Velleman Radio Kit - replace the trimmers?

[sign216]

I've got a Velleman MK118 FM radio kit. Tuning is controlled by two variable capacitor trimmers; RV2, CV1. They are small, and a little cheap. On a Dutch site the builder replaced CV1 with a quality trimmer, but it's huge. It's looks like a real improvement. Should I do that as well?

Joe

Link to the kit: http://www.velleman.eu/products/view/?country=be&lang=en&id=348222

Link to Dutch site: **broken link removed**

Instructions/circuit are attached.

 

[audioguru]

A real radio does not have coarse and fine tuning parts. I think this kit has the front end tuned by the potentiometer RV2 and the trimmer capacitor is set only one time for the least distortion.
On the Dutch site the cheap little trimmer capacitor CV1 was replaced by the tuning capacitor from a real radio.

 

[dr pepper]

I wouldnt have thought it would be neccessary, unless you will be tuning it a lot.
Still doable though.
RV2 sounds like a variable resistor not a capacitor.

 

[sign216]

So is replacing the capacitor CV1 with a larger, more involved tuning capacitor "overkill." Unnecessary? The Dutch site has CV1 being tuned frequently, but it that incorrect? Should it only be tuned once, when setting up the radio? If that's the case, then is the cheap little capacitor adequate?

 

[audioguru]

Why didn't Velleman say what the parts are used for?

 

[JimB]

So is replacing the capacitor CV1 with a larger, more involved tuning capacitor "overkill." Unnecessary?

Having studied the Velleman diagram and the Dutch website, yes I think that it is un necessary.

Should it only be tuned once, when setting up the radio?

Yes

then is the cheap little capacitor adequate?

Yes, the little trimmer capacitor is (should be adequate.

A technical explanation:

The RF sections of the radio are made up of a "front end" module which converts the incoming 88 to 108 MHz signal to the 10.7MHz IF.
(10.7MHz is a guess on my part, it is not obviously stated in the text.)
EDIT, a quick calculation of component values around CV2 confirms that the IF probably is 10.7MHz.

The potentiometer RV2 controls the frequency to which the front end is tuned.

The IF and demodulator sections are implemented in the TBA120 integrated circuit.
The little trimmer capacitor CV1 tunes the quadrature coil.
This should be adjusted for best audio signal. Do this once when first setting up the radio, it should not be necessary to adjust it after first set up.

JimB

 

[sign216]

JimB,
Thank you so much for the education regarding the trimmer capacitor. I might substitute the potentiometer, RV2, for a larger, better made unit, but that's merely to help tune it station to station. Unless you and board think that's a wasted effort as well.

Audioguru,
The documentation is inadequate, I agree. Just a sheet of paper would be great, talking about the theory, etc. But it is what it is. I actually sought out this unit, because it's no longer in production. The "new" Velleman radio kit has digital tuning, with only up/down scan. No dials. Just doesn't feel right, but what do I know.

Joe
USA

 

[audioguru]

The very cheap FM radio from The Dollar Store uses a Chinese copy of an old Philips IC. It scans for tuning.
I bought one for $1.00 to see how bad it is.
It receives only strong local stations, but the scanning misses a few of them. Most of the local stations overload the circuit because they appear at a few spots on the dial.

But my Sony Walkman radio has a variable tuning knob and it is also overloaded by strong local stations that appear all across the dial. They fixed it with a Local-distant switch that attenuates everything. I can never use it on the distant setting.

 

[JimB]

I might substitute the potentiometer, RV2, for a larger, better made unit, but that's merely to help tune it station to station.

Replacing RV2 with something larger, or, better still a 3 turn or a 10 turn pot may be a good idea if you can find one at a suitable price.

JimB

 

[sign216]

AudiGuru,
Realistically the performance may not be different between an analog dial and an electronic search, but the "feel" is different. There is something about turning the dial to tune in a station, versus a souless electronic search for the station.

JimB,
I will go that route and get a larger pot. With multiple turns I'd lose the ability to put a scale on the dial, but tuning would feel so much better.

Joe