Amplifiers contain voltages high enough to Kill you even
after the amp has been powered Off !!! If you don't have
enough electronic experience to identify and discharge the
Filter Capacitors which contain these Deadly voltages,
take your amp to a Qualified tech. If you don't know a
qualified tech in your area, post a request in the alt.guitar.
amps newsgroup, the folks there will be happy to help you.
(If you convert the 4210_mod.gif to RGB in Adobe Photoshop, you can view or print the orignal or modded schematic by alternately selecting the red and green channels.)
The presence cut switch in the clean channel becomes great for use with distortion pedals. (It's a subtle effect).
The 470K resistor on the grid of V2a is essential; don't try to eliminate it for more gain.
The cap that bypasses the gain control (the 'bright switch' cap) has a lot to do with defining the character of various vintage marshall models.
The mod puts a rotary switch in the back panel which provides a selection of values. Each position is described below:
N/C: "Tweed Bassman/JTM45 normal"
100p: "Tweed Bassman/JTM45 bright"
1n6 in series w/330k resistor: "70s Super Lead 1"
1n6: "70s Super Lead 2"
5n: "70s Super Lead 3"
This control yields more interesting variations than the gain control itself, which is best left between 2 and 4 (it starts sounding farty at higher settings).
The original resistor which passes the dry signal around the reverb is a 6M8. This value allows far too much signal loss. Change this component to a 1M. Now the clean channel is still won't overdrive the output section, and there's plenty of reverb available.
Adding a jack on the back panel provides a patch point after the treble control in the 'boost' channel. This gives you a preamp out, and allows you to experiment with external clipping diodes for simulating output stage distortion at low volumes. (See diode clipping explaination on The Analog Page.)
The 4210 has circuit board-mounted pots. If you make any mods to one of these amps, inspect the solder joints on the pots just before re-mounting the pots/PC board. They're prone to breakage from handling.