Fender Blues Jr. question.

[Danglin' Fury]

I have a Fender Blues Jr. I need to fix in the next couple of days as my buddy has a gig then. It has a nasty hum, but when I tap on V3, the ribbon cable for V3, and the El-84's it goes away and then comes back when the amp vibrates. I've changed all 3 12AX7's and it still does it. One would assume it would be a cold solder joint on the socket or ribbon cable but it's not. Is it safe to assume the ribbon cable on V3 is bad. I have to wait 'til tomorrow to get a meter and check continuity. If anybody has had this problem, please enlighten me. Also the reverb is out. I took off the tank and somebody had clipped the reverb wires to the rca inputs on the tank itself. I resoldered them to no avail. When I turn the reverb pot nothing happens.
Thanks for looking.

[Amp Kat]

Sounds like it is losing it's ground connection either through the socket or the cable. Try isolating which one really eliminates the hum. Or the tubes old you may be removing the connection to the EL-84 when you tap on it taking it out of the circuit killing the hum and could be a tube problem. New tubes can also fail so don't always trust them just because they are new.

[Enzo]

Did you actually resolder all the ribbon cable connections? Otherwise how did you determine it was OK? I don't know how a ribbon cable would go bad other than to have a wire break off at one end. With power off, get a small screwdriver and push on each wire in teh ribbon, just above where it is soldered. Look closely to see if you can push the wire away from the pc board hole. If you can the wire is broken off. So fix it. If at either end you cannot push the ribbon aside from the hole wheree the conductor is soldered, then I'd have to guess it was OK>

But why do we think it is the ribbon cable? Because you tap it and the hum comes? You can tap on the various tubes and it does it as well. WHen you do that, you also send the mechanical shock of tapping through the circuit board. How about if you tap on something like one of the larger filter caps? Or just tap on the circuit board?

Get a wooden chopstick or something insulated and gently poke at each part on the board - looking for something that responds.

Also, look very closely at the tube socket pins where they solder to the board. Look for any tiny cracks in the solder. Or just resolder all the tube sockets.

[Danglin' Fury]

Ok, fixed that problem. It was an EL-84. As I was testing it out, I cranked up the master a bit. When I turned the VOLUME past 4 or 5, I started getting some really weird harmonics. Almost like I had an octave pedal in front of it. Anybody know what this might be? Does anybodyknow where I can get a schematic for this?

Also my reverb is still not working. Does the BA4560 Dual op-amp next to the reverb circuit have anything to do with the reverb?

[CaptainKirk]

You can get a schematic at http://www.schematics.ca. Opamp U1 is the reverb amp. U1A, pin 1 drives the reverb pan. Check power supply voltages on pins 4 and 8 first. Then check pin 1 for signal at TP22 to see if the reverb pan is being driven. The schematic gives a voltage reading you should see when driving the amp with a signal generator. If you don't get a reading at TP22, move to TP21. If TP21 has a the correct reading and TP22 doesn't, replace U1 opamp.

[Danglin' Fury]

Ok, I got a schematic but it seems to be the wrong one. My model number is PR 295. The schematic I have says REV. A is the PR 295. Down at the bottom the rev level is B and the part number is EC 1476. Just a couple of discrepencies are: on the schematic, it lists R7 as being the VOLUME pot. On the board, it has R5 as the VOLUME pot and R7 is down by the ribbon cable that goes to V2. Also R1 and R2's values are reversed. Does anybody know what the deal is or does anybody have a rev level A, PR 295, Blues Jr. schematic? Any help is appreciated.

[Enzo]

I have the full set of schemtics posted over at www.ampix.org in the Enzo gallery. In fact I inadvertantly posted it twice, the two files are the same until I remove one.

Yours is one of the later ones. Look towards the end of the file for yours.

[Danglin' Fury]

Hey Enzo, thanks a million for the schemmies. They helped me a ton.

[bnwitt]

So Danglin, what were your problems with the amp?

[Danglin' Fury]

I had some weird harmonics almost like I had an octave pedal in front. The first cap after the rectifier was supposed to output 329V and I was only getting 245. Also R45 (I think) the resistor right after that cap got so hot it burned the board. I replaced them both as they caused alot of other test points to have about half the voltage. Also the line from my board to the reverb tank is dead. The one from the tank to board is live but my reverb is still not working. Any suggestions?

[bnwitt]

Danglin,
here is a good site for you to review on the blues Jr.

http://home.comcast.net/~machrone/bluesjunior.htm

It discusses feedback on the green Rev B board that can be eliminated with some aluminum tape shielding of the input jack in the reverb mod section. Sounds like this might be what you have going on. Also look at the "fixing the reverb tank section for your dead wire issue.

[Danglin' Fury]

Man thanks for the link to Bill's Blues Jr. mods. It turned out to be the "smaller than human hair" wires on the output transducer. They were'nt soldered properly and they read open. I used a magnifying glass, took a 22 gauge wire and clipped 2 single wires off of it and soldered them from the transducer to the clips that connect them to the RCA jacks and presto! The coil was separated from the clips. Now the reverb works perfectly. I thank everybody who helped me with this project. This is the best forum EVER!!!!!

P.S. When I was taking ohm readings across the 2 pins they wer reading about 60 ohms using a Fluke 74 III.