Posts: 909
Threads: 40
Joined: Jan 1970
I calculated the ratio of your three results to their average and got 84.9%, 109.9% and 105.2%. My 20S, Indonesia ID836 gave 4749, which is 109.2% of that average.
Here's a guess: I'll bet the frequency is set by an external capacitor and a resistor inside the IC, or possibly an external capacitor and inductor. A resistor inside an IC would probably have enough variation to account for the range of speeds; I think the Voyagers had an external capacitor and inductor, and I know the Classics and Woodstocks did. But however they did it, and assuming they had a design that gave a fairly consistant speed, the easiest way for a sizable reduction in speed to happen by accident would be for someone at the factory to substitute a lower quality capacitor for the one that sets the frequency. You can get capacitors in a tolerance called "Guaranteed Minimum Value", which is sometimes stated "+100 -0%". These are intended as bypass capacitors for low frequencies. Compliance engineers warn against using them in signal filtering applications because they can be self-resonant at the frequencies you are trying to suppress.