piepermd
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Yes, that was great! Thanks again.
If you have an S-meter, then you have a voltage and current signal proportional to signal strength. If that can be converted to a low voltage, it can be digitised by the autotuning processor during the peak search.piepermd said:I’m not sure how this might affect using the AVC voltage for tuning.
Now I see the circuit, the 250k S-meter pot, used I assume to adjust FSD of the S-meter, is such a divider chain and the voltage on the wiper contact will be at the lower end of the voltage range. Check out that wiper voltage with an oscilloscope. The S-meter is connected to the VT-207 plate circuit and the +HT supply, which is not good for A-D converters.piepermd said:Can I just use a pair of resistors as a voltage divider to decrease the voltage for input to the microprocessor pin?
It helps, thanks! I didn’t realize the voltage is negative, though. That tends to wreak havoc on a microcontroller input pin. There are some ways around it, but I think it will make the circuitry more complex.Tom.G said:The top of the 250k S_METER pot is the source of AVC voltage, negative-going with increasing signal strength.
The AVC voltage is tapped off thru the resistor going to the left of that junction. The AVC time constant appears to be 15ms, if i'm reading the blurry values correctly of 1.5Meg and 0.01uF.
The switch above those two components switches the AVC line to ground (OFF) to disable AVC, and to the 15ms RC filter (ON) to enable AVC.
The left half of the dual diode tube, 12H6, is the AM detector, the right half is the Noise Limiter, whose threshold varies with received signal strength.
Hope this helps!
Tom
There is a difference between a control voltage being negative with reference to common ground, and a positive AGC voltage that falls as the applied signal increases in strength.piepermd said:It helps, thanks! I didn’t realize the voltage is negative, though.
“Negative-going” was an unfortunate turn of phrase. A decreasing positive voltage is obviously not “more negative” or “negative-going”. Two hours of my life wasted by that, but I’m glad I don’t have to worry about it. Since the AVC voltage decreases with increasing signal, am I not then looking for a dip rather than a peak? Here’s the whole book:Baluncore said:There is a difference between a control voltage being negative with reference to common ground, and a positive AGC voltage that falls as the applied signal increases in strength.
A voltage may become "more negative" as it falls, without actually being negative relative to a reference voltage.
It is wise to use a Zener diode and series limiting resistors to prevent the AGC signal sensed, from exceeding the analog input range.
You need to measure some voltages to get an idea of what changes are needed.
Do you have a link to the Ham mods to the BC-624 ?
I really wonder how far you can safely extrapolate from the many interpretations of the existing documents, before you make an actual measurement of a real voltage.piepermd said:Thus I must look for the greatest negative value of the AVC voltage when applying my bias sweep to the varactors.
The S-meter circuit is difficult to model in that loop. The 12AH7 looks like it does a square-law conversion for the S-meter scale only, with the wiper used to calibrate the meter FSD. No matter how the S-meter components are adjusted, or change over time, the AVC voltage will remain a steady indication of the signal strength, so measure the voltage on the AVC line with a high impedance voltmeter.piepermd said:For the AVC voltage, do I probe between the S-meter adjustment pot wiper and chassis ground?
And it is hard, because you need a very high impedance voltmeter.piepermd said:That’s easy, because the AVC line goes right out to the Cinch-Jones connector pin.
Anything is possible at this stage.Tom.G said:Of course you could use the same approach of connecting a non-inverting op-amp buffer to the AVC line itself, with no additional delay/filtering needed (maybe).
My multimeter has a 7.8 MOhm input impedance. Do I just use 11/7.8 as the multiplier? What should I use as the source of the low impedance reference voltage?Baluncore said:And it is hard, because you need a very high impedance voltmeter.
The AVC line is driven through a low-pass filter that employs a 1M0 series resistor.
A digital voltmeter, with a typically 10M0 input resistance, will only read 10/11 = 91% of the actual voltage from ground.
You must compute the actual voltage by multiplying the meter reading by 11/10 = 1.100, and then maybe re-measure the AVC voltage relative to a low impedance voltage closer to the computed value, (maybe from a 10k pot). You are looking to make a zero current, null voltage measurement, then measure the low impedance reference voltage you used.
The AVC high resistance will have implications to the design of the circuit you employ to condition and convert the AVC voltage to the A to D converter input range.
Thanks, Tom. Good to have you back on board!Tom.G said:At least for testing and measurement purposes you can get a relative measurement of the AVC at a lower impedance point.
On the schematic, look at the S-Meter pot, notice there is a 270K resistor going upward. The other end of the 270K is connected to 1Meg and 50K resistors.
The voltage at this junction of the 3 resistors will be approximately twice the AVC voltage; more importantly it is much lower impedance, about 50K.
This point may even be adequate for your final configuration by connecting a non-inverting op-amp stage to it as a buffer. The advantage of a non-inverting stage is that the input impedance is close to infinite, thereby not loading the AVC line.
There is very little filtering at this point so a low pass filter would be needed somewhere after the buffer to match the 15mS delay in the AVC.
Of course you could use the same approach of connecting a non-inverting op-amp buffer to the AVC line itself, with no additional delay/filtering needed (maybe).
Either way, you still need a Negative supply voltage that is more negative than the voltage you are measuring.
