto switched to XT mode?), and now I get a RAM error.

It will count to about 512K and says:

40 RAM Bad 80040
Error. Press F1 key to continue_

MVIMG_20191013_172716.jpg

First of all, how do I put a model M into XT mode? Second of all, how do I figure out which RAM chip is bad and replace it?

Reply 8 of 33, by HanJammer

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RAM chip - use Landmark diagnostic (if you can) - it will show you the address with failed chip - it's pretty easy to locate it then. If you can't do that - take some other DRAM chip (which you know is good) and remove chips one by one replacing them with this good one (trying to boot after every replacement) - eventually you will stumble upon a bad chip (ie errors will stop). You can also leave as little RAM as you can and test single bank - if you stumble upon error - it contains bad chip - replace half of the chips with chips you removed - no more errors - bad chip is in the half you removed - then replace any chip with one of the chips you removed and test... Without proper software tools or DRAM tested it's basically trial and error.

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Reply 9 of 33, by Horun

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HanJammer wrote:

You can also leave as little RAM as you can and test single bank - if you stumble upon error - it contains bad chip - replace half of the chips with chips you removed...

That is what I was going to suggest. Take all out but bank 0 and test, if passes then install bank 1 and test, etc. Also you might try gently pushing down on each chip to make they are snug in the sockets first

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 10 of 33, by Caluser2000

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Does the Model M work in other systems? Could have some bad caps whitch is not uncommon. One of these might be useful in the long run https://monotech.fwscart.com/PS2_to_XT_Keyboa … 4_19714002.aspx Even if it's just for testing purposes.

There's a glitch in the matrix.
A founding member of the 286 appreciation society.
Apparently 32-bit is dead and nobody likes P4s.
Of course, as always, I'm open to correction...😉

Reply 11 of 33, by FAMICOMASTER

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Well, I tried the BIOS from my parts motherboard and now it... "Boots"

I think I like this "TURBO XT" BIOS better, it's pretty neat. It displays the list of expansion cards it can detect and the RAM counter at the bottom.
It doesn't tell me the keyboard is bad, but it does ignore any input from it. It also doesn't find any error with the RAM and gets through all 640K just fine.

With the DTK multi I/O board installed, it will seek both diskette drives and it will even attempt to boot MS-DOS from the 360K diskette I made! Granted, it seems to lock up after "Starting MS-DOS..." and gets stuck on a flashing cursor. The diskette drive is still spinning but I can't hear it seeking anymore.

The first 512K are all 41256 DRAMs, and the last 128K are 4164 DRAMs, so I'll try swapping the two banks of 4164s to see if it changes anything. If bank 3 is bad, then moving the chips to bank 4 should give me 576K before it throws the error with the Phoenix BIOS.

I don't have the Landmark BIOS, nor an EPROM programmer, nor the appropriate EPROM to put it on. If I had one, I would try. My hope is that I can make it boot into DOS with 512K and then I can use CheckIt or something to go from there.

However, to do that, I need it to:
A> Not lock up trying to boot DOS with the Turbo XT BIOS and
B> Get the keyboard to work somehow.

If I can get the keyboard working, I can try booting DOS with the Phoenix BIOS as well, since it halts with a memory error.

The model M works perfectly well with my modern Dell Optiplex 7010DT, which has onboard PS/2 ports, it works with my turbo AT clone using a PS/2 to AT pin adapter, and it works with my Compudyne 3SL/25 laptop no problem.

The problem with one of those adapters is that not only is it out of stock, but it's coming from quite far away, and I'd like to have something working right now. I can live with the wrong keyboard if I can just get it to work. Besides, I have a real model F somewhere, but it will take some digging in storage to find it. I bought it a long time ago to use with an AT class machine, which it didn't work with correctly, so I stuffed it in my attic somewhere at my parent's house and never went back for it. I'm thinking I'll grab it next time I'm in the area.

EDIT: I used some other diskette drives and it does the same thing regarding the MS-DOS diskette. Either it's incompatible with the BIOS or DOS 6.22 doesn't like an NEC V20 and an 8087-2.

IMG_20191013_175600.jpg
MVIMG_20191013_175549.jpg

Reply 12 of 33, by HanJammer

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There is so much you can improve with these XTs and related devices, but you really need some programmer (cheapo TL866-II from ebay and you are good!). Since I got TL866-II I'm able to do so much things and repair/test thing in a few minutes which otherwise took me several days (visitng friend with a programmer and so on) - worth every cent spent on it.
For example you can replace the BIOS with this: https://www.phatcode.net/downloads.php?id=101 which is pretty much the ultimate XT BIOS with many cool features.

Anyways - this replacement BIOS probably skssp the RAM error but it show up during DOS boot and system just hangs...

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Reply 13 of 33, by FAMICOMASTER

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I'm running "Turbo XT version 3.1" already, it shows all the same information at boot as the one you linked. The Phoenix BIOS is much different, but this unbranded one works fine.

I'm going to set it to 512K on the switches and see if it will boot properly from there.

I just want to get to a DOS prompt. Any suggestions for the keyboard? I've hears Model Ms can switch to an XT mode, but I don't know if this is true or how. Any help?

Reply 14 of 33, by Caluser2000

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Should auto detect but is isn't unusual for them to flake out because of a bad cap.

There's a glitch in the matrix.
A founding member of the 286 appreciation society.
Apparently 32-bit is dead and nobody likes P4s.
Of course, as always, I'm open to correction...😉

Reply 15 of 33, by FAMICOMASTER

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Caluser2000 wrote:

Should auto detect but is isn't unusual for them to flake out because of a bad cap.

Any way to force it into XT mode? I'm not opposed to taking it apart and putting a wire across something for now.

Reply 16 of 33, by FAMICOMASTER

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UPDATE: Pulling the chips from bank 3 and 4 and installing bank 4 into bank 3's positions got it to boot to DOS 6.22!

I guess one of my 4164s is bad or something. I'll put them into bank 4 and try the other BIOS again, maybe it will tell me what chip it is.

Reply 17 of 33, by FAMICOMASTER

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UPDATE (Again):

Putting the bad chips into bank 4 caused it to stop booting entirely. I pulled the 4164s from an old "Magitronic" MDA / Parallel card I won't be using and stuck them into bank 4. I was one chip short so I picked one at random from the original bank 4, and now I have a full 640K on the board! Both the Turbo XT and Phoenix BIOSes boot the machine to a DOS prompt now, but I still can't get anything from a keyboard. I think I'll put it back into it's case for now while I look around for advice on the keyboard situation.

MVIMG_20191013_192000.jpg

Reply 18 of 33, by Horun

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Good job FAMICOMASTER ! Think I would stick with the original BIOS until you get the KB thing figured out.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 19 of 33, by Anonymous Coward

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In my experience, the model Ms don't really work properly on XT computers. I had the same problem with my turbo XT board...the keyboard worked intermittently. I ended up having to use an AT to XT keyboard converter to get it working properly.

There is a special model M for XT computers that doesn't have LEDs. It was sold with a later XT motherboard that had a BIOS that supported the extra keys. Not sure if that particular keyboard works on normal XTs, or if that motherboard supports regular model Ms, but maybe both are required to work.

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