The 15W Fender Squier guitar amplifier
Paul Rako, Technical Editor - April 21, 2011
A friend gave me a Fender Squier guitar amplifier when he retired. The sound was terrible, and all the potentiometers made scratchy noises. I was able to pry into the vinyl-covered chip-board enclosure using only a Phillips-head screwdriver. By looking at the product photos on the Digi-Key site, I saw that the potentiometers were similar to those in the Bourns PDB181 series. Digi-Key had none in stock, but Mouser did. The company even had the “-A” audio taper for the volume. After disassembling the amp, I fired it up. Although it sounded weak, the scratchy noises were gone. After reassembling the unit, though, I found that the sound had become a tinny buzz. I again pulled the unit apart. I thought I had blown the output transistors. When they checked out OK, I noticed that the sound on the input jack seemed intermittent. I applied DeoxIT contact cleaner to the contacts and restripped and resoldered the wires between the jack and the board.
Finally, I noticed that a slight pressure on the PCB (printed-circuit board) made the sound cut in and out, meaning that I had done a bad job on soldering the joint on the gain potentiometer. Once I fixed that problem, the weak volume and buzzy noises disappeared.
1. I found that two of the potentiometers were broken in half and the rest were scratchy and intermittent—not good. I replaced the no-name cheapos with genuine Bourns units. You can see, feel, and hear the quality improvement.
2. The carry-strap screws secure the chassis inside the chip-board enclosure. Two tabs secure the lower part of the panel.
3. One of the common mistakes mechanical engineers make is using electrical components to carry structural loads. The potentiometers should not have to support the whole PCB.
4. Thanks to my poor soldering job, I had to yet again pry the PCB apart to find out why the sound was intermittent.
5. I had to cut down the shaft and grind off the antirotation lug on the replacement potentiometer on the right.
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