Logitech Squeezebox Boom VFD Display Repair Guide Part 2: Filament Power Supply

Please refer to Part 1 for instructions on VFD common information, Boom disassembly procedures etc.!

Fixing the "Filament Starvation" Phenomenon on the Logitech Squeezebox Boom

This entry is about a problem that apparently exists mostly in Boom units. I never found it in Classics or Transporters.
So what is filament starvation anyway? I got this term from the Noritake technicians - Noritake Itron is the manufacturer of the beloved VFD displays in the older Squeezebox devices such as SLIMP, Classic, Boom, and Transporter. It means that the heater wires which you can see horizontally across the display in the frontmost position are supplied with insufficient power so the display cannot show its full brightness. Even worse, the display may show shadowy sections especially at the left and right side. No matter if you just replaced the old VFD by a brand new one, you may be greeted with a sight that looks pretty rotten.
Here is an example I took of a unit in factory button test mode. Observe the inconsistent brightness of the blocks across the display:

Or even worse: this is a brand-new display on brightness level 2 / 5 (lower brightness makes the problem much more apparent):

The same display can also look like this (same brightness level):

So how is this issue identified vs. a generally-aged display?

Idenfying the Actual Boom Display Issue

When a display begins to look shoddy, there are multiple (mostly concurrent!) indications for the respective problem behind the symptoms:

• display shows 'shadowy' sections rather in its center --> probably burnt-in pixels --> display replacement advised. This can go as far as recognizable digits from the clock screensaver which appear as a permanent dark cast across the actual display content, of course mostly in the places where the clock would have shown its digits
• display gets darker beginning at the left or right side. The center is the brightest part until it fades away completely --> power supply failure
• display is completely dark, or quickly fades from barely readable to completely dark shortly after powerup (after being separated from the power supply) --> power supply failure. This recovers when the Boom has been sitting around without power applied for some hours, but quickly comes back after powering up again

Most Booms have both simultaneously, i.e. burnt-in pixels as well as a failing power supply. When you replace the display anyway, it's worth while applying the power supply fix described here. That is, if you find that the display replacement alone did not end in the expected bright and shiny new look.

Edit 2020-06-08: there is a new blog post about this matter specifically, to be found here: https://joes-tech-blog.blogspot.com/2020/06/vacuum-fluorescent-displays-how-they.html

About VFDs

Wikipedia.com gives you extensive information about how a VFD works. Please take a look to understand the basic principles of operation.
The Noritake displays employed in Squeezebox devices need mainly four supply voltages:

a) +5V operating voltage at low current for the embedded controller chip
b) +55V grid voltage
c) +5V filament voltage #1 (left side)
d) variable voltage between +1.5 and +3.0V for filament voltage #2 (right side)

A high current flows between c) and d), I estimate it about 800mA.

EDIT 2023-01-19: well, after all these years, I have finally come to measure it and actually it is far less. It's a unit where the the diode fix is already in place. With that in the loop, the amperage is around 105mA constantly. It's a little bit more when the display brightness is higher, and is zero when the display is off.

What we are going to fix here is d). A little theory for those who did not visit the Wiki article above. A VFD is very similar to a classic vacuum tube in that it has a heated cathode that emits electrons, a grid that controls electron flow towards the anodes, and eventually anodes to "pick up" the electrons which in a VFD are the individual pixels. If a pixel is positively charged, it attracts the electrons emitted and the phosphor coating causes the pixel to glow.
While the heater wires are supposed to permanently emit electrons, the grid sections between the heater wires and the pixels control which group of pixels receives electrons at all by setting all grids that should be blocked to the a positive potential so they basically catch all free electrons. Only the grid section where electrons should pass through is switched to a neutral potential so the electrons can get past it. The neutral part in the grid sections is scanning to enable all groups of pixels in a cycle. This happens hundreds of times per second so the human eye will not notice it. A slight flicker can be observed when the eye moves quickly across a VFD though.
Filament starvation causes the heater wires to reduce or even stop electron emission. There can be any degree between hardly noticeable to completely dead. In some cases, both heater wire connections are fed with +5V which means there is no voltage drop at all. While the display may be fully operational, it will act like it's dead.
So the thermic electron emission in a directly-heated cathode (such as the heater wires in the VFD) is caused by a voltage drop across the heater wires. The higher the voltage drop, the more the heater wires will glow and thereby emit electrons. In reverse, if the voltage drop gets too low, the electron emission will slow down or stop. A considerable current is needed to heat the wires up. In dark environments, you will even see the six horizontal wires glow. This is also a reasonable limit for the driving circuit. A balance should be found between sufficient electron emission that is just enough to have a good visual impression that is not disturbed by the heater wires glowing. The corridor between both is rather narrow.

