Tape recorder carrier wave oscillator not oscillating

Discussion in 'General Electronics Chat' started by Lambda, Feb 22, 2012.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
    28
    0
    I'm working on repairing an old tape recorder, a Webcor Regent IV. One problem that recently appeared is that the carrier wave oscillator is no longer working. When I first began working on it, I noticed that in record mode there was a 660hz sine wave on the ch. 1 speaker, but I didn't know quite what it was for. Now, as best as I can tell, it's common to modulate a high-frequency wave with the audio signal before it's recorded on the tape, and power the record head with that same signal. And now it's not working.
    Attached is a simplified schematic showing the oscillator circuitry when in record mode (I can post the whole schematic on request). The pentode shown is a ECL86, and is also channel 1's output stage, which is switched to form part of the oscillator when in record mode (and channel 2 becomes a monitor).
    I've put down all the values that were given in the schematic. The transformer-looking object near the center is an inductor, with two separate coils wound on a single ferrite rod. No value is given, it's just labeled L1. Both the record and erase heads are marked as "1/2 MI". I'm not sure if this means micro-henries or what. It could be some obsolete unit of inductance, the schematic uses "mmf" for picofarads and some odd switch symbols, so it's possible that "MI" is a nonstandard unit.
    So, any ideas?

    [​IMG]

    EDIT: Forgot to write in the value of the capacitor next to the 6.8k resistor. It's 0.001uf.
     
    Last edited: Feb 22, 2012
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Generally the resistors caps and wires you can test quite easily and the failure will probably come down to a faulty inductor. First check the wires to the erase and record heads, as then can break or go high ohms. With the head and inductor coils you can quickly rule out broken (open circiut) coils but the other fault, shorted turns usually means you need replacement parts to drop in or really good knowlege of the device and test procedures.

    Also (not shown on your circuit) there is always a multipole switch that will disable/enable the carrier oscillator when it is playing (not in record mode). That is a key place for failures in bad contact connections etc.
     
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