34401A VFD swap, some useful photos and notes - Page 1
Forum section: electronics / repair

author: robert.rozee
author_rank: Frequent Contributor
timestamp: December 01, 2020, 01:11:41 pm
content: the following is an account of swapping the VFD in a 34401A, hopefully there may be some useful points for others...a few weeks back i picked up an HP-34401A bench multimeter from trademe (local online auction site here in nz), but unfortunately when it arrived the VFD was very dim and uneven. still, it came from a reputable source and had a calibration that only expired a few months back, so i persevered and ordered a "NEW" replacement display from ebay:https://www.ebay.com/itm/184377935754when the display arrived, it didn't exactly look new - pins were a tad askew, it looked like they has been desoldered and there was even some rust on a number of them. i guess itcouldbe new-old-stock that had been languishing in a warehouse somewhere, however my pick is that the display had quite possibly been salvaged from a scrap 34401A. i was not convinced that it would be any better than the one already in the meter, so set about doing a quick test before doing any major work to remove the existing VFD.NOTE: i found getting the front panel off the 34401A a right pain! there is a red plastic clip at the top of the terminal block that needs removing, which is hard to spot as something removable due to it being the same colour as the terminals. even then, and with both switch bars removed, it took a fair bit of 'gentle persuasion' to get the panel free from the sides of the chassis. the cable to the display PCB should be disconnected before attempting this, disengaged by pressing down on the back of the connector on the main board.looking at the schematic of the 34401A display board, it quickly became clear that in order to test the 'new' display i only needed to connect up three or four wires. the heater pins (1 and 16) are located at opposite ends of the bottom row of pins, with the remaining bottom row pins (bar one) all grids. meanwhile, the entire top row of pins are all anodes. a testing jig could be made out of four bulldog clips, two squares of paper, and a few jumper leads. i tacked short extension wires onto the heater pins to make life easier, and used the bulldog clips (after filing clean the jaws) to short together the remaining pins:i also tacked a link across the 'shift' key on the display board so that the meter, upon powering up, would permanently keep all the segments of the display lit in a test pattern:i connected the top-row bulldog clips to a single anode pin, and the bottom-row ones to a single grid pin. in retrospect, i suspect that with the test pattern showing i could have just connected all four clips to a single anode pin. but as connected it worked:below is with camera stopped down to better highlight the difference in brightness levels:i checked the heater voltage before attaching anything extra, with it measuring about 5.1 volts. with the second set of heaters paralleled up this dropped down to about 4.6 volts - passable for a quick test. confident that the 'new' display was a vast improvement, changing it over was relatively straight forward. i had first removed the metal rails above and below the VFD (to get access to the pins while testing), so just had to work along the rows of pins with solder wick. after fitting the 'new' display, i soldered these rails back in place - they were clear of all the pins so didn't need any insulation added as others have reported. i did, however, remove the 'barb' from the middle pin on each rail (that fits the larger hole), which made life much easier.while not quite as bright as a fluke 8842, the display is pretty acceptable:i now have a spare 'old' display as a backup, unless anyone in nz or australia would like it?cheers,rob :-)

author: lowimpedance
author_rank: Super Contributor
timestamp: December 01, 2020, 11:30:09 pm
content: Thanks for the write up.Rather surprising is how bright the FLUKE 8842 display is still !. Not many units with that much life left in their displays out there, look after that one.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 02, 2020, 11:53:45 am
content: bought my 34401a vfd from Aliexpress, stiil going strong ... was around 30$ usd / ea 1 year ago ??

author: Runco990
author_rank: Frequent Contributor
timestamp: December 03, 2020, 02:57:49 am
content: Quote from: lowimpedance on December 01, 2020, 11:30:09 pmNow that I have a working (and non-working) 34401A, I decided to gift my perfect Fluke45 to a good friend of mine for HIS bench. Yes indeed.... good VFD meters are getting rare. The flukes are good.Nice to know the 34401A VFD is available, although mine are very low hour. Pretty happy!