Welcome, one and all, to the latest edition of Fix It Friday. Today we're going to be taking a look at a very popular amplifier, the Fender Blues Junior. These are fairly straight-forward circuits which make for some really versatile sounds, have the potential to be great pedal platforms, and are very commonly used by guitarists of all types.
I have a Blues Junior of my own which I've not been able to use in quite a while because I actually managed to blow up the power transformer somehow...lucky me! Naturally, I want to get this bad boy back in my touring rig, so I’ll need to swap out the power transformer and make sure it’s still the reliable old workhorse I used to know so well.
Mojotone offers a direct PT replacement for Blues Juniors; the replacement transformer is listed below along with all of the other items and tools we'll need for today’s project. This replacement transformer is made right here in the United States by Heyboer Transformers, and most of our customers who have implemented it have noted an improvement in their amp’s sound.
Alright, here is that list of tools and supplies I promised you…
Blues Junior Style Upgrade Power TransformerAlligator ClipsWire CuttersWire StrippersNut driver/Socket WrenchPhillips and flathead ScrewdriversMultimeterSoldering IronSolderSmartphone or Camera
Okay boys and girls, here we go on another whirlwind adventure to the mystical land of Bettertonium. First, we need to remove the cabinet’s back panel by loosening the screws seen in Image 1A below…
Note : The two screws closest to the bottom of the back panel are slightly shorter than the rest -- make note of this and be sure to use those same shorter screws on the bottom two holes when re-installing the back panel at the end of the project.
Now set aside the back panel, and be careful not to misplace any screws or washers. You can also pick up and set aside the metal cage covering the tubes. We should now be looking at something resembling Image 2A below…
Next, in order to extract the chassis from the cabinet entirely, we’ll need to remove the two chassis mounting screws on the top panel of the cabinet (Image 3A), as well as the lonely little screws found on each of the cabinet’s side panels (Image 3B).
Note : The screws removed from the side panels and top panel should be notably longer than those removed from the back panel. Make a mental note of this and be sure to reinstall these screws properly at the end of the project.
Before removing the chassis from the cabinet, we’ll need to disconnect the speaker via the jack on the chassis; we’ll also need to disconnect the reverb tank, via the plugs on the tank, and remove the screw from the white cable clamp which should be holding down our reverb connectors…
Once the chassis is out, we need to drain the filter caps as a safety precaution. My amp hasn’t been used in so long that my filter caps are already safe but let’s go over this procedure for posterity’s sake. Note the three large filter caps in Image 4A…
In order to drain these filter caps, we’ll need to use our alligator clips. Connect one end of the clips to the thin lip of the chassis (this will be the ground), and connect the other clip to the lead of the filter cap shown in Image 4A above. Connecting to this point should drain all three filter caps -- once the alligator clips are in place, you can test voltage by turning your multimeter to DC Volts, placing the black probe on the chassis, and successively placing the red probe on all of the leads of all of the filter caps to make sure you read 0V at all test points…
We are going to leave our alligator clips in place throughout the entire project today so don’t worry about moving them or disconnecting them right now. Since our circuit is safe, let’s turn our attention to the power transformer and its leads. You should notice there are (1) black and (1) white primary, as well as (2) Red, (2) Brown, and (2) Green secondaries. These are all coming out of the power transformer and running to their appropriate places within the circuit; in most Blues Junior amps these will be connected via a .25” fast-on which has been crimped onto the end of each wire (Image 5A).
Eventually, we will need to disconnected all of these leads so we can remove the old power transformer, but first we need to make note of where all of these leads are connected. Grab your Smartphone or camera and take a few pictures of the area as a reference. We’ll come back to these images once we’ve installed the new transformer. I’ve attached my reference image below and made note of all of the connection points (Image 6A).
Note : The printed circuit board should denote the connection points as “P4, P14, P15, etc.” You may prefer to make a list outlining which color leads go to which connection points on the board. The only lead that does not connect to a point on the board is the white primary lead -- this lead goes to a lug on the power switch.
You may have noticed a number of zip ties bundling your power transformer wires and other wires together; using our wire cutters, let’s go ahead and clip these zip ties in order to free up our leads. If your amp has been worked on in the past, the zip ties may have already been clipped; if this is the case, no worries, just keep moving along…
Once the zip ties are gone, we can start removing the fast-ons from their terminals. This may take a little extra wiggling as some will be tighter than others, but you should be able to do them all by hand…
Remove the white primary from the power switch, and remove the black primary from P4 on your circuit board. Then remove the red secondaries from P11 and P12 on the PCB, remove the green secondaries from P15 and P16, and remove the brown secondaries from P13 and P14.
Note : The above connection points may vary in your amp so it is important to make note of which points were used in your circuit.
Now it’s time to remove the old power transformer. Let’s grab our nut driver/socket and, one by one, remove the nuts that are currently fastened down to the transformer mounting screws (Image 7A).
