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  • #16
    well says 5v rms if set on high that is quite a lot of signal try the lower setting.

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    • #17
      Yes I have the fine adjustment very low, I'm aiming for 150mV. Now I'm measuring 46.6 on the mV scale on the DMM.

      R109 says 3.3K on the schematic, I don't find any 3.3K resistor on the PCB. I'm trying to follow the schematic but I don't see the resistor.

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      • #18
        well not sure where but if you find Q101 you should find the R109.
        so you have a clean output with 46mv?
        sounds like its probably ok voltage wise, if you got that output.
        150 mv might overdrive a tube amp, guess depends on the amp.

        it also occurs to me that someone possibly changed some components in there.

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        • #19
          What I don't get is why the sound is so low, I mean I'm in the x1K scale, the wave should be really high in pitch, and it sounds like a medium distorted note. Like an 80's synth note sustained (is the only description that comes to my mind).

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          • #20
            try x10 should kick up the frequency.

            I wonder how old that unit is you have?
            Last edited by dstrat; 04-19-2018, 12:18 AM.

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            • #21
              The solder points on the PCB look unmolested, pretty neat indeed.

              The 3.3K is changed for a 5.6K on the PCB. What kind of voltage should I measure there on R109?

              Can you explain "voltage above the both sides of R109, then at the source, drain of the FET." a little bit more?

              Thanks again.
              CE

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              • #22
                sure, the manual says the power supply should deliver 28 vdc to the unit.
                depending on how the fet, transistors are biased you would likely see about half the 28v more or less at the source drain, or collectors and emitters but not always.
                it would hard for me to guess what they should be beyond that.
                its more about what its got and then do the math and see if its in the ballpark.

                Edit: there are people here that could tell you more or exactly what to expect, beyond my skill level.
                Last edited by dstrat; 04-19-2018, 12:32 AM. Reason: corrected spelling

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                • #23
                  Probably 1980 from what I have been able to find.

                  I think I'll have to read about this FET devices, I know nothing about them.

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                  • #24
                    that age I would expect the caps needed checking at least, also the voltage set with that trimmer will likely change how the fet is biased and the voltages.
                    I mainly do tube gear so solid state stuff I leave alone, I know enough to be dangerous

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                    • #25
                      Hey guys, me again after 3 years.

                      To make the story short. I took the audio generator to the tech, and the guy made the corresponding repair (2018). I didn't put the unit to use because some health issues that prevented me to receive more tube amps for maintenance and repair.

                      This week I noticed some strange noises on my Fender Princeton Reverb 69 Amplifier, so I decided to give the unit a try, and to my surprise, it's back to the state it was when I took it to the tech. Just giving pulses as output.

                      Checking the circuit in detail I noticed that the guy replaced some capacitors with other of different value (all of them 10uf 100V) and made an strange bridge between the thermistor and one of the transistors with a 2p cap. The attached picture is the circuit with the mods that the guy performed. I decided to put the unit back to the original values, I soldered the correct caps and removed the strange connection cap at the back of the board.

                      Now I understand the circuit almost entirely, the supply is feeding 30V to the circuit. As dstrat mentioned before the problem is in the oscillator-amplifier section (q101-q102,q103), in fact I would say, the problem is in the oscillator, since the volume controls work fine for both Hi and Low switch values, you can hear the difference trough the amp.

                      The tech told me that this is a difficult type of circuit to troubleshoot and repair, but been such a few parts and simple circuit I decided to give it a try and check it myself. I took the voltages on the Fet and the transistors you can find them attached on one of the photos. The guy told me to that he believes that the problem should be something mechanical (I just don't know).

                      If somebody can give me some pointers about how to approach this thing with the information provided, it would be much appreciated!

                      Regards and be safe!

                      Click image for larger version

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                      Click image for larger version

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                      Leader LAG26
                      Leader LAG26 Board




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                      • #26
                        Ok, I have some time now, so I decided to give this thing a last try.

                        I attached a couple of pictures of the signal that the output of the unit is giving. First one Sine Wave, second one a Saw Tooth.

                        On the previous pictures you can see the VDC voltages on the FET and the Transistors. As I understand, this is a 3 stage amplifier using Q101, Q102, Q103. The problem definitely has to be on the oscillator. The voltages on Q103 seem incorrect, I'm not getting the proper voltage drop to Q103 Base, but watching the signal on the scope, it is a periodic signal, isn't it? Are the values for Q101 right?

                        There are some unusual things about the circuit, the manual calls for 2SC735G, the board has 2SC372 printed on it and checking the components it seems that the transistors installed in Q102, Q103 are C1815 (2SC1815??).

                        The Wein Oscillator seems like a simple kind of circuit, I hope to be able to find a solution. If somebody can give me some advice with the information provided, it would be much appreciated.

                        Regards.
                        CE


                        Attached Files

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                        • #27
                          I would suspect a bad electrolytic cap somewhere. You should have a good sinewave at the collector of Q102. If not, check C103.
                          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                          REMEMBER: Everybody knows that smokin' ain't allowed in school !

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                          • #28
                            Q101 gate should sit at 0VDC. if not, I would suspect bad frequency range switch contacts or some leaky cap.

                            BJT base to emitter voltages should be measured directly between base and emitter.

                            Some cheap DMMs can't correctly measure DCV with ACV/signal present.
                            - Own Opinions Only -

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                            • #29
                              You mentioned the trim pots were in rough shape. Have you tried cleaning them? I had unstable output in my generator and cleaning the trimmers was all it needed.

                              Also, are you 100% sure all those changes were made by the most recent tech? You would not want to remove what might be factory suggested updates just because they do not match the schematic.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


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                              • #30
                                The pots are fine, in fact the unit is very good and clean condition. The changes made by the last tech were the installation of the 10uf 160V caps you see marked on pencil. He installed 4 of them. Later I found out that the fix was actually pure coincidence because the guy said that the unit started to output a nice clear sine wave. So I don't know if I can call that a real fix.

                                The C1815 were in the unit when I bought it, and the 5.6K resistor too. I just installed the caps that the guy removed putting the circuit back to the original specs.

                                I'll check all you guys mention and come back with more information.

                                Thanks for the help.

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