Repairing A VFD Driver On A Car Stereo

We love seeing repairs and always marvel at the ability to track down the problem. [Todd] seems to have a knack for this. He was met with a lot of adversity when trying to get the Vacuum Fluorescent Display working on his car stereo. A lot of persistence, and a little bit of taking the easier way out let him accomplish his goal.

The head unit is out of his 1994 Jeep. He knew the radio functionality still worked, but the display was completely dark. After getting it out of the dashboard he connected it to a bench supply and started probing around. He established that the data lines were still working by setting the radio to auto scan mode and testing with a multimeter. When he went to measure the cathode pins he didn’t get any reading. It seems the driver which supplies that signal is burnt out.

One easy fix would be to replace the parts from a scavenged unit. [Todd] hit the junkyard and picked up one from a Jeep that was just one model year apart from his. Alas, they weren’t exactly the same, and although he swapped out a chip (using a neat heated solder sucker) it didn’t work. In the end he simply dropped in a power resistor to use the 12V rail as a 1V at 0.1A source for the filament.

You can see his repair extravaganza in the video after the break. If you’re looking for tips on scavenging these types of displays check out this post.

29 thoughts on “Repairing A VFD Driver On A Car Stereo

  1. Good to keep original gear in place, it has less theft appeal.
    Are you sure there wasn’t a bad solder joint in the original ckt. These kind of resistors often fail on the board joint.

  2. This or a similar video should be required viewing for consumers who pee& moan why repair shops can’t make a living out of repair the modern equipment most consumers are demanding. A Sams manual may it may not have made the trouble shooting faster, but they are a hell of an investment themselves. Very few hackers or shade tree mechanics actually trouble shoot Nice to see one to go through the process to teach noobs & old hands how it’s done. ACC= accessory? But would in mean a line from the accessory position on the ignition or a line to something like a power antenna of outboard amp that goes hot only when the radio is turn on? Wouldn’t be hard to figure it out if knowing was important.

  3. Side-effect of the repair is that you get an instant voltage reading of the battery and check that the alternator is working.
    Not sure if the controller actually did some current sensing and if the VFD is temperature-dependent. Anyway, just FYI, the display life decreases dramatically if overvolted.

    1. I’ve seen no VFD controllers that adjust anything according to temperature. LCD controllers often do this. The VFD won’t be optimal lit after this repair, since there now is a voltage gradient from left to right. VFD filament is meant to be driven via an AC waveform, so no voltage gradient occurs. There is very specially made VFD’s that is constructed for DC filament in that way, that a voltage gradient isn’t visible.

      1. @Per Jensen. true, an AC source would be correct for this display and if it dies my plan is to use the display from the wrecking yard unit and build a correct regulated AC power source if possible. I do see the brightness gradient issue you mention but after 10 years of being dark even this poorly hacked display working is GREAT! Others have posted on my channel to test a few things that might just fix my original equipment so I will try their ideas this weekend. I learn so much from others that follow my hacks and give great feedback. Thanks for the information.

      1. Yes it was a bit of an assumption being 100%. But it would be easy for them just to flip the silicone than make a new one.

        I enjoyed your cruise control obtopsy video. I’m currently diagnosing my jeeps cc and now I don’t have to upen it up to understand how it works.

        Nice work space btw.

        1. I posted a comment at my blog that I picked up a used servo module from the junk yard for $10 and my cruise now works again! It is nice having wrecking yards with so many old cars with good parts to salvage.

  4. Nice job. I love to fix items at the component level. It takes some patience, but can be worth it. My wife asked me what I wanted last Christmas, I told her I would like a nice variable bench power supply. Well Santa delivered. I used it the first day to trouble shoot a constant current LED power supply driver issue.

    Sometimes doing these things makes you think outside your box.

    1. There is so much to learn too. My day job is not electronics so I find when I’m trying to fix, make or hack something I spend a lot of time on the Internet just trying to learn the basics of how something works or why it was put together the way it was to make it work. The rabbit hole gets a bit too deep for me sometimes and I know it shows when I try to fix stuff something like this VFD. But I get better every day, learn something every day and being it’s just a hobby for me its still fun. I know I couldn’t do this hobby without the Internet and the endless information that is now out there, free and easily accessible. I tried to learn the “art of electronics” when I was young and in college but you only had a few textbooks and a couple of class to learn from. Most everyone around you knows the same or less and no Internet so if you got stuck or wanted to learn something you had no place to turn. The young people today have it made, they’re young and can study anything they want in grand detail mostly for free from anywhere they have a computer. Fascinating!

