Author Topic: Agilent 34401A VFD replacement  (Read 2082 times)

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Offline Veteran68Topic starter

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Agilent 34401A VFD replacement
« on: November 29, 2022, 04:10:11 am »
2-3 years back I purchased a very good condition 34401A from a fellow eevBlog Forum member, and it's been a great meter for my hobby use. I have over a dozen handheld meters ranging from a Fluke 87V and Dave's BM786 on the higher end to Aneng and Voltcraft meters on the low end, but I always wanted a quality high-digit bench meter and this 34401A fit the bill nicely.

While we were in between houses, I had it packed up and in (climate controlled) storage for nearly a year. After getting the basement of my new house remodeled into an office/lab/makerspace I got it setup but hadn't used it for some time. Recently I've been working on a laptop repair and noticed that the VFD is flaky in places. Some spurious extra segments are appearing and disappearing at random. See photo I just took showing the extra segment on the "V" making it look like a "K". They've also come and gone from different areas of the display. Sometimes, I think after it's been powered up for a long time and warmed up, they disappear and everything looks normal.



Is this as simple a repair as picking up a $50 VFD off of eBay? Or is there more to it? I've seen comments online that indicate old vs new models have different video driver boards that aren't compatible. Does this look like a driver board issue vs just a VFD issue? Looking for input from anyone who has seen a similar issue and undergone a repair.

Thanks!
David
 

Online daisizhou

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Re: Agilent 34401A VFD replacement
« Reply #1 on: November 29, 2022, 06:15:51 am »
Compatible displays are made of LED,It is recommended that you first disassemble the host for inspection before determining the maintenance plan
daisizhou#sina.com #=@
 

Offline JenniferG

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Re: Agilent 34401A VFD replacement
« Reply #2 on: November 29, 2022, 06:39:49 am »
I ran across the following mod for this meter.  It replaces the VFD with OLED display.  Uses an STM32 microcontroller.  THe HP uses SPI for the display I read in the article.  He shares the kicad files, gerbers and firmware source.

https://hackaday.com/2018/08/29/faded-beauty-dmm-gets-an-oled-makeover/

I have no clue how good the mod is. 
Test Equip: GDM-8251a, UT61E, Probemaster, Tektronix 2225
Power Supplies: GPD-3303S (w/o overshoot problem)
Soldering Station:  Hakko 926
 

Offline Hydron

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Re: Agilent 34401A VFD replacement
« Reply #3 on: November 29, 2022, 10:03:52 am »
If you're lucky it's just the VFD driver (scope it to check I guess), and you'll be able to get one off ebay or something. Schematics are available in the service manual.

IIRC the units with black LO connectors use the newer display design with separate driver and micro-controller, so you've avoided the ones with an un-repairable display. The VFD itself looks nice and bright (better than mine at least!).
 

Offline HighVoltage

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Re: Agilent 34401A VFD replacement
« Reply #4 on: November 29, 2022, 10:09:26 am »
The original front panel PCB with VFD is still available from Keysight.

There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #5 on: November 29, 2022, 11:18:47 am »
But they will cost around 200$ usd  if it's still available like @Highvoltage wrote

I was lucky to buy  VFD clones from Aliexpress, they worked really well, they where 35$ usd  at their launch  loll   now they have doubled in price. 

You need the right equipement to remove it
,  many vfd pins, and if you're not good in desoldering, you may bust pcb pads


With the black plugs  you have a latest version of drivers ic's in it,  but normally the serial number will tell  what you have in it ...  and in the service manuals  or search here i posted in the past  the serial number ranges who got the older or the newest driver ic's

But some checks have to be made in the supply sections for the electrolytics capacitors and tantalums ...   and theses too require very good soldering skills

And for the dissasembly  if you go that way

If you ever remove the front panel from the main pcb, removing the front case panel is tricky,  you have to remove the black special latchs on the  inputs connectors on the front and back/rear 

The front panel too is complicated  ......   


the white connector who hold the front pcb flat cable is tricky, you must be very careful,   (normally  you push on it),  stay pushed and gently pull on the flat cable to remove it while pushing on the white connector   it act like a latch,   and maybe if it was never removed,  the wire are getting  like glued by pression over time

ONCE AGAIN BE CAREFUL


I would say  looking at it   i would  not touch the vfd,  even with some slight garbage in the display, i've had way worse then that
« Last Edit: November 29, 2022, 11:42:57 am by coromonadalix »
 

Offline wasedadoc

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Re: Agilent 34401A VFD replacement
« Reply #6 on: November 29, 2022, 01:03:36 pm »
I'd suspect the problem is elsewhere, not in the VFD itself.
 

