Please help me with this potentiometer(?)

So, I've been trying to modify an AM/FM radio and am having some issue with one of the potentiometers (or at least I've assumed it as such).

It's 4-pin by the looks of things and whoever I wire it up I cannot get any response from it. I've only really used 3-pin ones in the past and I always assumed the 4th pin would either be either NC or a a duplicat eof one of the others.

Please see attached pictures. The 'Tune' dial is attached to it (using some cord to turn it from the offseted dial.

Any help on how I can go about wiring this guy up would be greatly be appreciated (assuming all traces to the board have been cut; I only want analogue values from it).

Apparently it's a dual variable capacitor, as used in many classical radio receivers. There seem to be four trimmer screws accessible through the PCB. In case of a superhet receiver (presume it is), the matching of both capacitors, respectively the capacitance ratio, is essential for receiver performance.

What are you trying to modify?

If that component is connected to the tuning dial it is more likely a variable capacitor. Unfortunately the pictures don't show the part too well to be 100% sure. However, nearly all mechanical tuning devices in radios are variable capacitors, especially for AM.

Btw. what is the purpose of the modification work you are trying?

Hey guys

Thanks guys!
Yes; I believe it's a variable cap. Makes sense!

I'm ditching the AM/FM part and making a digital 'internet radio' from this. I wanted to make use of the original tuning dial to provide a variable voltage to and ADC connected to my MCU.

I guess using this as a capacitive voltage divider will do?

You can use the dial with a potentiometer but the original circuits will be made useless. Make sure the voltage from the potentiometer is within range of the MCU, do not rely on anything from inside the original radio. Also make sure none of the potentiometer connections go to the original circuit board because the nature of LC tuned circuits is they are short circuits to DC voltages.

I'm not sure what you mean by using it as a capacitive divider. You can't substitute the original multi-ganged capacitor for any other component, at least not without difficulty.

Brian.

Yup I'm already using the potentiometer (on the voltage control). It works well.
I need to use the variable capacitor to the same effect (modify voltage over range for my ADC). SO I was thinking of connecting another cap in series with it, applying a voltage and thereby forming a voltage divider with which to feed my ADC. AS the capacitance of the variable capacitor changes, so should the voltage at the point between the capacitors. Am I wrong?

I need to use the variable capacitor to the same effect (modify voltage over range for my ADC). SO I was thinking of connecting another cap in series with it, applying a voltage and thereby forming a voltage divider with which to feed my ADC. AS the capacitance of the variable capacitor changes, so should the voltage at the point between the capacitors.

Click to expand...

The voltage driving the capacitive voltage divider would be an AC voltage, the varicap has rather low capacitance, some ten to maximum 100 or 200 pF. So you are building a high frequency circuit which can't drive an ADC without further signal processing, e.g. rectifying. It's much easier to make a variable frequency RC oscillator and count the frequency.

Capacitors do not conduct DC voltages so it would not work as you intend. The other factor is that if you add series capacitors the overall effect is to reduce the total so the existing tuining would no longer work either.

Your best solution, if mechanically possible, is to link the existing tuning control to a potentiometer so they both work in unison. That will keep the existing radio working and also allow you to produce a voltage from the potentiometer that is proportional to the tuning knob position. It appears from the photographs that there is a white cord linking the tuning knob and the variable capacitor, it may be possible to use that so it also rotates the potentiometer shaft.

To make it clear, the existing variable capacitor is not just a single variable capacitor, it is four on one shaft. Two are used for the AM tuning and two for the FM tuning but importantly, they all have slightly different values but change simultaneously as the shaft is turned. That type of receiver is called a Superheterodyne ("Superhet" for short) and it relies on two tuned circuits at different frequencies tracking each other, hence the capacitors having different values. The four small slotted 'screw heads' you can see are to trim the capacitor values to take out manufacturing differences, they are a 'fine tune' for each of the four main capacitors.

