HP34401A display with dots after every digit

[not1xor1]

Hi

I'm looking for a HP34401A on ebay and noticed that several of them have dots in the middle after every digit
e.g. zoom in
http://www.ebay.it/itm/122529702180

is it normal or just a symptom of problems with the display driver or other parts of the circuit?

thanks

[floobydust]

I've worked with the HP34401's and never seen that.  Would drive me nuts    You 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?

[xemax]

You need a new display.
I had two 34401 with the dots problem. After replacing the display they are like new.

[not1xor1]

I've worked with the HP34401's and never seen that.  Would drive me nuts    You 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?

thanks for your reply

I 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.

[not1xor1]

You need a new display.
I had two 34401 with the dots problem. After replacing the display they are like new.

so that is quite a common problem I guess
BTW does the display module integrate the driver circuit?
How much did you spend for those displays and where did you got them?

thanks

[xemax]

BTW does the display module integrate the driver circuit?

No. Just the display is faulty.

How much did you spend for those displays and where did you got them?

I ordered this:
http://www.ebay.de/itm/1x-New-vacuum-Fluorescence-Display-Display-VFD-For-HP-AGILENT-34401A-/131861099316

[not1xor1]

BTW does the display module integrate the driver circuit?

No. Just the display is faulty.

How much did you spend for those displays and where did you got them?

I ordered this:
http://www.ebay.de/itm/1x-New-vacuum-Fluorescence-Display-Display-VFD-For-HP-AGILENT-34401A-/131861099316

thanks for your reply

I think somebody replaced a similar display with alphanumeric LEDs, but do not remember what multimeter model...
I have to check with google
it should be cheaper, but would probably require more work

[coromonadalix]

it was for a fluke 45 if i recall   a thread is here

https://www.eevblog.com/forum/repair/fluke-45-vfd-display-broken/

[Gyro]

BTW does the display module integrate the driver circuit?

No. Just the display is faulty.

How much did you spend for those displays and where did you got them?

I ordered this:
http://www.ebay.de/itm/1x-New-vacuum-Fluorescence-Display-Display-VFD-For-HP-AGILENT-34401A-/131861099316

Just 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. 

[floobydust]

Being a VFD display fault doesn't make sense to me   
Segments 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.

[usagi]

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

[floobydust]

those dots are caused by leaky VFD drivers. they are TI parts. obsolete and replaced by pericom . i have posted the xref before on the forum.

these are 32 bit high voltage shift registers in plcc44 package. replace em and the problem is solved.

Note : if you have a really old 34401 then the VFD is driven by a upd75xxx custom cpu with integrated drivers . That one is unobtainium....
and the fronts are not interchangeable... a newer 8051 based frontpanel does not work in a machine that needs the upd75xxx based one.

This from that thread conclusion there is the VFD driver IC is leaky- not a VFD display tube problem

free_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/