Not every modification goes according to plan. Sometimes you break something, sometimes your soldering skills are not what you thought they were.
While you can always send your amp to me for repair or modification, you may be able to fix it yourself. Circuit board repairs are fairly easy if you have a steady hand, and we have a page that describes how to do them in detail:
The larger problem may be finding the break. It’s often hidden under a solder joint that looks perfect. If you put a signal into the input jack — any kind of audio tone — you can trace it through the amp and see where it stops. The usual tool for this job is an oscilloscope, but that’s an expensive proposition for, perhaps, a one-time use.
The video below will show you how to make this audio probe:
Update: I slipped the probe into a pen barrel, per a user’s suggestion:
It’s a simple device that you plug into another amplifier, so you can hear the test tone. In many cases it can be faster than an oscilloscope in finding the problem.
It’s simply a capacitor soldered to a piece of coaxial audio cable, with the shield lead brought out to a ground clip. Shrink tubing makes it rigid enough to use as a probe and a finishing nail on the end helps it stand up to wear and tear. The .01uF 400 volt capacitor I used is a Mallory 150, which was about the smallest diameter I could find. It fits nicely under the shrink tubing. If you get a male-to-male mono 1/8 inch cable, you can cut it in half and make a spare probe.
Note: There’s some indication (feedback from users) that even a small cap like the .01uF cap may send too large a signal to the Radio Shack mini-amp and blow it out. You should probably use a smaller cap, such as a .002uF.
The little Radio Shack utility amplifier has been in stores for as long as I can remember. Here’s the catalog link:
I like using something that’s both inexpensive and battery-powered so I don’t have to worry about ground loops, more wires, or potential hum problems from an AC-powered amp. But you can use another guitar amp for this if you want. If you get loud hum from the guitar amp, it’s probably because of a ground loop. Since you have a common ground through the electrical grounds on both amps, you may find that it will be quiet when you remove the ground clip.
You could also use the audio input of a notebook computer, but I hesitate to recommend it because, with the very wide range of loud and soft signals coming from the amp under test, there’s the outside chance that you could blow up something expensive.
This next video shows the audio probe in action:
When you set up your test tone, use your multimeter’s lowest AC voltage range and set the voltage from your signal source (probably a headphone jack) to around 10 millivolts (.010 volts). Even good multimeters are very sluggish when measuring small AC voltages, so be patient and give the meter time to detect and respond. If you listen to the tone with headphones, it should be “there” but not very loud.
After you’ve verified the proper working of the output section with the reverb wire test shown in the video, turn down the master volume. Keep a speaker plugged into the amp the entire time that it’s on, however.
Now is the point at which you say, “But I can’t read schematics!” Sure you can. It’s just like reading a roadmap. If you don’t have one, here’s the address to the Blues Junior schematic at Fender’s site:
The important part is knowing which parts are important and which to ignore in your trace. I’ll use cream board part numbers to describe the flow, with the green board part numbers in green:
R2 (R1): Grid resistor from input jack. Signal should be present at the top of R2; it comes directly from the input jack.
R1 (R2): Sets the input impedance. Not important.
R3 (R3): Plate resistor for first preamp stage. Will have around 200 volts DC on both sides, so be careful. The test tone should be loud on the lower (tube) side of the resistor.
C2 (C1): First-stage coupling cap. The tone and DC voltage should be on the lower side, just the tone on the upper side. From here the tone goes to the volume control. We’ll assume that the control is OK.
R7 is the grid resistor for the second stage. It gets the signal from the volume control. There is no equivalent to R7 on the green board Blues Junior. The signal on both sides of R7 should get louder or softer with the volume control. Test the green board at R5.
R8 (R10): Plate resistor for the second preamp stage. High DC voltage, as above, and strong signal that can be varied with the volume control. From here the signal goes into the tone stack. All tone from the stack exits from the center terminal of the treble control and flows to
R15 (R30), the grid resistor for V2. V2 re-amplifies the signal, which gets diminished in the tone stack. You should hear the tone on both sides of R15 and you should be able to vary it somewhat with the tone controls.
R16 (R39): Plate resistor for V2. High voltages again, but the signal should be present on the tube (lower) side of R16.
C8 (C16) is the coupling cap from the second stage. It strips off the high DC voltage and passes along the signal to R18.
R18 (R40) separates the signal that goes to and comes from the reverb and feeds the combined wet (with reverb) and dry (no reverb) signal to the master volume. If you don’t hear the signal at R18, you have a bad connection at C8.
There are other places where things can go wrong, such as the broken ribbon cable in the video, but following the signal through the first three tube stages, as we just did, should solve the great majority of problems that people encounter.