On eBay I bought a bunch of VU meters based on VFD displays build in Russia. The displays where rather cheap and looked nice. I thought I'd give it a try.
It seems that the display was used in a cassette deck build by Radiotehnika Model MP7301.
When the vu meters arrived I realized that on all devices there was one transistor missing.
To make them work again you need to "repair" the vu meter ... don't panic you can easily solder a substitute transistor onto the pcb.
Step 1: Introduction
On the Internet you can find a bunch of DIY vu meters, a few of them based on vfd displays. Vfd displays look cool ... much cooler than led or lcd displays. Some of the displays are NOT vu meters. Some of them are spectrum analyzers, calling them vu meters is wrong. Some of them also do not behave like vu meters. A vu meter does display the audio level of a audio amplifier. But it does not do that in realtime ... there are some time constants build in. You should have a look at this wikipedia article http://en.wikipedia.org/wiki/VU_meter .
I bought the vu-meters on eBay http://myworld.ebay.de/kalleb/ .
The vu meters are taken out of some russian tape recorder. They are rather old and not all that I bought where working completely. On one of them the counter did not work, but who cares? You don't need the counter. The schematics of this devices that you can find on the internet are wrong. They are simply for another pcb version. I needed to do some research to find out how to use them.
Basically I used the vu-meter, and some components I took from a PIR pcb, also from kalleb, and an old cd-rom drive from sony. The cd-rom drive is only used as a case. All the electronics and the mechanics are not needed here.
Step 2: Repairing and Preparing the Vu Meter
First of all you need to find out if the vu meter works. Therefore you need to have 3 voltages. +15V, -15V and +5V. I did some research on the connections but I could not find out about every connector on the board. The function of some pins are still unknown to me.
1 - Analog in right channel
2 - Analog in left channel
3 - Up/Down Counter Select (open=down, +15V=up)
4 - GND
5 - nc
6 - VCC (+15V)
7 - GND
8 - nc
9 - VDD (-15V)
10 - Heater 6.3V ac
11 - Heater 6.3V ac
- 8 is not connected. It is marked as 22V but not used. Maybe this comes from an old release of the pcb
- 10 and 11 are the heater connection. I used 5V dc here and it works very well.
1 - ?
2 - Discharge (open=count, +15V=counter reset)
3 - ?
4 - nc
5 - Count (Count +15V pulses)
6 - GND
Take a FET transistor like the BF256C or the BF245. The FET is not really critical, because it is needed as a constant current source with rather low current. Solder the transistor onto the pcb. You need to cross the D and S pin to make the fet fit into the holes.
Now you can test the device, if you got a power supply.
You need to put some capacitors into the audio channels because the input of the vu-meter is dc coupled and you amplifier may not like this.
To Test the device you should turn the turn R16 and see if you can adjust the zero point. Every dot but the first one should disappear when the display is adjusted. The first dot is lit permanent.
Next you should power up the music. Use R5 and R6 to find out if the display can be adjusted to show all dots.
If the device is working well and you are satisfied with the brightness you can do the next steps. If it is defect or it looks ugly, take another one and test repeat the tests.
Now you need to prepare the display. The way it is mounted it is a little bit to high to fit into a 5 1/4 case.
You need to cut the pins on the top that are left from production. Then you need to desolder the display and bend the pins 90 degrees to the back of the display. Be VERY, VERY careful doing this! If you aret to forceful the glass of the display could break. If that happens, take another display from another board and try again.
Now solder the display the display back to the pcb, directly on the soldering side. You need to put a plastic strip between the display pins and the pcb.
Remove R1, R2 and VD8 from the pcb. They are not needed when using 5V to power the heater. If you do not remove these parts they will melt, be warned ... :-)
Now the display is ready for mounting and you can concentrate on the psu.
Step 3: Building a PSU
There are many ways to build the psu. First I experimented with the +12V and -12V dc voltages that come from the standard computer psu. I found out that the devices work with only 12V but on some devices I was not able to adjust the zero point. Also the brightness of the display is not high enough using only 12V.
Next idea was to implement the psu using a transformer and linear dc-power regulators. This would be easy but then I would need to put 220V into the pc. That seemed to be a bad idea.
Finally I decided to generate the +-15V dc from the 5V of the pc psu. Therefore I needed step up converters. I used the components I found on other pcb boards that kalleb also offers on eBay. These are PIR boards, from a hands free kit for mobile phones build by com.n.sense http://www.greasemonkeyconversions.com/10609/Com_N_Sense_Hands-Free_Kit_(Nokia_3310_etc).shtml . I bought this pcb's for usage of the parts in future robotics projects.
The schematic of the psu is taken out of the data sheet of the MC34063A. This chip can be used to build positive and negative regulated step up converters. One problem using step up converters could be that the switching regulator could inject noise into the audio path. If I turn up the volume of the amplifier I can hear this noise. It is rather low, but you can here it.
The psu is build on an experimental pcb.
I build the psu on an experimental pcb board. You should have some experience in soldering when doing this. The components are very tiny. I did not optimize the design. The whole thing draws about 0.3A current in the end. The vu meter draws only about 0.16A current. This means that the psu is not very efficient. The main problem seems to be the coils I used. The positive and the negative switching regulators need different coils. I simply took the coils from the PIR pcb without taking care of their value. I could optimize that but efficiency of the psu is not important here.
The output voltage can be adjusted with small trimmers. It ranges from about 12V to about 17V. You can carefully experiment with voltage here ...
In any case, please test the psu before connecting it to the VFD's pcb!
Step 4: Assembling
Take the plastics mounting frame from the cdrom and cut holes in it that are big enough to make space for the pcb's. Be careful to leave enough material for the mechanical stability of the system.
The psu pcb is mounted with screws. The vu meter is mounted with hot glue.
The stereo plug of the cd-rom is mounted to the frame using hot glue.
The cables are fixed with hot glue to the frame.
After the pcb's are mounted take the front plate, fill the holes of the volume control, the led and the push button with resin. Do not fill the hole of the headphone socket. Do some sanding. After that spray new color onto the front plate. A white front plate doesn't look nice in a black pc ... :-)
While the front plate drys do the Wiring;
connect +15V to XS1-Pin 6, -15V to XS1-Pin 9, Ground to XS1-Pin 7.
Connect XS1-Pin 10 to Ground, SX1-Pin 11 to +5V.
Connect Audio via capacitors to XS1-Pin 1 and 2. 1uF capacitors will do fine.
Now adjust the vu meter. Switch on the power and do not apply an audio signal. Use R16 to adjust the display to the zero point. Only the first dots glow.
Connect the audio cable to you computer and put the mixer to maximum. Put in some music and adjust R5 and R6 to maximum. The 6dB dot should glow. Use some music with heavy bass in it. Bass appears on both channels at once and you can adjust the channels to the same level. Test some other music and do minor adjustments until you're satisfied.
Ok, the vu meter is not adjusted to the standards defined for vu meters ... but who cares? The idea is to have a nice looking device, nothing else ... :-)
Take the now dry front plate and put some acrylic window inside the opening of the cd-rom drawer. If you like you can leave the hole, I think it looks nicer if you close the hole with a window. The pictures are taken without the window because I left a mark on it while sanding the edges and I need to build a new one. I will put it in later ...
Put all parts of the case together and mount it into your computer ...
Step 5: Result
At the moment I have no idea what to do with the counter.
It can count up and it can count down. It can be reset to all zeros.
But what useful can be done with it? Should it count up steady? Should it display the typing rate of the keyboard? Should it display the number of unread emails? Should it be dark?
On the pictures it is displaying SOS ... but that is coincidence ...