Tuesday, September 30, 2008

Pro Jr Problem: The Top Middle Screw on the Back Panel

I’d read several places online that the top middle screw on the back panel of the Pro Junior amp caused the amp to be noisy, so removing the screw and leaving it out was the way to go. I thought it was just one of those weird online rumors and ignored it. My Pro Jr. sounded fine.

But after I’d taken the amp apart and put it back together several times to make modifications, I noticed during testing that the tone control did not turn as freely as the volume control. It was bound up somehow. Here’s what I found:

The picture above is looking upward at the back of the amp with the back panel removed. The screw hole you see is the top middle screw for the back panel. Below that you can see a rectangular piece of metal; presumably to shield the volume and tone pots from heat or RF signals. The pots are right above it, with the other amp components below.

The metal shield is attached to the circuit board using silicone sealant, and it will move pretty easily. What I discovered it that it can easily get bent upward toward the bottom of the pots when handling the chassis, particularly when you are coaxing the chassis into or out of the cabinet. (It is a tight fit.)

If the metal shield is deflected upwards its free end will be above the top middle screw on the back panel. As you drive in the screw it presses the shield up against the tone pot, causing it to bind up and possibly short.

Problem solved. Just make sure that metal shield is parallel to the control panel just above it and out of the way of the screw.

(BTW, the violet wire at the lower right of the photo is the Negative Feedback Circuit. Follow this wire to the speaker tap, unsolder it and tie it off to give your Pro Junior a slightly coarser harp tone.)

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