Vacuum Fluorescent Display Missing Characters - Page 1
Forum section: electronics / general

author: Pootle
author_rank: Newbie
timestamp: December 09, 2021, 10:02:17 pm
content: I have a synth with a vacuum fluorescent display and it is missing some characters. I wondered whether anyone had any experience of this phenomenon and knew whether it was likely a fault with the VFD itself, the controller chips or the connections. No schematics are available, so I don't want to start getting deeply involved if it is probably the VFD itself and therefore presumably not repairable.Any insight would be great.

author: floobydust
author_rank: Super Contributor
timestamp: December 09, 2021, 10:47:12 pm
content: It's not the VFD, with so many digits missing I would go after the IC's doing the digit-drive to the VFD's grids. One IC could do say 6-8 and that IC would be suspect. You'd have to post pics of what ICs are used.edit: a synth gets banged around a lot, so look at the VFD's soldering for fractured joints, although I would expect other problems like missing segments then.

author: james_s
author_rank: Super Contributor
timestamp: December 09, 2021, 11:18:11 pm
content: It's possible that it's inside the VFD, for example broken connections to some of the grids, but I agree it's more likely to be external to the display itself.

author: Benta
author_rank: Super Contributor
timestamp: December 09, 2021, 11:35:28 pm
content: The ICs are unlikely. But the pin-to-glass connection points on the display can be problematic, and are very difficult to repair.

author: floobydust
author_rank: Super Contributor
timestamp: December 10, 2021, 02:46:04 am
content: OP, I forgot to say welcome to the forums, and note there will be conflicting opinions all the time. It's part of the fun.Blaming the VFD or CRT is common and yet not always the problem. It's best to use troubleshooting technique to find out what the problem is, but with no service manual that takes tremendous skill. What make/model is it?Keyboards I've repaired are plagued with fractured soldering joints. I'll first spend 1/2 hour with a magnifier and soldering iron doing touch ups. The jacks and keyboard, buttons are common places. I would look at the display and driver IC soldering.VFD construction, mechanical failure it's not feasible to have a group of digits stay off so I suspect a driver IC or power to it. A disconnected grid leaves a digit lit, as vacuum tubes are normally on devices, they need -ve bias to turn off.

author: Pootle
author_rank: Newbie
timestamp: December 10, 2021, 12:54:56 pm
content: That sounds hopeful, thanks for the info. The synth is an Ensoniq ASR-10 - there is a service manual, but it doesn't contain schematics and they don't appear to be available anywhere. Ensoniq don't really seem to like to get below a board level of granularity, i.e. follow flow chart, if fault replace whole board.I read somewhere that the display driver is an MM58241N, but I'll open it up and have a look and get some pics and check all the joints as well.

author: james_s
author_rank: Super Contributor
timestamp: December 10, 2021, 09:02:35 pm
content: There's really no substitute for getting in there and taking a look. It's possible you'll find a cracked joint or some other issue fairly quickly, or it could be a bad driver IC, or possibly even some fault on the digital side that is corrupting the data to the display. Is it always the same segments/digits that are blank?

author: Pootle
author_rank: Newbie
timestamp: December 11, 2021, 01:51:16 pm
content: I think so, it appears to be digits 11,13,14,17,18,19 & 20.

author: james_s
author_rank: Super Contributor
timestamp: December 11, 2021, 08:19:42 pm
content: If it's always the entire digits that are out and always the same digits I would suspect something with the grids. Certainly measuring those is where I'd start.