80's TV UHF tuner

Mark

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

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:

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

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