Removing the Blues Junior Circuit Board

Many techs refuse to work on printed-circuit Fenders. It's too time-consuming and frustrating. The newer amps were definitely not designed with serviceability in mind. I hope this guide will help you.

The first thing you have to do before you can do any mods to your Blues Junior is get to the back of the circuit board. You don't have to remove it completely from the amp; it's more convenient, in fact, if you don't. Here's what I do to open up a Blues Junior.

Remember that tube amps run at temperatures that can burn you and voltages that can kill you. Work safely, and always think before you reach into an amp for any reason.

Disclaimer:
It's a shame to have to include this, but some people just like to make their problems somebody else's problem. Although all of these mods have worked well for me, you perform them entirely at your own risk. I do not warrant or guarantee that they will perform the same way for you or that you won't damage your amplifier, burn yourself, electrocute yourself, or stick an X-Acto knife through your palm. Tube amplifiers have components operating at high temperatures and lethal voltages. If you don't feel comfortable doing these mods, take the amp to someone who does.

These modifications will void your warranty. Peace and music, not lawsuits.

1. Unplug the amp. Pardon me for stating the obvious. Then remove the back. If the back panel binds on the sides of the case, you may need to loosen the two screws on the sides of the case that hold the chassis. Don't remove them; just back them out a few turns to relax the tension on the case. Here's what you see when you open the back

2. Discharge the power supply capacitors. They're the four large gray and black tubular objects. If you touch the positive terminal of the big one within 15 or 20 seconds after shutting off the amp, you'll get a spark. After 30 seconds, the amp has self-discharged enough to not spark, but enough to still shock you. Also discharge the three smaller caps. Touch the jumper to their lower leads, shown on the left in this photo.

 3. Clip the wire ties.
You need some maneuvering room, and you'll also want to retwist some of the wires later for hum control. Be very careful not to cut or nick any of the wires.

4. Measure output transformer resistance.
You'll need this to set the bias properly. This is a good time to do it. Five wires come from the output transformer, behind the circuit board. Two green ones go to the speaker jack. The red (center tap) goes to the circuit board, as do the brown and blue. Measure the resistance from red to brown and red to blue. Each should be around 100 ohms. Write down the numbers.

5. Remove the knobs and input nut.
The knobs are pressed onto the plastic shafts of the control pots. Pull up with even, steady pressure. If knob doesn't come off easily, hold the pot body with your other hand to reduce stress on the circuit board. If the knob really won't come up, use a pair of spoons as levers. Pad the faceplate so you don't damage it.

The input nut is 9/16". Use a nut driver or socket wrench. Please don't use pliers or an adjustable wrench!

6. Remove speaker/FAT nuts.
Use a 1/2" nut driver or socket wrench to remove the nuts that hold the speaker and FAT switch jacks. The lock washers are located inside the chassis, not under the nuts.

7. Remove the circuit board screws.
Seven screws hold the circuit board.

8. Unplug the output transformer leads.
Pull up firmly and steadily on the red, brown, and blue leads while wiggling the connectors side to side. They let go all of a sudden--don't tear out any wires. If you're working on an older (green circuit board) BJr, also remove the pilot light. Do this by prying up gently on the red lens or by squeezing the white socket to release the lens. The LED socket will pop free, but will remain attached to the wires.

9. Make sure circuit board is loose.
The black plastic standoffs may be stuck to the chassis. Pull gently at each standoff location to pop them free.

Now you're ready to actually maneuver the board so that you can get to the back. Move the speaker/FAT daughterboard out of the way, letting it hang over the lower edge of the chassis. Press the output transformer and reverb (black) wires flat against the chassis. The lower edge of the circuit card will want to catch on these wires, so keep pressing them down as you gently move the circuit board down far enough for the pot shafts and input jack to clear the edge of the chassis. Unkink the power transformer wires so that they don't resist. Keep wiggling and pressing and sliding, making sure that you're not stressing any wires. When the shafts clear the chassis, bend it out gently, lifting the lower edge of the circuit board, until it is nearly at right angles to the chassis. This will give you full access to the back of the board.

Don't pull on the big filter capacitor. Put pressure on the empty quick-connect stakes at the left edge of the board, instead.

This is what your circuit board should look like in "working" position. You have full access to the back of the board. and can operate the amp in this position after you replace the red, brown, and blue output transformer wires. For access to the component side of the board, it's easier and less wear and tear to lay the amp down on a soft cloth or carpet than to continually bend and flex the wires. The printed circuit traces are fragile! When you solder/unsolder, keep the heat on the component lead. Use a temperature-controlled iron. Too much heat on the board can cause the copper trace to lift and curl. Un-bend any bent-over leads with a jeweler's screwdriver or a thin knife blade as you unsolder, before you pull the lead out of the hole, but make sure that the solder is molten or that the lead is unsoldered. The unsuccessful mods that come to me for repair almost always have burns on the circuit board from a too-hot iron. You CANNOT use a crappy little plug-in iron with no thermostatic control! And definitely not a soldering gun! See my mistakes page for examples of how not to do it.
Update: If you have difficulty, it's likely to be when you unsolder the tone stack capacitors. The bent-over leads can be difficult to straighten without damaging the printed circuit. The traces and solder pads on the back of the board are rather small and  Instead, clip the capacitors from the other side and use a solder sucker to lift the solder and the stub of the lead. Flush cutters are a better choice than standard diagonal pliers. The picture shows the cream board tone caps, but the same technique works for the green board.
Update: After you've clipped the caps, you'll see that even with flush cutters, the upward force on the lead has caused a dimple in the solder. It's best to remove the solder and the lead stub with a solder sucker, as shown here. You'll have a clean hole with no damage.
When it's time to button everything up, reverse the above procedures. When you get the circuit board back into the chassis, pull the green filament wires from the circuit board, twist them tightly, and replace the connectors. Do the same for the red and brown pair, and you'll have less hum. Dress the wires so they arch over the circuit board and back down. Note the aluminum shield in front of the ribbon cable going to the phase inverter tube. The PI is very prone to oscillation--just bringing your hand near the cable during bias adjustments will send it into ultrasonic oscillations. The thin aluminum sheet grounds to the chassis and shields the PI wires. Oscillation is not a problem when the back is on the amp because the back is shielded and grounded to the chassis. See Cream Board Phase Inverter Oscillation for the proper way to dress this cable when you're done. Very Important! The plastic jacks for the input and FAT switch are fragile. Any attempt to tighten them snugly will strip the threads. Just bring them up to the point where the nut stops turning by hand, then give them a little bit more with the wrench. Be gentle. If you've loosened the screws on the sides of the amp, don't forget to retighten them.

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