Royce 1-641

[brandon7861]

Before that can be answered, it is important to know how that part of the circuit works. This will be my first look at an ALC circuit so I might get this wrong (im just learning too).

It looks like C74 samples a small amount of the RF output and D25 does a half-rectification on it to provide a DC voltage that is negative with respect to ground (which is why the electrolytic cap has its + side on the gnd foil). That negative voltage acts as bias for the ALC PNP transistor. The more RF that gets rectified, the more negative that ALC voltage gets, and the more Q14 shunts the incoming audio to ground. VR8 is connected to the 9v regulator during TX and it acts as a voltage divider to set the voltage at the cathode of D24. The higher the voltage at the cathode, the higher the RF voltage must be to forward bias that diode. But when it does, the RF goes through C76 to ground. That reduction in the sampled RF is what controls the how negative that ALC voltage gets.

[Edit: I think the more D24 conducts, the more negative the voltage at C75 gets, which adds to the next cycle through D25 further lowering the voltage. Not sure. Gotta think about it longer lol]

With that out of the way, it becomes clear that the voltage at that point will be dependant on the amount of audio coming into the mic jack. If, for some reason, you suspect D25 is bad, just verify the voltage drop across the diode rather than what the ALC is doing. To set the ALC, you need an AF signal generator.

 

[Stuck in the mud]

Before that can be answered, it is important to know how that part of the circuit works. This will be my first look at an ALC circuit so I might get this wrong (im just learning too).

It looks like C74 samples a small amount of the RF output and D25 does a half-rectification on it to provide a DC voltage that is negative with respect to ground (which is why the electrolytic cap has its + side on the gnd foil). That negative voltage acts as bias for the ALC PNP transistor. The more RF that gets rectified, the more negative that ALC voltage gets, and the more Q14 shunts the incoming audio to ground. VR8 is connected to the 9v regulator during TX and it acts as a voltage divider to set the voltage at the cathode of D24. The higher the voltage at the cathode, the higher the RF voltage must be to forward bias that diode. But when it does, the RF goes through C76 to ground. That reduction in the sampled RF is what controls the how negative that ALC voltage gets.

[Edit: I think the more D24 conducts, the more negative the voltage at C75 gets, which adds to the next cycle through D25 further lowering the voltage. Not sure. Gotta think about it longer lol]

With that out of the way, it becomes clear that the voltage at that point will be dependant on the amount of audio coming into the mic jack. If, for some reason, you suspect D25 is bad, just verify the voltage drop across the diode rather than what the ALC is doing. To set the ALC, you need an AF signal generator.

I am not sure this is a good radio to learn on LOL

 

[nomadradio]

Back in the bad old days of average-reading wattmeters I just told anyone who didn't have a 'scope to start with the ALC all the way up, modulate it with "Aaaahhhh" and turn the ALC downwards slowly. There will be a sharp threshold where the average power falls abruptly, usually to half or less the "wide open" reading. Told 'em that when the wattage drops like a rock, the ALC is set right.

And if you have a 'scope, that's cheating.

73

 

[Stuck in the mud]

Back in the bad old days of average-reading wattmeters I just told anyone who didn't have a 'scope to start with the ALC all the way up, modulate it with "Aaaahhhh" and turn the ALC downwards slowly. There will be a sharp threshold where the average power falls abruptly, usually to half or less the "wide open" reading. Told 'em that when the wattage drops like a rock, the ALC is set right.

And if you have a 'scope, that's cheating.

73

Brandon do you by any chance know what pot changes the clarifier

 

[brandon7861]

Brandon do you by any chance know what pot changes the clarifier

I used to have a schematic of the sardine can, but I cannot find it. The only pot on the schematic that goes to the PLL is VR1, so good chance thats it. I would, however, ohm out both sides and write it down before you change it because I do not know for certain.

 

[Stuck in the mud]

I used to have a schematic of the sardine can, but I cannot find it. The only pot on the schematic that goes to the PLL is VR1, so good chance thats it. I would, however, ohm out both sides and write it down before you change it because I do not know for certain.

Ok thank you very much