VC1Q noise problem

[photocat]

Hello. I have a pair of VC1Q studio channels (The original, not the “cs” version) and have have really enjoyed using them since I bought them new years ago. Unfortunately one of the two now has an elevated broadband noise signature from 30 to 1K that I would really like to fix. The enclosed image was taken with the compressor, eq and enhancer off. There was just a single output connected and both input and output gain adjustments where completely off. I tried the same setup (including wiring) on the good unit and it was clean down to 110db driving into a reasonable good 24b ADC. Grounding inside and out all look good and I tried both the 1/4 phono and XLR.

Any idea what could be causing this? Any and all help would be appreciated. Thanks.

[Ted Fletcher]

Hmmm…. This is an odd one.

Looking at your spectrum analysis the form of the peaks is very characteristic of a low level square(ish) wave. the fundamental frequency is at the mains power frequency, so the cause of the problem must be the power supply. The cause might be ageing of the smoothing capacitors….. there are two 470uF capacitors in the 15V regulator circuits; it would be worth replacing these with 1000uF 40V caps. Another possible cause could be the phantom power generator, but this is a bit more tricky; I doubt if it is worth replacing all the electrolytic caps, but the main ones in the power supply will probably fix the problem.

BTW, can you hear it??

[Author]

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