quartz crystals

[old_tv_nut]

Originally Posted by Yamamaya42

Since the the subject of crystals was brought up, there was something that I have always been wondering, IF a colorburst crystal has to be replaced in a vintage TV, can one go out and get any tiny 3.58 MHz crystal that can be found all over the place? Or will the operating voltages ( 80v – 150v ) and so on be a problem to it?

I'd guess it's OK, but I don't know. Also don't know what the amplitude of the AC waveform is in typical tube sets. Looking at a few schematics, the CTC-5 has a DC blocking cap in series with the crystal. Later RCA chassis have DC across the crystal. The one Zenith schematic on the earlytelevision page has one end of the crystal at -0.5v (grid) and the other end floating DC-wise on a couple of capacitors.

Maybe someone who has done replacements can comment.

By the way, there were at least two different nominal 3.58 crystal tunings used in the industry. RCA and most others used a crystal cut to resonate at the exact color subcarrier with an external 22 pF capacitance. That is, the natural resonance of the crystal was offset so it acted like a large inductor at subcarrier frequency. This inductance resonated with 22 pF, which was supplied by fixed components plus the reactance tube circuit. Zenith, for some reason, came up with a design resonating with 18 pF. So, Zenith crystals were not cut identically to everyone else's. This crystal spec carried through for decades into their solid-state sets. When Zenith supplied a low-cost proc amp to go with their pay-TV headend gear, they re-used their TV IC to recover subcarrier, but it had a transient phase pull-in after vertical sync. That was OK in TVs (hidden in overscan) but did not meet FCC specs. I was called in to redesign the PLL, and determined that a tighter crystal tolerance resonating with 32 pF was needed. I had to call up the retired engineer who had designed the 18 pF circuit back in 1953 to ask if there were any potential gotchas of making the change. He said that 18 pF was just the way the design worked out in the original Zenith prototype, and I could do whatever was necessary.