Help Repairing VFD display - Page 1
Forum section: electronics / repair

author: dark_hawk
author_rank: Regular Contributor
timestamp: March 06, 2019, 05:17:56 am
content: Hi,I'm trying to repair an old AND GX-2000 Scale. There was some water damage on the board and I was able to repair the voltage lines.My problem now is that the VFD display is only illuminated in one half of it. The other half is dark.I don't think the display is bad since if i applied a low voltage to some of the bins the dark portion of the display illuminates, and also the dark part of the screen flashes when I disconnect the power.All the voltage going to the display and the driver seem to be correct.Here is the schismatic and a picture of the display:How do I go about diagnosing and hopefully fixing this issue?Thanks.

author: helius
author_rank: Super Contributor
timestamp: March 06, 2019, 05:59:51 am
content: These displays are multiplex: there are N anodes and M grids, so up to N*M segments can be selected. When some of the anodes won't light up, it means that those anode drivers aren't functioning properly, or the connections between the anode drivers and the VFD pins is bad.You can test continuity between the anode leg of the VFD and the corresponding leg of the driver chip: if they are high or intermittent then the board traces or solder joints need repair.If the continuity is fine, but the anode outputs of the driver chip never go high (high is from Vdd, +25 volts), those drivers are dead and you need a new driver chip. They are hard to source.

author: Chriss
author_rank: Frequent Contributor
timestamp: March 06, 2019, 06:06:38 am
content: I would focus around U7 and U8.Maybe some of the outputs are bad on U7.Sent from my GT-I8260 using Tapatalk

author: dark_hawk
author_rank: Regular Contributor
timestamp: March 06, 2019, 06:54:36 am
content: Outputs (50, 51, 52, 53) of U7 (The display driver D16310gf) are shorted.Is that normal in these chips or does that mean this chip is dead?Also continuity between all bins from the driver to the display and elsewhere are ok.Output of U8 is correct.

author: picburner
author_rank: Frequent Contributor
timestamp: March 06, 2019, 11:19:57 am
content: Quote from: dark_hawk on March 06, 2019, 06:54:36 amNo, it isn't absolutely normal, every output is independent and there must be no short circuit from others pin or vcc or gnd.Probably the scale has been turned on still wet and this has caused some damage.You can find the UPD16310GF (if it's the QFP-64 version) here:www.ebay.com/itm/NEC-UPD16310GF-3L9-QFP-64-HIGH-VOLTAGE-CMOS-DRIVER-FOR/401509929978?hash=item5d7bdb53fa:g:hfwAAMXQdGJRz8h4There is also a version of the same IC in QFP-80.

author: dark_hawk
author_rank: Regular Contributor
timestamp: March 06, 2019, 12:01:40 pm
content: On closer inspection, it's not a dead short. Every two adjacent have a resistance of about 3.3 ohms that's why the meter was peeping a short.Is that still considered a short?I just want to exhaust all the other obvious reason before getting that Chip.

author: picburner
author_rank: Frequent Contributor
timestamp: March 06, 2019, 12:19:18 pm
content: Have you tried if the scale, apart from the missing digits, works?If it works you could also proceed with the repair, otherwise I would also wait to buy the IC.For a driver that can supply about 50mA @ 80V, 3.3ohm can be considered a short circuit.

author: dark_hawk
author_rank: Regular Contributor
timestamp: March 06, 2019, 11:06:33 pm
content: Yes, the scale does work, after fixing a few bad traces of the voltage rails it boots up and pass the self check.I can't test it's accuracy or the other parameters without the screen though.

author: dark_hawk
author_rank: Regular Contributor
timestamp: March 07, 2019, 01:59:59 am
content: I unsoldered the 4 shorted pins of the display, and turns out the short is not in the driver, it's actually between the VFD display bins.Anything I can do to repair it? The display is custom made for the scale and I don't think I can source one.

author: picburner
author_rank: Frequent Contributor
timestamp: March 07, 2019, 06:47:50 am
content: Quote from: dark_hawk on March 07, 2019, 01:59:59 amI have never experienced a similar failure with a VFD but perhaps, on this forum, there is certainly someone who knows more than me.I agree that finding a custom display is a more difficult task, it would have been better and cheaper to replace the driver...