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Three random questions on this piece of radio guts! (ID-ing parts and the mysterious wax)

So i opened this old Crown Japan stereo and found somethings i've seen before, but never had the initiative to ask their raison d'être (why are they there?).


1 - What is the blocky component with a colored screwdriver notch on them?
2 - Why do they always use this specifics pots for the tuners of radios?
3 - Why do they apply wax (often in generous amounts) over some components?

(i know all 3 sound pretty noob, but i had to ask someone)


Picture of Three random questions on this piece of radio guts! (ID-ing parts and the mysterious wax)
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framistan7 years ago
Most Radio receivers work by "beating" two signals together which generates a third frequency... For example, if you listen to 1400 khz am radiostation, then
an internal oscillator generates a 985 khz tone.  the resultant OUTPUT would be 1400 minus 985 =455 kHz.  Here's another example... if you listen to 630khz station.... then the oscillator generates a 175 kHz tone.... subtract  again... 630 minus 175 = 455 kHz...  The answer is always 455kHz for ANY station you listen to on the AM dial.  NOW you know what  the little red "coil" is.  They are Intermediate Frequency (IF) filters.  They work with a parallel capacitance to ONLY ALLOW 455 kHz frequency to pass through.
If you look at a VERY OLD tube-type radio, they had separate tuners for the
oscillator that you had to adjust manually every time you changed stations...
plus you had to manually change the TUNING capacitor also.  Someone got the idea to GANG these two VARIABLE capacitors together into ONE. Then they added FM to the same little radio which works on similar tuning principles.  Now the GANGED VARIABLE CAPACITOR has 2 sections for AM and 2 sections for FM... plus little alignment "tweaking" capacitors.  NOW, you know what the big white box is with all the little tuning screws on it.  Notice the TOP of the tuning capacitor has FOUR tuning screws on it. 2 of them are for AM band and 2 of them are for the FM band.  If you turn them you will RUIN the alignment of either the AM or FM section of the radio, depending on which one you turn. Dont turn them. The WAX on the board is used to keep wires and parts from moving because any movement would cause a CHANGE in the alignment of the components. FM radio uses a 10.7 mHz   ( I.F.) intermediate frequency section. This process gets more involved and i have simplified some of this for the purpose of  making it
understandable for someone just beginning to understand radio. So please don't criticize my explanation as leaving out details.... i left the confusing details out on purpose.
vinacarv (author)  framistan7 years ago
Hey, that was a very detailed explanation over only two components! Thanks!
Is very interesting the theory of the frequency clipping and filtering.

I wonder if the "ganged" variable capacitor has uses other than tuning frequencies. OR even other applications, in other devices, doing the standard work of frequency tuning (ie. a RFID kind of device).
lemonie7 years ago
The large thing is a variable capacitor for tuning, I guess some one makes / made a lot of them and they're standardised
The wax keeps things in place, anti-shock I think.
The other things are inductors, variable by screwing the ferrite core in and out.
That's an old radio?

L

The screws in the tuning capacitor are trimmers and allow the capacitor to be tuned so that the dial is accurate.

The wax is there so that those components don't move.  They are part of the tuned circuit and if they move any the tuning changes.  They use was since there might be a need to retune later and wax is not permanent.
I agree.

L