Make sure you set the nuts down in a safe place, as we’ll be using these to mount our new upgraded power transformer. Carefully pull the old transformer out of the cutout in the chassis, and bring the leads gently with it. Be careful not to get the leads snagged on any other wires as you are pulling the transformer out. Now we should be looking something like Image 8A below...
It’s time to install the new power transformer. We are essentially just going to do the reverse process of the old transformer’s removal. Start by feeding the leads up through the chassis cutout -- colorful secondaries closest to the circuit board and the black and white primaries closest to the outside of the chassis. Then get the mounting screws into their respective holes and fasten the nuts back down. You should be looking something like Image 9A…
Okay, it’s time to connect all of our leads. A couple of things can happen here: for those of us who have a crimping tool and some extra fast-ons lying around, we can simply cut each lead to its respective length, crimp on our fast-ons and quickly connect the leads back to their corresponding terminals; or, for those of us who do not have a crimping tool or the extra fast-ons, there is the option to solder the leads directly to their corresponding terminals.
I am opting to go ahead and fully solder in my leads. In this case, I will still need to cut my leads to the appropriate length and strip the insulation off of the ends, so let’s start there. In whatever order you prefer, run the leads out to their connection points and cut them to length (Image 10A).
Note : Some may prefer to twist certain groups of wires together before cutting them. I will be twisting together all wires of the same color, just to clean things up a bit.
Now that our wires are cut to length, we need to strip off just enough insulation from the ends of the wires so that we may solder them to the terminals. Use your wire strippers to do this to the ends of all your transformer leads. In the end, your wires should look like Image 11A below.
We’re on the home stretch, everyone! The only thing left to do is solder these puppies in, so let’s pull up that reference image we took earlier, get some solder and a hot iron, and start soldering in these leads. Just follow your reference image or the list of connections you made at the beginning of the project -- these connections should all be exactly the same on the new transformer as they were on the old one…
Once all of your connections are soldered in (or connected via fast-on) you should be ready to test this bad boy out. If you just want to play through the amp to test out the power transformer, make sure you connect the circuit to a load to avoid damage. If you’d rather test voltages, make sure you remove all of the tubes from the amp first. Here is a picture of the inside of my chassis with the new transformer installed…
If you’re looking good, you can fasten your back panel to the chassis (smallest screws go in the bottom two holes), and then mount the chassis back inside the amp. I’m actually going to leave my chassis out for the time being so I can make a few more modifications to my Blues Junior...but that’s an article for another day!
Thanks again for joining us. If you have any projects you’d like to see us run through, please send your suggestions to logan@mojotone.com. Tune in next time as we install our Blues Junior into a custom speaker cabinet, you absolutely have to see how it turns out!
I have a Blues Junior of my own which I've not been able to use in quite a while because I actually managed to blow up the power transformer somehow...lucky me! Naturally, I want to get this bad boy back in my touring rig, so I’ll need to swap out the power transformer and make sure it’s still the reliable old workhorse I used to know so well.
Mojotone offers a direct PT replacement for Blues Juniors; the replacement transformer is listed below along with all of the other items and tools we'll need for today’s project. This replacement transformer is made right here in the United States by Heyboer Transformers, and most of our customers who have implemented it have noted an improvement in their amp’s sound.
Alright, here is that list of tools and supplies I promised you…
Blues Junior Style Upgrade Power TransformerAlligator ClipsWire CuttersWire StrippersNut driver/Socket WrenchPhillips and flathead ScrewdriversMultimeterSoldering IronSolderSmartphone or Camera
Okay boys and girls, here we go on another whirlwind adventure to the mystical land of Bettertonium. First, we need to remove the cabinet’s back panel by loosening the screws seen in Image 1A below…
Note : The two screws closest to the bottom of the back panel are slightly shorter than the rest -- make note of this and be sure to use those same shorter screws on the bottom two holes when re-installing the back panel at the end of the project.
Now set aside the back panel, and be careful not to misplace any screws or washers. You can also pick up and set aside the metal cage covering the tubes. We should now be looking at something resembling Image 2A below…
Next, in order to extract the chassis from the cabinet entirely, we’ll need to remove the two chassis mounting screws on the top panel of the cabinet (Image 3A), as well as the lonely little screws found on each of the cabinet’s side panels (Image 3B).
