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saba: 125 Stereo; Meersburg Automatic: Stereo and Gram

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » saba: 125 Stereo; Meersburg Automatic: Stereo and Gram
           
Michael Watterson
 
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Schem.: 650
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25.Feb.12 14:10

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Obviously the output stage (the Beam tetrodes of the 2 x ECL86) is either two separate outputs for Stereo or Push Pull on Mono.

But strangely while tracing the switches on the Schematic (circuit diagram) I find:

  1. St1 is simply the Mono/Stereo lamp. That's OK.
  2. Operating Stereo OR Gram will cut power to the RF tubes/valves (St7 and T1 are in Series to the +4 HT line)
  3. St5 logically connects the two secondaries of Tr2 and Tr3 (Audio outputs) in parallel on mono and disconnects in stereo.
  4. St2 logically switches to AF preamp in Stereo and input of unity Inverter triode driving other output in Mono.
  5. T4 disconnects the Search motor, which makes sense.
  6. T5 disconnects U7 100nF feedback and changes effect of Sp3 to match Sp1 on the 2nd Stereo channel (Sp4 is Speech/Music lamp switch)
  7. T6 switches from Radio to  Tape pin 3 tied to Gram Pin 3 via 47K.
  8. T3 connects to Tape pin 5 (tied to Gram Pin 3 via 47K).
  9. The Audio amp preamps fed from T6 and T3 "common" via a pair of 47K resistors.These "common" points feed Tape pin 4 & 1 via 2M2 & 100K (x22) attenuators.
  10. St6 logically feeds 2nd Gram input to 2nd audio Preamp Triode in Stereo or shorts it to first Preamp in Mono. 
  11. I can't find St3 and St4 used anywhere
  12. I can't find Sp2 anywhere

The Stereo switch is obviously meant to be used to switch between Mono or Stereo Gram. But it also switches off the Radio, But leaving Tape Pin 1 connected to FM or AM detector. In this mode with Stereo pressed and Gram not press the only audio input path is Pin 1 & 5 of Tape socket via 2M2 Resistors.

It doesn't make sense to me that:

  1. T2 is decreasing cathode bias resistor (i.e.More DC Anode Current) when in Gram, not St switch increasing it. The Bias would expected to be switched for lower current in Push Pull Mono and higher in current in Stereo.
  2. I  can't find St3 and St4 being used anywhere.
  3. T3 and T6 connect Tape socket pin 4 to 5 via 2M2 and pin 1 to 3 via 2M2, Though disconnecting the Radio. (T6 to C47 open)
  4. Why Gram Switch T5 rather than St"x" switch?
  5. Are T2 and T5 really the missing St3 and St4? That would make more sense. Are the Schematic labels a mistake? I will check next time I open the set and I'm temped to wire St3 in place of T5 and St4 in place of T2 if the schematic is correct.

[Edit:

Examining the "dots" on the piano key diagram suggests that St3, St4 and Sp2 contacts are not actually fitted on the switch mechanism, hence they have to use T2 and T5. A mistake in specification  of the switch mechanism?]

Any thoughts?

 

This article was edited 25.Feb.12 14:21 by Michael Watterson .

This question was moved automatically to the board "Talk" to get more possibilities for answers.
Moved from board »RADIO MODELS DISPLAYED« on 02.Mar.12 14:10 from automatically 
Michael Watterson
 
Editor
IRL  Articles: 970
Schem.: 650
Pict.: 2497
17.Aug.12 21:03

Count of Thanks: 3
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Some posts by Konrad Birkner and discussion have clarified the operation of the SABA. 

Moved from board »* TALK - visible for members only« on 18.Aug.12 10:16 from Martin Renz 
  
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