Heathkit LG-1 as AM Transmitter Help

Discussion in 'The Projects Forum' started by PrincessWoona, Mar 23, 2016.

  1. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
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    image.jpeg image.jpeg image.jpeg I got a heathkit lg-1 signal generator. I read some articles online about using similar generators as am transmitters. I want to do so with mine when nothing good is on fm. I love in Wyoming so most music is country. Not my preference. Don't worry, FCC rules allow me 200mw and 200ft range for unlicensed transmit equipment. I replaced most old capacitors and resistors. I replaced all capacitors except the ceramic(aside from 2 of those), the electrolytics and a 2uf molded paper. I don't have a 2uf on hand. I don't have a 40uf or 80uf or close for those. But I don't get a modulated am signal out of it. There is a ceramic capacitor near the output right before the step attenuator that I replaced. I used my oscilloscope and saw what looks like a modulated signal before the capacitor(all in reference to ground) but after I saw a "blob" with no modulation. I replaced it. I may replace my 1k 5w resistor back out with an original 1k 2w Carbon comp resistor and check again.

    I'll attach pictures of my scope findings and the schematic with my parts in question marked if I can. I'm asking what may be done here to correct this. I originally bought this not as a transmitter but for alligning vintage radios should I need. And also when testing its running through my isolation transformer. The first oscilloscope picture is before the capacitor and the second after. Notice a undefined blob? Also my meter when set to modulation the meter pegs regardless of the modulation knobs position. I don't have a vtvm to calibrate that. Thanks
     
  2. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
    53
    2
    Here's a link to the schematic I'm using. The 12au7 is split into two tubes. That confused me at first until I remember it's a dual triode tube. I replaced the 0b2 and the 6au7 with some others I had. It wasn't the tubes causing this. For the modulation so I've been using my iPod mini connected through the external modulation playing music.

    http://www.vintage-radio.info/download.php?id=226
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If I understand how it works correctly you can set it to CW, 400Hz tone, or External modulation. So maybe a scope picture of the CW output with voltage and frequency, then the same frequency with the 400Hz tone, then with the Iphone.
     
  4. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
    53
    2
    Unfortunately I would show my scope with frequency and voltage measurements. But my scopes a Tektronix type 317 and very inaccurate. It's really only good for checking what kind of signal exist not for measuring. But I do got a Tektronix 2232 for my birthday coming up in a couple months.

    I'll try with the scope to show what the different modes look like. If I remember right it's just all that one blob or close. I'll get some images soon.
     
  5. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
    53
    2
    image.jpeg image.jpeg image.jpeg image.jpeg image.jpeg image.jpeg No difference between cw, mcw and external mod. It's all the same blob. Even terminated with a 50ohm BNC connector and even with a iPod mini playing music. I did get a interesting thing when I took my oscilloscope ground probe to the positive external mod in. That's the only wave which was different.

    First is the cw with my scope at approx- 5-1ms/div and the second at approx: 0.2us/div.

    Next is the mcw at approx: 5-1ms/div and the second at 0.2us/div.

    And now the external modulation at once again, 5-1ms/div

    The last is when I feed the oscilloscope ground into the external mod positive in and the oscilloscope positive to the generators output.

    Notice a pattern? There seems to be no difference between modes. And looking at the blobs up close it shows a repeating pattern. But then those blobs stop and wait. This is all analog so I don't know why it would do so. It's almost like something is killing and then restarting the oscillations. I'm just guessing. I'm open to suggestions. And willing to probe anything else. Maybe I'll open the signal generator up and probe across my capacitor with the negative on one side and e positive the other. See what's happening across it and that 1k resistor I changed. I would expect this behavior from the before I replaced parts. What ever. I'll fix it or use it for it intended purpose.

    Ps my oscilloscope is really only inaccurate on the volts/div. I assume the time is ok but not sure. I had the signal generator set to between 2 and 2.5mhz. It's just a random frequency I did. Read the brightest out of band C.
     
  6. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
    53
    2
    I forgot to mention but some of my images I had the scope in the mag position. Images 1,2 and 5. The ones with the large green in the background. In that position I'm not sure on the time base. It goes from a max of 0.4s/div to a minimum of 0.04us/div. But I guess any frequency measurement is not really important right now.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There are probably some tube guys here that are much better at this than I am, but until they come I'll give it a shot. :D
    Set it to about 300Khz and CW. It should run all the time and one cycle should take about 3 microseconds.
    With your voltmeter measure the DC voltage on the 6AV5 pin 5. It should be around 120 volts.
    If ok scope 6AF4 pin 10 or 7. It should be a sine wave at 3.3 usec. Use AC coupling on the scope.
    If okay 6AV5 pin 1. Sine wave
    6AV5 pin 5 bigger sine wave.
     
  8. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
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    I've kinda decided that I'm going to not try this as a transmitter right now. I don't feel like tracking down weird little issues on this. I will later. This is more because I'm trying to fix the radio out of my 1973 dodge powerwagon. I do still want to figure this out just not right at the moment. Thanks for the help ronv
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    287
    Greetings:
    Ahh...that generator gives me such fond memories...it's a true classic.
    You should note that most signal generators of the era had no interest in super clean modulation...the point was to generate enough of a modulation envelope to do effective signal tracing through a receiver.

    However....you can greatly improve the performance by, first, being sure the output is properly terminated. Secondly, be sure the audio modulator is really putting out a sine wave...not that it matters if you're going to use external modulation...but it WILL allow you to see if the modulation process is clean.

    If you can't get adequate modulation envelope with the existing circuitry, you can add a newfangled doubly balanced mixer to do the job for you, and use the last r.f. stage as a linear amplifier.

    Hope this gives you some ideas.
    Eric
     
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  10. BR-549

    Well-Known Member

    Sep 22, 2013
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    I agree with Eric. You would need a modulation transformer in the plate circuit.

    These units only gave ~ 30% modulation.

    I wouldn't ruin this unit to try it.
     
  11. PrincessWoona

    Thread Starter Member

    Nov 30, 2015
    53
    2
    I wouldn't want to mod the circuit unless it was minimal. But it's fully(somewhat) working at the moment and almost dead on accurate. When I get my better oscilloscope(Tektronix 2232) in less then a couple months for my birthday then I may really go though this circuit trying to find the faults. By then I know more about electronics to troubleshoot it.
     
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