Very early revision HP 34401A - Page 1
Forum section: electronics / testgear

author: nueron
author_rank: Newbie
timestamp: May 11, 2023, 10:11:25 am
content: I thought I might share this, some people might find it interesting.I purchased this HP 34401A back in 2009. It is a very early revision, with a serial number of 3138A00042.The internal serial sticker is just marked as "#42".The unit may have been used for test purposes as there were some internal modifications (e.g. couple of 8-pin DIP devices soldered by wires to U420).Both U150 and U501 have gold plated pins and are marked as "PROTOTYPE".The U502 EPROM seems to be running some test code with a version of "DCz ENMR 001".U700 is marked as 34401-88802 rev 01, 12-6-91.I had removed the DIP devices attached to U420, which are two 8-pin packages soldered one on top of the other. I didn't separate these, but the top DIP is an Linear Technology device with a HP part number of 1826-1511.I also made a copy of the EPROM. The unit would come up with errors related to 74x (calibration) and 604 (ADC analog hardware). After updating the EPROM, the 604 error went away and the unit appeared to work fine, although I never did a full test on it.I had intended to perform some backdating to bring it up as a possible spare unit, but never got around to it.

author: daisizhou
author_rank: Super Contributor
timestamp: May 11, 2023, 11:54:57 am
content: :-+very nice

author: coromonadalix
author_rank: Super Contributor
timestamp: May 11, 2023, 12:18:39 pm
content: i would keep it as a museum piecebut for functionality mmmmmmmmm unless some calibration or extensives tests are done i would not use it ...seeing the 2 chips floating around give me chills loll

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2023, 01:41:19 pm
content: The extra DIP chips attached to U420 are indeed strange. I the type of chip readable ?

author: TheSteve
author_rank: Unknown
timestamp: May 11, 2023, 03:06:14 pm
content: As far as I know having a number like "42" on the internal board does indeed mean it was used in some sort of test/development role.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 11, 2023, 05:20:26 pm
content: see the patchs around the ic on the back pcb / back panel loll even wired in the socket loll

author: factory
author_rank: Super Contributor
timestamp: May 11, 2023, 05:48:49 pm
content: I'm seeing quite a few minor layout differences between this rev A1 board and the standard Rev A board used in older 34401A.David

author: floobydust
author_rank: Super Contributor
timestamp: May 12, 2023, 04:16:53 am
content: U402 ended up being AD711JR but in the pic I can sorta see the p/n end in __27 possibly.I see only 3 wires connecting from integrator op-amp U402 pin 3(+in) to the two DIP-8 (and power). That could mean it's likely a voltage follower for observing U420 output but no idea why two op-amps are dead bug like that. 1826-1511 is OP-27EN."Amplifier U420 improves the offset voltage characteristics of integrator amplifier U402".Wild guess optimizing the A/D including choice of op-amp.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2023, 06:48:05 am
content: The integrator with the 2 OP amps U420/U402 (OP27 and AD711) is a well understood and good solution. Just the choice of the OP amp types is a bit strange (rather high current noise of the OP27 and relatively slow AD711). Part could be the parts available at that time, though they have the slightly faster MC34081 already in the circuit.It may work to replace the OP27 with a combination of something like OP177 and AD711 to have a 3 OP-amp integrator. The 2 OP amps could be directly linked for this. This would be a bit similar (but slower) to the integrator in the HP3458 that was developed at about the same time. So I could immagine that they tested a 3 OP-amp construction instead.

author: nueron
author_rank: Newbie
timestamp: May 12, 2023, 11:25:31 am
content: Quote from: floobydust on May 12, 2023, 04:16:53 amYes, U402 is actually marked with 6-1991 which would be HP part 1826-1991 or AD711. U420 is an OP27.There is still an SMD cap across pins 2 & 3 of U420 which I hadn't removed. The original images I took in 2009, so I have taken some better ones.

author: nueron
author_rank: Newbie
timestamp: May 12, 2023, 11:31:04 am
content: Quote from: coromonadalix on May 11, 2023, 05:20:26 pmIt looks like they were still working out the external trigger circuit. And U700 pin 4 is wired to pin 8 (piezo signal), although pin 4 is NC in schematics I have.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 12, 2023, 05:15:47 pm
content: and the socket is cracked .... or seems to be ??ma main question would be loll does this meter works ??loll fw hw versions displayed when you power it up if you did, or not ?unobtanium vfd drivers ?thks to show us the internals of your #42 maybe slipped out of the company loll

author: mawyatt
author_rank: Super Contributor
timestamp: May 12, 2023, 06:00:25 pm
content: Could this have a development unit at hp way back that somehow found its way out instead of the grind up trash bin? Maybe a development engineer got to take it homeSeems hard to believe the old hp would actually release something like this with kludged up dead bug style parts in production, of course today anything goes as long at influences the bottom line for the executives quarterly bonusesBest,

author: TheSteve
author_rank: Unknown
timestamp: May 12, 2023, 06:47:06 pm
content: If you were local I'd trying to acquire it for my HPAK proto collection

author: dimmog
author_rank: Regular Contributor
timestamp: May 29, 2023, 11:08:20 pm
content: Wow. Very interesting piece of equipment. It’s such a good meter!I also have a very early revision of the non-working 34410A meter when they were without a rear fan. There are also a lot of float mods on the PCB.