Agilent/HP 34401A VFD bad Segments. - Page 1
Forum section: electronics / repair

author: naimns
author_rank: Newbie
timestamp: December 21, 2024, 02:01:17 pm
content: I've recently had a problem with a MY41XXXXXX serial# unit with the VFD which wasn't related to a leaky driver or bad segments. See image.All characters that used any top 3 middle segments "\|/" were corrupted. These are SEG_I, SEG_K and SEG_L, not identified in any order.In the process I could not find any reference for U606 VFD SEG_XX pin mapping to display segments. The 18 segment signals (SEG_B, SEG_C, SEG_D, SEG_E, SEG_F, SEG_G, SEG_H, SEG_I, SEG_J, SEG_K, SEG_L, SEG_M, SEG_N, SEG_O, SEG_Q, SEG_DP1, SEG_DP2, SEG_COM) will map to each element of a given GRID. The "4W, diode and buzz" annunciators I expect replace the two dots and comma for the right end digit, where these are redundant. Each digit also has its own annunciator which I expect is one of the SEG_XX lines.Attached are a few findings which are related to this fix which ended up being debris under IC U603, the VFD driver, specifically around pins 34/35/36. These pins were clearly shorted on the VFD side when testing with a meter, cutting one of the VFD pins isolated this to a driver side issue. On reflection there is enough space around the VFD PCB holes to extract the solder and isolate the pin from the PCB.When I find some time I will reverse engineer the full segment mapping.

author: Swake
author_rank: Frequent Contributor
timestamp: December 21, 2024, 03:59:37 pm
content: QuoteIn the service manual dated October 2005 there seems to be a pin to segment mapping. What I read for example is that SEG_B maps to pin 30 and is named Anode 16. Is this what you're looking for ?

author: naimns
author_rank: Newbie
timestamp: December 21, 2024, 04:18:35 pm
content: Thanks, Thats where I found the names of the segments from.What would be useful is knowing which segment (SEG_XX) pin maps to which segment of the display. There are 14 segments on each alpha and then the 4 other for each GRID, so the 18 segments on the VFD pinout. This will help to identify which driver or VFD pin to observe.I am hoping to short some pins and check which corresponds to which on the display. I've not found that information anywhere and I do not expect the names to follow a standard.

author: Swake
author_rank: Frequent Contributor
timestamp: December 21, 2024, 04:38:43 pm
content: ho, I see now.And changes are they are not following standard protocol starting above left with the 'first' segment.This might be of interest for you: Someone made a converter to allow for the connection of other display types in the 34401A:https://github.com/ragges/hp_display

author: naimns
author_rank: Newbie
timestamp: December 21, 2024, 04:49:59 pm
content: Yes, I did see that.Most of those solutions are intercepting the messages that interact with U602 the 87C51 u-controller, so no information on how the U603 driver outputs map to the 18 segments.Its Sunday tomorrow and this is bugging me so I may have to unpack my soldering iron.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 21, 2024, 05:52:09 pm
content: a good search would have answeredi participated with an member qu1ck about an oled subst lcd, there is a github repo on that projectyou can put another vfd (mcu oled lcd) for not to much cashhttps://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg5715245/#msg5715245https://www.eevblog.com/forum/repair/reverse-ingeeniring-hp-34xxx-display-panel-serial-protocol/https://hackaday.com/2018/08/29/faded-beauty-dmm-gets-an-oled-makeover/

author: naimns
author_rank: Newbie
timestamp: December 22, 2024, 01:01:44 pm
content: If you find you have a segment display issue it is worth checking that there are no shorts around the VFD (U606) or the Driver (U603) first. In my case I observed shorts.Attached is a diagram showing the schematic component VFD U606 map to the segments on the display. This helps to check pins on the display board.

author: Swake
author_rank: Frequent Contributor
timestamp: December 22, 2024, 01:24:17 pm
content: Excellent.The naming is a bit curious: The usual thing to name the segments is to start counting top left. Here they used a 'Q' to name that first segment. As if someone using an Azerty keyboard was blind typing on a Qwerty keyboard and didn't notice.

author: naimns
author_rank: Newbie
timestamp: December 22, 2024, 01:39:11 pm
content: That's a good observation. Maybe the outcome of some stray lookup.I thought there may be more logic in the Anode mappings, which is again different, but decided this is the most useful for fault finding.It would help if they annotate the schematic with this information.I had a quick look at the 33120A and 53131A schematics and they have a similar odd allocation.