Identify this circuit

[stevef]

I found this installed in my NC-101X receiver between the 2nd IF can and the detector stage.  The stock 6C5 detector was replaced with a 6SQ7.  The hambone put a switch in the front panel to turn it off or on.  Is this a product detector?

- Thanks,
Steve KK7UV

[K7MCG]

Probably intended to be a pulse noise limiter, but the circuit doesn't look right.  The left end of the diodes lacks a dc path to ground.
73
Chuck K7MCG

[N3GTE]

Do not think it'd pd as there isn't any bfo injection point.I'm thinking it is a diode detector or nl circuit but that .1 cap is going to kill off just about everything that is going though it and bypass to gnd.
Terry N3GTE

[N3GTE]

Hmmm  On second thought the bottom diode is the AM detector. When you close the switch the top diode acts as a nl passing all high amplitude signals though the .1 cap to gnd.
It took me a second to figure it out it's drawn going the wrong direction.
Terry

[AB2EZ]

Expanding on Chuck's post:

Any current flowing from right to left through either diode will charge up the capacitor they feed current into. There is no path for the capacitor to discharge through.

Therefore, after a brief time, this circuit will be an open circuit, with both diodes biased off.

Stu

[KA2DZT]

Could be a noise limiter but I'm thinking the top diode is backwards.  Just a guess, what do you think?  Strong pulses would forward conduct through the diode to ground via the .1 cap.

Make any sense??

Fred

[Opcom]

If that were done, the 0.1uF cap could take on a more and more negative charge. Perhaps this would reverse bias the lower diode, in such a way as to act as a gate with a time constant. similar to an AGC but cutting off the bottom of the waveform, not the top?

Could it be intended to allow SSB via BFO without having to fool with the RF gain as is a usual case on an AM deceiver?

I guess it could be simulated.

[W3RSW]

Where do the diode plates of the 6sq7 go?  It looks like the PI rf filter ( 2 ea. 50 pf and 47k) is ahead of the SS diodes instead of after detection. 

What is hooked to the bottom side of the last IF winding?  Back around to one of the 6sq7 plates and an AVC detector circuit?

The one meg and .1uf is definitely a long time constant and may be long AVC for SSB, again on wrong side of detection unless somehow the 6sq7 diode plate/s is active back up through the bottom end of the IF transformer.

We need the rest of the circuit and assume the 6sq7 was used for its diode plates, else why change out the 6j5?

[W1RKW]

it looks like a detector but why would it be before another IF stage?

[KA2DZT]

it looks like a detector but why would it be before another IF stage?

The schematic is drawn backwards.

[WU2D]

You have heard of the second detector? This is the rare third Detector.

[stevef]

OK gang, I redrew the circuit and expanded it a bit.

[AB2EZ]

For a possible clue:

Look up US Patent 3005910 on line.

The inventor discloses the use of germanium diodes in receiver circuits to improve their performance (I haven't attempted to understand his claims).

In the circuits included in his patent, he shows diodes feeding into capacitors.

Apparently he is depending on the reverse saturation current of the germanium diode to leak off the charge that accumulates on the capacitor when the diode is conducting in the forward direction.  For these diodes, the reverse saturation current is a few micro amperes... which is consistent with the high value resistors in the mystery circuit.

Stu

[KA2DZT]

OK gang, I redrew the circuit and expanded it a bit.

 You need to double check the circuit to be sure you have it drawn correctly.  I notice the switch is now a double pole type yielding a different circuit.

The 1meg resistor along with the .1ufd cap make little sense in the location you have them.  It seems they would be just shorting any audio to ground.

Are you sure the diodes are drawn correctly,  could the upper one be reversed?

Fred

[stevef]

Here are photos of the board and switch.
I think you can make out the connections OK except for one spot where I should point out that the yellow lead from 6SQ7 pin 2, the lower lead of the .05 cap, and the left lead of the 470k resistor are common to same tagboard terminal.