Offline Dr. Frank

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Re: Agilent 34401A VFD replacement
« Reply #7 on: November 29, 2022, 01:31:25 pm »
I as well assume, that the VFD driver IC is having a problem, maybe caused by humidity, as I read your description.
The replacement of the VFD itself probably won't fix the problem at all, so maybe you live with that effect, if it does not lead to mis-readings.

If that extra segment vanishes after a very short time, maybe the driver IC has some beginning problems with spikes on its supply, or the supply voltage is faulty, i.e. a burnt diode, or a faulty capacitor. I'd suggest to check some repair descriptions here in the forums first.

Btw.: I also started my private metrology lab with a 34401A, but that's 30 years ago, and my instrument still works fine and stable.
Frank
 

Offline Hydron

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Re: Agilent 34401A VFD replacement
« Reply #8 on: November 29, 2022, 01:58:27 pm »
Before going any further, try and get the revision number from the system menu. This will confirm which display PCB version it's running. If it's rev 1 (v. unlikely) then I think the KS replacement assembly is the only option, though I'm no expert on this as my meter is running a rev 2 board.

If you do end up replacing the whole board with the Keysight assembly rather than just trying to get at the driver IC (SN75518FN thanks to the following post for the correction of package type, probably under the VFD itself) or find that a PSU or something is the cause, then I'd be interested in the old board for potential VFD salvage. Been tossing up whether to risk getting an ebay VFD and trying to replace my faded one, but there don't seem to be any sources with known-good-quality VFDs (sellers previously recommended here aren't selling any more). I've got a US mailing address btw.

Otherwise, good luck with repairs (and if you really want to get adventurous with some EEPROM hacking while it's open, see here: https://www.eevblog.com/forum/testgear/hp-agilent-34401a-hidden-menu/)
« Last Edit: November 29, 2022, 04:50:17 pm by Hydron »
 

Offline pqass

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Re: Agilent 34401A VFD replacement
« Reply #9 on: November 29, 2022, 03:03:32 pm »
Judging by the faceplate, your model likely has the separate VFD driver chip (vs. the MCU+VFD single chip); this is good. According to the Agilent/HP Service Manual 6th Ed, the VFD driver chip is a 1820-5330 which cross-references to a SN75518FN which is a PLCC44 packaged part (the one on the right in image below).



I didn't see any PLCC versions on eBay but there are many 40 pin DIP versions available there.  If you're desperate, you could always solder some fine [enameled] wire to the PCB and route them around the back to the 40 pin DIP (dead-bug style).


Agilent/HP Service Manual 6th Ed (see last page for the VFD schematic): https://www.ecb.torontomu.ca/guides/instrument-manuals/Agilent-HP_34401A_Service_Guide.pdf
Component cross-reference (search for 1820-5330): https://www.sphere.bc.ca/download/hp_xref-free.pdf
SN75518FN datasheet: https://datasheet.octopart.com/SN75518FN-Texas-Instruments-datasheet-117963.pdf
eBay.com: https://www.ebay.ca/sch/i.html?kw=SN75518
« Last Edit: November 29, 2022, 03:10:08 pm by pqass »
 

Offline Veteran68Topic starter

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Re: Agilent 34401A VFD replacement
« Reply #10 on: November 29, 2022, 03:33:31 pm »
Thanks all for the responses!

I like the idea of an OLED hack but I'd need to see some trustworthy reviews to risk such an invasive mod.

When I get home I'll dig a little deeper into what specific revision I have.

Yes, the VFD is still plenty bright, and as far as I can tell it's still reading normally. I have had occasion to misread at a glance and have to do a double-take to realize there were extra segments. Not the end of the world, just annoying. Especially to my OCD.

If this proves too difficult or risky, I'll just live with it as long as it doesn't get worse.
 

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #11 on: November 29, 2022, 03:44:00 pm »
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact

Same problems for many vfd based instruments, since some of them are powered for very long periods ....


There was a project here for a remote display for the 34401a,

https://www.eevblog.com/forum/testgear/external-display-for-agilent-34401a-(or-any-dmm-with-rs232-output-stream)/25/

You put the 34401a  in talk mode over serial port  and use an external display  to help visualize the meter ???   it could be more practical for you ???

