Removing the Circuit Board
Some 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. Actually, if you turn the amp off while the tubes are warm, it will self-discharge in 20 seconds or so. Once you’ve turned it off, don’t turn it back on. Ensure that all the stored charge is gone from the caps by touching the positive terminal of the big one to ground with an insulated jumper. The other caps are all connected together through resistors and will discharge any remaining charge at the same time.
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. 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.
5. Remove speaker/FAT jack 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.
6. Unplug the power transformer and output transformer leads.
Do this before you unscrew the board. Pull up firmly and steadily on the red, brown, and blue leads while wiggling the connectors side to side. Pull the push-on end, not the wire. Note where they go. They’re marked for color on the cream board, not the green board. The green, red, and brown pairs from the power transformer are AC, there’s no polarity when you reassemble. They let go all of a sudden–make sure you’re clear of the thin wires that run from the speaker jack board. 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 pulling gently on the “petals” of the white socket assembly. I use a bent-jaw long-nose pliers for this. The red lens may pop off; be careful not to lose it. The white LED assembly will remain attached to the wires.
7. Remove the circuit board screws.
Seven screws hold the circuit board. 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.
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. (I haven’t moved this page to the new site yet.)
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. Do not try to salvage the old parts by removing them intact! It’s far more likely that you will destroy the traces on your board. 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.
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, twist the green, red, and brown wire pairs together to reduce the possibility of hum. Secure them with wire ties. Dress the wires so they arch over the circuit board and back down.
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.
Bill,
If I were to get a temp adjustable iron, what temp should it be set at?
I set mine at 650° F
Bill,
just seeking some further clarification on discharge of the capacitors before I commence work on the amp.
You say above: “2. Discharge the power supply capacitors. They’re the four large gray and black tubular objects. Actually, if you turn the amp off while the tubes are warm, it will self-discharge in 20 seconds or so. Once you’ve turned it off, don’t turn it back on.”
By the bit about “if you turn the amp off while the tubes are warm” do you mean:
1. You switch the amp off at the on-off switch on the amp itself when the tubes are HOT, or
2. You firstly do as in “1″ above, but then let the tubes cool down to warm, and at that point THEN switch the mains power off, or
3. Something else I haven’t covered.
Sorry to labor the point, but where electricity is concerned….
If the tubes are hot enough to play through and make a sound, the amp will self-discharge in 20 seconds or so when you turn it off. Running a jumper from ground to the + side of any of the big gray tubular capacitors is just for insurance.
Will a solderiong iron at 770 be too much heat to do the twin stack mod?
As I said above, I keep mine on 650°F.
Bill,
What type of solder do you recommend?
Todd
I use Alpha 60/40 solder.
bill,
i’m sorry i can’t find the right place to ask this question, so i post here.
in the bias mod, i’ve changed the resistor to 27k(1/2W) instead of trimpots, will there be a difference of using 1/4W or 1/2W?
thanks very much!!! i enjoy the mod u designed very much!!!
mike
No difference. Either 1/4 watt or 1/2 watt will be fine.
Bill, what iron/soldering station do you recommend? I have a 15/30w switchable radio shack right now, but want to get a better iron. From what I hear, Wellers are the best. Any recommendations?
Thanks
Scott
I prefer the Hakko 936-12. It has excellent thermostatic control, it’s light, well made, and a better value per dollar than equivalent Weller irons. Hakko 836-12.
I have done the tone stack, change over to JJ tubes, and rebiasing per your instructions and my BJ wails. Others that hear it are amazed. And finally the best part, since I rebiased the amp it runs much cooler and the ice in my drink sitting on top of the amp doesn’t melt as fast.
OK, newbie question. I’m thinking of following your recommendation in buying the Hakko 836-12. Does it come with the tip I need, or do I need to order the tips separately? If so, which tips? Thanks!
The standard tip that comes with the 836-12 is fine for everything you need to do.
i need to change the fuse, plugged it into 240 in africa. what size fuse???? help me !!!
For 120V, 2A 250. For 240V operation with international transformer 1A 250.
Hi Bill,
I play the harmonica and did your mods, can help for feedback???
If not, what is the simplest mods to do ?
I do things a little differently for harp. Some things are the same–basic mods kit, TwinStack–but I also increase the input impedance to 10M ohms so you can use a crystal mic or a dynamic. It also makes the high end a little more airy.
I also do a “super mids” control which gives you a much broader range of adjustment on the mids, from zero to more than stock. The tone controls are less interactive, so you get more control and make it easier to dial out feedback frequencies.
I unbalance the phase inverter a little for more crunch at the same volume and recommend the Clean Boost module for more output tube drive tone.
The presence control is an option if you want more bite and cutting tone at the same volume.
Bill,
I have just done a series of the mods and it sounds great. Just a few things that I would like to ask you please, …..
First, when putting the circuit board back in the chassis, I am finding that the shafts of the pots are not centered in the chassis holes. This is leading to a stiff spot on the various pots when they are turned. To reduce this I removed a screw that holds the circuit board down, so that it allows the board to move slightly allowing the shafts to turn more freely. It almost looks like there should be a washer behind the black spacers that the screws go through, so that the circuit board would stand away from the chassis. This would work but it would be a nightmare to try and re-fit the circuit board with washers, and I am thinking that if they were not there originally then they should not be there now.
