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De-Modding a Marshall

Tim restores a modded Marshall to its original vintage glory.

Amplifiers have always been close to the electric guitar players’ heart. It’s often a love/hate relationship. Love, when your amp is performing as it should, never letting you down no matter how hard you push it. Performing night after night without hesitation, giving you the exact tone and feel that you ask of it. And hate, when your amplifier is doing things that you would never ask it do, often, totally on its own. From a faint crackling sound reminiscent of a distant fireplace at your grandfathers cabin on a wintry night to the blood chilling screeching that a harpooned pterodactyl might make in the troughs of death.

Sometimes the issues that the player may have with his/her amp are a little subtler. Perhaps they’re not getting the quality of gain that they are looking for or maybe the tone controls don’t seem to have the variation that is desired. The overall tone of the amp could be a little too bright or sound too flubby in the bottom end. Regardless of the issues, many folks at some point over the years have thought about making some modifications to their beloved amp. Most anything can be done (whether it makes sense to do it or not is a totally different issue).

With the prices and scarcity of “vintage” amplifiers these days, more and more people are removing mods that either they had put in many moons ago or have purchased gear on the used market that they would like to restore to its former glory. Such is the case with a Marshall 2203 JMP MKII amplifier that had recently been brought into my shop.

It’s a fairly typical story. The owner, a middle aged fellow, at one time was a member of a fairly well know rock band until the all too familiar responsibilities of adulthood reared their ugly head. The band was put on hold, the day job was found, the mortgage and diapers paid for and the cool gear stashed in the back of the closet. Recently, he had been bit by the music bug again and was interested in having his instruments setup and cleaned and his amplifier returned to the way it was before it had the 1980s mega gain shred mod and variable slave-a-doodad installed.

Believe it or not it is often more difficult to determine what the original circuit was then to mod an existing one. Fortunately, since the amp was a Marshall there is always a fair amount of information available to us these days. Whether it be schematics or photos of the front, back and insides, they can be found in books, our extensive personal shop files or online. One often finds gray areas though. Sometimes an amp was made during a transitional period and the service data leaves much to be desired. The circuit board and its component value labeling may be taken from a European model but converted to US specs at the factory. Remember, we're trying to get this particular amp back to the way it was when new.

So it began.

Starting with the obvious, I removed the additional preamp tube and socket that had been installed to accommodate the extra two stages of gain. Unfortunately, this left a hole in the chassis, which I covered using a small plate made specifically for this purpose. Associated resistors, wires and caps were next. As original resistors and caps are often used in the construction of mods, care was taken not to discard any parts needed in the rebuild.

Then came the variable slave-a-doodad, which had been installed in the back plate via two more holes drilled into the chassis. Aftermarket labeling was removed with the appropriate solvents and the holes plugged using hole plugging caps. Also, the modder had cut some of the traces off of the bottom of the board so as to use the holes and space for his/her creation. Big fun! The board was removed from the chassis and the circuit traces repaired/jumpered with buss wire. Now that all of the mods had been removed, it was time to do the reassembly. Several different schematics were studied, as were the clues on the circuit board itself. Since not all of the component spaces were used on this model, solder joints were looked at to see if the solder was factory or not for additional clues (this was made easy by the fact that the modder had less-than-perfect soldering chops).

The board obviously did double duty in a couple of different models as a key was found on one of the corners for what the symbols on the component values meant. For example, a resistor value marked on the board with an “x” next to it meant “lead” model. With the help of the tools at my disposal it was fairly easy to determine how the preamp was to be rebuilt to the original specs.

It became slightly trickier when it came to the power amp section. While Marshall has been pretty consistent over the years with the values of components to be used for correct balance of voltages for bias etc. this case was not so clear. For the 2203 model, a couple of the values were fairly unique. The schematic was telling me one thing, while the value markings on the circuit board another, and the amp had EL34s installed without the bias section being accounted for (the client wanted to keep the EL34s). Some educated guesses were made as were some calculations, mixed with a little trial and error. Remember, were trying to get this amplifier as close as possible to spec (with the addition of EL34s).

The final outcome was an amplifier that once again had that “mojo” of an original 2203. The classic grind was back with the preamp juiced up a bit with the touch sensitivity and punch that 100 watt Marshall’s are know for. While the mods installed years ago had served their purpose at the time, the amp now had its original tone and feel back. A great circuit returned to its former glory made a very happy player of its owner.

Tim Schroeder is the owner, master luthier and chief designer of Schroeder Guitar and Amplifier Repair in Chicago Illinois. There he oversees the daily repair operations of the shop as well as designs the amplifiers and effects that they manufacture in house.