Thursday, June 13, 2013


     A reader complimented the schematic I posted yesterday.  Of course, that's the "clean copy."  Here's the one I drew as I traced it:
     You will note several layout errors that are corrected in the later version -- don't know why I omitted the headphone jack wiring in that one, though.  Schematic drawings can nearly always be improved after the first draft gets everything on paper (or on the screen, if you work that way).  The rules are simple, applying them sometimes isn't.

     And here's what I traced:
     The Thordarson "Tru-Fidelity" mic transformers are not original -- the fellow who gave me the mixer salvaged the original UTC transformers.  The Thordarsons are at least as good.  They're also larger.  I'm not happy with the length of unshielded wire in the grid leads, it will want changed.  Looks like I took most of the wire harness lacing out when I traced it, too, so that needs redone as well.  I wonder how those Sprague "Atoms" electrolytics are holding up?  (They're on the terminal board behind the right-hand mic jack.)
     Back panel:
     Blank and ready for call letters!  (It's traditional.)

     2" Cannon connectors, with a penny for scale and ancient masking tape stuck on the chassis:

     12-pin Jones plug, holes where a previous user had relocated the output terminals, fuseholder:

     Rannie the cat loves my fancy screwdriver set:
      This made disassembly a little more difficult.

     (Here's another take on a small mic mixer, kind of interesting...if you have the parts.)


  1. You forgot the colored pencils or felt tip pens to "creative Color" the ckt as to B+, GND, Signal path.

  2. I was never fond of that, except for very complex things. Besides, if you drew it, you knew it.

  3. Nice!
    What do you suppose is the purpose of the jumper between pins 9 & 12?

  4. Thanks for the photo. One of my very first home brew projects was a single tube amplifier to convert a HI record player to a stereo . . . . kinda miss the tube days. Nice job on the schematics.

  5. The jumpers (there should be three) originally allowed the user to select between AC and battery supply and still have benefit of the off-on switch in either mode. 9 - 12 and 1 - 3 connect the switch to the transformer in AC mode, and 4 - 6 complet the heater circuit between tubes and transformer. Presumably in battery mode, there's a jumper between 9 - 11 and B+ connects to pin 7, B- to 11, while 5-6 are jumpered and heater ("A Battery") voltage connects to 3 and 8. ...Or at least it worked that way until a later user jumpered the equipment side of the connector.

  6. It's been my experience that the Sprague Atoms last until they leak.I am still using a Williamson audio amp I built in the '50's and the electrolytics are fine. The Black Beauties have long since been replaced with orange drops, though.


  7. Yes -- they were among the best of the waxed-paper/foil caps, but that's kind of like being the skinniest Fat Lady at the County Fair....

  8. Some retro news for you:
    To Avoid Cyber Espionage, Russia's Switching Back to Typewriters

    Max Headroom was unavailable for comment.