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IBM XT TM100 Floppy Drive Repair (Twice)

Cyberstein

Experienced Member
Joined
Dec 26, 2021
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64
I recently purchased an IBM XT 5160 with dual 360K full height Tandon drives. When I first received it, I found it had multiple shorted capacitors on both drives, as well as the motherboard. The defective ones were the common defective tantalum capacitors on the 12V rails. I have indicated in the attached picture the two defective capacitors on the drives. I should note this is not a picture of my drive but one I got off the internet. I just got foot surgery, it was easier to grab a photo off the internet. And my wife just yelled at me for fixing the second issue I will tell you about shortly. As I am supposed to be doing nothing with my foot elevated. I literally just got home from hospital but the part came in today. I was excited to repair the issue and get my PC back fully functional.

As I said, when I first received the PC, I replaced all the shorted tantalums with new equivalent or higher rated capacitors. The computer then worked great with both Tandon's fully functional. It had no hard drive though, it never had one. Going off subject now I know, lol. But I must share my experience with you guys, nobody else wants to hear it that I know. To remedy the hard drive issue, I installed an IDE XT card of course, the deluxe version from Blue Lava Systems. I also replaced the standard monochrome card with a Hercules Monochrome graphics card. I love playing games in phosphorous green :) It did have a SixPakPlus card already, the short version, but the RTC battery was dead. I replaced it and setup my autoexec.bat to configure the clock on bootup.

Okay, so now back to the point, failure two. It worked great for a few weeks like this, until one day, Tandon drive B: just stopped working. When you went to access it by typing B: <ENTER> it would just attempt to read for awhile and state that it fails. I went through all the normal troubleshooting steps first to name a few...

1) Check supply voltages to make sure no other components shorted.
2) Swap drives to make sure it isn't a controller or cable issue.
3) Clean heads and check drive belt.
4) Verify head moving freely and not stuck.
5) Verify all wiring/connectors secured, making good contact/not corroded and in good condition.
6) Verify micro-switches working for disk detection and stepper motor home detection.
7) Visually check head and disk spin motors working.

After doing this I decided it must be an issue with the control board. Most likely the read head since it would attempt to read a disk when I tried to access the drive. The red light on front would come on for awhile and the disk would spin. But after some time, say 10 seconds or so, it would state drive read failure. I was able to find the service manual for this drive online at "deramp.com" which was amazing. It contains the schematics for the drive which is also amazing. After some troubleshooting, I found the Texas Instruments UA733CN differential video amplifier was defective. I believe the bond wire on pin 8 has opened internally based off my resistance measurements. You can identify this component as defective in circuit by looking at pins 1/14, 7/8 and 4/11 with the drive powered. The voltages on pins 7/8 should be nearly identical and the same should be true with 1/14. I believe the voltages at 1/14 should be around +8VDC when idle. The voltages at pins 4 & 11 should be ~+4.5VDC. In my case, the voltage was ~+0.115VDCat pin 8, and ~+2VDC at pin 4 if I remember correctly. (I use the '~' symbol to indicate approximately)

I was amazed to find this part was available at "mouser.com" which is the number one place to order parts in my opinion. I ordered it and received it within 2 days. I simply removed the old defective component and installed a 14 pin socket. I almost didn't install the socket, but I do notice a lot of the vintage cards I get that are repaired people do this on. After thinking about it, I decided this is good practice to keep these old vintage electronics alive. Think about it, every time you rework these boards, no matter how good you are, it degrades them some. If we socket failed components then we will no longer have to degrade the board in the future if that part needs replaced again. And it will be a much quicker repair. After installing the socket and new component. I reassembled the drive and installed it back into my XT 5160. The drive now works flawlessly again and I once again have a fully functional IBM XT 5160. More off subject comments just to share lol, I also installed my new external SYSGEN 3.5" 720K drive and it works great too. This being said, my XT 5160 now has X2 360K 5.25" Tandon drives and a single external 3.25" 720K SYSGEN drive. I hope this post provides help to others or at least some vintage reading pleasure. I enjoy sharing this experience with the few of us left out there. As this was my first experience working on a TM100 Tandon drive.
 

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cyberstein,
There are a couple of other things you should check on your Tandon TM-100 Series drives.
The two 10uF 16 Volt Caps in the RPM Board have been known to leak acid and some PCB
traces on the RPM Board will be gone. I'd suggest taking a quick look to see the status of those
two caps. You can save yourself a lot of rework on the RPM Board by replacing those caps.
Also verify the RPM for each drive is 300 RPM.

Your Hinge points on the Floppy drive door(s) need to be looked at. If you have a full length
Brass rod for the hinge point, you are fine. (Your Drives are likely old enough that they still
have the full length Brass rod for the hinge part.) But, if there are two short White Nylon Pins
used for the hinge point, they need to be removed and replaced by a piece of 3/32" Brass rod.
If the small hinge part is broken (by the use of the two small White Nylon Pins) it also needs
replaced. Those parts are available from 3D Resin Printed parts for the Tandon TM-100
and Texas Peripheral Incorporated Floppy Drives (TPI).


Larry
 

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Thanks for the info Larry! I do luckily have the brass rods on my drives and they are fully intact. It's also good to know there is already 3D models for the hinges in case I need them! I will be proactive and replace the two additional capacitors on the motor controller board you mentioned. Thanks again!
 
I re-checked the Caps that are on the RPM Board and they are 16 VDC and 47uF.
I was going from memory, and at my age that wasn't the correct thing to do. I bought
my replacement Caps from Digikey.

Larry
 
Thanks for sharing @Cyberstein. I do enjoy reading about the old 5150 & 5160 machines.

Weird though that you got a 5160 without a hard drive. Wasn't that an XT to begin with? The XT had a hard drive from factory, no?

My FIL had an early 5150 with the two 360k floppies and we added a HD later. I have a 5162 (AT Class) and it had an old RLL/MFM hard drive in it when I got it and a 5-1/4 360K FD. I could not get the 360K drive to work (not as smart as you and Larry on floppy drive repair) but I did get a 1.2MB drive to work. Then I low level formatted the HD using debug and it now boots PC-DOS 3.3 from HD.

Seaken
 
Sorry just saw this. Yep, it came with two full height Tandon drives, no hard drive. It's badges state XT and do not look to be tampered with and it definitely has XT motherboard. I have a 5150, so I know the difference. Jealous of your 5162, I have been looking for one of those. ;]
 
I guess they sold the XT without a hard drive as an option. I did not know that. I am happy to have an original IBM in the 51xx series. But I would love to find a working 5150 since it was the first one I worked with. Ours was upgraded also with a Hercules graphics card but still running the original green monochrome screen. I would like to have that setup again. I upgraded the XT-286 with a VGA card. I have another clone with an Amber screen but Mono text only.

Thanks for sharing.
Seaken
 
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