80's TV UHF tuner
 

Hi All,

I have an older TV set with the UHF tuner going up to channel 83. The
tuner is a mechnical type.

The output of the tuner goes back into the VHF tuner and you select the
U on the VHF tuner to operate the UHF channels.

In down conversion of the UHF band, what is the IF frequency of the UHF
tuner? The set is an old RCA.

pdrunen

44 MHz, same as the VHF tuner, the VHF tuner is configured as an
amplifer when in UHF mode.

Mark

wrote:
Hi All,

I have an older TV set with the UHF tuner going up to channel 83. The
tuner is a mechnical type.

The output of the tuner goes back into the VHF tuner and you select the
U on the VHF tuner to operate the UHF channels.

In down conversion of the UHF band, what is the IF frequency of the UHF
tuner? The set is an old RCA.

pdrunen

Same IF as VHF: 45.75 MHz.

When the VHF tuner is set to "U", its local oscillator is disabled,
and its RF amp & mixer amplify the IF out of the UHF tuner
without doing any frequency conversion.

Those mechanical UHF tuners have no RF amp stage, and
use a diode mixer, therefore no gain. The VHF tuner
makes up for this by acting as an IF preamp before passing
the IF signal to the IF stage.

This configuration is likely due to the lack of cheap, low-noise
UHF transistors in that time period, along with there being
much fewer UHF stations on the air, so the UHF tuners
went unused in many sets. By the time digital-controlled
tuners became commonplace in the late 80's, the UHF
tuners got real RF amp stages, and didn't pass their
IF outputs thru the VHF section for amplification.


Mike
WB2MEP

wrote in message
oups.com...
Hi All,

I have an older TV set with the UHF tuner going up to channel 83. The
tuner is a mechnical type.

The output of the tuner goes back into the VHF tuner and you select the
U on the VHF tuner to operate the UHF channels.

In down conversion of the UHF band, what is the IF frequency of the UHF
tuner? The set is an old RCA.

pdrunen

What country are you in?

What is the problem you are trying to solve, or were you just curious
about how it worked?

Bob Hofmann


Barney wrote:
wrote in message
oups.com...
Hi All,

I have an older TV set with the UHF tuner going up to channel 83. The
tuner is a mechnical type.

The output of the tuner goes back into the VHF tuner and you select the
U on the VHF tuner to operate the UHF channels.

In down conversion of the UHF band, what is the IF frequency of the UHF
tuner? The set is an old RCA.

pdrunen

What country are you in?

Thanks All,

No problem with the set, just trying to play around with the UHF tuner.
It appears to have an input for 18VDC and line for the AFT. The TV
had some other problems and I removed the tuner to see if I could do
anything with it.

I don't see anything on the O'scope out of it but will try a scanner at
44 MHz to see if I can hear any stations. Guess I will need a wideband
FM scanner.

) writes:
Thanks All,

No problem with the set, just trying to play around with the UHF tuner.
It appears to have an input for 18VDC and line for the AFT. The TV
had some other problems and I removed the tuner to see if I could do
anything with it.

I don't see anything on the O'scope out of it but will try a scanner at
44 MHz to see if I can hear any stations. Guess I will need a wideband
FM scanner.

If you're talking about putting the scope at the output of the UHF
tuner, that's not likely to show anything.

As someone pointed out, those tuners used a diode mixer, so you not
only see no gain, but there is a loss between the input and the output.
The signal is going to be very weak with that sort of tuner. I'm not
sure when active tuners kicked in, but even those won't have too much
of an output.

Better to make sure the oscillator is oscillating. Use the scanner
to tune in the tuner's oscillator signal. After that, either the
diode is good, or it's not. Beyond that, if there's a problem it's
not an active component, because you've just accounted for them
all.

Michael

yep, the uhf tuner outpout will be too weak to se eon a scope but you
may be able to use a scanner tuned to 41 to 47 MHz. The UHF tuner
ocsillator will be very unstable compared to the narrowband signals in
a scanner. The drift is not a problem for 6 MHz wide TV signal but
might be a problem for narrow 30 kHz signals etc.

If you work at it, you mat be able to generate an AFT signal from the
scanner to stablize the tuner. Have fun

Also, most cell phone signals are digital these days so you won't be
able to use this method to hear them.




Mark