HP 33120a - Display Missing Segments - Page 1
Forum section: electronics / repair

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 05, 2019, 08:45:01 pm
content: 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 youSent from my Moto Z (2) using Tapatalk

author: bitseeker
author_rank: Super Contributor
timestamp: January 05, 2019, 11:03:44 pm
content: 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.

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 06, 2019, 12:21:00 am
content: I believe it is supposed to display this screen after startup like this one attached to the same main unit.Sent from my Moto Z (2) using Tapatalk

author: bitseeker
author_rank: Super Contributor
timestamp: January 06, 2019, 12:38:51 am
content: 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?

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 06, 2019, 12:47:19 am
content: 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.Sent from my Moto Z (2) using Tapatalk

author: bitseeker
author_rank: Super Contributor
timestamp: January 06, 2019, 02:35:02 am
content: 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.

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 06, 2019, 02:58:26 am
content: 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...

author: coromonadalix
author_rank: Super Contributor
timestamp: January 06, 2019, 03:09:54 am
content: 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 ??

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 06, 2019, 03:15:22 am
content: 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?Sent from my Moto Z (2) using Tapatalk

author: floobydust
author_rank: Super Contributor
timestamp: January 06, 2019, 04:31:24 am
content: Microchip HV518PJanother repair here:https://www.eevblog.com/forum/repair/hp-agilent-53181a-display-error/msg1399630/#msg1399630

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 06, 2019, 05:02:32 am
content: Quote from: floobydust on January 06, 2019, 04:31:24 amGood stuff here on another display.. Thanks!

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 31, 2019, 03:20:39 am
content: Update for this issue...I did the VFD driver chip swap.. no change, same display error. The display panel is defective.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 31, 2019, 02:19:46 pm
content: Maybe add a new oled display interface like qu1ck did on a 34401a meter

author: chekhov
author_rank: Regular Contributor
timestamp: January 31, 2019, 07:06:20 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: January 31, 2019, 09:11:52 pm
content: 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.

author: chekhov
author_rank: Regular Contributor
timestamp: January 31, 2019, 09:20:35 pm
content: Quote from: floobydust on January 31, 2019, 09:11:52 pmI've read this threadhttps://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.

author: mr.fabe
author_rank: Regular Contributor
timestamp: January 31, 2019, 09:23:24 pm
content: Quote from: floobydust on January 31, 2019, 09:11:52 pmIt's worth a shot before I begin to have a 33120a parts sale...