It depend of your electronic knowledge / experience(s)  you can add anything for a display  with arduino's

I tried it  with an 4x40 display  loll   but it is kinda a bit slow refresh rates  vs the VFD ...  you have some arduino libraries to double de digits sizes over 2 characters lines

But enough for normal use,  @CellularMitosis  give the arduino code


« Last Edit: November 29, 2022, 03:56:34 pm by coromonadalix »
 

Offline pqass

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Re: Agilent 34401A VFD replacement
« Reply #12 on: November 29, 2022, 03:57:24 pm »
GOOD NEWS..... 
I was reading this thread https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg2690097/#msg2690097  and saw the attached image (see below).

Notice the VFD driver is a HV518PJ which is still available from digikey!   The function and pinout appears to be the same as the original SN75518FN.
See: https://www.digikey.com/en/products/detail/microchip-technology/HV518PJ-G/4902550

 
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Offline wasedadoc

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Re: Agilent 34401A VFD replacement
« Reply #13 on: November 29, 2022, 05:57:21 pm »
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact
Don't count on that not applying to OLEDs too.
 

Offline Veteran68Topic starter

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Re: Agilent 34401A VFD replacement
« Reply #14 on: November 29, 2022, 07:03:48 pm »
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact
Of course. I realize that, but the issue with mine isn't dimming. As you can see in the photo, mine is still very bright. It just has these random spurious segments that come and go. And when they appear, they seem to be brighter than surrounding segments, as if they're being overdriven (or driven by a short).

This meter looks to have been very lightly used, as it was in largely pristine condition when I bought it 2-3 years ago and it's seen very light use since then. So I believe the VFD itself still has a lot of life left in it, particularly as it seems from the responses and further research that this issue is likely not a VFD issue, but a driver or supporting component (bad cap?) issue.

GOOD NEWS..... 
I was reading this thread https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg2690097/#msg2690097  and saw the attached image (see below).

Notice the VFD driver is a HV518PJ which is still available from digikey!   The function and pinout appears to be the same as the original SN75518FN.
See: https://www.digikey.com/en/products/detail/microchip-technology/HV518PJ-G/4902550
Thanks for the follow up! I'm at work right now but am gambling on that being a compatible driver for mine. Since I had some stuff in my DigiKey cart already that I hadn't pulled the trigger on yet, I just added one before placing the order. If it's not the issue, it's only 7 bucks, and I'll toss it in the spare parts bin for a potential future project. :)
 

Offline alm

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Re: Agilent 34401A VFD replacement
« Reply #15 on: November 29, 2022, 08:10:42 pm »
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact

Same problems for many vfd based instruments, since some of them are powered for very long periods ....
And OLED displays don't even need to be powered on to deteriorate ;)

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #16 on: November 29, 2022, 11:38:09 pm »
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact
Don't count on that not applying to OLEDs too.

You need to check some oled meter displays substitutes threads,  some of them are dying faster than the 34401A  VFD  itself ...
 

Offline Veteran68Topic starter

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Re: Agilent 34401A VFD replacement
« Reply #17 on: November 30, 2022, 12:48:02 am »
The revision of mine is 10-05-02. So only 20 years old, not pushing 30+ like some of them out there. :)

As I'm going through the menus tonight, I noticed a pattern in this "leaky" segment behavior. At least for the moment, it's happening whenever either of the bottom slanted segments are used -- either "\" or "/". So V, R, and N all show the extra segments (see photo). Whenever one is displayed, so is the other, and it doesn't matter the digit's position. So those two segment lines seemed to be tied together.

I don't know if that's enough of a clue for anybody that's seen this behavior to narrow down the problem or not. When I get time here in the next few days I'll take it apart and look for bad caps or transistors, and if I don't see anything obvious I'll replace the driver IC when it gets here.

