Bizarre Mystery Circuit added into Wire Recorder

Aerochrome

Active Member
I recently made a thread about capacitors in my wire recorder, before realizing that there was a tube/capacitor package that was not on any of the schematics. [Please note that there are four tubes in the upper left hand corner here, making a total of six]
uTfrNnq.jpg


It seems entirely home made, and is built upon a flimsy piece of metal, and screwed into the chassis in a way that screams home bodge.
dcDgjv3.jpg

Ci6CgFH.jpg

Does anyone have any ideas what this might be? I can easily restore the wiring to the schematic, but before I do I want to get some opinions on it first as well as field guesses as to what it is. My best guess is a mod to make one of the pin pairs an audio out and the other an audio in, stock the audio jack swaps between an input and output based on what mode it is in.


One of the tubes is unmarked, but the other is a 6C4 tube. Before I give an annotated schematic, it is worth mentioning that the schematic is for the 78 model, not the 78-1 model that this unit is. However based on how bodged the extra "board" is I can not believe that this is was due to the slightly different version. The capacitors in the added board are also from a much more recent era than the caps in the rest of the unit.

From the added board: One wire [Brown] goes to pin 7 of the 6x4 rectifier, the black and red wires go to the pinout on the back of the unit (not the audio out, but to the pair of pins that would be shorted when the unit turns on normally), and the striped wire goes to the ground pin of the audio input on the front.
gDcjU9q.jpg
 
Last edited:
Ah, couldn't see those. :oops:

If it was hacked by a previous owner, you can never know how good or bad in electronic repair that person really was unless you knew him personally and acquired it from him.

Does it work? The 6C4 and the other tube looks to be a 6AQ5 or equivalent which could be an extra driver and output stage. Maybe the person was trying to make stereo or more power or something else! :dunno:
 
Last edited:
You'd need to draw up a schematic of the added bits and show where it ties into the rest to have some idea of what it does. It does look like a 6aq5 though, so 6c4 + 6aq5 could be an audio amp. Is there an output transformer somewhere in the mix?
 
Modification candidates that come to mind would be:
An audio buffer circuit that either level-adjusts, isolates, or impedance matches existing I/O to more-current specs.
(Balance of list removed on edit.)

Upon closer look at the schematic and review of your post, I think this is what's going on. It's likely a 'line input' mod that allows the unit to record outside feeds (like from a radio) without re-injecting that signal back into the radio on playback, which may have happened if hooked up with original design.

I would also verify that all the switches still do what they're supposed to, and one hasn't turned into a mic/line switch or something.
 
Last edited:
Back
Top Bottom