2907 TUBE DRIVER [Replace Q3, P channel MOSFET]
Display operation has been discussed via the forums in detail, I wanted to offer the above as something that may be over looked.
Some have either jumpered or changed the 22 ohm resistor to supply higher current to filaments. Also there has been some debate of alternating current vs direct current to drive the display segments. Often this type of display is driven with alternating current. But true sine wave alternating current is different than a push pull circuit, producing a modified, square sine wave. Can't say the display will notice this difference. John Archie has covered this in his post: Ice Tube Clock IV-18 To-Spec Hack
and says a small improvement is visible in segment display evenness. If you are having problems or just want to test the MOSFET, a simple jumper test is all that's needed. The clock needs to be working, so remember all the safety from the kit directions when testing a live board. Touch the board by the sides where the power jack is connected. I doubt touching the higher voltage points will be your demise, but when that muscle reflex from the ' ZING ', sends your board sailing across the room...
Take your jumper and carefully connect the two outside pins of the MOSFET, through the jumper. Do not short the middle gate pin, just the two outside pins, drain and source together. If your display gets brighter and evens out, MOSFET is not holding up it's end. Since a transistor uses current to switch, a current limiting resistor is needed. MOSFETS do not need a current limiting resistor.
I've attached photos for the basic construction of the modification. Cut the center, base pin, of the PN2907 transistor to about a 1/4 inch or so. Also cut one lead of the resistor the same way, there's some leeway here for your personal preference. Clamp the parts together with tape and lightly solder together. Cut the remaining resistor lead to match transistor leads. New modified part mounts directly where MOSFET was, rounded contour of transistor matching round contour of board etch. Be careful not to overheat base pad connection during soldering, and unsoldering resistor modification made earlier. You may not experience the exact same result of total segment evenness as I have, there are other variables, including slight differences in tubes, but decent filament current is an absolute. Very pleased with my tube driven by the 2907. By the way, if you want to see the filaments in action, Look at the display in total darkness, especially at low brightness, they should appear, slightly glowing red. Now if I can just figure out this C language, the SUN will really be shining.
Hope this is of help
Anyone having a problem with battery backup not functioning or a VFD display that's uneven or with dim digits, I offer this Transistor MOD. The sole purpose of the ZVP3306A MOSFET, is to disconnect tube filament power at power loss, so backup battery is not supplying current there, instantly draining the cv1220 button cell, which is then unable to power AT168 to keep current time. The MOSFET is supposed to switch fully off at power loss and fully on at power up. This was not the case on my board. MOSFETS are switched by voltage, and I'm not certain there is a high enough voltage potential at the gate to fully and reliably turn on and off, so it can be in a PARTIAL state. The PN2907 is an old work horse, uses current to do switching, and has worked perfectly in my tests. If you've ever closely examined your tube, you might have noticed the two hair sized filaments stretched across the entire display, about 3/16 an inch above the segment screens. Current flowing through these is what enables the phosphor in the segments to illuminate. Pin 1 and 13 are the filament connection pins of the VFD tube. One other note of operation, filament current is constant, regardless of tube brightness, segment brightness is handled by the MAXIM chip via the Atmega 168. A reliable, good current, on the filament, is essential to a full, bright, even display. Tube specs are; 5 volts @ 75-95 milliamps. Measurements I made : current measured directly from + 5 volt rail to tube filament anode (pin 13) read 65 milliamps. Current measured from transistor collector to filament anode pin read 64 milliamps, no current loss through transistor. voltage at +5 rail is 4.73 volts and voltage at transistor collector is 4.6 volts. No real voltage loss through transistor, .33 volt loss on +5 rail is due to voltage drop of MBR160 schottky protection diode. Tube is a little under driven according to specs. I'm not criticizing, just showing the readings I made of available power compared to transistor operation, which is very good. I can't say this will improve everyone's display. It's made a marked difference in mine. Bright, full, even digits throughout, and battery backup is now money, Just by using the transistor.
Last edited by Russell 27
on Tue Feb 04, 2014 5:44 pm, edited 2 times in total.