Repair to front panel of HP34401A - help please - Page 1
Forum section: electronics / repair

author: AudioFile
author_rank: Contributor
timestamp: October 04, 2025, 03:18:48 pm
content: I wonder if any of the experts here can help with my attempt to repair my HP34401A.When powering on (with or without Shift button), all display segments are immediately lit and the unit sounder comes on continuously. The unit is then completely unresponsive so I can’t get error messages. Luckily I have a second working unit and by swapping the front panel displays I can be sure that the problem is with the front panel itself.I’ve checked all DC voltages on the 12-way ribbon cable and they are identical between good and bad units. Also the resistance to ground (with power off) is the same at all pins too. Checking with a scope, I find that all pins are showing the same traces, except for pin 10. The images here show good and bad unit traces at pin 10. (Note different timebase settings).Now I’m a bit stuck as I don’t really understand the way the circuit works! Pin 10 is ‘IGFPINT’ and goes to the non-inverting pin of U602, and there are only two other components connected here: R623 and C623.I can remove the display to get to the other components on the board, but wanted to ask here in case there is something else I should check first.Note: The display boards are type 66502 Rev. A. I note that later service manuals show the circuit for a 66512, and the above chip is labelled U607.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 04, 2025, 06:03:40 pm
content: i would say check all main voltages, schematics are on the webYou have many voltages in the meter, some tantalums love to short, main capacitors may have lost effectiveness over the years etc .....The flat ribbon is very stiff and love to get damaged if you manipulate it too often, and can get stuck in the white connector even if it is well pressed against the pcb to release the flex cableyou could have a stuck keypad touch pressing on the pcb

author: AudioFile
author_rank: Contributor
timestamp: October 04, 2025, 08:00:02 pm
content: @coromanadalix, thanks for this.I have already replaced all the electrolytics and the voltages (and ripple) are all fine. It's certainly the front end at fault as I've put the front panel in a working meter and the fault is seen, and vice versa.Had checked continuity across the ribbon, and cleaned all the switches with IPA.I'm looking for something that could cause the odd waveform on pin 10.Rgds

author: coromonadalix
author_rank: Super Contributor
timestamp: October 04, 2025, 10:33:09 pm
content: mmm if memory are good or not, there was a zenner on the front panel i think who get badok if i understand its the old vfd board you have, cr606 would be to check, and maybe the lm339 could be bad ?some had to change the display mcu, some display mcu image are there :https://www.ko4bb.com/getsimple/index.php?id=manualsyou can not exchange the front panel on other meters if they have a different 66502 vs 66512 vfd driver chip, the meter FW is different

author: Kwakerman
author_rank: Regular Contributor
timestamp: October 05, 2025, 07:08:07 pm
content: Does your 34401A have all red banana sockets (x5) or 3 red and two black? All red types are the early version and the VFD driver chips appear to be no longer available if it is duff. The red & black are the later version.

author: AudioFile
author_rank: Contributor
timestamp: October 05, 2025, 08:27:04 pm
content: Quote from: Kwakerman on October 05, 2025, 07:08:07 pmYes, it's the all-red type sadly.Quote from: coromonadalix on October 04, 2025, 10:33:09 pmThat makes sense. I'm going to remove the display itself and check those components, they're cheap to replace and I don't have anything to lose.

author: pqass
author_rank: Super Contributor
timestamp: October 05, 2025, 10:46:38 pm
content: Before taking the VFD off the board, have you tried to communicate with meter via RS232 or GPIB?Also, consider temporarily removing R513 next to the P500 connector on the main-board then checking if the FPINT signal improves. It should be a CMOS/TTL logic level coming from U500 pin 19. If it's more robust (than your screenshot), then U602 or the C623 cap is the problem.Be wary of the goofy power supplies on the front-panel board (+18, +13, FP, -18) and where you put the ground lead of your oscilloscope. To measure signals from the main-board, your ground lead should be on W601(9) (ie. FP). U602 is acting as a level converter to the front-panel MCU. +13V is generated by CR606 zener + CR605 which is the "ground" of the MCU. Checking voltages on the output of U602 or the MCU should be referenced to +13V. BUT DON'T put your oscilloscope GROUND LEAD HERE as it is connected to PE and where the main-board ground is also likely chassis connected too. Use an isolated meter/oscilloscope instead to measure the front-panel MCU signals.Rant: All the goofy level conversion and clamping to VLB just to avoid a -31V DC-DC! And of course, they did it again in the last revision except now it's +18V, -13V, -18V.FYI: See thefollowing excerptdescribing the wire protocol. It may be useful to you as you probe around."34401a CPU uses a 3 wire serial protocol to facilitate bidirectional communication with front panel MCU ...Schematics available in service manual revealed these lines to be labeled asSCK, SO, SI, which immediately suggests SPI....that is definitely SPI with clock idle high and trailing edge data capture, bytes sent MSB first.However a curious observation can be made: theslave pulls it's output low before the master starts the clock, which is not a part of standard SPI protocol. This can be explained by the fact that FP also uses the same output line to do the scanning of the input buttons state.Whenever CPU has data to send it first has to notify FP by pulling INT line (not pictured above) low, at which pointFP stops polling buttons and pulls SO low to signify that it's ready to receive next byte. Similarly if there is abutton press, FP notifies CPU to start the clock by pulling SO low."

author: indeterminate
author_rank: Regular Contributor
timestamp: October 06, 2025, 05:17:32 am
content: Quoteyou also need to chk the firmware versioni have a late model front panel and all 5 red bananas

author: AudioFile
author_rank: Contributor
timestamp: October 07, 2025, 10:11:51 pm
content: Thanks guys for your excellent help, I think we're getting there. I disconnected pin 10 on the ribbon cable (same as removing R513) and saw a good 5v / 0v transition signal from the main board as expected. C623 and R623 seem ok tested in circuit so I have a LM339 comparator on order.As confirmation, with pin 10 disconnected, the meter works fine on all settings and passes its self test! The only glitch is that every five or six seconds, the display goes blank for about a quarter of a second. I guess the lack of the FPINT signal causes a lack of syncing.I haven't yet got to grips with GPIB etc., and maybe I've escaped again this time! Seems a bit of a steep learning curve.

author: pqass
author_rank: Super Contributor
timestamp: October 07, 2025, 11:46:18 pm
content: Quote from: AudioFile on October 07, 2025, 10:11:51 pmGPIB is very accessible with just an Arduino and anIEEE488 connector. Seethis topicfor guidance, andthis githubfor the code & manual.You can also automate data collection usingthis logging programor have a batch script write strings to your serial port and log responses via terminal program logging feature.