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Acoustic 370 Repair Low Output

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  • Acoustic 370 Repair Low Output

    Hey ya'll,

    I'm working on an acoustic 370 that isn't sounding too hot. I haven't worked on these before, and in general, have a much easier time troubleshooting tube amps! It has very low output into a 4ohm load, and seems to mainly be passing predominantly midrage frequencies. Can someone who's more familiar with these offer troubleshooting suggestions? Thanks!

  • #2
    Here's the schematic.

    Click image for larger version

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    Do the line outs work normally? That will tell you if it's a preamp or power amp problem.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      I am NOT suggesting you just change them all, but my immediate suspects are those little 1uf caps strewn through the preamp.
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        I would inject 1kHz 18V RMS or 50mVpp at high gain input, J401, set all tone controls to 5 and follow voltages along the path. Just as suggested in the schematic.
        Yes, we can find missing voltage and it may be caused by those capacitors, in which case the test will show that.

        Check all voltages; those enclosed in triangles are Audio RMS voltages; if using a scope Vpp will be "almost" 3 times as high (actually 2.83) but as a rough check itīs close enough.

        You should reach 250mV RMS or 700mVpp at preamp output jacks.

        Also check power supply, you should have some +90V DC, then half that, some 45VDC at speaker out rail, on the positive lug of C404 and if all is well, some 20VAC at speaker out jacks.

        Take those measurements and report any abnormality.

        Being within 20% of expected value is fine.
        "Low output" by itself tells me nothing, "numbers" are way more precise.
        Juan Manuel Fahey

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        • #5
          Thanks for the schematic link. They aren’t working at all!

          Comment


          • #6
            Originally posted by The Dude View Post
            Here's the schematic.

            [ATTACH=CONFIG]52398[/ATTACH]

            Do the line outs work normally? That will tell you if it's a preamp or power amp problem.
            Originally posted by J M Fahey View Post
            I would inject 1kHz 18V RMS or 50mVpp at high gain input, J401, set all tone controls to 5 and follow voltages along the path. Just as suggested in the schematic.
            Yes, we can find missing voltage and it may be caused by those capacitors, in which case the test will show that.

            Check all voltages; those enclosed in triangles are Audio RMS voltages; if using a scope Vpp will be "almost" 3 times as high (actually 2.83) but as a rough check itīs close enough.

            You should reach 250mV RMS or 700mVpp at preamp output jacks.

            Also check power supply, you should have some +90V DC, then half that, some 45VDC at speaker out rail, on the positive lug of C404 and if all is well, some 20VAC at speaker out jacks.

            Take those measurements and report any abnormality.

            Being within 20% of expected value is fine.
            "Low output" by itself tells me nothing, "numbers" are way more precise.
            I took some measurements in the suggested test points on the schematic. I injected a 1k 18mv RMS signal and on the positive leg of C107, I only read 139mV. 161mv on line outputs, and 104mv on speaker outs. All DC Voltages I took looked normal, and power supply is supplying all the correct voltage

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            • #7
              OK, you put signal to the input, and checked the output (of the preamp) Now go through the preamp stage by stage still using that input signal. See where the signal path seems to lose it.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by Steelwitch View Post
                I took some measurements in the suggested test points on the schematic. I injected a 1k 18mv RMS signal and on the positive leg of C107, I only read 139mV. 161mv on line outputs, and 104mv on speaker outs. All DC Voltages I took looked normal, and power supply is supplying all the correct voltage
                Cool, now we are working

                Keeping all tone/EQ controls to "5", please rise volume to "10" ... do you now have expected 250mVAC on C107? (both legs should show the same.

                If so, continue with other indicated test points.

                You should have about same VAC value on C116 and C127 , since those controls set to 5 are unity gain.
                And same on line outputs.

                Which I already pre-confirm as "good" , since:
                read 139mV. 161mv on line outputs
                , showing about unity gain.
                Small discrepancies can come from pots not being *exactly* on 5 when set to that number, but in any case, I donīt see any significant problem with the preamp, letīs concentrate on following stages.

                That power amp has about 80X gain so even 160mV at its input should put out some 15V RMS at the speaker out..

                It does not, we must find why.

                1) rise volume to 10, check now you have 250mVAC at preamp out.
                If somewhat less, rise input signal slightly until it does.

                2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
                Be careful with the latter, measuring is safe but if meter slips and touchd what it shouldnīt it may damage the power amp.

                How are pre out and power amp in connected?

                If by a couple connectors and wire, some of them might be open or corroded.
                Juan Manuel Fahey

                Comment


                • #9
                  Originally posted by J M Fahey View Post
                  Cool, now we are working

                  Keeping all tone/EQ controls to "5", please rise volume to "10" ... do you now have expected 250mVAC on C107? (both legs should show the same.

                  If so, continue with other indicated test points.

                  You should have about same VAC value on C116 and C127 , since those controls set to 5 are unity gain.
                  And same on line outputs.

                  Which I already pre-confirm as "good" , since:
                  , showing about unity gain.
                  Small discrepancies can come from pots not being *exactly* on 5 when set to that number, but in any case, I donīt see any significant problem with the preamp, letīs concentrate on following stages.

