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How do I properly signal trace and check voltages?

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  • #16
    Did you test the transistors before installing them? I bought some NTE102a's from Parts Express and got several bad ones.
    Jim

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    • #17
      I did, the new and old ones all read out at about .2 volts, I just replaced them just to see if that would fix it but no luck!

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      • #18
        Trace the signal from the square shaper transistor to the 10k resistor through the 1nfd capacitor to the base of the first divider transistor.
        Compare the strength of the signal with a known good generator.
        Compare voltages with a good generator board.

        td

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        • #19
          I will do that! How do I properly and safely check voltages on the generator board? I have a digital multimeter and an old heathkit volt meter.

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          • #20
            Clip the ground lead of the digital meter to the "-" terminal on one of the boards. Check the voltages on the same transistors on the good vs bad boards. They are marked c, b, & e. work mainly around the first divider. No need to move the meter's ground clip between boards.
            Ideally a scope would be used to troubleshoot this problem.

            td

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            • #21
              Thanks so much, I will do that. To check the voltage that is going directly to the board would I clip the ground lead to the negative terminal and then touch the positive terminal with the other lead?

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              • tucsondave
                tucsondave commented
                Editing a comment
                Yes.
                No need to move the ground from one board to another.

            • #22
              So I compared voltages between the good and bad boards and they all read out to be about 9.6 vdc. I did some signal tracing comparing the d# board with a perfectly working board and noticed that the 100k resistor (circled in the photo) produces a weird squeal sound, where the same resistor on a good board produces the correct tone. I think it reads out properly, at about 108k. Should I start by replacing that?

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              • #23
                Have you replaced the transistor pair in the oscillator?
                Jim

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                • #24
                  Where are those located? Sorry, I’m new to schematics and al of this stuff lol.

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                  • #25
                    If those are the two to the top left of the card, then yes.

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                    • #26
                      Under the circled resistor?

                      Jim

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                      • #27
                        Oh yes I did, I replaced the sft 352’s with some other 352’s I had. The replaced ones and old ones all read out at about .2 volts, I just replaced them because I figured it was worth a shot.

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                        • #28
                          It would help if you could tell us where that resistor is connected. There are 3 100k's on each divider.
                          If it's the one I think it is I would start by replacing the (10nfd ?) capacitor that is connected between that 100k and the 5.6k resistor. The 8.2 nf cap would be my next choice.
                          I'm assuming you did compare the signal from the square shaper through the 470k and 1nf cap to the base of the first divider transistor.

                          td

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                          • #29
                            I won’t be able to check till tomorrow morning but when I do I’ll report back! Thanks for all the help everybody.

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                            • #30
                              I noticed that the F terminal 1uF capacitor is weaker than the same cap on a good board. I also noticed that the signal on the top transistor by the tuning coil and the transistor under that are weak as well. What could cause that?

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