2. Circuit Tracing

There are different ways to do this.

  • The easiest way is to get your hands on a schematic and trace it through the chassis.  Can’t do that with the S 950!
  • Folks who are more experienced and less memory-challenged may simply eyeball the circuit boards and commit it to memory.  To do this you need a mental database of  similar circuits.
  • Get out the paper and pencil.  I go through a lot of erasers when I do this.

I used a new approach (for me) on this amp.  I pasted photos of the board into Visio, a MS graphics app commonly used for org charts, process flow diagrams, engineering layouts, etc.  At its core, the program is good at creating lines and boxes and connecting them.  Once the photos were pasted in, I then added a object on top of the picture for each component.  In other words, I drew my component object, a resistor for example, right on top of that object in the photo.  I then connected the objects with lines replicating the connections in the amp.  This was time consuming, and I’m sure there are better methods, but the advantages of this method for me are:

  • It leaves a record of the process I can go back and check when something doesn’t make sense.  That is, when I’ve made a mistake I can find it.
  • It creates a crude layout picture, similar to the second page of Fender schematics from the 50’s and 60’s.  Those are useful for finding the physical components represented on the schematic.
  • When I delete the photograph, I’m left with a sort-of schematic which is then relatively easy to rearrange into a conventional schematic ordering of components and circuits, which can create a usable schematic document in Visio.

Here are examples of what I did.  It’s not pretty, but it shows the thought process, and the amount of work, and will probably discourage anyone from trying this at home!  This is a photo of the bottom circuit board with resistor and capacitor objects added on top.  The line connectors to the tube sockets at the bottom of the picture have yet to be added:

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Here is detail from the above.  Brown rectangles are resistors, capacitors are yellow, eyelets are black circles,  wires are lines.  As always, you can click on the picture to enlarge:

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Here’s most of the amp with Visio drawing objects (parts) overlayed on photos:

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Deleting the photos, and rearranging the order of the parts produced a rough sketch of the general architecture:

This is just a worksheet from which I could create a usable schematic.  You might be able to see where I created “real” schematics next to the circuits copied from the photo/overlay effort.  One lesson learned was that, to avoid this intermediary step,  I should have used component symbols on the overlay instead of colored rectangles.

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Next step was turning the above into a decent schematic.  Here is an example of that:

The resolution of this picture does not allow you to read the values, but it doesn’t matter because there are many problems in this schematic –  no voltage to the screens, mucked-up power supply, etc.  Please don’t use it as a basis to build anything!  We’ll get it all straightened out as we move forward.  We’ll analyze each part of the circuit and get a working amp put together!

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Next Page:  Block Diagram

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