2:ninety humming

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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Bottle Rocket
Posts: 3
Joined: Tue Oct 24, 2017 11:20 am

2:ninety humming

Post by kilgoretrout5447 » Fri Nov 02, 2018 12:57 pm

Sup gents. I just picked up a 2:ninety with issues for dirt cheap, and I am having trouble killing the hum. Hoping another tech might have run into this. There's threads out there, but none seem to have the answer. It is waaay worse on channel A, but slightly present on channel B. It goes away when you turn down the level control. I have swapped all of the tubes around, and tried new tubes on both channels and preamp. I recapped the power supply. Not the bias caps though. When I scope the input of channel A at the jack, it seems clean, but at the other end of the wire, I get sort of a clipped half wave 60 hz ripple. Bias caps? But that would affect both channels equally right?
Only other place I can think to check is the 10k uf 16v caps in the switching matrix. Has anybody run into this?

Bottle Rocket
Posts: 1
Joined: Tue Sep 04, 2018 10:51 am

Re: 2:ninety humming

Post by stevemp » Wed May 01, 2019 10:05 am

Did you get anywhere with this? My 2:90 also hums but I haven't even started to look into it. I have a full complement of new tubes in the amp.

Mark II
Posts: 102
Joined: Wed May 18, 2016 11:05 pm

Re: 2:ninety humming

Post by Henz » Wed May 01, 2019 11:39 am

I doubt that the 10,000uf caps in the matrix are doing that . Are they leaking on the pcb? How are the presence controls acting? If there are no red plating tubes, I might suspect caps. The .1 coupling caps or the .047 cap at the input stage. And of course, visually check for bad connections, solder connections. What about the plastic clip that goes to pcb from Power tranny? They are known to go bad and melt.
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