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cGil

Hum and Squeal

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The following is an edited excerpt from an old thread regarding Hum and Squeal and Sizzling Sounds that was referenced in the VJr Mod Training and Reference 101 thread. If anybody finds any others tips in old thread that might be helpful in the search for gremlins, please post 'em here.

 

 

Hum, Squeals at high volume, and Sizzling Sounds from the amp.

 

I'd say that would indeed be a couple of problems, and if your amp has a black board, one problem is a carryover from the green board Jr's.

 

Okay, but let's start in reverse with the squeal you hear when the volume knob is above 2(pm). That's an oscillation that usually happens from wires being where they shouldn't. Make sure leads running to tone pots and gain switches are short and sweet. These are not controls easily done on the back panel, even with shielded cables. Shielded cables, nor even twisting the cables shouldn't be necessary for noise rejection if everything else is done right. If everything looks right, and you still get the squeal, it'll be in a bad solder joint on one of the control leads. That usually where it gets into the circuit.

 

Where it comes from is another story. The output transformer's primary wires are most likely squeal inducers. They're the yellow and white wires to T3 and T4. Make sure they stay close along the back wall, come in from the side and drop down from above to the connectors. A twist in the wires is always appropriate here until they need to split to their respective connections. Don't run any non-OT related switches or controls anywhere near the OT primary wires. An NFB mod switch is okay back with the output jacks, but not the tone pot. Most stuff will pick up a squeal within an inch of them. A bad solder joint will pick it up from anywhere inside the chassis.

 

 

The power transformer's wires are hum inducers. The primary wires between the power cord jack and the switch should be bundled as neatly as possible along the side wall, away from everything as best as possible. The three extra blue, brown, and white wires connected to the board are spares for different country's wall voltages, so you can disconnect those, insulate the ends real well with tape or shrink tube and bundle them to the side as well. Don't trust the boots as insulation for this... they're open on the dangerous end, and they will slip off. The only transformer wires you need to the board are the 2 red, 2 orange, and 1 green wires, plus the OT's yellow and white primary leads.

 

The PT's secondary red wires are the main high voltage AC supply to the board. Twisting these AC lines is appropriate, too. Keep them away from the board, and again, come in from the side and drop in from above.

 

 

The orange wires are the 6.3v AC lines to the filament heaters. They're big time hum inducers and should be twisted tightly for the least noise, and approach the board as described above. If one of these orange leads goes out for a while, your sound dies with it. I suspect you've either got a dirty orange wire fuse holder contact pad, a loose orange wire quick connect, or a bad solder joint underneath one of those quick connect lugs. One of the three is probably what you hear sizzling. My bet's on the solder joint.

 

But first, check the two easy ones. Use a Scotch brite pad to scuff up the contact pads inside the fuse holder to be sure it's making good contact with the fuse. Then check and see if the orange quick connects are loose. A word of caution, those quick connects just cant take ANY stress. You may ultimately have to get under the circuit board and reflow the solder joint, so watch out for the rest of the connectors when you're working for the sake of the one. Go easy, pry 'em apart, slide them gently off and on, and gently crimp them snugly back into place when you're done. Then power up and check it out.

 

 

Good luck and keep us posted.

 

Gil...

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