Ok I have enough information that your "Test" of the channel selector seems ok. The issue of the "lockout" can be put aside for the momnet - wanted to point out that they used the Resistor Color Code until they got to #5 - they lost GREEN
So they have the wires, Black is Ground and goes up from there.
You're getting a "low voltage" on L19 - yet you're able to adjust frequency - so that tells me the DIODE seems to want to work,.
What about the bias voltage? You won't get much, looking at Pin 6 BUT Pin 5 the Diode in the SIPP when done up right, is reverse biased, so it looks more like a voltage controlled variable capacitor - so those frequency readings are correct. Its seeing (pin 5 and Pin 6, the L19 - the UNBANDED side - so it wants the capacitance oscillating there - to shove it into TR20. Pin 5 is the Pressure - the PLL pushes on the Varactor by a voltage component.
So yes, find out why the voltage is low, you have to look at TR20s' Bias circuit to know for sure you don't have an open resistor...C90 is the tricky cap that is a TANTALUM type - if that went bad, there's your ballgame and why the voltages on Pin 5 appear low.
So if the Diode is bad, as in blown open you're seeing the blown open as a capacitance L19 resonates with - for if you refer back to this ...
- Got sidetracked...
- IN the above, Pin 5 supplies the Varactor with voltage - so if your Pin 5 is low - there is a buffer resistor INTERNALLY to keep the outside world from skewing (loading down) the oscillator resonation going on between 5, and 6 - so the low voltage problem would be EXTERNAL to this.
- Pin 5 on the IC - is the Charge Pump are we talking the same thing?
- To be able to tune L19 and see it "vary" can only presume look at TP 10 not TP9 for the TP9 is the input voltage and it's filtered to Pin 5 - and it's buffer resistor - again influences from the outside world are trying to be avoided.
- You got 1.0MHz at Pin 17 - means you're seeing predominately the low-pass filter as working - you'd have a bunch of stuff you'd never be able to get a lock on except for the ~35MHz the L21 runs at. otherwise.
Your photo though, hmmm.. although not the best, you are aware that the IC is SOCKETED not soldered..to me I would verify you don't have bent pins under it, but if you're getting voltages to and from it, then those pins seem to be ok.
The evidence is starting to pile up, per your suggestion - but IF the VCO is bad and you can swap in another then try it you have nothing to lose at this time...
But here's the Truth Chart, when you turned off the Radio and back ok, you said you got 1.0MHz at 17 which tells me that you would have been close to channel 19 - close to channel 12 though, you lose a pin - it should go open - or Zero volts -
IT's the Channel 12 and lower stuff that has me scratching my head...that looks at Pin 11 of the PLL so you might want to look over that wiring harness color code and see if you're got a broken or missing wire...if Pin 11 is "active" and you go below channel 12, the Pins' voltage goes into another direction when you rotate the channel selector. Verify that this is happening - for if Channel 12 and below - Pin 11 being shorted to high for some reason would show up here and throw the wrong BCD codes and make the PLL got into LOCKDOWN - wrong pin programming...
Just some thoughts...
