The reverb tank is definitely a weak spot, but its most common problem is fixable if you have the time and patience.
Check the continuity at the tank itself by measuring the resistance at each RCA jack (plug in a jumper cable for ease of access). The input side (towards the center, where the red wire goes) should measure around 60 ohms. The output side (black wire, on the right, looking at the back of the amp) should measure around 200 ohms.
It’s not uncommon for the wires that go from the input and output jacks to the reverb transducers to break right where they go into the white plastic push-on connectors. The green and black wires are pressed into the back of the push-on connector, and the connector has an insulation-piercing blade that slices through the insulation and makes contact with the wire. Unfortunately, the blade sometimes nicks the wire and it breaks right there.
Track down the broken wire by looking for continuity with a ohmmeter on each wire, from the pins to the jack. When you find the bad one, pull the wire out of the push-on connector with long-nose pliers, trim the broken portion, and push it back in again with a thin screwdriver so the blade re-pierces the insulation.
If that doesn’t do it, the problem may be with the tank itself. The very fine wires from the transducer coil to the push-on pins may have broken. You should have continuity through the pins–If you don’t have continuity on both sides of the tank, one of the fine wires is broken. These are very hard to resolder–I have to work under a magnifying lamp to do it.
At this point, you may want to just replace the tank. They’re less than $25 from suppliers such as Antique Electronics and Mojo Musical Supply. The stock replacement tank is the Accutronics 8EB2C1B. If you like long, surfy reverb and darker reverb tone, the Belton and MOD tanks available from Antique Electronics(www.tubesandmore.com) are functionally similar to the Ruby tank, which was just a relabeled Belton tank. I think the long-decay version, the 8EB3C1B, lasts too long, but that may be a matter of taste. The replacement tanks are all more sensitive than the stock Accutronics tank, so you may want to replace the reverb control with an audio taper (available from my Mod Kits page) to control it better. One combination you definitely don’t want is the long-delay tank with the stock reverb pot. It’s barely usable on 1 or 2.
The longer tank, used in the Hot Rod Deluxe and Blues Deluxe, is electrically compatible with the Blues Junior, but it doesn’t fit inside the BJr cabinet. Some people have hacksawed the ends off the tank to make it fit, but the Ruby tank will probably give you all the reverb you’d ever want.
This is the back of the plug that connects the RCA jack to the reverb assembly. Sorry for the blurry picture, but you’ll see a pair of knives that cut the insulation when the black and green wires are pressed into the connector.
They often nick the wire, however, which breaks. Everything looks good, but there’s no connection or it only works occasionally. The fix is to pull the wire out (find out which one with an ohmmeter), trim the broken part, and reinsert it. Push it in with a jeweler’s screwdriver or other small blade. You can also eliminate the white plug entirely and solder the green and black wires directly to the pins on the reverb unit.
Reverb IC failure
Rarely, the reverb integrated circuit can fail. It’s a very inexpensive dual op amp (operational amplifier). In the green board it’s a TL072, in the cream board it’s a 4560. They’re completely interchangeable. In the little 8-pin package there are two amplifiers, one for sending the guitar signal to the tank, the other for recovering the reverb signal from the tank.
It’s very easy to test the recovery side. Turn the amp on. Set the reverb and master volume to 4 or so. (If you have a stock green board, the master volume won’t matter.) Pull the black wire from the tank and touch the center pin of the RCA connector (there is no danger of shock). You should hear a buzz that gets louder and softer with the reverb control. If you don’t hear the buzz, the connector, wire or chip is bad.
It’s a little more difficult to test the send side, but if you connect a small speaker to the red RCA connector, you should hear your guitar through it. Turn down the master volume so you can hear it. The speaker impedance is wrong for the op amp, so don’t keep it connected for long–just enough to verify that you have a signal going to the tank.
When the reverb IC fails internally, it sometimes pulls down the supply voltages. Check the voltage on pins 4 and 5, which should be -15VDC and +15VDC respectively. If one reads low, perhaps 9 or 10 volts, the chip is likely defective.
If you decide to replace the chip, the traces going to it are thin and closely spaced. Cut the leads off the chip, close to the board, on the component side. Then desolder the stubs, using a vacuum spring plunger type solder sucker to remove the solder and the stubs. Install a low profile machined socket instead of soldering in another chip. If the second one fails, the board probably wouldn’t survive another desoldering. Machined sockets are made far better than the simple spring type, and low profile sockets prevent oscillation.