In article ,
Pat wrote:
Hello,
I am have a Sony STR-KS360 5.1 AV receiver I purchased 10 or 11 years
ago. It still works well except the display is very dim. It can only
be seen if the room is completely dark. The left 10% of the display
is a little brighter than the right 90%, but even the brighter part is
very dim. A few years back, I did some Google'ing and found I wasn't
the only one having this issue. If I recall correctly, someone
suggested a bad electrolytic as being the likely cause, but I was busy
at the time and dropped it. Now, I'm retired and have some time to
investigate.
Anyone know which cap is the likely culprit? I can't find the old
post I had read a few years ago. (My only available test equipment is
a Fluke 87, but I can solder small components). The receiver has a
"dim display" function, but it is set to off. The display gets even
dimmer when I turn it on.

Unfortunately, this may turn out to be unrepairable in practice. This
receiver uses a VFD (a vacuum fluorescent display). These are, in
their essence, small specialized vacuum tubes, with a filament, a
bunch of control grids, and a bunch of anodes coated with fluorescent
material. These tubes wear out with time and use - the filament ages
(its ability to emit electrons decreases) and the anodes age (the
fluorescent material emits less and less light when struck by
electrons). Eventually they go dark.

The problem is worse for products which leave the VFD powered up
(filament heated and drive voltage applied) even when the product is
"turned off" - to show a clock, or an "Off" indication, or something
like that.

The VFD tubes were usually custom-made for one specific product, or
for a product family. Finding an off-the-shelf replacement is
probably not going to be possible. It's doubtful that Sony would even
sell you a replacement VFD tube if they have one - their "smallest
field serviceable spare part" is probably the whole display-panel PC
board (which might still be available).

Now, it's possible that the problem could be capacitor-related, I
suppose. This receiver derives the high voltage for the tubes using a
DC-to-DC converter (oscillator plus transformers and rectifiers and
filter caps). There are a couple of electrolytic filter caps on the
display board (C703 and C706) as part of this circuit - if one of them
has "gone leaky" it might be dragging down the voltage supply to the
tube and its driver IC and compromise the brightness.

Checking and replacing these caps _might_ help, but I don't think the
chances are very good.

HiFiEngine.com has the service manual (free registration required).