Ebay Classic organs

Collapse

Announcement

Collapse
No announcement yet.

How do I properly signal trace and check voltages?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • How do I properly signal trace and check voltages?

    Hey guys, I am very new to fixing organs but have had some decent success with a few repairs. I’m trying my best to learn but unfortunately I don’t have a mentor, so I resort to this forum. I am currently working on a farfisa combo compact that has a problem with the D# and A cards, even after replacing some caps and transistors. I was wondering how to properly use a signal tracer to see what went wrong in the path. I build a tracer out of a quarter inch cable and two alligator clips going from the shield and center wire. I’m also wondering how to properly and safely check voltages on different parts of the organ as well. Thanks for all the past and future help everybody!

  • #2
    Are the D#'s and A's missing in any or all octaves? Out of tune in any or all octaves?
    Farfisa Mini Compact V1, Fender Rhodes 73 Mk. 1, Hammond B2, Hammond L-102 "El Choppo", Hammond M-101, Hohner Cembalet CF, Hohner Cembalet N, Hohner Favor Combo, Hohner Pianet L, Hohner Pianet T, Hohner Symphonic 30N, Leslie 145, Leslie "430" (former 130 cab with horns and light show added), Nord Electro 3, and an entire village of guitars and harmonicas.

    Comment


    • #3
      Maybe this will help.
      td

      Audio Signal Tracing.pdf

      Comment


      • #4
        Measuring voltages, Normal meter leads don't work very well for measuring voltages on crowded circuit boards. Make up a couple of meter leads with mini grabber hooks on the ends. On the real tight spots I will turn the organ off, clip the grabbers on the leads to be tested, then turn it on and make a reading. This is slow, but it avoids the fear of shorting two leads together while probing with a standard meter lead.
        Signal Tracing, In addition to the audio signal tracing that Dave mentions above, you can trace with an oscilloscope. Those used to be expensive, but look on eBay for "pocket oscilloscope". I have not used one of these (I have the full size version) , but they look like they might be good for basic audio tracing.
        Ed Kennedy
        Current Organs - Conn 645 Theater, 1861 Smith Melodeon (being restored)

        Comment


        • #5
          Thank u guys for the awesome responses! The top D# only works on the 4’ footing. It makes a 4’ footing too on the 8’ tab. The 16 makes some different note. The ones below make g# and c#, it’s different for each footing and note. I recapped the power supply and both of those boards, also changed the top 3 transistors on the d#. Also thank u for the PDF but I’m more wondering exactly how to signal trace one of these boards. What is safe to touch with the tracer and what isn’t? Where am I looking for faults?

          Comment


          • #6
            The capacitor will protect against touching the wrong thing on a tone generator. If you could post a picture of the board it would help us point you in the right direction. Normally the divider outputs are obvious and those would be the ones to probe. They are normally wires of the same color exiting from the sides of the tone generator boards.

            td

            Comment


            • #7
              Okay, I’ll post photos of the board in a couple mins. Can I use a 1uF 50volt capacitor for the signal tracer?

              Comment


              • #8
                Here are the boards.

                Comment


                • #9
                  1 mfd at 50v is fine. The divider outputs would be the widely spaced terminals to the right. They would have a different color on each board depending on the note.
                  The master oscillator output might be just to the left of those.
                  Start probing from the rightmost terminal and move to the left until you get a signal. That will show which divider is defective.
                  Compare test points on a bad board with the same points on a good board.
                  td

                  Comment


                  • #10
                    I built this little amp specifically for Farfisa's.
                    Kemo M033N 18 Watt Universal Mono Amplifier Module - from Parts Express driving a small speaker. Inputs are banana and 1/4 jacks with a 10uf NP cap (for DC Protection) going to a 10K pot. Runs on a 9 volt battery although the input voltage is 8-20VDC.

                    Use it all the time.
                    Jim

                    Comment


                    • #11
                      When I touch the 1st divider it plays the highest footing at a d# but after that they’re all messed up. Hey be replaced the all the capacitors on that board and the first few transistors but still no dice. What else can I look for on this generator board?

                      Comment


                      • #12
                        Have you replaced the pink 50uf cap by the tuning coil?
                        What were the original transistors and what are you replacing them with?
                        Jim

                        Comment


                        • #13
                          I just found a schematic for that generator so I can more easily follow along.
                          So You get the top octave at "f" output and an octave lower at "1d" and no sound at "2d" and lower outputs?

                          Th OC71 transistor crosses to a pnp germanium 2N1307 or 2N1123.

                          td

                          Comment


                          • #14
                            I did replace that 50uF cap but I did nothing, I only had one new 50uF so I switched it back and put it on the preamp board. Also sorry let me rephrase my previous post. When I touch the F output I get the correct d#, all the other outputs produce random notes such has g# or c#. The highest 4 footing d# plays correct but the same key plays a g# on the 8 footing.

                            Comment


                            • #15
                              Also I replaced the transistors with sft 352’s.

                              Comment

                              Working...
                              X