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 Post subject: 12SQ7 operating voltages?
PostPosted: Jan Mon 24, 2011 5:37 pm 
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Joined: Nov Fri 26, 2010 2:04 am
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I have a M-W Airline Model 84KR-1520A radio that I am trouble-shooting. It uses the old time TRF technology rather than the newer superhet/oscillator/if technology.

It uses a 12SG7 rf amplifier, followed by a 12SQ7 af detedtor/avc/af amplifier.

My question is, what would you expect the operating voltage to be on two diode plates(pins 4 and 5)? Mine is about 60 volts, with the triode plate at 90 volts.


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PostPosted: Jan Mon 24, 2011 6:42 pm 
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The diode plates should never be positive, one would expect a few volts negative when tuned to a strong station.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject:
PostPosted: Jan Mon 24, 2011 7:15 pm 
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12SQ7 diodes are off the volume control, used as detector. They return to B- through the control. Hard to be that positive with pin #3 of 12SQ7 grounded. Miswire, short?

http://www.nostalgiaair.org/PagesByMode ... 011830.pdf

Pin #3 may not be grounded with 90 volts on the plate? 12SQ7 is not drawing current. Maybe bad?

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 Post subject:
PostPosted: Jan Tue 25, 2011 11:17 pm 
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Thanks Dennis and Norm. Upon really close inspection I did find one of the hair-like strands of the tuning coil wire causing a short between the two coils of the input tuned circuit to the 12SQ7 detector, thus causing the high voltage on the diode pins.

I now have about a negative .9 volts on the diode pins.

AS a followup question, should I now be able to insert a modulated rf signal to the diode pins (through a small coupling capacitor) from my rf signal generator, and hear the demodulated af signal on my radio speaker? I know the af stage works if I insert an af signal on pin 6 (the triode plate) of the 12SQ7.


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 Post subject:
PostPosted: Jan Tue 25, 2011 11:32 pm 
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Yes, you would expect that signal to be heard, but since the coil tunes the circuit to a specific frequency the RF signal may need to be pretty close to that frequency.

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Dennis

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 Post subject:
PostPosted: Jan Sun 30, 2011 1:39 am 
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Dennis thanks for the reply. If I tune the radio to the rf signal generator frequency I can hear an audible tone when I insert the signal on the diode plates (pin 4 and 5) of the 12SQ7, but not from any where prior to that in the rf stage.

This raises a question I have about the 12SG7 rf amplifier tube. My diagram of the 12SG7 shows pin 3 (G3) internally connected to pin 5 (cathode). Yet my schematic for the radio shows pin 3 (and thus pin 5 also) connected to B- of the power supply, while G3 is connected to pin 1 and to chassis ground (which is not actually B-).

Any idea what I am missing?

Dick


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 Post subject:
PostPosted: Jan Sun 30, 2011 4:23 pm 
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Dennis: I guess I answered my own question. The schematic drawing of the 12SG7 in my radio is wrong. The G3 grid is not connected internally to pin 1 in the tube itself. Since pin 1 is not used in the tube, my radio just uses it as a convenient post to connect a couple of components and the pin itself to chassis ground.

I still don't know what my problem is however.

Thanks Dick


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 Post subject:
PostPosted: Feb Tue 01, 2011 5:07 am 
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Location: Long Island NY
What else have you checked? Do the coils show proper resistances for their primaries and secondaries? Are you seeing B+ voltage on the screen grid and plate of the 12SG7? Is the 2.2 meg grid resistor for the 12SG7 good?

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 Post subject:
PostPosted: Feb Tue 01, 2011 3:32 pm 
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Joined: Nov Fri 26, 2010 2:04 am
Posts: 74
Chris, I finally got the radio working. The last trf stage in front of the detector was way out of tune. I had made so many changes from the time I started, and some one had worked on it prior to my starting on it that it was a real challenge.

It didn't help either that I didn't even know what a trf receiver was when I started.

Anyway, thanks for all the advice along the way on this and a couple of the other forums. I'm sure I'll be back in the future with some other questions.

Thanks again. Dick


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