Fixing a Fender Frontman 15B

I was given a Fender Frontman 15B amplifier to repair. 

For as long as I've been fixing equipment, I've been fascinated by how people describe problems and what they assume the fix to be.  I'm sure we've all seen the ads on many popular sites about how they are selling an amplifier that just needs one "easy" fix.  And somehow, despite this fix being so easy, it is never done.  I've gone off on a bit of a tangent. 

The amplifier was given to me with the problem description being that the input jack is bad and you need to wiggle the cord to get it to make sound.  Truly an easy repair! 

I disassembled the amplifier which is the bass variant of the Frontman amps (hence the B designation).  It comes apart pretty easily.  There are 6 screws on the back which  remove the back panel.  The back panel also has the power jack on it.  I removed the terminal connections from the power jack.  There are 2 screws inside on the under side of the chassis which hold it in place as well as the 2 screws which hold the handle.  I disconnected the terminals from the speaker and carefully slid the chassis out of the box. 

The board is mounted in such a way where you can see the trace side of the PCB.  It was clear that the input jack was free to move. Additionally, I spotted a broken solder joint on the volume pot.  Worse than the input jack being broken, the threads for the plastic nut were also stripped and I do not have any spare Fender style jacks.  I did have some Marshall style jacks though from when I was working on a friend's plexi RI. 

I decided that it would be a good replacement jack.  I soldered it in and fixed the bad solder joint on the volume pot and reassembled ready to pat myself on the back for a job well done. 

Only problem is, the amp was motorboating now.  Admittedly, that was a step forward for this amp which had previously been dead quiet, but not the end result I had been shooting for.  I had to reassemble it and take a closer look. 

While I had the amp disassembled I turned it on with the speaker disconnected.  The output IC is a TDA2050 (just like I put in my Marshall VS15R!) so I know it can handle no load.  However R22 (a 5.6 ohm resistor) could not handle the load and promptly began to smoke.  I turned off the amp and decided to just kind of replace everything quickly. 

R22 was replaced with 2 10 ohm resistors in parallel giving me 5 ohms which is close enough.  I probably could have picked different values to get closer to the original 5.6 ohm load, but the value hardly seemed critical. 

I also replaced U1 and U2 which are the input IC and post gain/eq buffer/driver IC (respectively).  U3 is the output ic.  U4 and U5 drive some sort of compression circuit. 

I reassembled and crossed my fingers.  Powered the amp on and was now ready to pat myself on the back!  Success! 

I'd like to take a moment to talk about some of the pros and cons of working on this amp. 

Pros:
-Fender provides the schematic for this amp free of charge on their website!   All companies should do this and most used to.  Now schematics are like secrets from most companies. 
-The circuit was fairly straight forward with the only complicated part going to the compression circuit.
-It came apart extremely easily.

Cons:
-The board had traces which were very easy to damage.  I managed to break the LED trace off of the board entirely just by touching the LED in the wrong way. 
-The schematic provided was close but had a couple of glaring and major differences from the actual unit.  Most striking was the speaker output on the schematic instead of a CD player input. 
-While the amplifier came apart easily, I had to do it multiple times as the tolex managed to get in the way a couple of times during reassembly and I don't like sloppy tolex.
-And finally, I didn't get paid for this amp at all because it was a favor.

Because I was not getting paid for my work, I did make a few parts substitutions with what I had on hand.  For example, the 5.6 ohm resistor that burnt up was replaced with 2 10 ohm resistors in parallel for 5 ohms.  U1 and U2 were TL072 op amps which I replaced with MC4558 op amps.  They will sound close enough but they are not the same.  I just didn't have any TL072 op amps around.  And the input jack was replaced with a marshall style jack (which is a much nicer jack anyway!). 

I know a lot of people might not waste their time repairing an amp like this when the used value is probably worth less than the time I put into it, but I have the time and the grand total in parts was well under 5$ to repair the amp. 

Another thing I'd like to note about the amp is how much additional circuitry Fender put into the amp than is absolutely necessary for a bare bones amplifier.  U4 and U5 could be eliminated entirely and the amplifier would probably sound pretty close at lower volumes and would distort at higher volumes a bit more.  But for a practice amp, I am actually impressed with how much engineering they put into it.