Cheers,
Tom
The series resistor is 1M5, not 1M0 like I thought, so the ratio would be (7M8+1M5)/7M8 = 1.19, but it is not really important. For an initial estimate, measure the AVC voltage and let the AVC loop partially compensate for the load. Then you can decide how to generate a reference voltage that straddles the possible range of AVC voltage.piepermd said:My multimeter has a 7.8 MOhm input impedance. Do I just use 11/7.8 as the multiplier?
Find an external DC reference voltage source, greater than the extreme AVC voltage. Maybe then a 10k pot to ground, adjust for null voltage between the wiper and the AVC, then move the meter to measure the wiper voltage that was set at null.piepermd said:What should I use as the source of the low impedance reference voltage?
It is the DC voltage we need, so capacitors can be ignored. The 50k is not grounded at the top end, but is connected to a current sink, the plate, which has an extremely high impedance. The grounding is in series through the 270k and the two 1M0 resistors of the 'T' low-pass filter, giving 2M270 in parallel with the 250k pot = 225k.Tom.G said:The voltage at this junction of the 3 resistors will be approximately twice the AVC voltage; more importantly it is much lower impedance, about 50K.
That sounds about right. At the small signal end, the response will flatten off in the noise floor, while at the high signal end, the response should flatten as the receiver saturates.piepermd said:What should be the range of the RF signal input at the front end as AVC is measured? A few microvolts up to one volt?
No. The LO signal to the mixer should be big and stable, with amplitude independent of the receiver input signal. The LO signal came previously from a harmonic generator. There will be an optimum LO level based on mixer performance, gain and noise. Insufficient LO drive, and the receiver will be deaf at the microvolt end. Too much LO drive, and spurious signals and noise will flood the audio. You must experiment, or become an expert in VT mixer design.piepermd said:Should the LO be set to the same signal level as the carrier?
The LO is injected through coil 225, which is one winding of a three winding RF transformer at the input to the mixer.piepermd said:To which coil should I apply the carrier signal?
LO at 225 and carrier at 221, correct? Or would it be better to inject carrier at 222?Baluncore said:That sounds about right. At the small signal end, the response will flatten off in the noise floor, while at the high signal end, the response should flatten as the receiver saturates.No. The LO signal to the mixer should be big and stable, with amplitude independent of the receiver input signal. The LO signal came previously from a harmonic generator. There will be an optimum LO level based on mixer performance, gain and noise. Insufficient LO drive, and the receiver will be deaf at the microvolt end. Too much LO drive, and spurious signals and noise will flood the audio. You must experiment, or become an expert in VT mixer design.The LO is injected through coil 225, which is one winding of a three winding RF transformer at the input to the mixer.
Great, thanks. For the first tests of magnitude of the AVC voltage I don’t need to apply any tuning capacitance, correct?Baluncore said:Coil 225 is the LO drive current to the mixer.
Coil 221 is the received RF input from the antenna, or your attenuated test signal generator. Avoid calling "RF" the "carrier" or it will get confused with the transmitter modulator. Coil 221 is coupled loosely into 222 which is the tuned circuit at the input to the front-end RF amplifier. Avoid direct connection to coil 222 as it will lower the Q. Tune coils 222 and 223 with capacitance to pass the RF test frequency during AVC tests.
You need to trim the caps on those coils to peak the RF tuning prior to the tests.piepermd said:For the first tests of magnitude of the AVC voltage I don’t need to apply any tuning capacitance, correct?
Yes.piepermd said:The 216 and 217 mechanically variable capacitors have been removed and would be a pain to reinstall. Can I put a fixed value capacitor across the coils, selected to approximate resonance at the calculated inductance of the coil and use the 218 trimmers to adjust?
All old VT electronics smell a bit when first powered after a break. Dust on the heated components such as valves and resistors. Gasses from oils or electrolytes used in paper and electrolytic capacitors.piepermd said:When I brought it up to power, I detected a bit of electrical smell and immediately shut down. I replaced the 9002 and 9003, re-powered, and now all is fine.
Did you see that instead of re-installing the 216 varicaps I placed a fixed value 22 pF cap across those three RF stage coils? With that value and up to an additional 10 pF from the trimmers I should be able to receive 126 MHz as my test RF. I am going to modulate it with a 1 kHz test tone.Baluncore said:Sorry, I was getting the trimmers confused with the tuning caps.
Replace 216 for my 218.All old VT electronics smell a bit when first powered after a break. Dust on the heated components such as valves and resistors. Gasses from oils or electrolytes used in paper and electrolytic capacitors.
It is possible that removal of a VT resulted in higher voltages across a capacitor.
You should repeat the experiment and see if it happens again. If it does, try to find the faulty component.
It should be OK.piepermd said:So removing the two tubes in the LO circuit is an acceptable way of disconnecting those stages?
Baluncore said:Sorry, I was getting the trimmers confused with the tuning caps.
Replace 216 for my 218.All old VT electronics smell a bit when first powered after a break. Dust on the heated components such as valves and resistors. Gasses from oils or electrolytes used in paper and electrolytic capacitors.
It is possible that removal of a VT resulted in higher voltages across a capacitor.
You should repeat the experiment and see if it happens again. If it does, try to find the faulty
OK, I’ll give it another go.Baluncore said:It should be OK.
If the +HT supply rises before those VT filaments warm, then they will appear to be open circuits for a while anyhow.
That description is a bit unclear, but the earth ground by the transformer center tap is suspicious.piepermd said:...the computer that USB- powers the Arduino unit with the Si5351 oscillator is on a different outlet. My radio power supply is connected to earth ground by the transformer center tap, and so is the radio chassis.