Kenwood KR-V75R Receiver

6v sounds quite a lot to be across the heater. It is usually in the region of 1.5 to 3v AC. Most common reason for failure of VFD to light, is the main negative supply being missing. This is typically -32v, but can be anywhere from -20 to -40v. The heater supply is usually floated on this negative supply, so you can measure if it's there or not, by clipping your meter +ve probe to chassis, then measuring on the VFD heater pins with the meter -ve probe. You can check on the heater pins at whichever end of the display is easiest to get to - it doesn't matter, as both ends should be at about -32v, with respect to chassis.

Assuming that the supply is missing, the cause will be an open circuit electrolytic capacitor. The supply is usually derived from an AC coupled voltage multiplier, fed straight from one of the windings on the power transformer. Most commonly, it is the input capacitor which fails. Value is usually around 47uF at 50v working. You will be looking for a small cluster of caps ( 3 or 4 ) with a few small diodes scattered around them, in the power supply area. The caps will often be very " tired " looking.

If you have a 'scope to hand, this is often the easiest way to locate the caps. Find some likely looking candidates, then check on either end of them with the 'scope. Sooner or later, you will come across one which has a dirty great AC waveform on one side, and virtually nothing on the other.

Good Luck

Arfa

I went through the power supply and didn't find any bad caps. Seems to be a good 30V supply there in the PS area, but without a schematic I can't follow it any further. Would it be right to suspect logic problems or a bad display at this point?

manual

main

+ve

about -32v,

is

cluster

them

dirty

It is very rare to get either logic faults or bad VFDs to cause total failure of the display. VFDs normally wear out gracefully over a couple of years - very common on VCRs. One notable exception was a particular model of Sony, which used to suffer a display destroyed in a matter of days when a cap in its heater supply went bad, but that is the only bit of kit that I have ever known this to happen to.

When you say that you have a " good 30v supply in the PS area " are you talking a separate minus 30v, and not one half of the + / - 30v supplies that are probably there for the output stages ? Did you find the voltage multiplier that generates this supply. If so, did you check the caps' coupling performance with a 'scope, or their " goodness " with an ESR meter ?

Have you checked that there is minus 30v at the display panel heater pins, as I detailed ? I'm willing to bet that the problem is still supply related ...

Arfa

Yes this 30V supply is separate from the two others that drive the output stages. There doesn't appear to be a voltage multiplier, it seems to be directly derived and rectified from a dedicated winding on the power transformer. The customer couldn't remember if the display just quit, or gradually got dim over the years, so I don't have that info to go on.....

a

of

meter

OK. Some models from some manufacturers do derive it direct from a dedicated winding. On some that I've seen, they then follow this either with a simple regulator, or sometimes a series resistor. Is the supply making it as far as the display ? Chances are, if it's present at the display heater pins ( you still haven't told me this, and it's an important pointer as to where the problem is ) then it's also going to be present at the display controller IC. Report this info, and we can determine wher you should next be looking.

Arfa

Please read the whole thread, where I have given a definitive description of how VFDs usually go bad ...

Arfa