One segment of vintage alarm clock's VFD aberrantly lighting - Page 1
Forum section: electronics / beginners

author: lxlbacon
author_rank: Newbie
timestamp: June 04, 2021, 07:48:31 pm
content: To start, I'm a complete electronics noob, virtually no experience beyond what little I learned about basic concepts in an intro physics course I took. With that out of the way, here's the problem:I recently bought a Panasonic RC-200 alarm clock radio, and quickly saw an issue with one of the 7-segment VFDs of the clock. The bottom left segment (the only difference between displaying a 6 and 5) isn't lighting correctly. When the clock's displaying a number that the segment should be off for (1, 3, 4, 5, 7, 9), the segment is incorrectly on, and at the fullest brightness possible, independent of how bright I have the display adjusted (there is an adjustment knob on the side of clock to control brightness level of the clock). Additionally, when it's displaying a number that the segment should be on for (2, 6, 8, 0), it is correctly lit, but weirdly enough is not at full brightness, and is actually dimmer than the rest of the segments making up the number. This issue isn't always obvious, like if I have the brightness turned all the way down, or all the up, it can't be readily seen. But I can tune the brightness to make the issue very obvious. It's only the rightmost individual VFD that is giving this problem. The other ones that make up the clock display don't have this issue at all. I did take briefly take the radio apart, just to see if I could find anything blatantly wrong with it, but didn't really see anything. As far as I could tell, all the capacitors and resistors of the VFD driver were intact, no bulging or explosive stains. My best guess right now is that some gunk or crud is messing with the connection between some traces, but as I said before, I know next to nothing about these things.Here is a video demonstrating this problem that I uploaded to youtube, to give the clearest picture possible of what I'm trying to describe:So, my question for you guys is what do you think is going wrong here? And is it something that I'd be able to potentially fix? Thanks in advance for any help!P.S.Since I'm already making a post, I might as well throw this in here too:One of the two bulbs back-lighting the radio tuner is burnt out. This isn't a huge deal to me, but I'd also be curious to know how difficult/expensive it'd be to obtain a replacement and swap it out. If anyone would happen to have a general idea about that process, please let me know as well!

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2021, 09:06:25 pm
content: I'd start by looking at what IC the clock is based on and then grab the datasheet, that will probably show you something very close to the circuit used in the clock. It's possible that there's a leaky driver which may be part of the IC although you may be able to work around it by adding a pullup resistor.The lamps for the tuner should be trivially easy to replace, how difficult they are to get to depends entirely on the design of the unit. You'll need to measure the voltage in order to have an idea what lamp to buy, and it would be very helpful to know the current too, which you can measure while you have one working lamp. You can probably substitute the lamps with LEDs too but often incandescent lamps in stuff like this are driven with AC which will make an LED flicker visibly. You can usually tap off DC from somewhere else in the device though.