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Old 1st April 2012, 05:26 AM   #1
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Join Date: Mar 2012
Location: Tulsa, OK
Default Flea market find: Russound TBL-75 - What parts?

I was at the local flea market today, looking for likely objects to buy cheap and disassemble for educational purposes (or, as my wife says, cluttering up the basement with more useless junk). I bought a Russound TBL-75 device (seen here at Crutchfield and in this manual).

As I understand it, this device uses autoformers to adjust the volume level of a pair of speaker inputs, and then outputs that signal to 2, 4, or 8 speakers, matching the impedance of that output accordingly.

Given the Crutchfield price, I'd be tempted to sell it here on the Swap Meet, except that, once I opened it up, I discovered one long transverse crack all the way through the PCB.

So, that leads me to the idea of parting it out. Not counting jumper wires, there are eight parts:
  • 1 bank of speaker inputs/outputs
  • 1 4-position impedance switch
  • 2 autoformers
  • 1 12-position rotary switch
  • 1 1/4" stereo phono jack
  • 2 resistors
I have a plan for the phono jack and possibly the speaker terminals. My first question is, can the autoformers be used in any way as an amplifier? In use, it seems like this would only attenuate signals, not boost them, but can they be repurposed?

Secondly, the rotary switch. As far as I can tell by tracing the board, it's not a potentiometer, but a switch that just determines which path through the autoformer the signal takes.

Finally, the impedance switch. Does this act in a similar way to a variable resistor, such that each position of the switch produces a different level of impedance within the switch itself? If not, I don't see any other way it can be affecting the signal, unless that also happens within the autoformer?

I can't locate a schematic online, but I'll try to post some photos tomorrow if it will help.
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