H.H. Scott 130 Preamplifier channel bleed help

tdwpgtp

Active Member
Good afternoon everyone,

I recently acquired a Scott type 130, and after getting it up and running, I noticed that if I turn the volume on the preamp down, one of the channels plays a fair amount of mid-to-low frequency audio, and changing the volume has no effect. It seems that regardless of the settings, this spectrum of the sound still comes out. If I set the volume control to an appropriate level and play music, everything sounds great, but if I adjust the balance to the other side, I STILL get those same frequencies out of the other channel.

If necessary, I could probably film this phenomenon and post it to youtube for reference.

Does anyone have any idea what would cause this, or how I should go about figuring it out. I'll be honest that I have yet to ever open up any tube gear, but I am not opposed to reading and learning how to troubleshoot. I also have a competent electrical engineer friend that I can use for help.

All input is appreciated. Thank you.
 
It's a problem I've seen a number of times in Scott units. Invariably, the answer is better shielding of the components used between the wiper of the volume control, and the input grid it ultimately supplies a signal to. I've even had to go so far as to take a piece of small insulated wire, wrap it around a sensitive component, and grounding a free end of the wire -- this to complete shield sensitive components from picking up stray signal. A goofy approach, but very effective. In the case of the 130, the rumble circuits are between the wiper of the volume control, and the grid of the next stage, so there's lots of components there that could act as an antenna to prevent the unit from fully turning down.

Good luck with it!

Dave
 
On a lot of Scott gear, the unused inputs are grounded through the input selector switch to eliminate crosstalk. Your input switch might just need cleaning with Deoxit. Also possible that a ground connection on the input selector is disconnected. Recheck the schematic.
 
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