Kaypro 4/83 Floppy Disk Woes.....

I have written in earlier posts of my project to interface the FreHD hard disk emulator to my Kaypro 4/83.  

The standard Kaypro ROM does not include hard disk support so a third party ROM is required, the most common being KayPlus from MicroCode Consulting or the Advent TurboROM from Plu*Perfect Systems.  

The initial project work was done using KayPlus because there was no known source for the Advent TurboROM compatible with the Kaypro 4/83 (the version for the 4/84 was readily available but the two machines are sufficiently different that it can not be used).  The KayPlus ROM worked but there were a few odd behaviours running some CP/M applications that needed further investigation.

In May 2014 a Kaypro II was purchased from eBay by phorgen on the Vintage Computer Forum that included the rare '83 version of the Advent TurboROM and phogren was generous enough to share a copy of the ROM to help with this project.

Between June and September 2014 the Kaypro sat untouched while "real life" got in the way and I finally got back to it on October.

Working with FrankS (also of Vintage Computer Forum fame) we got the 4/83 + FreHD to boot using the Advent TurboROM (details will be included in a future post) but I had problems reading and writing floppy disks.  

The initial symptoms where that Advent CP/M would not recognize standard Kaypro format floppy disks.  Nor would it read a disk that it just formatted.  

My initial thoughts were that the Advent ROM was not compatible in some way with the KayPro 4/83 so I swapped back to the standard Kaypro ROM.  That wouldn't boot either.

Now suspecting the floppy drive I tried a number of different drives and none would read a disk.  

Finally after closely watching the drive while formatting a new disk I realized that the heads were not stepping even though the formatting program was counting up the tracks.

Given that track stepping is directly controlled by the Western Digital WD-1793 Floppy Disk controller, I replaced the Kaypro WD-1793 with a known good part from a TRS-80 Model 3

The known good WD-1793 worked fine in the Kaypro which was a big relief.  Not sure if it was old age or caused by my handling of the Kaypro PCB but at least now I knew what to replace.

On eBay I learned a few things about the WD-1793 (or FD-1793).

  • The WD-1793 (or FD-1793) is expensive.  
  • They are also frequently described as "collectible" or "super rare" which doesn't inspire confidence that a working part will be had for the money.  
  • The Soviets made a compatible part called a KR1818VG93.  This was even more expensive and collectible.  They may also be metric pin spacing so even if a working part was obtained it wouldn't fit well into a Kaypro socket.
  • Mitsubishi made a compatible part called a M5W1793 which were readily available as NOS parts from China at a much better price. 

The M5W1793 seller (no relationship.... just a satisfied customer) had good eBay feedback so I ordered 2 units hoping they would be genuine.

Two weeks later (very quick.... normally it takes 3-4 weeks from China to New Zealand) and the parts had arrived when installed in the Kaypro worked perfectly.  



The Model 3 can have it's FD-1793 back.



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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn