VFD Display Dim At Power Up

[kiheiman]

I have a friend with a Kenwood TS-870 radio.  The VFD display is dim at power up.  The display gradually gets brighter.  After about 10 minutes, the display is very bright.  I am assuming that the VFD is OK.  I have had HP test equipment where some segments on the VFD display were dim - due to long hours of use.

I assume that something is heating up on the display board.  Earlier models of this radio had problems with leaking electrolytic capacitors on the display board.

Any recommendations on what I can check for VFD voltages?

[texaspyro]

It could be the cathode (filament) in the VFD has weak emission and is taking a while to warm up enough to throw off enough electrons to light the display.   This is a common wear-out mechanism of VFDs.

[shakalnokturn]

In the case of a VFD check filament voltage of course, however most of the time this is a symptom of bad small (22-100µF / 50-100V) electrolytics on the cathode-anode drive voltage (usually 20-30V).

Looking at the TS-870S service manual the display is an LCD with CCFL backlight (or electroluminescent sheet) in that case I'd suspect a weak CCFL, but it is also worth checking capacitors around the inverter.

[texaspyro]

EL backlights are very well known for their finite (and usually rather short) lifetimes.  But, they don't have a warmup change in intensity.

[james_s]

CCFL backlights certainly do get dim with age though and require some time to warm up. The first step here is to determine exactly what sort of display this is and if it has a backlight what type.