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Hartke Model 3500 Mosfet compressor issues

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  • Hartke Model 3500 Mosfet compressor issues

    I have a Hartke 3500 Mosfet (not HA3500) that has compressor issues. When I turn the compressor on the volume significantly drops down. Turning the the compressor control does very little to affect the sound.

    How should I go about troubleshooting it? BTW, I am not exactly sure how the compressor works in this unit.

  • #2
    What signal level are you running into the amp? Is this with a bass, test tone, other? The compressor will knock off the peaks of the signal and the control sets the threshold. If your input signal is too hot, it will be hard to tell what the threshold knob is doing.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #3
      I am testing the amp out with a bass. When I turn on the compressor the volume drops off so that I can barely hear the sound. Turning the compressor potentiometer does not seem to do anything.

      Attached is the same schematic in JPG and PDF formats.
      Attached Files

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      • #4
        Do you have a scope? If so, follow the signal through that compressor circuit and see where it gets lost or goes low. Start at the input to that DBX 2252.
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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        • #5
          Dude, Of course I have an oscilloscope. I use it to prop the door open! ;-)

          I took your advice and traced the signal chain. I think the DBX2151 is faulty since the signal goes in but nothing comes out. This voltage controlled amplifier IC is obsolete but luckily THAT Corporation makes a better performing replacement.

          http://www.thatcorp.com/datashts/THA..._Datasheet.pdf

          http://www.thatcorp.com/2150-series_...plifiers.shtml

          I am not sure if this recommended external component change will affect my circuit if I just replace the IC without making any changes. Hopefully I can just plug and play. Advice please.

          http://www.thatcorp.com/datashts/dn137.pdf

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          • #6
            Could someone please explain how the compressor works on this amp? It's kind of hard to troubleshoot the circuitry if I don't know how it should operate. My guess is the DBX2151 chip is malfunctioning but I could be wrong. Thanks.

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            • #7
              Is there a cash prize?

              I'll have a go but it'll be pretty broad brush strokes.

              The DMX2252 generates a DC signal on pin 7, the bigger the input the more positive pin 7 will go. The range is -300mV to +300mV. The threshold control V103 plus all the mess of those four opamps slices and applies an offset with gain to that signal that is appropriate to drive the VCA chip, the DBX2151, on control pin 2. As the control voltage goes more negative, so the gain is reduced on the main signal into pin 1. So the overall effect is for small input signal the compressor doe nothing. As the signal increases over the threshold to the gain is reduced to control the amplitude of the signal.

              You could first scope the output IC106 pin 7 to check that varies linearly with the input amplitude. The check the IC109 control pin 2 to see what it is doing.
              Experience is something you get, just after you really needed it.

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              • #8
                Nickb, no cash prizes but I will sing your praises! On second thought you have not heard me sing so maybe I will just speak your praises. ;-)

                Thanks!

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                • #9
                  This amp may be the death of me. I am still trying to figure out what is going on.

                  When I first turn on the amp and turn on the compressor everything works as it should. The compressor works and is adjustable by the control. The green LED flickers and turns sold red as the control is turned and more compressor is added. Then after about 30 seconds the output starts to fade (the volume decreases). The volume gets to the point where I can barely hear it unless I have the preamp volume turned up and the master volume turned up. When I switch off the compressor (by turning the knob counterclockwise) the volume resumes to normal (and loud).

                  Any help would be appreciated.

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                  • #10
                    OK, so something fails after it warms up. Or ther is a charging path not complete, so a voltage drifts until it crosses a threshold. This is what freeze spray is made for, spray circuit chiller on the circuit and see if it wakes back up.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      That, plus a hairdryer to accelerate the warm up time when it's on the bench.

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                      • #12
                        Have you actually measured anything?

                        From post #7: "You could first scope the output IC106 pin 7 to check that varies linearly with the input amplitude. The check the IC109 control pin 2 to see what it is doing. "

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                        • #13
                          When I put a scope probe on pin 7 of IC106 I get a sine wave that I can increase and decrease the amplitude with the compressor control. But after a few seconds the sine wave slowly diminishes in amplitude until it is a flat line. The same thing happens on pin 1 of IC109.

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                          • #14
                            Originally posted by Enzo View Post
                            ......This is what freeze spray is made for, spray circuit chiller on the circuit and see if it wakes back up.
                            ^^^^^^ That!
                            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                            • #15
                              And how is Pin #1 of IC 106 behaving?

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