DBX 160 VU distortion - is this normal?

AusTex64 · Sep 12, 2016

The attached plot shows distortion artifacts I've never seen before on a THD plot I've done. Specifically the noise all around the 1K test tone. I can hear this fuzz too. Used it on snare drum a couple of weeks ago and it was quite noticeable. I recapped the entire unit when I bought it at the first of this year. Maybe calibration or a bad VCA?

Evertide · Sep 12, 2016

1st test : + 15 Volt +/- 50 mV , -15 Volt +/- 200 mV

PRR · Sep 12, 2016

Is there a cursor on that thing?

Check the "1KHz" and then get the exact freqs of the first several spikes either side.

Compute the differences. 1,010Hz and 890Hz is a 120Hz difference, which suggests Texas Power ripple sneaking into a control path.

AusTex64 · Sep 12, 2016

Thanks guys! Sounds like I need to have a close look at the power supply. And here I'm guessing (accurate term usage) it's calibration or the VCA. Will report my findings.

JohnRoberts · Sep 13, 2016

AusTex64 said:
Thanks guys! Sounds like I need to have a close look at the power supply. And here I'm guessing (accurate term usage) it's calibration or the VCA. Will report my findings.

If the power supply is dropping out of regulation hum could get into the VCA control port and AM modulate the signal.

JR

AusTex64 · Sep 13, 2016

Reporting back.

+15V and -15V are spot on at C7 and C8.

I found the rather long factory calibration instuctions online, that's the first thing called for to check.