Joerg said:
Hello Phil,
ROFL.
Actually when you turn AFC on it gets even worse. Then the tuner hangs
on to a strong station for more than 400kHz. In my opinion AFC was a
kludge, probably invented because some folks couldn't figure out how to
design a stable oscillator.
Regards, Joerg
I have a 1986 Ford Laser with the standard radio.
It is a rotten performer, and no doubt the
radio and audio sections are minimalist chips, the
worst that chip technology can offer, and just
what the fucking bean counters at Ford are delighted with.
But nearly all the domestic FM tuners made after 1970 with chips are
fairly good, although I far prefer my all tube
home made fully tubed FM tuner which includes a tubes stereo decoder.
The FM medium is inherently suited for good separation of stations
since they are 300 kHz apart and the maximum deviation is +/- 75 kHz.
But with AM, the stations are at 9 kHz apart, and the RF sections of AM
sets are receivers are almost all limited to about 6 kHz of RF bw.
This results in 3 kHz of audio bw, and this was standard in all the old tubed
AM sets.
The modulation applied to some station transmitters is -3 db at 9 kHz,
maybe not all though, but the RF bw of the receiver cuts the possible response.
If you try to listen to to distant stations only 9 kHz apart,
then the set with reduced RF bw will give good separation.
But two distant stations both modulated with 9 kHz of bw would interfere
badly with each other if you have a wide bw receiver.
To avoid the interference, the station count would have to be halved,
so stations were 18 kHz apart.
Anyway AM radios are a compromise on selectivity needed for
long distance listening, and fidelity need for locals.
A wide bw RF section in an AM radio is harder to build,
but they only suits local station reception with stations at 45kHz minimum
apart.
Ppl here have cited other fancy tuners, but nearly all the
AM sections of AM/FM tuners are also quite hopeless with
3 kHz or less of measured AF bw due to their
small RF bw within.
Why anyone would ever bother with the crackly hissy
interference ridden sound of distant AM station listening is beyond me.
They should all be made with a wider band width RF section.
In fact the RF section should consist of a double tuned bandpass circuit and
the
IF section should have a ceramic filter with 18 khz of BW,
and then we get the best from AM with modern devices.
The radios I have made for myself use double tuned two section
LC inputs before the frequency converter tube,
then have R strapped across the IFT coils to lower
the Q of the IF transformers, and one has variable IF coil spacing, and then I
get the
18 kHz of IF bw needed before one can get 9 kHz of audio bw.
Then I have a +/- 6 dB tine control, mandatory with AM radio
since HF audio content is variable.
Radio National here in the ACT on 846 kHz sounds magnificent.
Music programs are fine.
With old standard sets, boosting the treble with a tone control might possibly
stretch the response to 4 kHz. But old tube sets usually had only a treble cut
adjust knob.
The speakers were current driven from a
beam tetrode, a 6V6, and the response of the 6" speakers of 1960
gave increasing output as F rose, so some slight further extension
of the HF due to speaker rise compensated the response, so
maybe 5 khz on a good day, wind permitting was obtained from
a tube AM set.
I have measured dozens of them and repaired and tweaked many.
I don't treasure any old AM sets as they were presented as standard.
All were crap.
All need serious mods before one removes the horrendous thd levels
and bw limitations.
Most of the SS AM tuners which followed the tube sets were far worse
than tube sets because their RF/IF pass bands were
even narrower than the tube sets since they used single coil auto
IF transformers, with one tuned LC, rather than two,
critically coupled to give a flatter pass band.
Only 3 tuned circuits were involved, and this gave very poor
"skirt" selectivity, which means that attenuation of powerful local stations
45 kHz apart is inadequate.
The SS tuners using discrete transistors had a limited dynamic range compared
to valves,
and the distortions were worse in the amplifier stages and the
use of a single transistor as an oscillator and frequency converter wasn't
as good as the heptode or or triode hexode like the 6BE6 or 12AN7
used so effectively when tubes were king.
The effect of the invasion of Australia by cheap Japanese transistor
radios made people even more used to the worst possible sound.
I never ever purchased such crap.
If you want really good A reception, you have to search carefully
for tuners which guarantee at least 9 kHz of audio bw
for local station reception.
Unless a tuner specifies the AM audio bw id 9 kHz, assume is the usual
2 kHz so often found.
While some kits were offered in the past, I know of no standard sets available
right now
which do the business properly on AM.
To easily get 25 kHz bw from each of IFTs in an old tube set, one would really
need to have 2.5 MHz IF frequency rather than 455 kHz, the standard F for
over 60 years. And you'd want 3 IFTs, but this was something that
no accountant would ever allow a 1950's radio maker provide.
The history of AM is one about how to keep most people fooled most of the time.
Patrick Turner.
