"H. P. Friedrichs" ) writes:
Hi, all--

I picked up an old cable box at a recent hamfest. I'd like to use the
tuner module as part of a receiver project.

I'm fairly good at reverse-engineering things, but that can be a time
consuming process. I wonder if anyone might have service schematics for
this box:

How much trouble can it be? Those modules are generally self contained,
with only some peripheral circuitry needed.

Often the metal on the modules will be stamped with the function of
the pin, though it may be slightly cryptic. Sometimes the circuit board
may be marked.

"B+", ground, tuning voltage, a sample of the VCO that has been prescaled
via an internal divider, how many other pins can there be? The input
and output are generally on Type F connectors, and will be apparent from
the markings on the outside of the unit.

Open the case, turn the unit on, and measure the voltages. Change channels
and see which one varies. That will be the control voltage.

Or just trace the wiring. There'll be a regulator at the output of the
transformer/rectifier, and the output of that will supply a constant voltage
to the module. You can either read the voltage off the regulator, or measure
it at the output.

The prescaler output will go to some sort of divider, a simple logic counter
of some logic family which further prescales it down in frequency.

The control voltage to the VCO will be connected to some really good
capacitor, I guess it's before a good op-amp. This would be the "memory",
keeping things on channel when it's powered off.

Ground is pretty obvious, though at least some units I've looked at
used a negative supply voltage, so the ground was where the positive
output of the power supply went.

Those modules are really easy to use, which is why they've ended up
being used in various projects over the years.

TV tuners in tv sets and even VCRs tend to be more complicated, since
they are single conversion, and they have to be switched from VHF-lo to
VHF-hi to UHF, and there is usually a pin on those modules for AGC.

Michael VE2BVW