HEATHKIT SG-8 WON'T MODULATE FREQUENCY - Printable Version
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HEATHKIT SG-8 WON'T MODULATE FREQUENCY - zboltman - 01-18-2021
Hello, Have and old Heathkit SG-8. I replaced both tubes.
It still won't even think of changing the frequency with the dial.
It will modulate the amplitude.
The steps switch functions.
I get the same sine wave on b & c. a is unique and d & e are the same, (but different from "a" and "b&c")
Any suggestions where I should start with this?
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - Radioroslyn - 01-18-2021
I'm not sure I understand your question.
The SG-8 is not really designed to generate a frequency modulated carrier. It should generate an amplitude modulated carrier w/a 400 cy tone.
How do you know that it's not changing frequency? Is the the tuning cap moving when the tuning knob is turned?
I take it that you are trying to see what it's doing with a scope? If so hook the generator to a receiver and listen for it. It's NOT a precision device so you will have to tune around a bit near the frequency the dial sez to find the signal.
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - morzh - 01-18-2021
SG8 is a multi-band generator that will produce an RF output of VARIABLE frequency on each band changed by rotating the dial from Min to Max of the respective bad, modulated with the fixed 400Hz frequency.
Does it do it or not?
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - zboltman - 01-19-2021
About the 400Hz Frequency; Decided to take a look at the AF output and it appears to be a mess...
Scope is set to 2ms. Looks like I have about 58Hz???
Oh boy, looks like I have to really get into this.
OK, what advice would you give for me to at least get this 400Hz going right?
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - mikethedruid - 01-19-2021
I have a somewhat later Heathkit sig gen. It is very similar. With ANY sig gen, as with any piece of ancient electronic equipment, the first thing to do is replace all the paper and electrolytic condensers, and check all the resistors for value. This is what I did, and mine works like a champ. FAR better than a brand new Chinese one I bought a few years ago, which died within a year. Yes, I DO have to double check the frequency output with a digital readout shortwave radio I have, but it is actually close enough so I could get away without doing so. It's a belt and suspenders sort of thing.
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - morzh - 01-19-2021
Your RF frequency does not look like it is what you should be getting at the 33MHz.
RE: HEATHKIT SG-8 WON'T MODULATE FREQUENCY - RodB - 01-19-2021
With the scope set to 2ms, one complete cycle is a little more than 1.5 divisions making it around 350Hz. Close enough for government work. But you should replace the caps and bad resistors to get rid of that ripple and maybe get closer to 400Hz.