Brian.

betwixt said:

Capacitors do not conduct DC voltages so it would not work as you intend. The other factor is that if you add series capacitors the overall effect is to reduce the total so the existing tuining would no longer work either.

Brian.

Click to expand...

I don't need existing tuning to work. Yeah I know that caps in DC won't conduct and therefore there will be no energy transfer. But I was under the impression that there would be a voltage drop across each one proportional to the ratio of their respective capacitances? So if I have C1 = 2uF and C2 = 1uF in series and the voltage applied across both is 5V for e.g., I'd drop ~ 1.7V over C1 and 3.3V over C2. Varying one of them with this variable cap (using just a single one of the variable caps it provides as C1 or C2) should influence the capacitance and hence the voltage drop surely? Am I completely wrong here?

The ADC doesn't require much energy to be transferred afaik. It'd work like a 'scope probe in this case I'd think?

Yes, sorry, you are completely wrong!

It's true that the voltage drop is proportional to the ratio of the capacitances (XCtop/XCbottom) but only if energized with AC at constant frequency and voltage. The voltage at their junction would also be AC and would need to converted to DC before the ADC could measure it. At frequencies that allow the existing variable capacitor to be used that would be tricky.

If you want to sense the tuning knob position without being too invasive, the simplest way would be to leave the radio in AM mode and read the local oscillator frequency. It would give a measurement ranging from about 1000KHz to 2000KHz from one end of the tuning to the other (assuming the local oscillator is on the high side of the received frequency). You might be able to read that with an MCU without needing to use an ADC. Your software can easily convert the frequency reading to something the internet radio can use.

Brian.

Thanks for correcting me before I wasted time doing it! Makes sense I guess an inductive divider would work (dividing over resistive values). Don't know where I got the capacitive divider idea working for DC from; it'd just act like an open circuit after it's fully charged.

I should have just gone the way of measuring the AM frequency as you suggest. It's pretty trivial for those frequencies. Alas I cut the traces and gutted the board lol

Any suggestions on how to turn this back into something useful? make it into a RC oscillator?

Inductive wouldn't work either. You would get the opposite problem, even at low frequencies the inductors would have very small resistance so they would short out the voltage. The inductances you would typically find in radio receivers like that only have a resistance of a few Ohms, almost a complete short across the voltages.

If you are asking for what to do with a dead receiver - it depends on how 'gutted' it is. If the audio stages are intact, it might make a good signal tracer and if enough of the tuning parts are still there, you could turn it into an RF signal generator. An RC oscillator is certainly possible but you can build one of those in a matchbox so it would be more profitable to utilize the available space for something a little more advanced.

Brian.

I'm still looking to build my Internet radio from this thing. I've got everything else working; software, volume control pot, audio out etc. just want a way to use the original tuning dial so I need to have it produce some sort of signal to feed my MCU so I can 'tune' channels (in an absolute fake way).
So I liked your suggestion of using it to vary frequency which I can measure pretty trivially. Thing is its just currently 4 capacitors (2 x 2 caps in parallel right?) I need to get it generating a waveform somehow which has it's frequency varied as I turn the 'tune' knob. So I was thinking of creating some sort of RC oscillator using the variable capacitors + fixed value resistors with an opamp...

You would use the two AM (or all four AM+FM) capacitors in parallel to get maximum capacitance.

Simplest RC oscillator can be build with a Schmitt-trigger inverter 74HC14 and a resistor.

Cool I could do that. Or maybe using a 555 timer as I'm not sure I have a 74HC14 lying around? I guess I could build a schmitt trigger from opamps but using a 555 (which I have) may be easier
The capacitance would still be in the region of pFs I'd think right? (I dont have spec for my variable capacitor). I guess I'd have to pick suitable resistor values accordingly,,,

So I went ahead and built an astable multivibrator from the 555. Works a charm! The tuning dial (all 4 caps now connected in parallel) gives me a nice square wave freq of around 1.8Khz-7.2Khz using 2.2k and 1M resistors.
Thanks for your help guys! I learnt a lot!