Kenwood KR-V75R Receiver

[Chris F.]

The unit has sound and seems to be functioning, but the display is totally
dead. The filaments in the VFD look OK and are getting about 6V across them
(not checked for continuity though). The power supply seems OK, but without
a service manual there isn't much testing I can do. Are these displays known
for going bad? If not, does anyone know where I can find a service manual
for this?
Thanks.

 

[Arfa Daily]

The unit has sound and seems to be functioning, but the display is
totally
dead. The filaments in the VFD look OK and are getting about 6V across
them
(not checked for continuity though). The power supply seems OK, but
without
a service manual there isn't much testing I can do. Are these displays
known
for going bad? If not, does anyone know where I can find a service manual
for this?
Thanks.

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

 

[Chris F.]

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?

 

[Arfa Daily]

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?

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

 

[Chris F.]

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.....

 

[Arfa Daily]

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.....

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

 

[sck0006]

In my limited experience with VFD's, they don't usually go bad, just
start to lose brightness on pixels. This is of course just what I've
seen in very few things, and really don't know that much about them,
so they may very well go bad like you say. Actually, I'd like to
know. Anyone have a more definitive answer?
Steve

 

[Arfa Daily]

In my limited experience with VFD's, they don't usually go bad, just
start to lose brightness on pixels. This is of course just what I've
seen in very few things, and really don't know that much about them,
so they may very well go bad like you say. Actually, I'd like to
know. Anyone have a more definitive answer?
Steve

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

Arfa