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Peavey 701R Mixer- What is this part??

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  • Peavey 701R Mixer- What is this part??

  • #2
    So I can’t figure out what these silver block guys are by the xlr jacks. I apparently did something wrong when I plugged into the Sub B out xlr (2nd from right). I need to test the silver things but don’t know what they are. Can anybody help?


    • #3
      Click image for larger version  Name:	UnBalanced to Balanced.jpg Views:	0 Size:	53.0 KB ID:	923571
      Click image for larger version  Name:	Master Outs PV 701.jpg Views:	0 Size:	83.8 KB ID:	923572 They are hi to low impedance unbalanced to balanced matching transformers.

      You have 4 it seems so it should be possible to measure the DC resistance (not the impedance) of the other three and compare the readings to the suspect one.
      You may know why microphones have 3 wire circuits Usually a ground (shield) and a positive and negative wire where the tiny AC voltage generated by the microphone travels back to the mixer.
      With this 3 wire arrangement any outside electrical noise that gets into the cable on its way to the mic preamp gets cancelled .
      The same apply's to the outputs although not as likely as the ac voltage has been boosted and is much higher that the mics's output.
      This 3 wire system is commonly known as balanced.
      You may notice that out of the line amp ,possibly an IC , there is only one wire which goes to the leds ,the tip of the unbalanced jack socket and the primary of the transformer.
      This obviously is an unbalanced output and only has one "hot" or live wire and ground which is usually a shield.
      With a multimeter set to ohms if you have one lead connected to chassis you should be able to check the very low resistance of all the transformer primary's by simply putting
      the probe on the tip of the unbalanced outputs.The secondary's just as easy just measure between pins 2 & 3 .. they all should measure the same.
      Cheap guitar leads are known for picking up noise and some expensive guitars too as they are unbalanced.
      I have often wondered why someone hasn't come up with a workable balanced guitar output.
      Les Paul did but it really didn't catch on.
      There is one other aspect to these transformers and that is impedance change.
      They convert what would be a high impedance output from the line amp to a low impedance more suited to long runs of cable.
      This is achieved by the turns ratio of the two separate coils inside the transformer.
      A bit like a small wheel driving a large wheel and vice-versa.
      You would need to read up on transformers - impedance matching - and converting unbalanced to balanced using a transformer.
      A tube guitar amp uses an output transformer to match the inner high impedance tubes to the much lower low impedance speakers also physically separating the high voltages from the speaker socket.

      I doubt any of your transformers have failed.
      Last edited by oc disorder; 01-27-2021, 10:27 AM.