Sweep Generator Waveform Question

[Jon1967us]

I have a sweep signal coming out of a Heathkit TV4, centered on 45mc with a marker at that freq.

Its going straight into my homebrew demodulator without going through a tuned circuit, straight into a scope in XY mode.

The pattern looks a little lopsided. Is this shape normal?

https://ibb.co/pwcQZ4j

[old_tv_nut]

In general, the waveform should be flat if your test gear has flat frequency response, but you need to give more details of your lashup including the cable and the demodulator circuit before concluding something is broken.

If your lashup is not loading the generator properly, it could cause problems.

You may need a matching pad to prevent an uneven frequency response due to the loading by your demodulator.

Another thing to note is that your scope should be set to DC coupling, otherwise the slow rate of the waveform could cause distortion. It looks to me like it is set to DC (the baseline is flat), but you didn't say.

[Jon1967us]

Switch the apparently unshieldedcable from sweep out with a shielded one and we now get something like this....

https://ibb.co/T09k88c

Probably as flat as its gonna get but...

The lashup connections are as follows to scope in xy mode

1. Generator (60hz) Horiz out -> scope X input

2. Gen sweep out —> clipped to rf dmod board—> scope DC in to Y input

[old_tv_nut]

Originally Posted by Jon1967us

Switch the apparently unshieldedcable from sweep out with a shielded one and we now get something like this....

https://ibb.co/T09k88c

Probably as flat as its gonna get but...

The lashup connections are as follows to scope in xy mode

1. Generator (60hz) Horiz out -> scope X input

2. Gen sweep out —> clipped to rf dmod board—> scope DC in to Y input

Well, since you didn't post a circuit for your demodulator, I can't say if it's hypothetically loading things, BUT the waveform is quite flat enough for alignment use now. Note that many alignment specs say something like "this part of the response should be +/- 10% from that part of the response" so you need to start with less than 10% tilt in your gear.

[Jon1967us]

Sure I can provide the demod schematic. If theres a better one, please let me know. I spotted a couple of different ones which i may try building for comparison.

https://ibb.co/88NZ8Dm

[old_tv_nut]

This should be fine.

I can't find info on the TV-4, but assuming the output is 75 ohms, the final check of your lashup is to put 75 ohms (or the nearest resistor value you have, 68 or 82) across the end of the cable (input and ground leads of your detector), and see that the response is still flat. (The 75 ohms will match the cable and swamp out any loading the detector does.)

[old_tv_nut]

I found this video that shows some probe schematics. For what it's worth, many of them appear to use a 100 pF capacitor as the input instead of 1 nF.

https://www.youtube.com/watch?v=Rl_e7lpPqNA

[Jon1967us]

Old TV Nut - much appreciated

apparently the manual states a 50 Ohm output

https://ibb.co/R6Y5wL0

and this seems to be reflected in the test leads that they give build instructions on:

https://ibb.co/9sPpySs

https://ibb.co/bWG82qg

i’ve built the second one with the little RC network on the “scope lead“, but it looks to me this circuit is another way of demodulation in place of the D mod board I am currently using. I might be wrong there though.

looks like I still need to add the parallel 50/47 ohm resistors on both of the output leads... It seems like thats the only thing missing in the lashup?

[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.

[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.

[old_tv_nut]

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?

[Jon1967us]

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.
makes sense. I’ll be mindful.

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).
yes it does appear to flatten the response a bit As far as what I’ve observed

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

here’s what the particular reference to Sam’s photo fact says about that circuit see link to picture, row 10

https://ibb.co/tXLL6fy