Pre-Teardown

It wasn’t until after I called around and showed the amp to a few local pros that I realized it wasn’t worth paying to have it fixed. If I could do it on my own however, I could probably make it worthwhile and the experience would be priceless. So it took about a week before I got the nerve to commit to doing this on my own.

Before I even touched the soldering iron I did a TON of research. Lots of forums, lots of asking beginner questions on lots of forums, talked with local pros and basically just anyone who had pieces of information to offer. I’ll admit, it took me a fair bit of time to even wrap my head around what is going on inside these amps, and I still don’t know 100%. 55% maybe. And tube theory? Forget it! I just know which ones to order. But I really thought it was necessary to familiarize myself as much as possible.

One of the first things I did was download the “showman_ab763” Layout and Schematic.
I stared at that layout every chance I got, and eventually started physically tracing each and every wire down, over and across to every component on the board until I could identify all the leads and more-or-less see how each stage of the amp interacted with the others.
Then I took it a step further, I wanted to familiarize myself with the schematic, and as you’ve all seen, the schematic is an electronic copy of a Xerox print of an old-school hand-written blueprint. So I took the schematic PDF and imported it into Microsoft Visio. Then, using the Electrical Design tools within Visio, began the painstaking process of redrawing a software diagram of the schematic over the hand written schematic, complete with voltage test points and the values of each component. But the true beauty of this effort is that it really helped me understand the circuit a lot more than just looking at the layout.
I was also able to add notes and details to the schematic that the original print didn’t offer (i.e. tube pin numbers, wire colors, personal notes, etc.).
I’ll post it up to share if any of you are interested but honestly this was more of a personal benchmark than anything else; and for the most part (save for my own notes and other finer details) it’s fairly redundant next to the original schematic.

Here you can find the software version of the print (this is ONLY for this model and AB763 circuit Showman and Dual Showman’s):
AB763 Showman Schematic

This is a screen shot of the original hand-drawn schematic imported into software and transparency turned way down. I basically just copied over the existing text.
AB 763 Schematic w_Underlay

The other thing I did was take a high resolution photo of the top of the chassis. I was then able to print a 1:1 scale copy onto color paper so I had the untouched layout in front of me the whole time. Actually my original plan was to glue the print down onto a sheet of Styrofoam, that way as I started removing components from the board I could stab them into the paper image and it would stick into the foam. Once I started unsoldering lead wires however, I realized I wasn’t going to bother removing the individual components. I’ll explain more about that later.
IMG_3976

Turned out pretty good and looks just like the real deal. At this point all I’ve laid out were a couple pairs of green heater wire and the bais pot. You’ll notice I was careful to number each lead on paper and affix the matching number to the actual wire once unsoldered. Note: for those of you paying attention. The “H” and “I” labels on both the bias pot and the yellow/red leads on the print is obviously wrong. I was in the process of relabeling a few things because I didn’t have all the letters in my wire label booklet.

The original photo which later became the print. BTW, prints nicely to two 11×17 sheets. Have to tape the two halves together.
IMG_3961  IMG_3961