                  That power amp has about 80X gain so even 160mV at its input should put out some 15V RMS at the speaker out..

                  It does not, we must find why.

                  1) rise volume to 10, check now you have 250mVAC at preamp out.
                  If somewhat less, rise input signal slightly until it does.

                  2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
                  Be careful with the latter, measuring is safe but if meter slips and touchd what it shouldnīt it may damage the power amp.

                  How are pre out and power amp in connected?

                  If by a couple connectors and wire, some of them might be open or corroded.
                  Ok, so we have 250mv on the preamp out. and 250mv going into the power amp. There is a significant decrease in ac voltage going across R301 and C301. Across them, - to + for C301, it decreases to approximately 50ma. I may test them out of circuit to see if they are functioning properly, I could not get a reading on R301 in circuit at all.

                  Preamp and power amp are connected via plastic terminal strips and held onto the board with zip ties... which I really don't love. The wiring does look a bit corroded, but conclusively there are no bad solder connections. But it I guess that doesn't mean on of them couldn't be shorted.

                  Wow!! Thanks for all your help! I'm not as familiar with solid state circuitry as I should be, and the layout of this amp has been difficult for me to lock down what's going on here. Anyway, the decrease in signal across them is abnormal, correct?

                  Comment


                  • #10
                    You are doing fine

                    Sorry maybe I was not clear enough with instruction details.

                    Now we checked preamp is doing fine (may have other, minor problems such as hiss, hum, buzz, non working controls, but at least itīs amplifying and passing signal as expected) but now I must expand my
                    2) follow preamp signal path, measure at *both* ends of C130 - R301 - C301
                    to mean "measure at each end (left/right or top/bottom) of C130 - R301 - C301 relative to ground"
                    So 6 values in total.

                    Signal voltage should be same at each side of a coupling capacitor (or both sides would not be "coupled" and somewhat less, say 20%, after going through R301, but going from 250mV to 50mV is too much loss.

                    But letīs have the proper measurements first .
                    Juan Manuel Fahey

                    Comment


                    • #11
                      Here’s a list of all voltages I’ve gotten per your suggestion, some of them have changed. Anyway:

                      Input - 17.8mv
                      Line out - 1.5 v
                      Speaker out 968mv
                      C130- 280mv, 1.04V
                      R301- 40mv, 1.2 mv
                      C301- 40mv, 0v

                      It seems that I have no signal past this coupling cap?

                      Comment


                      • #12
                        Yep. Try changing the C301 cap.
                        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                        Comment


                        • #13
                          Originally posted by Steelwitch View Post
                          Here’s a list of all voltages I’ve gotten per your suggestion, some of them have changed. Anyway:

                          Input - 17.8mv
                          Line out - 1.5 v
                          Speaker out 968mv
                          C130- 280mv, 1.04V
                          R301- 40mv, 1.2 mv
                          C301- 40mv, 0v

                          It seems that I have no signal past this coupling cap?
                          Ok, fine, letīs pass an even finer comb.

                          Please post the VAC to ground at the following points.

                          Click image for larger version

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                          Notice that *many* are in Theory the same, since they are directly connected by a solid line ... but such line in the Real World is a piece of wire, may include a connector, a soldered connection or a PCB track ... any of which may be damaged.

                          And some of your measurements are inconsistent, such as having 4 times larger signal exiting C130 than entering it, or having R301 getting 1.2mV, exiting 40mV , or having 280mV or 1V at C130 (which is already weird) but onlt 40mV reaching R301 ... although both are joined by a solid line (a piece of wire).
                          Such as 4 and 6 or 7 and 8 ... "the same point" ... but on opposite sides of a connector.

                          Sorry for the annoyingly OCD way of measuring things, but it usually leads me *straight* to the problem, without random parts replacement.

                          You do have old parts in poor state, no doubt about that, but letīs find the real problem first.

                          Which often doesnīt even lie on a bad part, but bad wiring or connection.

                          And my "bakerīs dozen" list of tests, looks long but is what I would do on my own bench, and goes "tick ... tick ... tick ..." end to end, just 5 or 10 seconds each ... until I find something WEIRD that is.
                          Last edited by J M Fahey; 02-08-2019, 11:43 PM. Reason: forgot image
                          Juan Manuel Fahey

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                          • #14
                            Thank you for showing me where ac signal exists! my results are as follows:

                            input 18mv
                            1. 1.5v
                            2. 1.5v
                            3. 1v
                            4. .97v
                            5. 370mv
                            6. 1v
                            7. 1v
                            8. 1v
                            9. 9mv
                            10. .1mv
                            11. 36mv
                            12. .7mv
                            13. 2.5mv
                            14 .1mv

                            So it looks like we have significant signal loss after R301. This is what we're looking for! It seems though that there is some cumulative signal loss across some of these 1uf caps. Should those be considered for replacement?

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                            • #15
                              Also, as OCD as it seems, I agree with you it's ultimately the quickest way

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