HP 34011A 66502 board VFD replacement - Page 1
Forum section: electronics / repair

author: Tango1668
author_rank: Newbie
timestamp: August 19, 2021, 04:58:59 pm
content: Dear all,The VFD of the 34401A is getting dim. Looking to replace it with one from eBay or AliExpress. The display board is the 66502, old version.The service manual has different part number for 66502 and 66512 boards. The pin out looks different in the schematics as well. Anyone has any idea are the VFD interchangeable between the two board versions?Has anyone replaced the 66502 VFD with a MIC one? In eBay and AliExpress nothing mentioned about their compatible part number. I figure what's available there are for the newer board.Thanks very much.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 20, 2021, 12:00:47 am
content: never heard of an old driver board vfd replacement ...I did replace the new version vfd with an Aliexpress clone and worked very wellsorry can't help you moreMaybe you'll have to buy one and try to compare them thru the windows glass ??

author: Tango1668
author_rank: Newbie
timestamp: August 20, 2021, 12:13:39 am
content: Hi coromonadalix,Thanks for the reply. I misread the part number. Both 65502 and 65512 use the same VFD. I read the wrong line, was sleepy at 3:00 AM.The pinouts' nomenclature in the schematic are different though.Thanks for the info as well.

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 12:19:01 am
content: http://literature.cdn.keysight.com/litweb/pdf/34401A-06.pdfApparently the VFD tube itself is the same either way, but the problem is that the drivers go bad in the 66502 PCB version (the display gets freckles), no replacements are available and the new two PCB versions are not compatible. Even if you don't see it now, you may have the issue once you replace the VFD. OTOH as cheap as the are, it's probably worth a try.

author: floobydust
author_rank: Super Contributor
timestamp: August 20, 2021, 05:48:43 am
content: 1st generation 34401-66502 front panel board uses NEC uPD7527 4-bit MCU which ages poorly giving the polka dot syndrome and ghosting segments. On S/N 3146A59641 and below.https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segmentsand OLED replacement.2nd generation 34401-66512 front panel board uses 8051 MCU and HV driver IC.The VFD is the same between them but the main board 80C196 firmware is different, and soldered-in EPROM U502 I believe.I started making a replica front panel board using the VFD but a modern MCU and driver- yet compatible with the old 34401a firmware.Took all the mechanical measurements and I left the project at reverse-engineering the protocol and recoding.Replacement VFD's from china are OK but segment problems (due to fading NEC MCU) can still be there.

author: Tango1668
author_rank: Newbie
timestamp: August 23, 2021, 07:15:38 am
content: Thanks for everyone's reply. The display seemingly not yet develops polka dots and ghosting. It is getting dim and uneven brightness between segments. The LSD is brighter because of long time usage at 5 digits mode. Since the MIC replacement is so cheapo I will try that first.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2021, 07:28:02 am
content: That picture looks pretty good given the age of these.. Smaller brightness variations between segments and digits is usually due to the phosphor aging. The phosphor will fade with the digits and annunciators that are lit most of the time.The power-on display test, it's crappy because the tube isn't fully warmed up yet. I'm not sure if there is a command to run it.edit: I have successfully rejuvenated 34401a VFD displays. If you have unevenly lit segments, sort of dim around the segment's edges it can be due to low emissions- as well as an aged NEC MCU having weak outputs, phosphor aging, or VFD power supply issues.I pull the tube out, CAREFULLY apply DC across the filaments starting around 6VDC until it warms up in a few seconds. Cold resistance is 10R and hot 39R around 150mA. Then I slowly ramp up to about 12VDC hold for 30 seconds then disconnect power. DON'T JOSTLE OR BUMP THE TUBE when its heater is glowing orange. Don't just connect 12V to it, it will blow due to cold inrush current. Ensure your power supply doesn't spike up when you switch it off, instead I just pulled out the banana plug to turn off and wait for VFD to cool down.The theory is the filament has oxide buildups which cause low emission and the high temperature (overvoltage) run burns these off.I also tried with another power supply applying +30VDC/1k ohm to each digitgridsalone, did get weird effects for a few seconds that went away, so that did something to the stainless steel grid. It might also get some strange oxides on it.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 23, 2021, 02:32:36 pm
content: quick1 or qu1ck .... has made an oled display variant for the 34401a .... do an eevblog search ..I helped him getting the last code bits between the old vfd board and the newest one for his mcu coding