John,
What is the part number and manufacturer of the oscillator?
Under what condition did you measure the voltage? If open circuit, the oscillator is likely defective or expecting a pullup of some sort, or it could be a remarked marginal oscillator. If in circuit, it still could be a bad FF input. One other thing to check. If the replacement oscillator is not the same make/part number as the old one, that extra forth pin might actually be an output enable, instead of a No Connect pin. That is probably a long shot.
I just looked at some photos I took of my A19 board and see it has a Dale XO-33A oscillator on it. It is not what the manual/schematic shows and this part has different specs than the K1122A oscillator. Looking at the spec, it indicates it can sink 16ma of current to make a low and source .4ma for a 2.4V high. It might be worthwhile to repeat the oscillator output test with the load resistor connected to +5 rather than ground. The oscillator on my board appears to pull a high level down rather than drive a low level up to a TTL level. I assume that would mean the oscillator is actually pulling down the high level on the FF pins, not trying to drive the FF pins up to a high. I had suggested much earlier that you try pulling pin 8 of the oscillator up through a resistor to +5 to see if it improves the clock level. Did you try that? Might try a 1K pull up with the scope monitoring the pin and see if you get a clock with a better TTL signal.
Another long shot to try when you have the FFs and the oscillator out, ohm the trace to ground and +5 (power off) and see if there is any resistance. Should be an open circuit. Proceed to add parts and measure as you install them. Could also measure the resistance of the trace before you remove the old FFs to compare with later. Could be an internal short in one of the FFs, some problem between nearby traces or a cold solder joint on one of the FFs. Are the voltage levels all ok on all the pins of the FFs (with the oscillator removed and/or installed)?
However.... Something is not right here as I would say you (and us) are spending too much time analyzing and speculating on this one problem. If you have a good new oscillator and have good new FFs, the circuit should just be fixed and work as it did 30 years ago. I am sure there are hundreds of these scopes, with this design, still working today. Mine is. I would not spend time designing new circuitry unless you wanted to design a new circuit to try to improve Tek's design. You should not have to design anything unless the parts are defective and cannot be replaced. Just my opinion.
Regards
Tony
