So I practically stole this Silverface Twin Reverb I don't even know how long ago, at least 5 or 6 years. The guy sold it for $250 because it was so beat up even though it worked and sounded pretty good. I probably only plugged it in once or twice after I got it home because between the Master Volume barely clinging to the front panel and the output transformer dangling from the chassis by 2 screws, it obviously needed to be gone through and since I have a pile of other amps it wasn't a big priority, just something I knew I wanted to get to some day. I've owned dozens of amps and worked on a lot of them, but this is the first legit hand-wired Fender I've ever owned and probably only the 2nd or 3rd one that I've cracked open, so I've read plenty about them and watched lots of restoration videos getting to this point where I finally pulled the chassis and put this thing on the bench last night. Before doing that I powered it up slowly with the variac and dim bulb and was surprised to find it still sounded pretty great, although it did seem the output became a bit less sparkly after it got good and warmed up.
So I proceeded to pull the chassis and wow, does this thing look abused. The photo is not distorted; the front and rear panels actually are that bent. Everything is positively filthy, though I'm happy to report I couldn't detect any conductivity across the turret board. I didn't really exhaustively check for DC leaks but I intend to do a very thorough cleaning either way.
There are so many different schematics for the Twins and the mid 70s SF era has always been the most confusing to me. I thought once I started comparing this to the different versions on this site it wouldn't be too tough to figure out which one I've got. I guess the 100uf filter caps would point to the 100w amp with the CBS logo in the title block, but I quickly realized the bias circuit has been modded and there are a few other inconsistencies so I'm not 100% sure about that. I don't think it's been extensively modded but things like the MV is not push/pull, a freaking 470uF bias cap (an odd cap too, no voltage marking I can find?), the inexplicable extra 5k trimmer pot in series with the balance pot, etc., have me scratching my head.
Beyond that there's just the atrocious hack work ethic in here, lots of weak looking solder joints and places where a big honkin' soldering iron was allowed to melt wiring insulation, just super sloppy work.
In any event, despite the fact somebody did a lot of poking around in here, it doesn't look like a lot of parts have been replaced. The main filter caps are all Mallory dated 1974, and none of the coupling or bypass caps have been replaced, save for possibly a couple of ceramics connected to the grids of V5 - and not for nuthin' but neither the vibrato or reverb are currently working. I can see the neon bulb flashing but haven't investigated beyond that.
In any event, I think I want to begin by just restoring this to as original as I can to begin with, new F&T filter caps, all the cathode bypass caps, etc., verify resistor values haven't drifted too far, get the effects working, straighten the chassis and correct any other mechanical issues, etc. Beyond that I'll consider whether I want to do a master volume mod to Uncle Doug style Presence control or anything else.
But first and foremost I just want to make sure I'm looking at the right schematic or even if I am, should I consider using the AB763 as my guide?
Topic: What in the wildly swinging soldering iron am I looking at here? modded '75SFTR (Read 1388 times)