The HP-85 tape drive was never going to be a daily drive proposition and the HPDrive emulator, good as it is, is a bit unwieldy for a convincing demonstration. I really needed (wanted?) a disk drive.
I kept an eye out for a disk unit. They are usually very expensive and i’ve only seen them listed for sale in the states, from where the cost of postage is horrendous. I did find a cheap 3.5” unit, but the thought of paying nearly $200 for postage prompted me to scream for help on one of the facebook groups.
To my surprise a gentleman Ben G happened to have a M82901 Dual Drive uniit out in a container at his property in NSW. He was kind enough to send some pics, explain the unit’s shortcomings, offer it at nominal cost, and arrange for its dispatch. That was a generous thing to do.
When it arrived it was clear from some dents and gouges that it had landed very heavily at some stage. Ben said that he’d recovered it from a gold recycler.
There were rattles in the box and once opened some damage was apparent. Both of the drives had parts of the plastic broken. The interface card was only secured by a couple screws used to secure the circuit card on the right drive. Not a great effort from HP really but I guess they weren’t planning on the unit being thrown on the scrap heap.
I attempted to glue things up but it will always be fragile. I pulled it all apart, cleaned it up, checked the supplies and put it all back together.
The drives are PC style double height double sided units. The heads were not moving easily so they needed some lubrication and bit more rough treatment to get them moving again. Once they were free, i connected them to a PC for alignment using ImageDisk.
One was pretty good on the PC but the other had some issues formatting; successful but with lots of retries so I subbed in a known good spare drive.
There’s a set of dip switches that set the HP-IB address. The drives are referenced by “:D720” where the second digit is the HP-IB address and the third is the disk drive number.
Format with:
INITIALIZE “<label>”,”:D720”
The drive unit failed to communicate.
Fortunately, the manual was available online and the unit has built in self tests. It took a while to work out how the tests worked and they seemed to produce very erratic results. It seemed to consistently show that the RAM test was failing so I ordered some new RAM. 128 byte static RAMs.
I also read back the ROMS using an EPROM programmer and that suggested that one ROM didn’t seem to be working. I scrambled to see if the binaries were online. Fortunately there were.
The natural replacement would be 2716 EPROMS but I only had 2732 EPROMs. They have a compatible footprint but with the upper half being the active portion. My EPROM and cheap EPROM programmer did not play nicely together but eventually i convinced myself that i had two working EPROMs.
After spending a long time searching for a problem which turned out to be incorrect configuration, i was able to get all of the tests to pass with a known good drive. That meant the controller was good. Even better the unit worked with the HP-85!!
Neither of the two original drives wanted to play the game. I had a good look at the writes on the first drive. Everything looked good except the signal on the top head was different to the bottom – about half the size. That made me have a closer look at the head which I discovered had been damaged – it looked like a piece of ferrite was missing.
The other drive’s heads seemed ok, and I’d checked the first drive’s electronics pretty thoroughly, so I thought I might get a second drive working by swapping the electronics card. That worked.
Sure enough the HP 85 could happily operate two drives.
The other original drive now has an electronics fault as well as a drive head fault!
I needed some disks to try it all out. The teledisk images I had seemed to often be for 3.5” drives and wouldn’t write to 5.25” disks. Instead i used the HPdrive emulator and copied them across:
COPY “:D700” to “:D720”
It’s not fast.
I found many of the disks would not run unless labelled correctly. The failing line of basic revealed the required label.
VOLUME “:D720” IS “STDPAC”
I could copy from the tape drive to disk:
COPY “:D700” to “:T”