OK, OK, I know... the HP-85 is really not a calculator. It is a desktop computer.

But it's an HP, it is beautiful, and I have one in good working condition, so it deserves to be mentioned here. Besides, back in the late 1970s when I was a high school geek, I actually had a poster of this machine up on my wall; an unattainable dream of course, but that's typical of posters you see on the wall in the rooms of high school boys, isn't it?

Anyway, this dream is no longer unattainable; recently, while on a trip to Hungary, I came across an HP-85 in good working condition, which I obtained for a reasonable price. And I was able to haul it back to Ottawa without damage.

The good news was that the machine worked out of the box. The bad news was that it wasn't fully working; both its printer and its tape drive had problems.

The printer had a problem that is apparently a well known issue not unlike the problem with HP-67 card readers: the rubber material of its drive belts has disintegrated over time. Fortunately, a fellow calculator enthusiast sent me a set of new belts, for which I am grateful. That was sufficient to bring the printer back to life.

As for the tape drive, my main problem was that I had no compatible tape cartridges. Once again, a fellow enthusiast was able to help: I received a set of tape cartridges. But when I tried to use them, all I got was an error message. Upon closer examination, I found out why: the rubber drive wheel in the tape drive also disintegrated. Ouch.

In the end, I was able to fix the tape drive by using a particular type of model airplane fuel hose that I obtained earlier as possible repair material for those faulty HP-67 card readers. It wasn't very good for that job (another type worked much better) but its flexibility allowed it to be used on the drive wheel in this HP-85, despite the wheel's much greater diameter.

At this point, I found that Murphy's law is alive and well. Just when I thought that my tape drive is ready to be tested, the HP-85's power supply gave up completely. Not a sign of life anymore. I suspected a blown fuse, but I found none. I looked hard for a fuse on the circuit board but there wasn't any. Someone, however, suggested that there is one. So I checked. And checked again. On the third try, I found the fuse and it was, of course, blown. After I replaced it, the machine was working beautifully. The tape drive, however...

Well, the tape drive seemed to be working, but my (brand new, "new old stock") tapes seemed to be damaged. At the spot where the tape was pressed against the drive belt inside the tape cartridge, the oxide material just came off. (Probably 15 years in the same position was too much.) To make a long story short, resurrecting a skill I first learned when I was a teenager and cassette tapes were my prized possessions, I spliced the tape and removed the damaged portion. Now I have a working tape cartridge.

Apart from the fact that it's a bit slow, the HP-85 is a remarkably powerful computer for its age. 16 kB of memory makes it quite useful; the integrated printer is reasonably fast and graphics capable, and the tape drive is surprisingly reliable. I/O cartridges allowed control of external devices, so the HP-85 was used often in instrumentation environments.

I have yet to revive an HP-IB cartridge that I have for this HP-85. With that cartridge, I may be able to control external instruments or, possibly, use (through an HP-IB/HP-IL interface) an HP disk drive for storage. Unfortunately, I do not have a necessary component: The I/O ROM, so for now, I store my little programs on my recently spliced tape.

One of my little programs, needless to say, computes my favorite, the Gamma function. It also plots the Gamma function graphically, thus demonstrating many of the capabilities of the HP-85:

 10 G0=0
 20 R=5
 30 GRAPH
 40 GCLEAR
 50 SCALE -R,R,-R,R
 60 XAXIS 0,1
 70 YAXIS 0,1
 80 FOR X=-R+.001 TO R-.001 STEP R/128
 90 S=1
100 IF X>0 THEN 130
110 S=-1
120 X=-X
130 RESTORE
140 READ D
150 READ N
160 READ G
170 FOR I=1 TO N
180 READ P
190 G=G+P/(X+I)
200 NEXT I
210 G=LOG(G/X)+LOG(X+D)*(X+.5)-X-D
220 IF S>0 THEN 240
230 G=LOG(PI)-LOG(X)-G-LOG(ABS(SIN(PI*X)))
240 G=EXP(G)
250 IF S<0 THEN G=G*SGN(-SIN(PI*X))
260 X=S*X
270 IF G0*G<=0 THEN MOVE X,G ELSE DRAW X,G
280 G0=G
290 NEXT X
300 BEEP
310 END
860 DATA 6.5,6
870 DATA 2.50662827477
880 DATA 573.853447696,-859.2982735,379.464193178
890 DATA -50.1620150656,1.1578192022,-3.05418838789E-4

Almost the same accuracy might be obtained with the following values:

960 DATA 5.65
970 DATA 5
980 DATA 2.50662827563,225.525584619,-268.295973841
990 DATA 80.9030806935,-5.00757863971,1.14684895435E-2

Er, did I mention that Murphy's law is alive and well? Not long after I finished this article, when I was about to pack up the HP-85 and store it away, I decided to test its tape drive one more time. Buzz, buzz... STALL. END OF TAPE. CARTRIDGE OUT. What the...? Even worse, it managed to despool my tapes as I repeatedly tried to bring it back to life. Very frustrating. I suspected a faulty ribbon cable (those are 25+ year old mechanical connectors after all) but no amount of reseating and reconnecting fixed the problem. I had to store my beautiful HP-85 away in a sadly broken state.

That was over a year ago. Recently, however, I received a spare tape transport from Mike Davis (thank you, Mike!) with which to try and revive my HP-85. I tried... and found out that the fault was not electronic after all.

You see, the tape drive's little motor has a transparent rim attached to its shaft, with a fine, printed grating on it, matching the grating on a window attached to the motor housing. A small incandescent (!) light and a photodetector are used to measure the motor speed and detect when the drive stalls for any reason. When I reassembled the tape drive after replacing the transport wheel, at first I managed to put the shaft on too tight, so there was so much friction, the motor didn't even turn. I then loosened it a little, but apparently not enough; there was still friction, enough friction in fact to damage the plastic window, rubbing off the printed grating, so after some use, the unit could no longer detect when the motor was rotating. Once I noticed the problem, the fix was relatively easy: the plastic window still had areas that were undamaged, so I was able to remove it and reattach it so that an undamaged area was above the light source. Once again, the tape drive works (almost) like new.