HP 33120a - Display Missing Segments

[mr.fabe]

Hello,

I have a HP 33120a that fails to display segments in the VFD panel.  The main unit is confirmed working properly by verifying a display from another 33120a unit.  The cable has been tested for any breaks.  Has anyone successfully repaired this type of fault?  I'm hoping it could be just a VFD driver chip failure but I am looking for some confirmation before I order a replacement and start removing the panel.

Any guidance would be appreciated!

Thank you

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[bitseeker]

Looks like a bad driver/controller on the front panel. It looks like it's even lighting up multiple indicators.

What is the display in the photo supposed to be showing? I don't recognize it.

[mr.fabe]

I believe it is supposed to display this screen after startup like this one attached to the same main unit.

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[bitseeker]

Ah, if it was just powered up, then the original panel is pretty scrambled. It also shows that one or more errors are present. Is that error indicator for real? What errors are stored?

[mr.fabe]

I believe any errors are exclusive to the defective front panel.  I get the same error when connecting to two different main boards.   When I connect a good front end to the same main boards, I get no errors running self tests.

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[bitseeker]

OK, that's what I was expecting. Do you have a donor unit to replace the faulty controller/display module? As far as I know, most parts for this model are no longer available from Keysight.

[mr.fabe]

No donor yet.  I'll have to start tearing the front panel to verify the VFD driver IC that I'll need to replace.  It will probably be an unobtainium part that no one stocks...

[coromonadalix]

Look very much as the 34401a meter front panel,   maybe the famous nec unobtanium,  or the newest ic SN65518, SN75518  ?? 

Check if your instrument has an service note for an display upgrade(s) vs a new firmware like the 34401a did ??

[mr.fabe]

I'm hoping it's the TI SN75518 like in the newer 34401a panel.  I think there was a suitable replacement for that chip.  518PJ or something like that?

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[floobydust]

Microchip HV518PJ
another repair here:
https://www.eevblog.com/forum/repair/hp-agilent-53181a-display-error/msg1399630/#msg1399630

[mr.fabe]

Microchip HV518PJ
another repair here:
https://www.eevblog.com/forum/repair/hp-agilent-53181a-display-error/msg1399630/#msg1399630

Good stuff here on another display.. Thanks!

[mr.fabe]

Update for this issue...

I did the VFD driver chip swap.. no change, same display error.  The display panel is defective.

[coromonadalix]

Maybe add a new oled display interface like qu1ck  did on a 34401a meter

[chekhov]

Reverse-engineering is cool thing, but it seems that better to have second unit if you are going to decrypt communication protocol.

I wonder what is the difference in front panel's MCU firmware (U1101 33120-88813 if I'm not mistaken) comparing to the one for other instruments like 34401A (for which firmware is accessible, and its MCU is mentioned as 34402). E3646A seems to have the same one, but not sure about 33120A - it has even more buttons as I see.

[floobydust]

I'm not completely sold on the VFD being defective because the SN75518/HV518PJ has push-pull outputs (unlike the NEC VFD MCU), so it can overcome leakage currents which seem to occur somehow within an aging VFD.
I would confirm the defective display board voltage regulator is OK for -13V, and -12V bias on the filaments from the power supply board CR1010 is holding up.
The SN75518/HV518 does get damaged (latch up) if the power-up sequence is wrong. It is +5V logic power before HV and on power-down turn off HV then +5V logic power.

[chekhov]

I think the firmware difference between multimeter, PSU, counter etc. is the encoder wheel.

I've read this thread https://www.eevblog.com/forum/repair/agilenthp-e3646a-repair/ and guys mentioned that firmware is the same between multimeter and PSU. But I also think that this coincidence likely does not apply to 33120A.

[mr.fabe]

I would confirm the defective display board voltage regulator is OK for -13V, and -12V bias on the filaments from the power supply board CR1010 is holding up.
The SN75518/HV518 does get damaged (latch up) if the power-up sequence is wrong. It is +5V logic power before HV and on power-down turn off HV then +5V logic power.

It's worth a shot before I begin to have a 33120a parts sale...