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5. Removal of parts that are to be replaced or become superfluous after the
mod
Heat up the soldering iron and keep de-soldering wire and needle nose pliers handy. Preferably wear safety
goggles: when applying some force to a component that suddenly comes loose, the soldering iron or a blob of
molten solder may suddenly jump towards your face.
A lot of parts will have to be removed, particularly those that are part of the op-amp tonestack and effects
loop; we're going to replace the tonestack, remove the effects loop and remove the ICs. The power supply that
was originally used for the ICs will be used to power the preamp tube heater filaments with DC, to reduce hum.
Figure 7 shows the schematic of the stock Harley Benton. If you're not so confident in your soldering and
electronics skills yet, it may be best to start with removing only the parts that are replaced in mods 1.-4. (see
p.2), namely R2, R4, R5, R6 and C4. Then start with implementing those mods only.
For Mods 5 (passive EQ and purely tube-based signal path) and 6 (wiring a DC power supply for the preamp
heater filaments and rearrange the heater wiring), all parts inside the red fields (Figure 7) need to be removed.
From experience; leaving them in place may affect sound quality and even result in fuzz-like background
sounds. So, take note of each component's number in the schematic (e.g. R4), find the number on the actual
circuit board of your GA15 and remove the component. If you haven't done this before: remove the solder from
the components' legs underneath the circuit board by holding de-soldering wire against it with one hand, then
with the other hand heat the spot with the soldering iron, until the solder melts and is absorbed by the de-
soldering wire. With most of the solder removed, try to gently pull the components away from the top of the
board by gripping them with the needle nose pliers. The following components should be left in place:
1.
The phones jack (front right of the board), because it provides an important mechanical connection
between the board and the chassis. Just remove R41 to ensure that the phones jack is no longer functional:
its design seems questionable and might present the output tubes with an incorrect load impedance. Also
remove the wires going into connections P9 and P10 to the right of the phones jack.
2. The input jack (front left) should be left in place, of course.
3.
Leave the wire bridges J3, J4, J9, J18, J19, J21 in place;
4. Remove connectors SCN1, CN2 and CN11 and the little circuit board in the rear of the chassis, which holds
the effects loop and line out jacks.
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www.cosmicampworks.com
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
®
all other wire bridges can be removed.
8
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