Fixing a Defective Drive Circuit in a Fender Hot Rod Deluxe

I just finished repairing another Fender Hot Rod Deluxe. A lot of problems that people have with this amp often somehow involve the Drive circuit and a couple of resistors that are prone to overheating (R78 and R79). The issue here was that the Clean and Drive channels both worked fine, but when the More Drive switch was engaged, there was no output and the LED turned off instead of turning red.

The first thing I noticed was that the amp had been worked on before, clearly because of the two aforementioned resistors. (If you're interested, this Premier Guitar article about the overheating resistor problem will bring you up to speed better than I can.) The first sign of previous work was that all of the bolts holding the circuit board to the chassis were missing. Oopsy. The only thing holding it in place is the board-mounted pots jacks, which are bound to fail eventually because of this. The second sign was the clearly non-original solder joints on R78 and R79. There was also hot glue or silicone under the resistors (Fig. 1a), which I don't think was stock in the 90s USA Hot Rod Deluxes (but I'm not positive about that). The back of the board all around those resistors was blackened as well (Fig. 1b), which I'm sure is the reason it was serviced.

Fig. 1a. Hot glue or silicone under R78 and R79

Fig. 1b. Blackened board and non-original solder beads on R78 and R79

After trying to figure out what had been done already, the first thing I checked was the solder joints on the Drive/More Drive LED (Fig. 2).

Fig. 2. The Drive/More Drive LED.

I looked at the joints and poked the leads (labeled R, C, and G in Fig. 2 above). G(reen) and C(ommon) were fine, which I expected, since the Drive channel worked fine, and when that's engaged, the LED is green. When I poked the red lead, I could see the tip wiggling loosely in the bad solder joint (circled in Fig. 3). Even without wiggling it, you can see that it's a bad joint. I touched that up and the amp worked fine.

Fig. 3. The culprit: a bad solder joint on the Drive/More Drive LED.

Most indicator lights have no effect on output, but in this amp, the LED is part of the Drive circuit. So if the LED isn't not functioning correctly, the Drive circuit isn't either. The LED's location in the circuit is outlined in red in Fig. 4.

Fig. 4. Unlike a lot of LEDs, the Drive/More Drive LED is part of the circuit, rather than just an indicator.

It doesn't take to much research to realize that this is not an isolated case of this particular LED causing issues with the Hot Rod Deluxe, so if you're having issues with the Drive channel, it's definitely worth a quick peek.

The Curse of Broken Traces: Repairing a Visual Sound Route 66 Overdrive

A music store owner in the area occasionally sends work my way (for which I am quite grateful—check him out at http://www.payettesmusictraders.com). One recent project was a Visual Sound Route 66 Overdrive, which is an overdrive and compressor in one pedal. The compressor side did not turn on. When you pressed the switch on that side of the pedal, nothing happened—the LED did not turn on and the sound did not change. He said it was probably the switch, and I went with that, a mistake that fortunately only set me back $6. 

I got it on my bench, checked it out, and it sure seemed like the switch, so I ordered one for $6 and didn't think much about it. The next day, though, I decided to have another look at it, because there was a sloppy jumper on part of the board (see pic below). Since a jumper often means a lifted solder pad or broken trace (or both), I started checking continuity along the circuit.

What the hell is this? Did the previous owner spill molten solder on the board?

When I got to the switch, there was very spotty continuity between two points that were clearly supposed to be connected. I poked the solder pad a bit with the meter probe and it moved. I think the copper trace was broken and disconnected, and being held in place only by the the thin green coating of the circuit board.

Two other nearby points were in the same condition, so I installed a couple of jumpers and replaced the solder blob in the picture above with new jumpers. That completely did in the solder pads (as you can see by the big brownish circles in the picture on the left below), so it wasn't as neat as I would have liked, but at the very least it is clear what has been done, since the jumpers are now lines that basically follow the circuit path, not just masses of solder.

Left: I replaced the solder blob with jumpers that will make the circuit much easier to follow for the next person who repairs this pedal. Right: The jumpers around the compressor-side switch that caused the problem.

As you can see, it sure isn't pretty on the inside (or on the outside, for that matter—this pedal's been ridden hard and put away wet a few times too many), but it works now and sounds great.

Ibanez TS-10 Replacement Switch

I recently bought an Ibanez TS-10 Tube Screamer with a non-functioning switch, but could not find an exact replacement. I bought a Cherry MX1A-11NN for 98 cents at Small Bear Electronics: http://www.smallbearelec.com/Detail.bok?no=588. The listing warns that the switch "fits the opening, [but] needs a piece of wood glued to the actuating lever to press its smaller shaft." That sounded sloppy, so I hoped to find a different fix.

I installed the switch with no problems, and saw that it was indeed too short. I tried small rubber caps (including two that I pulled from the end of bungee cords). They worked, but didn't look good. Cutting them straighter would have helped. The sizes I tried fell off and would have needed glue anyway.

I grabbed two pieces of shrink tubing -- one about 4mm and one about 5mm, give or take a mm. I placed the 4mm tube on the actuator, which was a tight fit, but not a struggle. The 5mm fit over the smaller one no problem. I cut the length to about 8mm and very, very briefly heated them with a lighter, just enough to taper the end and firm them up a bit. Here is the result, which looks good and works perfectly.  Hope this has helped someone. I'd be happy to hear about other fixes for this switch.