Sencore SG165 FM IF Alignment

[bbow01]

I recently purchased a used Sencore SG165 and I am doing my first FM IF alignment with it. I have done peak alignments using a VTVM but this is my first with the SG165 using the sweep generator/marker function.

The instructions in the SG165 User Manual confuse me on the set up. It says "Turn the oscilloscope horizontal frequency switch to the 60 cycle line sweep position, and adjust the line sweep phase control for a pattern with no evidence of foldover."

What is meant by this? I have a Tektronix digital scope and the only setting that I know of that relates is the 60Hz line trigger. And what is meant by the "line sweep phase control"? Is it talking about the scope's trigger control?

Thanks from anyone who has set one of these up before using a digital scope.

 

[amptramp]

The SG-165 has a provision for modulating the sweep at 60 Hz from a separate winding on its power transformer that drives a varicap in the FM oscillator section. There should be a sweep phase control on the scope you are using that allows you to synchronize the phase of the scope with that of the signal generator. If it goes out of phase, the rising part of the 60 Hz sine wave on the scope will not correspond with that of the generator, so there will be two waveforms the same shape as each other but shifted in phase and it will look like the same response curve but repeated side-by-side with one sweep corresponding to the rising side of the 60 HZ waveform and the other corresponding to the falling side.

I am not familiar with your scope but the instructions are meant to be used with a scope that has direct 60 Hz sweep from the AC line. This function is usually provided with a phase control on the scope to maintain lock between the source and the scope.

 

[bbow01]

Thanks. I have not found a "sweep phase control" on my scope, but do have a 60Hz internal trigger as an option. Selecting that does not seem to make a difference to the trace. I'll keep playing around and see what I get. The resulting 60Hz waveform coming from the ratio detector is more like a square wave with the markers visible (see pic). (This was after I tuned the primary of the input transformer to the ratio detector.)

 

[David r Doty]

I recently purchased a used Sencore SG165 and I am doing my first FM IF alignment with it. I have done peak alignments using a VTVM but this is my first with the SG165 using the sweep generator/marker function.

The instructions in the SG165 User Manual confuse me on the set up. It says "Turn the oscilloscope horizontal frequency switch to the 60 cycle line sweep position, and adjust the line sweep phase control for a pattern with no evidence of foldover."

What is meant by this? I have a Tektronix digital scope and the only setting that I know of that relates is the 60Hz line trigger. And what is meant by the "line sweep phase control"? Is it talking about the scope's trigger control?

Thanks from anyone who has set one of these up before using a digital scope.

I am curious, has anyone come up with a viable solution for the 75-ohm L-pad?