    1. Thanks for the link. I need to learn this! When I was reading up on VFDs I found that small VFDs where DC and some pulse duty cycle DC. I had read that larger VFDs used AC but I never found any details on what was small or not small. My testing showed I had a ground, maybe it is a ground fault :) so I tried DC started low and well as you see it worked. I figured I must have a small DC VFD but maybe I just have a ground fault in my driver chip that is letting me get away with my hack. I will use the details from your site to build a proper driver circuit if my DC solution ever fails. THANKS for sharing!

  5. A power op amp (single sided supply) would be much more efficient and reliable. I would not want to rely on filament resistance as part of a resistor voltage divider. Plus the op amp would let you do the headlight detection, or even better put a brightness pot somewhere.

    Also, there was a 5v supply to that board, why would you not use that? It is less power to burn away in a regulator.

    1. I was trying not to complicate the final hack solution and just get it working. I had a lot of complicated ideas spinning around in my head. The power op amp would be another good one. I did think about the 5v source but didn’t want to risk pulling 100mA from it and burning out the 5v regulation supply or killing off something using the 5v.

  6. I stripped down a kenwood head unit for its VFD the other day and was pleasantly surprised to see that both the 10V pk-pk AC filament and ~30v grid voltage were generated by a nifty separate 5 or 6 pin module that simply took 10v in and did all the hard work for me. I like kenwoods now :-) The purely dot matrix VFD was also SPI interface…

  7. 1… I never saw very many Sam’s in the digitally tuned car radio era. I just now dug up a Delco book circa 89. A friend wanted an audio jack on his beater radio so as to keep it low profile. Knowing at the time (15YAG) it used the electronic volume control I told him I’d need a schematic. Kokomo is closer than Indianapolis, I don’t know where he got it, but it helped me hack one of a similar model I had.
    2… It uses DC on the filament, bet you it is a shorted Zener diode or open tx. A simple current regulator from the ign power side powers the filament.

    1. “the digitally tuned car radio era” Respectfully I have to ask if that’s a bygone era? As far as digital displays for the radio station tuned in. I really don’t know what Sams provides anymore because it’s been a long time I had accessed to someone’s library of them, or needed to try to order anything.

  8. Like the fact he decided to repair it, dont like the way he did it. Why not google the chip and check what it does?

    This is like me uploading a video where I replace polyfuse with a glass fuse/wire to fix a laptop or LCD panel. Yay it works instead of “oh, so this was the reason”.

  9. I get the idea of DIY and the great feeling of accomplishment when a fix works. But, uh, why not just get a used radio of the same model from the junk yard and replace the whole radio? I know the old timer argument of “disposable stuff vs. fixing it” but I work with old timers who will tell you the art really is knowing when to throw it away and when to bother fixing it, which comes down to time & $$$. I guess you have both? ;)

    1. I know the video was long but about 1/2 way through I made just that decision. I went to the dump and got one for $17. It was busted up but it should have had the board I needed. Turned out that board was not a fit. I wanted the radio to be OEM because it fit in my dash with no ugly bezels. I may take another trip the wrecking yard if this hack fails.

  10. Ryan7777 – He does mention that he tried to find a replacement, and came as close as he could, but could not find the exact model. I’m not sure an article on replacing a car stereo with an identical replacement would be viewed as informative – otherwise we’d have car audio jockeys from Worst Buy’s car audio garage posting to “”

    Sometimes it’s the journey, not the destination…

    1. I actually enjoy fixing things. And it’s obvious Todd does too. Not to mention the things you learn. I learned a lot about these displays watching the video and reading the comments. The knowledge is where the value is at, not the stereo.

  11. i love VFD’s, great work on the fix!

    but personally i would have combined the LM317 and the resistor… 317 bolted and resistor for 1.25v -> 1.0v … that way the exact voltage of the “12”v does not affect brightness.

    car 12v can be 10 – 14v
    while starting, 12v = 10v
    while not running, 12v = 12v
    while running (idle), 12v = 13v
    while reving, 12v = 14v

    1. In the top photo left to right:
      Fluke 322 AC current clamp meter.
      Radio Shack DMM which was my first auto range DMM my wife got me in college (LOVE IT STILL).
      Fluke 16 DMM spec’ed for HVAC work.
      Fluke 117 DMM spec’ed for elecrical work.
      Fluke 87V DMM spec’ed for electronics work.

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