Boom Failure Mode

While I never had any Boom with failures regarding the filament voltage #1 which is a stable +5 Volts whenever the Boom is connected to the external power supply (yes, even in the deepest standby mode), the other side of the filament is apparently the output of a power supply circuitry that has some still-unknown component that fails over time. It is meant to supply variable voltages depending on the selected brightness level, so there is a more complex circuitry behind that that is designed to handle the high current... or rather not apparently.
If you have a multimeter at hand, you will be able to measure +5V at any of the three leftmost pins of the display - positive test lead going to the display, negative test lead put to GND which can be found at any of the screws that hold the board in place while mounted. If not mounted, use the gold-plated screwholes instead. You should also measure approximately this between GND and one of the three pins on the right side:

Level 5: 1.418 Volts
Level 4: 1.414 Volts
Level 3: 1.413 Volts
Level 2: 2.513 Volts
Level 1: 2.511 Volts
Level 0 (off): circuit open

By the way, if you switch the display 'off', both filament voltages might be GND or dropping towards GND. There seem to be releases of the Boom where the display is actually turned off, in contrast to what I assumed. Just found this out. Interesting. I will probably investigate deeper here because that is what I would like to see in all Squeezebox devices when their displays are not showing anything. Just imagine how much power might be saved, and most Booms are pretty wasteful in this respect by keeping the heater wires hot all the time, no matter the operation mode.

So levels 5 and 4 mean a voltage drop of about 3.6 Volts which I think is pretty extreme. The voltage drop in levels 3 thru 1 is about 2.5 Volts.
When a Boom begins to show filament starvation, what happens is that the variable filament voltage rises too high. I measured up to 5.5 Volts in defective units which means the right side is even supplied with more volts than the left side, reversing the electron flow, but the resulting 0.5V voltage drop is not getting you anywhere.
So the failure mode regarding the right-side power supply is not that it fails and is eventually 'open' or falling to the potential of GND, but is reaching or even exceeding +5 Volts instead. So what fails is apparently some kind of pull-down circuit that is designed to provide stable voltages below 5V at a higher current.
We have +5V on the display's left side so the right side needs at least 1V more or less for a voltage drop big enough to start the magic, or better 2 Volts. The higher the voltage drop, the more the heaters will glow, so there is a voltage drop where undesired effects begin to show. Heater wires glowing red is probably not what you want. They can withstand a *lot* of power, so they will probably not take damage, but will emit a ton more electrons than actually needed. This will make the active pixels shine brighter which will more quickly consume them. Consequentially this will cause shadows in the display caused by pixels which burnt down quicker than others. Which puts you back to square one, having to replace the display again.
So let's assume that we want to define and limit the voltage drop. Otherwise, you could set the right side of the filament to GND, creating an effective voltage drop of 5V, and you'd be done. But that would be too easy, wouldn't it? A display might last some weeks or even months under this condition but it would cook itself to death.
Unfortunately, to this day, there is no technical documentation available from Logitech, and it is uncertain if it will ever surface. The majority of the Boom consists of tiny SMD parts which are hard to figure out, mounted to a multilayer board (at least 4-6 layers, I think). It's practically impossible to know what part of the circuitry is failing, and what could be replaced to fix it.
Basically, even if we knew, the design has an apparent flaw. A brand-new component will just solve the issue for a time before it starts failing all over again.

The (Pseudo-)Solution

IMPORTANT: You should not apply this fix unless you are more or less convinced that your Boom is suffering from the filament starvation phenomenon! Because there is no way of knowing if an intact power supply circuit could be damaged by the fix so things get even worse.

I have a more 'brutal' approach to this. Assuming that the right-side power supply has failed and generates +5 Volts or more continuously, why not pull it down to a more reasonable level with the help of just a few additional parts? While this will eliminate the ability to create arbitrary voltages, I found that a constant voltage drop will still allow you to fully control brightness levels. And it's really a very simple addition:

What you see here is the right side of the display. The three pins at the top are linked together (a tribute to the higher current that passes them). This is by design. Even if the three pins are not connected to anything, they share the same potential. My idea is to use three simple diodes in series from these pins towards GND, shown in yellow.
The diodes used can be run-of-the-mill 1N400x-type diodes which are available for just a few cents each. Just be careful to order ones which are capable of handling up to 1A of current. Typical diodes have a voltage drop of around 0.7 Volts each, so a chain of three creates a voltage drop of ~2.1 Volts. Assuming that the original voltage is around 5 Volts, the diodes would pull it down to about 2.9 Volts.
That is in the range of the voltage that an intact circuitry would generate; a voltage drop of 2.1 Volts is enough to give you a nice clear display in all situations, and even though this is less than the up to 3.6V voltage drop in the original circuit, giving the display a lower voltage drop is likely contributing to a longer display life. Visually it's negligible.

Current flows in the direction of the arrow towards the 'bar' in the schematic, and the bar is what you can also find on one end of the diode. That is the cathode (or minus) end, whereas the other end of it is the anode (or plus).
The nice thing about the diodes is that they cause a voltage drop but do not convert this drop into a lot of heat like a power resistor would. Furthermore, the current direction is still forced to be in sync with the original design.
This has proven to be a cheap and reliable solution in many cases (up to 60 at this point). Whenever I do a display replacement, many times I'll include this fix because the power supply decay becomes visible. Newer displays at low brightness are particularly good at helping to discover this.