 

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #18 on: November 30, 2022, 01:05:29 am »
eh  i could be wrong, the  vfd controller could play tricks ???  but the problem will be, remove the vfd carefully,   change the chip, put back the vfd

But  i would personally check before assuming its the vfd controller that  all the supply lines and noise  juuuuust to be sure they are clean

Some people reported bad capacitors on the vfd  pcb supply lines ???  you're dealing with  old puppies from 20, 25 ... years old in some cases ??

my 2 cents,   sure  the vfd controller  is not too pricey


the 34401a  sucessors  aka  34410a and 34411a   just had their end of life made in febuary 2022 ???  no original keysight vfd+pcb are available  only clones ... or keysight repairs who cost like a new meter loll
 

Offline robert.rozee

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Re: Agilent 34401A VFD replacement
« Reply #19 on: November 30, 2022, 02:02:23 am »
hmmm... when one lower diagonal is on, so is the other. looks like it could be a short. do you see ANY characters displayed where one lower diagonal is ON, while the other lower diagonal is OFF?

i'd be checking the display PCB for the dreaded tin dendrils. these are fine whiskers of tin that can grow between electrical contacts, developing over a period of many years. microscopically thin, but can grow up to half an inch or so long.

these dendrils can be removed simply with a paint brush, although be cautious when brushed aside that they do not land somewhere else where they can cause mayhem!


cheers,
rob   :-)
 

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #20 on: November 30, 2022, 02:07:34 am »
do the short display test



Hold shift and power up the meter and release shift,  it will briefly light all the vfd sections ..
 

Offline Veteran68Topic starter

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Re: Agilent 34401A VFD replacement
« Reply #21 on: November 30, 2022, 02:27:07 am »
Ok, so further observations of interest. Now that it's been powered up for over 2.5 hours, those leaky extra segments have gone, come back for a few minutes, and are now gone again. But when gone, multiple segments appear "hot" or over bright, particularly the slant segments both top and bottom, but only while in the menu. At the DC voltage measurement mode, for example, brightness is consistent across all segments including the slants in the V in VDC.

Looks like some thermal variation causing segment lines initially shorted to break contact, and voltage or current to fluctuate. That or the brightness variations are just a characteristic of an aging VFD, but not sure why only when in the menu. Maybe because the menu display is static vs refreshing like in measurement mode? The brightness difference doesn't show as significantly on camera as it does in person, but this pic captures some of it. Notice how bright the slant segments on the R and V are:



hmmm... when one lower diagonal is on, so is the other. looks like it could be a short. do you see ANY characters displayed where one lower diagonal is ON, while the other lower diagonal is OFF?
Nope. When it happens, both lower slanted segments show whenever either one should be displayed. I agree it looks like a short on those segment lines.

i'd be checking the display PCB for the dreaded tin dendrils. these are fine whiskers of tin that can grow between electrical contacts, developing over a period of many years. microscopically thin, but can grow up to half an inch or so long.
That's an interesting notion I hadn't considered. If such a thing occurs over time, it would certainly make sense in a 20 year old instrument. And with such a delicate short, thermal expansion could easily break the short.

I have a sneaking suspicion it's a faulty component or driver IC, but I will look at everything suggested and will give it a good brushing just to cover the bases.

do the short display test

Hold shift and power up the meter and release shift,  it will briefly light all the vfd sections ..
Yeah I've done the tests. All VFD segments light up, but that's to be expected -- I'm not having any issues with segments NOT displaying, just those displaying when they shouldn't.
« Last Edit: November 30, 2022, 02:28:57 am by Veteran68 »
 

Offline coromonadalix

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Re: Agilent 34401A VFD replacement
« Reply #22 on: November 30, 2022, 03:33:15 am »
you could hold for a long period the shift button and see over a long long time  how the vfd would react, it would be the maximum stress given to it ?

once again i have my doubts over the vfd drivers,    i had an 22 ish  year old meter, no dendrils  but the unobtanium driver ic  with some of the symptoms

and put an aliexpress clone  and everything went fine  .. the vfd as you know  have filaments, plates, heater  etc ...  it can create deposits inside the vfd

You have some trick like overdriving a bit the filaments to clear some internal residues   ...


another thread of vfd  here

https://www.eevblog.com/forum/repair/vfd-unwanted-segments-glowing/

some even tried this

https://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/
« Last Edit: November 30, 2022, 03:45:46 am by coromonadalix »
 

Offline james_s

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Re: Agilent 34401A VFD replacement
« Reply #23 on: November 30, 2022, 04:12:14 am »
Your VFD looks great, I really don't think that's the problem, so you already have the hard part taken care of. I suspect a problem with either the driver or the digital signal to the driver, maybe as simple as a bad connection. As far as displays go, VFD is one of the most robust and longest lived there is, second maybe to LED but some of those are not as reliable as you'd expect.

OLED displays look great as far as modern display tech goes, but they have a spotty reliability record, there are many cases of failing OLED displays.
 
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