Note : The screws removed from the side panels and top panel should be notably longer than those removed from the back panel. Make a mental note of this and be sure to reinstall these screws properly at the end of the project.Before removing the chassis from the cabinet, we’ll need to disconnect the speaker via the jack on the chassis; we’ll also need to disconnect the reverb tank, via the plugs on the tank, and remove the screw from the white cable clamp which should be holding down our reverb connectors…
Once the chassis is out, we need to drain the filter caps as a safety precaution. My amp hasn’t been used in so long that my filter caps are already safe but let’s go over this procedure for posterity’s sake. Note the three large filter caps in Image 4A…
In order to drain these filter caps, we’ll need to use our alligator clips. Connect one end of the clips to the thin lip of the chassis (this will be the ground), and connect the other clip to the lead of the filter cap shown in Image 4A above. Connecting to this point should drain all three filter caps -- once the alligator clips are in place, you can test voltage by turning your multimeter to DC Volts, placing the black probe on the chassis, and successively placing the red probe on all of the leads of all of the filter caps to make sure you read 0V at all test points…
We are going to leave our alligator clips in place throughout the entire project today so don’t worry about moving them or disconnecting them right now. Since our circuit is safe, let’s turn our attention to the power transformer and its leads. You should notice there are (1) black and (1) white primary, as well as (2) Red, (2) Brown, and (2) Green secondaries. These are all coming out of the power transformer and running to their appropriate places within the circuit; in most Blues Junior amps these will be connected via a .25” fast-on which has been crimped onto the end of each wire (Image 5A).
Eventually, we will need to disconnected all of these leads so we can remove the old power transformer, but first we need to make note of where all of these leads are connected. Grab your Smartphone or camera and take a few pictures of the area as a reference. We’ll come back to these images once we’ve installed the new transformer. I’ve attached my reference image below and made note of all of the connection points (Image 6A).
Note : The printed circuit board should denote the connection points as “P4, P14, P15, etc.” You may prefer to make a list outlining which color leads go to which connection points on the board. The only lead that does not connect to a point on the board is the white primary lead -- this lead goes to a lug on the power switch.
You may have noticed a number of zip ties bundling your power transformer wires and other wires together; using our wire cutters, let’s go ahead and clip these zip ties in order to free up our leads. If your amp has been worked on in the past, the zip ties may have already been clipped; if this is the case, no worries, just keep moving along…
Once the zip ties are gone, we can start removing the fast-ons from their terminals. This may take a little extra wiggling as some will be tighter than others, but you should be able to do them all by hand…
Remove the white primary from the power switch, and remove the black primary from P4 on your circuit board. Then remove the red secondaries from P11 and P12 on the PCB, remove the green secondaries from P15 and P16, and remove the brown secondaries from P13 and P14.
Note : The above connection points may vary in your amp so it is important to make note of which points were used in your circuit.
Now it’s time to remove the old power transformer. Let’s grab our nut driver/socket and, one by one, remove the nuts that are currently fastened down to the transformer mounting screws (Image 7A).
Make sure you set the nuts down in a safe place, as we’ll be using these to mount our new upgraded power transformer. Carefully pull the old transformer out of the cutout in the chassis, and bring the leads gently with it. Be careful not to get the leads snagged on any other wires as you are pulling the transformer out. Now we should be looking something like Image 8A below...
It’s time to install the new power transformer. We are essentially just going to do the reverse process of the old transformer’s removal. Start by feeding the leads up through the chassis cutout -- colorful secondaries closest to the circuit board and the black and white primaries closest to the outside of the chassis. Then get the mounting screws into their respective holes and fasten the nuts back down. You should be looking something like Image 9A…
Okay, it’s time to connect all of our leads. A couple of things can happen here: for those of us who have a crimping tool and some extra fast-ons lying around, we can simply cut each lead to its respective length, crimp on our fast-ons and quickly connect the leads back to their corresponding terminals; or, for those of us who do not have a crimping tool or the extra fast-ons, there is the option to solder the leads directly to their corresponding terminals.
I am opting to go ahead and fully solder in my leads. In this case, I will still need to cut my leads to the appropriate length and strip the insulation off of the ends, so let’s start there. In whatever order you prefer, run the leads out to their connection points and cut them to length (Image 10A).
Note : Some may prefer to twist certain groups of wires together before cutting them. I will be twisting together all wires of the same color, just to clean things up a bit. Now that our wires are cut to length, we need to strip off just enough insulation from the ends of the wires so that we may solder them to the terminals. Use your wire strippers to do this to the ends of all your transformer leads. In the end, your wires should look like Image 11A below.
We’re on the home stretch, everyone! The only thing left to do is solder these puppies in, so let’s pull up that reference image we took earlier, get some solder and a hot iron, and start soldering in these leads. Just follow your reference image or the list of connections you made at the beginning of the project -- these connections should all be exactly the same on the new transformer as they were on the old one…
Once all of your connections are soldered in (or connected via fast-on) you should be ready to test this bad boy out. If you just want to play through the amp to test out the power transformer, make sure you connect the circuit to a load to avoid damage. If you’d rather test voltages, make sure you remove all of the tubes from the amp first. Here is a picture of the inside of my chassis with the new transformer installed…
If you’re looking good, you can fasten your back panel to the chassis (smallest screws go in the bottom two holes), and then mount the chassis back inside the amp. I’m actually going to leave my chassis out for the time being so I can make a few more modifications to my Blues Junior...but that’s an article for another day!
Thanks again for joining us. If you have any projects you’d like to see us run through, please send your suggestions to logan@mojotone.com. Tune in next time as we install our Blues Junior into a custom speaker cabinet, you absolutely have to see how it turns out!