Hello -

Noise - random very spuratic - noticeable at Headphone Jack ( left channel more predominate than right ) while No Input to Amplifier.

Looking for suggestions on the Attached File #1 - Audio Pre-Amp Circuit.

Noise peaks of > 20 mv instantaneously.

Attachment #2.
Oscope waveform capture Channel 1 is Circuit Board Edge Connection Pin # 8, Channel 2 is Q107 collector.

I can see Noise being originated at Q105 collector - well it appears to be generated there.
To attempt to remove the Noise I have : Replaced Q105, Q107, all the electrolytic caps in this circuit portion. Tested values of the remaining Capacitors.

What are some suggestions of removing Noise here ?

Thanks in advance
cferry7

 

I have come very close Eliminating most of this Noise.
Replacement of two 2SC1344 Transistor's Q105, Q107. A few Electrolytic's replaced.

Also have found two additional Transistor's in the MIC input amp - P/ N 2SC1000 and 2SA___ ( Q101 , Q103 ) which their Bias Voltages have been desparriaged. I will be replacing these when replacements arrive.
May do some more Electrolytic Cap replacements.

 

Do you have an ESR meter to check those capacitors?

What model is your preamp?

 

This has turned out to be the Notorious Noisy TRANSISTOR issue which several of these Japanese models experience after 25 years of age. P/N 2SC1000, 2SA672, 2SC1344's and others in this class of Audio Amp Transistor's.