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Kaypro 4 Tandon DSDD Drive B problematic

Armitage

Member
Joined
May 31, 2020
Messages
40
Hey all, I recently was given a Kaypro 4 Plus 88 machine in good working order. It boots and runs CP/M and DOS just fine from drive A. Drive B, however, is another story.

Symptoms:
- Can sometimes display a directory listing from original Kaypro disk
- 1/10 chance that original copy of Wordstar will load to the title screen but never gets past that point
- Can format and verify a blank disk
- Cannot display the directory listing of a disk it just formatted (just spins)

What I have done:
- Cleaned read/write heads with cotton swab and IPA
- Cleaned and lubricated drive rails
- Ensured stepper motor moves through its range during format operation
- Calibrated motor speed for 60hz mains power
- Swapped cable around so drive B was using drive A's connector

Any suggestions what I should try next? Scratching my head here because it seems to work correctly when formatting and verifying but seems to have a lot of difficulty reading.

Thanks!
 
1. Take a look at the RPM Board on the back of the Drive. Check the two 16 Volt 10 uF caps connection
to the RPM PCB. Lots of these caps leak acid and eat away the PCB traces. If that's the case you can
rebuild the PCB with copper Hollow Rivets, and lay wire wrap wire over the missing PCB traces.

2. Check the Door Hinge point by having the door Open and look for two short White Nylon Pins that are
used for the hinge points. If there are two short White Nylon Pins (on either Floppy Drive) these need
to be replaced with a piece of 3/32" Brass Rod that is full length. The two short White Nylon pins cause
the Hinge to break an ear off, making the door not close properly. When the door closes the arm should
preload the hub a bit, so the hub grabs the floppy Media. If the Hinge is broken there are spares available.

3. Check the Hub that connects to the stepper motor which moves the Taunt Band. It should be snug
enough that it doesn't slip on the motor shaft causing mis-alignment. Just don't over tighten the screw.


Larry
 
Great stuff, thanks! I will start on #1 and work my way down the list tonight!
 
I checked items 1-3 and they all seem okay. The caps tested out fine and I even swapped the RPM board over to drive A and it continued to boot and work normally. I guess the next step is to look at radial alignment unfortunately I don't have a scope (yet). I did swap another similar drive from a TI99-4/A and it worked fine. Even read the disk my bad drive B formatted but couldn't read itself.
 
Also make sure there is no visible brown oxide on the Read/Write head(s). Gently wipe
across the Head(s) with a cotton swab dipped in Alcohol to remove any deposits. The
older floppy's may be leaving deposits of brown oxide as they scrub across the heads.
If it is a Single Sided Floppy Drive, make sure the pad on the side opposite the head is
intact. They sometimes disappear with age.

What is the Manufacture Name and Model of the Floppy Drive? Tandon TM-100's?

If you Power Supply has a single row of Pins for the Low Voltage side (Output) that
uses a Single long connector, remove the Power Supply and check the Pins solder
joint on the backside of the Power Supply's PCB.

REF: https://forum.vcfed.org/index.php?threads/the-tale-of-the-good-kaypro-and-the-bad-kaypro.1238494/
Posting #14.

Larry
 
It's a Tandon TM-100-2A. I toned out all the pins on the PCB, checked the solder joints, and power supply voltage. Still no luck. Then I swapped in a known good spare drive for the bad drive and wouldn't you know it, it had the exact same problems! So I put the bad drive B back in, this time as Drive A and... it worked just fine. So not a drive or cable problem at all. Reseated the WDC FDC chip on the main board with a little contact cleaner and wouldn't you know it, I have two fully functional drives now :) Mystery solved!
 
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