Steve

[N3GTE]

On my  NC-100X someone just popped out the 6C5 det tube and replaced it with a 6SN7. Hardly have to crack the volume control for room filling audio!
http://www.flickr.com/photos/42774321@N08/3941396777/
Terry

[stevef]

After looking at my HRO-W schematic, it now appears someone possibly tried to convert it to an HRO - which uses a 6SQ7 as the 2ndDET.

[AB2EZ]

Okay:

Let's assume that each diode has several microamperes of reverse saturation current (i.e. if it is reverse biased, several microamperes of reverse current will flow through it)

.........................

As an aside, the relationship between current and voltage for any diode is (approximately):

i= i(saturation) x [exp(V/0.026 volts)-1];

where i(saturation) is the diode's reverse saturation current (a positive value of current); V is the voltage across the diode (i.e. V is a negative voltage for reverse biased conditions); and i is the current flowing through the diode (i will be a negative current for reverse biased conditions)

......................

With the switch in a position where the upper diode is not connected... the lower diode, in conjunction with the 0.05uF capacitor it feeds into, will act as a "fast" acting peak detector. I.e. the capacitor will charge up to approximately: (3/5 of the positive peak value of the i.f. signal coming out of the low pass filter that is to the right of the 2nd i.f. transformer) - the voltage from grid-to-ground across the 470k grid leak resistor. The factor of 3/5 is due to the 330k ohm / (330k ohm + 220k ohm) voltage divider between the output of the low pass filter and the left side of the diode. The 0.05uF capacitor will (relatively slowly) discharge via the reverse current of the diode.

The reverse biasing voltage across the capacitor (from left to right) results in an added current (the reverse saturation current of the diode) that flows up from ground through the 470k ohm grid leak resistor, which adds to the negative bias on the grid of the tube. Note that this added negative bias does not increase if the reverse voltage across the peak detector diode is larger, provided the reverse voltage across the diode is greater than around 3 x 0.026V = 0.078V. If this reverse bias current were, for example, 5uA... and if the voltage from left to right across the 0.05uF capacitor were large enough to keep the diode reversed biased at more than -0.078V, then the added bias on the grid of the tube would be -5uA x 470k ohms = -2.35V. Note that the voltage across the diode (from right to left) is equal to: the voltage (from left to right) across the capacitor, plus the voltage from the grid of the tube to ground, plus the voltage from ground to the point where the 330k resistor connects to the left side of the diode.

Almost pure speculation: the circuitry posted here is a peak detector that is part of the overall AVC circuitry of the receiver.

[Note that most peak detectors that we run into, and use, have a resistor to ground on the left side of the capacitor... to discharge the capacitor. The time constant (resistor value x capacitor value) also sets the speed at which that type of peak detector can track the varying peak level of the input signal]

I think that the purpose of the switch positions that add in the upper diode is to modify the behavior of the overall AVC circuitry ... but I haven't thought that through. The time constant of the 1M ohm x 0.1uF capacitor is so long, that the input to the top 1N54A diode would be only the residual positive (with respect to ground) DC (average) component (if any) of the voltage at the output of the 2nd i.f. transformer. Closing the switch (upper position) would ensure that the 0.05uF capacitor is charged up to a value that is at least that DC (average) voltage (if any). Closing the switch (lower position) would ensure that the 0.05uF capacitor is charged up to a value that is at least some portion of that DC (average) voltage (if any).


Stu

[stevef]

Thanks for that Stu.   What sort of reception improvement would this circuit change have offered?  Perhaps I'll save this circuit for the last step in the restoration and see how it works on received signals.  Then I can decide whether to restore the original 6C5 circuit or not.

[AB2EZ]

Steve

I still have no clue as to what improvement (if any) this circuit would make in the overall behavior of the receiver. I see how this circuit behaves in isolation, and I can speculate that it plays some role in the receiver's overall AVC loop... but that's about it.

Stu