I bought one of these recently (I have an entry in the Test Equipment board under 'List your test equipment "scores" here! - post # 2095 on page 84 (yikes!)).
It's been suggested I do a teardown but I won't for 3 reasons.
1 - I'm a lazy bastard!
2 - My camera is sadly lacking in ability to take good photos of the kind expected here (or maybe I'm just no good at taking photos!)
3 - There's not a lot to see. Just two boards full of TTL. And this is your grandfathers first generation TTL. None of this new-fangled LS TTL!
The instrument I received seems to have been manufactured in 1979 - the chips have 78 and 79 date codes. It seems to have been originally released in 1975 - at least the manuals have a release date of January 1975. They didn't make all that many of these. Mine, from 1979, is serial 1914A00955. On the other hand, the manual is confused as well - the HP serial number scheme back then was that the first two digits indicated the year of the latest revision to which this item was built, yet the 1975 manual says it applies to instruments with serial prefix 1914A.
Anyway, as received it didn't work. The front panel contains a bunch of toggle switches, a 6 position LED matrix display and 10 other leds. The LED matrix display didn't display anything, no matter what switch settings were chosen. The individual LED's did work and their state followed that of the appropriate toggle switch. I.e., set EOI to on and the EOI LED lit up. And so on.
One Artek manual purchase later and I'm ready to start. If the matrix display isn't working the place to start was on the display board and work backward. And so it proved. No activity whatsoever anywhere within the multiplexers driving the display. I traced it back to lack of a clock signal driving that part of the instrument.
They used a SN7414 hex schmitt inverter to generate the clock. Well, one sixth of one anyway. I'd completely forgotten that one can generate a clock with such a simple circuit. A feedback resistor between the input and the output and a capacitor from the input to ground. Simple. Anyway, there was no output from this chip. Short the input to ground and I'd expect the output to go high but it didn't. Short the input to Vcc and I'd expect the output to go low. It didn't. So there it was. A dead SN7414. (The chip in question is U113).
To confirm it whilst I waited for Digikey to ship the replacement chip I set up my 3325A to a 2.5V p-p 6 KHz square wave and injected that signal into the output side of the inverter. The display sprang into life.
They actually use two of the inverters in the same configuration to generate two clocks. Neither one was clocking.
Now this is generating clocks to drive a display multiplexer and one wouldn't expect that it needs to be particularly accurate. So I had a chuckle at the resistor value they used in the feedback. 392 ohms! Not 390, nope, 392. Yes, that's a value in the E96 range but really, does it need to be 1%? Couldn't they have gotten by with 10%? And no, I don't think it needs to be in sync with other clocks on board - there's no attempt in the circuitry to sync with the primary clock.
Interestingly enough, there's a second SN7414 on the board and someone has been there before me and replaced it. It's date code is from 1984 and the soldering job, whilst neat enough, was let down by them leaving burnt flux all around it. I cleaned that off.
So now the instrument is working again. Another dinosaur rescued. So it displays everything in octal. I can cope with that. It's easy to forget that these date from the early era of desktop computers and they were designed to accommodate the conventions of the time. I'm sure if they'd been designed in 1983/4 instead of a decade earlier they'd have been in Hex. They'd probably have more than 32 bytes of memory as well. I half expected to find a small core matrix memory instead of semiconductor based memory when I opened it up!
Notes to anyone wanting to repair one of these. Don't remove the front panel from the display assembly unless you really need to. It's one of those deals where you seem to need 5 hands to get it back on. The problem is that whilst most of the toggle switches have 3 positions and can be easily lined up with the holes, there are 4 that only have two positions and this makes it difficult to line things up so that they'll pass through the holes. Add in the fact that the power switch is mounted on the front panel and the amount of slack in the leads is minimal and it adds up to a mildly frustrating job getting it back together again.
Other notes - the manual indicates that the 1914A serial range above number 490 should have an A3 assembly p/n 59401-66503 Rev B. Below that serial it should be 59401-66503 Rev A. However my instrument contains 59401-66505 Rev A. It has the same layout as the 66503 Rev B board and it seems to match the circuit diagrams but it has a later part number. Is this significant? No idea!
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