Uneven VFD Vacuum Fluorescent Display - UK Vintage Radio Repair and Restoration Discussion Forum

GaFordUSA · 15th Oct 2018, 6:27 pm

The display on my oven is a VFD - Vacuum Fluorescent Display. The display is uneven. It is brightest in the horizontal center of the display, but vertically above or below the center the display is dim. Attached is a picture. So far, I have changed all the caps with new low ESR caps and tested all the diodes, but the problem remains. The horizontal lines (cathode) all appear to work and I can see faint red lines (there are 6 cathode lines). I also tested the cathode with a 9v volt battery for 1 second and all cathode lines turned red. The display is about 20 years old. I am trying to determine if the display is worn out and I should throw in the towel, or if there is an issue that can be fixed. Thanks for any thoughts.

AC/HL · 15th Oct 2018, 8:01 pm

Hello and welcome. Coincidentally, this was posted yesterday. See post 4: https://www.vintage-radio.net/forum/...11#post1083411
How are your nerves!

G6Tanuki · 15th Oct 2018, 8:29 pm

I had a "Micronta" clock [bought from Radio-Shack/Tandy in the 1970s] which started doing that.

Turned out that one of the 3 diodes in the PSU had gone open-circuit, and though things struggled on for a while the resultant half-wave-rectified AC overworked the [rather underspecified] smoothing-cap.

A diode that actually worked as a diode, and a new electrolytic of somewhat-more-generous capacitance solved the problem.

GaFordUSA · 15th Oct 2018, 9:34 pm

Thanks, I just replaced all the caps but with the same size. I tested all the diodes and they tested properly using the voltage test, both forwards and backwards. One tested funky but was ok after I removed it from the circuit. I am not an expert (rather I am a weekend warrior), and I think the diodes by the transformer are the rectifier. Which cap do you think is the smoothing cap? There are two boards that are joined with pins. One side has the PSU, the other has the display. Thanks.

G8HQP Dave · 16th Oct 2018, 4:09 pm

If it was a UK oven I would suggest replacing the capacitive mains dropper, but you seem to have a transformer. My oven weak display was caused by the capacitor losing value.

crackle · 16th Oct 2018, 5:02 pm

My Hotpoint EW81 oven display had interminable sections that came on when the right buttons were pressed and went off when the left buttons were pressed.
It was caused by bad joints on the PCB where the wires to the display were soldered.
Down to poor design where they had used solid wires rather than flexible wires between PCB and display, so when buttons were pressed the flexing of the PCB put a strain on the solid wires.
Mike

Mike

15th Oct 2018, 6:27 pm	#1
GaFordUSA Retired Dormant Member Join Date: Oct 2018 Location: Atlanta, Georgia, USA. Posts: 2	Uneven VFD Vacuum Fluorescent Display The display on my oven is a VFD - Vacuum Fluorescent Display. The display is uneven. It is brightest in the horizontal center of the display, but vertically above or below the center the display is dim. Attached is a picture. So far, I have changed all the caps with new low ESR caps and tested all the diodes, but the problem remains. The horizontal lines (cathode) all appear to work and I can see faint red lines (there are 6 cathode lines). I also tested the cathode with a 9v volt battery for 1 second and all cathode lines turned red. The display is about 20 years old. I am trying to determine if the display is worn out and I should throw in the towel, or if there is an issue that can be fixed. Thanks for any thoughts. Attached Thumbnails

15th Oct 2018, 8:01 pm	#2
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Uneven VFD Vacuum Fluorescent Display Hello and welcome. Coincidentally, this was posted yesterday. See post 4: https://www.vintage-radio.net/forum/...11#post1083411 How are your nerves!

15th Oct 2018, 8:29 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Uneven VFD Vacuum Fluorescent Display I had a "Micronta" clock [bought from Radio-Shack/Tandy in the 1970s] which started doing that. Turned out that one of the 3 diodes in the PSU had gone open-circuit, and though things struggled on for a while the resultant half-wave-rectified AC overworked the [rather underspecified] smoothing-cap. A diode that actually worked as a diode, and a new electrolytic of somewhat-more-generous capacitance solved the problem.

15th Oct 2018, 9:34 pm	#4
GaFordUSA Retired Dormant Member Join Date: Oct 2018 Location: Atlanta, Georgia, USA. Posts: 2	Re: Uneven VFD Vacuum Fluorescent Display Thanks, I just replaced all the caps but with the same size. I tested all the diodes and they tested properly using the voltage test, both forwards and backwards. One tested funky but was ok after I removed it from the circuit. I am not an expert (rather I am a weekend warrior), and I think the diodes by the transformer are the rectifier. Which cap do you think is the smoothing cap? There are two boards that are joined with pins. One side has the PSU, the other has the display. Thanks. Attached Thumbnails

16th Oct 2018, 4:09 pm	#5
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Uneven VFD Vacuum Fluorescent Display If it was a UK oven I would suggest replacing the capacitive mains dropper, but you seem to have a transformer. My oven weak display was caused by the capacitor losing value.

16th Oct 2018, 5:02 pm	#6
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Uneven VFD Vacuum Fluorescent Display My Hotpoint EW81 oven display had interminable sections that came on when the right buttons were pressed and went off when the left buttons were pressed. It was caused by bad joints on the PCB where the wires to the display were soldered. Down to poor design where they had used solid wires rather than flexible wires between PCB and display, so when buttons were pressed the flexing of the PCB put a strain on the solid wires. Mike Mike