It is likely that the display filament power supply circuitry in your
Boom has failed. Many units suffer from this after some years. The
display itself is also wearing out, but if it goes all dark or starts to
fade from the outside edges towards the center, and sometimes recovers
after a few minutes of being powered-off, it's the filament supply with
95% chance.
A VFD is basically an electron tube, albeit a pretty complex one. In
operation, it needs the filament (the six thin wires close to the front
side of the display) to emit electrons, so it needs to be heated. This
happens at a low voltage and a rather high power. The filament is fed +5
Volts on the left side and between 2 and 3 Volts on the right, creating
a voltage drop between 2 and 3 volts. The higher this voltage drop is,
the more electrons are emitted (and the brighter the filament wires are
glowing). Besides the filament glowing brighter, the display's pixels
will also become brighter.
In 9 out of 10 cases, the failure is in supply for the the right side of
the filament. Instead of keeping a voltage around 2.5V, it ramps up to
5.5V so it's even higher than the left side. This stops electron
emission gradually or completely. VFD experts at Noritake (the
manufacturer of the displays used in the Squeezebox range) call this
"filament starvation".
A Boom in operation has some hidden SMD parts behind the display that
get red-hot. I am pretty sure that the failure point can be found
somewhere in that area, however, it is practically impossible to find
suitable replacement components because the SMD markings don't tell
exactly enough what each component is for.
While I could not find a way to repair the root cause yet, I found a
workaround that turned out to work fine for many people who sent me
their Boom for a display replacement. The fix is this:
22025
I am using three run-of-the-mill diodes, each of which has a known
voltage drop of around 0.7V. Three of them in series make a total
voltage drop of about 2.1V. I am using them to pull down the voltage on
the filament's right side. Diodes are needed because current must not
flow from GND towards the filament pins. GND can be found in each of the
screwholes so that is an ideal point to solder (as long as you leave
enough room for the screw).
The diodes can be packed together closely, shrink-wrapped, and hidden on
the backside of the board.
This little trick ensures that voltages on the right will never exceed
~3V as 2.1V are always subtracted by the diodes. So even if the
gone-crazy Boom circuitry is sending +5.5V in, we will make 3.4 out of
it which is in the acceptable boundaries. 2.9 Volts are still fine,
actually we should just stay below ~4 Volts on the right to ensure a
voltage drop big enough for the whole thing to work.
The fix works for all brightness levels also. I found that an intact
Boom is regulating the right side of the filament with different
voltages, depending on the selected brightness. It does not make much
difference though if we use diodes to make kind of a fixed voltage out
of it.
So you might give this a try, because this will help you a lot more than
a new display that would also stay dark if the power supply circuit is
broken :D
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5x Squeezebox Classic SB3
2.5x Squeezebox Boom
1x Transporter
1.5x Controller
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