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This just highlights the end result of a 1200S Restoration project that I began in mid-Nov 2015 and completed in late May 2016. Details are in the 1200S Restoration Project thread found in the Maintenance, Troubleshooting and Repair category. A couple weeks ago, I finally got around to creating a new updated 1200S schematic with the revisions, and have added thiat to this post.
The above are outside views, after thorough cleaning of the chassis surface, getting all of the crustiness off, as best I could. you'll see all that in the other tread.
The insides after restoration and upgrades have new supply filters, added a second bias pot so each pair of output tubes have their own bias adjust. Added Hum balance. Revised the previously non-grounding input jacks with a pair of switching transfer jacks (Switchcraft type 13A) and the usual type 12A jacks, so the 'bright' input networks are still there as well as non-filtered inputs, now set up with 0dB and -6dB input levels typical of Fender and other amps. Circuit is in the new schematic
The LDR Cell for the Tremolo circuit was moved off of the Tremolo PCB into a mu-metal and brass enclosure and installed between the Volume and Treble pot, moving it's high impedance out of the high hum field of the power supply/power transformer. The single transistor Reverb Return circuit was scrapped, now using a TL071 op amp circuit to mix the reverb tank back in, with the voicing in the Reverb path adjusted to taste. A lot more power supply filtering was added to the half-wave rectifier filter circuit that runs the Tremolo and Reverb circuit from the original.
I replaced the original Gibbs reverb tank with an Accutronics 4AB3C1A, as the Gibbs was sounding very dark and murky.
The re-composed and revised 1200S schematic follows.
Sunn 1200S Guitar Amp-3.pdf