Quote:
Originally Posted by Jon1967us
Looks like there are multiple variations on the demod circuit. I also found another from a Sams RCA tv alignment guide that only has 1 diode. This guide is for a the little 1956 8”.
https://ibb.co/kKDr15t
I did tack in a 50 ohm between demod board inputs. It did load down the signal a bit, so I may remove it.
Going to experiment with a couple more circuits just to see what couples the 60+ year old Heathkit with the scope the best. |
The 50 (or 47) ohm resistor belongs on the generator cable, not on the detector input, but since you are testing the generator connected directly to the detector, those two places are the same, temporarily.
The purpose of the 50 (or 47) ohms is to properly terminate the cable from the generator so the cable response is flat. It should be there when you use the cable as the input to the TV under test, followed by whatever coupling network is recommended by the service procedure (for example, sometimes just a small capacitor in series).
The detector should not have a 50 ohm across the input in normal use, as it will be connected to the radio or TV circuit, which is generally a much higher impedance, and putting a low resistance across the probe input would load down the circuit you are trying to tune.
The "compensated isolating scope lead" is not a detector, but only a simple low pass filter that will pass audio frequencies below about 3 kHz (if I did the math right) and roll off any IF frequency that may be present where it's connected. What does the manual say about how to use it and where to connect it?