Volume control
The volume control is a quite important control on the radio. You should always keep the level as low as possible since this will increase your signal to noice ratio. If you turn up the volume too high you can even damage your hearing.
When you use the control often it will soon be weared out. Potentiometers will never last for ever since it's a mechanical part sliding over some piece of plastic with carbon on it. As with the electrolytic capacitors in this radio – remember it was made back in 1978-1980. If the radio is used in a location that isn't totally dry you can also get corrosion on metal parts.
Change of the potentiometer
First turn off the power and let the FT225 be without any power for a couple of minutes before you start opening the chassis.
Tools needed;
- Screwdriver PH2
- Screwdriver small flat for knobs
- Wrench 11mm
- Soldering iron and solder wire
It's a good idea to clean all the knobs and the front panel now. Use water and dishwashing solution. A toothbrush is good since it'll help you to really clean all small parts and corners. Try to not get any water into the knobs since you can get rust on the screws. Afterwards you need to dry all parts very well. The “glass” on the front-panel can be removed. Be careful it's some acrylic plastic and can be very fragile. The red part is glued ot the front-panel so don't remove this!
Behind the plastic front-panel there is a metal-panel where all the potentiometers and switches are mounted. You have to undo this too and it's four screws, two on each side.
This metal-panel can now gently be removed a couple of centimeters. With a wrench, remove the volume potentiometer. Keep track of the different wires and their colours. This picture show you how it looks in my FT225RD, but there might be differences in the colour used. Unsolder three wires at the time and solder them to the new potentiometer.
Mounting
When all the knobs and the front-panel are dry again it's time for mounting. First check all the other nuts holding potentiometers and switches. Then mount all the small knobs.
The VFO-knob can be tricky. To get the knob evenly mounted I put three pieces of paper between the knob and the panel. Three to four layers of ordinary printing paper is good enough. Press the knob against the panel and tighten the two screws. I had to do this several times before I got it well.
Modification
As mentioned the potentiometer is often used, at least by me. It's also a little strange that the constructor has decieded to use a linear potentiometer for the volume control. In this way there is very much gain difference in the beginning of the potentiometer turn. Our ears are ”logaritmic” and we can only obeserve differences that are about a doubbling of the volume. A logaritmic potentiometer would have taken care of this naturally.
Since the potentiometer is very special we can probably forget to find one that will fit and be logaritmic. The solution is to change the part of the turn of the potentiometer that is used. It's not a perfect solution but it will probably give you slightly longer potentiometer life.
This can be done in several ways. One could put an extra resistor together with the volume potentiometer, but I decieded to change R14 in the audio amplifier, AF AMP UNIT. Increasing R14 will decrease the gain of the amplifier from about 43dB to about 7dB. This sounds as a lot, but the signal will be enough for most environtments.
Gain is calculated as R15/R14. Originally this is 3300/22=150 times. After the modification it'll be 3300/1500=2.2 times. To get this in dB-values you'll get 20*log(150)=43dB and 20*log(2.2)=7dB. (Remember this is voltage and then we always use the factor 20. For power it's 10.)
Originally you probably use about 10-15 degrees of the full potentiometer turn and after the modification I use about 50%, so I still have some headroom. The radio will also be more quiet since with the high gain you get some noise from the audio amplifier. Now when I turn the volume control to the lowest then the radio is quiet.