Have you seen this before, and have you any ideas as what I have done wrong?
Secondly, when you remove an original master volume pot, are you able to remove the solder on all of the connections allowing the old pot to come away easily? When I removed the master volume pot I found that I had to cut the original pot away from the circuit board and treat each of the points of solder like individual connections. Only then could I get the solder sucker to free the original points soldered to the board. Is this normal, or is my technique not up to scratch? I want to replace the plastic input socket but only if I have the capability.
Thirdly, the shaft of the new master volume pot is very short and the knob that fits onto it is only just on the shaft. Is this normal?
Finally, I have seen a solder sucker on Ebay that incorporates a heat source so it melts the solder then sucks up in the same unit. Are these any good?
Thanks in advance.
Graham
It’s possible that the plastic standoffs got a bit crushed from tightening the circuit board, which could cause the misalignment with the front panel. I actually like a little friction on the knobs–it prevents them from moving too easily if I brush one while adjusting another.
Using a plunger-style solder sucker, you should be able to remove a pot easily by desoldering the three terminals after desoldering the two snap-ins and bending them slightly away from the edges of their holes.
I don’t recommend the heated desoldering tools, at least not the AC-powered inexpensive ones with the squeeze bulb. They get very hot and can lift a trace very easily. You would need a dimmer to control the power and a lot of trial and error to get the right setting. This type of desolderer is best suited for circuit boards with through-hole plating. The Blues Junior circuit board is single-sided, with no plating. So all of the solder is right on the surface. The spring plunger-style solder sucker is best.
Hey there, just ordered all of your mods.. 1st question I have is about the LINE OUT mod.. Is it just there when you want to use it without interfering with amp operation, also.. is it a balanced output? My other question is if you knew were I could get a clear crystal for the LED indicator instead of that red one.. I really want a blue light!
Thanks for your time
The line out is not balanced. It’s the speaker signal attenuated down to line level. It does not cut off the speaker, doesn’t affect the amp in any other way. I have not found a source for a clear lens (or any other color) that would fit the BJr LED.
Ok well that still sounds awesome, I want to go out from my BJ to my tube-preamps and record like that.. Have you any experience doing this? How did it sound? What if I went with a whole new fixture that would fit the stock hole where the LED was.. Do you know what size this hole is? I was thinking about doing some hunting.. Thanks Bill!
This may sound like a silly question, but is the lineout dependent on the master knob, so, more speaker volume = more line out volume, or is it a flat, unchangeable volume dependent on the recording device’s intake/input level?
Also, what does balanced mean when referring to the lineout?
Thank you.
The line out is the same signal that goes to the speaker, but it is reduced through some resistors so that it is low enough for recording equipment or a PA amp. Any change that you can hear through the speaker, tone or loudness, goes to the line out also.
Balanced signals are the kind that use an XLR jack, with 3 prongs. Balanced signals cancel out noise and hum and can go a long way–hundreds of feet. Unbalanced signals are like your guitar cable–a center conductor and a shield. Despite the shield, unbalanced signals are prone to noise and should only be used for short runs.
A 10mm LED w/holder fits with little modification (if you’re here looking at mods..most likely your warranty is already void anyways
). The LED output from the Blues Jr is about 2.7v and runs a 3v led just fine. I have a very bright blue LED in mine…looks awesome.
Matt, wher can one acquire the neccessary items to do the blue led mod like yours? Would like to do the pjr also.Thanks
Found it ! Pedalpartsplus.com 5mm LED and Holder
Ya, found nothing
Hi Bill,
I just purchased you basic kit for my BJ, I am wondering is there anywhere I can find images of how to do these mod, kinda a step by step. The amp is sounding great the the moment. I put a Greenback in there and some Harma Cryo Retro valves in the preamp, based on the Mullards, really sweet tone and woody at the same time….I have a pair of JJ’s in the power section and want to wait until I can adjust the bias before using the Retro Mullard 84′s. Your site is fantastic btw, really learned a lot from it. Sure let me know about step by step, if there is one. Best wishes
The mod kits come with photo instructions. They should provide enough guidance if you have decent mechanical/soldering skills. If you’re a novice, you should get help from someone with the tools and experience.
Hey Bill,
I was wondering if I’d be able to disconnect the internal speaker on my blues jr (green board, made in the USA) and hook up the amp via speaker cable to my Avatar 212 cabinet. I was just curious to know if that would cause any problems. The cabinet is 8 ohms. Thanks a lot.
Abi
Yes, that will work very well.
Bill, thanks for having this site and offering your mod kits. I was repairing a recent Blues Junior for a student of mine when I noticed the cheeseball pots in the amp. His had a cracked midrange pot that I replaced with a real pot. I have a Blues Deluxe (not a reissue) that has real pots although I had to replace the input jacks with Switchcraft jacks. Also i had to resolder every joint on the two circuit boards. Now it is finally reliable. The tonestack cap change that you suggest (.1 and .015) did the trick on the Blues Junior. Much cheaper than a Jenson Alnico Speaker.
Now try the new coupling caps! It’s even better. If the knobs are mounted correctly, the stock pots (admittedly cheesy) will last a long time.