Help troubleshooting Neumann pv76n

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jakobz

Active member
Joined
Oct 2, 2011
Messages
26
Hello :)

I have a issue with a Neumann microphone preamp .I have a feeling i have all the clues necesary to figure this out, but i guess my understanding of how the circuit works is not adequate, so i hope some of you would take a look and point me in the right direction Here is what i know:

  1. The sound from the output is thin and distorted.
  2. When i run a testsignal through the the unit (1Khz sinus) the negative cycle is clipping, measured at the output. Tracing back the signal it seems the clipping occurs by Trs2/Trs3.
  3. I have noticed that when the unit is powered up, for 2 seconds or so the signal looks good, and then slowly the negativ cycle flattens out. I have also noted that on power up the collector dc voltage on Trs2/Trs3 starts on 14vdc and then slowly drops to around 9,5vdc. Am i right in assuming that the collector voltage should be around half of the supply voltage, and that it being 9,5 would explain why the signal is being clipped?
Please let me know if you need anymore information or additional measurements.
 

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