Radio Tuning

[alg2468]

I have a memory blank...I'd like to know the name of the part of a radio that is in the circuit board, made of ceramic and metal, and that if rotated with a screwdriver on its center adjusts the frequency and signal strength of the signal.  There are usually between 4 and 10 of these in a radio and each either adjustc AM or FM.  What's the name, any idea?  Thanks.

[SamBuca]

> I have a memory blank...I'd like to know the name of the
> part of a radio that is in the circuit board, made of
> ceramic and metal, and that if rotated with a screwdriver on
> its center adjusts the frequency and signal strength of the
> signal.  There are usually between 4 and 10 of these in a
> radio and each either adjustc AM or FM.  What's the name,
> any idea?  Thanks.

Resistors, capacitors and inductors.  It's a tuned RCL circuit.

______________

[dybas]

> I have a memory blank...I'd like to know the name of the
> part of a radio that is in the circuit board, made of
> ceramic and metal, and that if rotated with a screwdriver on
> its center adjusts the frequency and signal strength of the
> signal.  There are usually between 4 and 10 of these in a
> radio and each either adjustc AM or FM.  What's the name,
> any idea?  Thanks.
>

What you're describing sure sounds like a Variable Air Capacitor.

Dr. Dave

[westlife]

> What you're describing sure sounds like a Variable Air
> Capacitor.

No, he's talking about the IF (Intermediate Frequency) "cans".  An average manually tuned AM/FM radio has the "4 to 10" IF cans liked he mentioned, but only one variable capacitor for tuning.  Some radios, like the GE Superadio III, don't use a variable capacitor at all, and instead use a varactor for tuning.

______________

"This is the New York Emergency Broadcast System satellite channel.  They took the crosstown bus."

[John_U]

> I have a memory blank...I'd like to know the name of the
> part of a radio that is in the circuit board, made of
> ceramic and metal, and that if rotated with a screwdriver on
> its center adjusts the frequency and signal strength of the
> signal.  There are usually between 4 and 10 of these in a
> radio and each either adjustc AM or FM.  What's the name,
> any idea?  Thanks.
>
Hi,

There are two things that make up the tuned ciruit in any radio and that is an inductor and a capacitor. The inductor is one part of the transformer that makes up the IF stage. That is a metal can and tuned from the top using a hexagonal tool. It is fairly big.

What you are most likely discussing is a trimmer capacitor. In the old radios they were ceramic on the bottom with a mica layer between the two plates. They were tuned with a flat blade screwdriver.

John
K6JHU

[dybas]

> > What you're describing sure sounds like a Variable Air
> > Capacitor.
>
> No, he's talking about the IF (Intermediate Frequency)
> "cans".  An average manually tuned AM/FM radio has the "4 to
> 10" IF cans liked he mentioned, but only one variable
> capacitor for tuning.  Some radios, like the GE Superadio
> III, don't use a variable capacitor at all, and instead use
> a varactor for tuning.
>
Yeah...his description kinda threw me off a bit...cause IF cans don't really adjust the frequency.

[RadioFish]

Does anybody have an idea on how susceptible a modern receiver can be to interference from a nearby signal transmitted on the IF frequency (10.7 MHz + carrier frequency)?

[CW]

> I have a memory blank...I'd like to know the name of the
> part of a radio that is in the circuit board, made of
> ceramic and metal, and that if rotated with a screwdriver on
> its center adjusts the frequency and signal strength of the
> signal.  There are usually between 4 and 10 of these in a
> radio and each either adjustc AM or FM.  What's the name,
> any idea?  Thanks.
>
Could be a tunable capacitor (Which does adjust the freq of the radio and the signal strength of the rcvd signal) or an IF Xfmr (usually a square metal can with a tunable slug inside)...the IF transformers will usually only affect the signal strength of the signal and not the frequency.
The IF stages are at 10.7 MHz in the FM side and 455kHz on the AM side.

[CW]

> Does anybody have an idea on how susceptible a modern
> receiver can be to interference from a nearby signal
> transmitted on the IF frequency (10.7 MHz + carrier
> frequency)?

The IF of a FM radio IS 10.7 MHz. Unless you are close to a transmitter on 10.7 MHz, it likely wont cause a problem (the case is too small to allow signal ingress at 10.7 via leads, caps and filtering on wiring also prevents it, etc)..The IMAGE freq would be at 21.4 MHz above the displayed freq..Ex: a rcvr tuned to 93.3 say would have an image freq of 114.7...thats in the aircraft band which is low power AM...so not much of a problem...with the FM band only being 20Mhz wide, any image falls outside the band (you would think it was designed that way?? The only problem is the LO interfering with a local FM freq...the rcvr tuned to 93.3 will have its LO at 104.0...and can cause blanketing problems for a rcvr nearby trying to listen to 104.1 or 103.9...which is why the FCC has spacing requirements for station spaced 10.6 or 10.8 MHz apart.
(Yes Virginia, there IS some logic to the madness

[_Jim]

> cause IF cans don't really adjust the frequency.


Actually, they do.

One could/can shift the IF frequency from its nominal 455 Khz
or 21.4 MHz value by a noticable, but perhaps not enormous
amount, but the shift in any case in a standard, non-ceramic
filter IF-equipped AM radio, is enough to force touch-up on
the tuning dial.

Additionally, I have staggered-tuned some IF 'strips' with
the goal of broadening up the response, and yes, it made
a definite improvement.

One radio in particular I tried this trick on was a solid-state
sixties model Sears 4-band 'Portable' that was equipped with a
5 x 7 speaker, ran on 4-D cells, had a built-in AC adapter and
had a pair of 455 KHz IF 'cans' lightly coupled to each other
as the first stage in the IF chain and the tuning of those cans
markedly affected the resulting IF bandwidth and AM fidelity.

FM 'alignment' in older gear can require the use of a sweep
generator in order to properly align an IF that needs to have
a 'flat' passband on the order of 200 KHz wide.

Good TV 'work' of yesteryear required one of thise B&K Dynascan
TV sweepers to align those MASSIVE 4.5 MHz TV IF video strips
that had double-tuned IF cans that required peaking at half
a dozen different freqs! Not to mentioned the one or two
suck-out filters (traps) that required artful tuning.