Tools Needed

What you need for this repair is:

• all the tools for disassembly / reassembly (see Part 1)
  
• soldering iron with fine tip (~ 285°C, no more than 40W)
• 3 1N4001 (or 1N4002, 1N4003, whatever) silicon diodes with 1A rating
• 1 pair of pliers
• 1 small piece of heat shrink tube ~6mm diameter
• 1 small piece of heat shrink tube ~2mm diameter
• highly recommended: a "3rd / 4th" hand tool to hold the diodes in place for soldering

Creating Your Own Fix

So here is a guide on how to integrate these diodes. Please consider reading it through the end first before you start your own repair. There are some tips hidden here that could appear out of sequence.
From your collection of diodes, select three:

Keep in mind that we want a chain where one end of the overall construct is the anode and the other is the cathode. All diodes between need to be in line, the anode of one connecting to the cathode of the next. Reversing any diode in this chain will probably end in something nonfunctional that acts like an open link.

Pick two, bend the anode (unmarked) wire of one 90° to the side. Do the same with the cathode (marked) wire of the other.

Now let's join these two with a little drop of solder. First cross the bent pins, put the diodes as close together as possible, and solder:

The result is a 'Pi' pair of diodes:

Now bend one of the pins away 90° again. The third diode will be soldered to this wire. In this case, we bend the anode wire of one diode:

Also bend the cathode side of the third diode that comes in now:

Life gets easier if you clip the wires of the diode contacts that we already soldered together:

Now cross the two bent leads again and solder them together. Ensure that the anode of one diode is connected to the cathode of the next diode, or vice cersa. This needs to be consistent, otherwise the fix will not work.

Finished:

The end result is a series of three diodes in the smallest possible form factor:

The brave of heart could try a little dry test now. If you power up the Boom board, holding the diode cascade's anode to one of the three rightmost display pins and the other lead to the gold plated screw hole next to them (see schematic in "The Solution" section above), this should immediately light up the display to a much brighter level if it works. But make sure you do not touch anything else on the board with any side of the diode cascade! If nothing happens, or it gets blindingly bright and you see the heater wires getting red-hot, there is something wrong. Please stop immediately, recheck, and do not continue because this is not recovering by itself. In doubt, better send me a message and I will try to help you.

We need to isolate the diodes a little to ensure that we won't create any short circuit in the precious Boom board. I recommend heat shrink tube, one piece of ~6mm diameter (yellow in the following images) to cover the barrel of diodes, and another of ~2mm diameter (blue in the photos) which we will see shortly.

Prepare two pieces like so:

Mark one piece of the 6mm heat shrink tube so we know where the cathode of the chain is. It should correspond with a cathode marking on the last diode of the chain (it's the one closest to the camera in the following picture):

Push the 6mm heat shrink tube over the diodes, and the 2mm one over the cathode lead:

Then apply heat (150-200°C):

The anode wire (the unmarked side) will go across the three pins to the right of the display. As there is not enough space on the main board's front side, we will place this piece in the back.
First, bend the anode wire 90 degrees off:

 
Place it across the three pins as shown here and solder:

Please make sure the lead you attached to the display's pins does not touch the Wi-Fi antenna metal pad! Missing this point may damage your Wi-Fi board, or worse. The antenna pad has some degree of GND level. If both pieces make contact, you have around 5V voltage drop, so its way too high, and goes through the Wi-Fi card, too, which can't be good. Please avoid from the start. Place the lead as far right as possible. In doubt, put some isolation tape in between, like Logitech did in the original design (this strip of black plastic adhesive tape is missing in the photos). The diode contact should still touch all three solder pads of the display but this is not vital. It also works if you solder the diodes to just one of these pins as they are shorted together in multiple places on the board as well as in the display itself.

Clip the excess wire and bend the diodes so the cathode wire points to the right:

You can see in the previous picture that there is not much clearance between the diodes and the antenna. The gap should be as big as possible. Or, if you are even smarter, you first read up to here and remember to put another small piece of 2mm shrink wrap to this end of the diode cascade to isolate this properly :o)
I helped myself by bending the entire diode setup after soldering which becomes apparent in the next pictures but it's certainly not the nicest solution.

You may have to adjust the length of the 2mm shrink wrap tube. The wire needs to be soldered to the screw hole. Bend the lead to go there the shortest way. Prepare the diode's lead as well as the outmost section of the screw hole gold plating with a bit of solder:

Now join both:

Make sure the contact is stable because there will be some (mechanical) load on it once you reassemble the Boom.
You should not see any indication of the fix on the front side:

But on the back - see top left corner!

That's it folks! You're done!

I'd be overjoyed if you share your results and thoughts in the comments. Thanks for reading and for your feedback!

Final Words

Some legal stuff because you never know: please bear in mind that I am writing this as a hobbyist, not a professional. I describe personal ideas here which is only one of many ways such a repair can be achieved. I cannot guarantee that following this guide will lead to a good result, and cannot be held liable for any personal, physical, or monetary damage anybody suffers by following this guide.
I am open to advice if anything described here is wrong or can be done better. Please let me know in the comments if you find there is anything left to be desired.
Thank you!