HP34401A display with dots after every digit - Page 1
Forum section: electronics / testgear

author: not1xor1
author_rank: Frequent Contributor
timestamp: June 08, 2017, 05:19:53 am
content: HiI'm looking for a HP34401A on ebay and noticed that several of them have dots in the middle after every digite.g. zoom inhttp://www.ebay.it/itm/122529702180is it normal or just a symptom of problems with the display driver or other parts of the circuit?thanks

author: floobydust
author_rank: Super Contributor
timestamp: June 08, 2017, 06:18:31 am
content: I've worked with the HP34401's and never seen that. Would drive me nutsYou can program the comma separator on/off.In the eBay ad, looks like segment E shorted to colon? The colon is on with any digit that has segment E on.I would say trouble with the VFD driver IC or solder bridge on the PCB. NEC uPD7527 to drive the VFD.Hope I'm wrong?

author: xemax
author_rank: Contributor
timestamp: June 08, 2017, 08:16:10 am
content: You need a new display.I had two 34401 with the dots problem. After replacing the display they are like new.

author: not1xor1
author_rank: Frequent Contributor
timestamp: June 09, 2017, 04:42:45 am
content: Quote from: floobydust on June 08, 2017, 06:18:31 amthanks for your replyI also noticed that the 4W indicator is on too while it should be off since the multimeter is likely set in V measurement.So I guess all those multimeters should be considered as defective even if they are sold as in full working condition.

author: not1xor1
author_rank: Frequent Contributor
timestamp: June 09, 2017, 04:45:24 am
content: Quote from: xemax on June 08, 2017, 08:16:10 amso that is quite a common problem I guessBTW does the display module integrate the driver circuit?How much did you spend for those displays and where did you got them?thanks

author: xemax
author_rank: Contributor
timestamp: June 09, 2017, 07:43:09 am
content: QuoteNo. Just the display is faulty.QuoteI ordered this:http://www.ebay.de/itm/1x-New-vacuum-Fluorescence-Display-Display-VFD-For-HP-AGILENT-34401A-/131861099316

author: not1xor1
author_rank: Frequent Contributor
timestamp: June 10, 2017, 02:04:49 pm
content: Quote from: xemax on June 09, 2017, 07:43:09 amthanks for your replyI think somebody replaced a similar display with alphanumeric LEDs, but do not remember what multimeter model...I have to check with googleit should be cheaper, but would probably require more work

author: coromonadalix
author_rank: Super Contributor
timestamp: June 10, 2017, 02:53:25 pm
content: it was for a fluke 45 if i recall a thread is herehttps://www.eevblog.com/forum/repair/fluke-45-vfd-display-broken/

author: Gyro
author_rank: Super Contributor
timestamp: June 10, 2017, 03:58:53 pm
content: Quote from: xemax on June 09, 2017, 07:43:09 amJust the display? That's a bit weird. Surely that must indicate an internal short (assuming it's not a soldering fault).If you can work out which pins and are actually able to measure a low impedance path (after removing it), then there ought to be a good chance of clearing it with a bit of current.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2017, 10:15:21 pm
content: Being a VFD display fault doesn't make sense to meSegments are basically a phosphor-coated electrode with a trace out to the external pins.If we assume electrical short between segments, the NEC VFD driver MCU should have push-pull outputs? Oddly it MCU runs off +13V, -18V, +18V rails.

author: usagi
author_rank: Frequent Contributor
timestamp: June 12, 2017, 07:21:52 pm
content: this should be stickied at the top of the forum as a FAQ:https://www.eevblog.com/forum/buysellwanted/ebay-many-faulty-hpagilent-34401a-multimers/msg811170/#msg811170

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2017, 07:47:24 pm
content: Quote from: free_electron on December 01, 2015, 01:00:23 amThis from that thread conclusion there is the VFD driver IC is leaky- not a VFD display tube problemfree_electron believes it is the VFD driver IC.xemax says replacing the VFD fixed the problem, and it was the VFD, in another thread below:https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/https://www.eevblog.com/forum/repair/hp-34401a/https://www.eevblog.com/forum/buysellwanted/ebay-many-faulty-hpagilent-34401a-multimers/msg811170/#msg811170/