Pretty much anybody who runs an amplifier for AM wants to control the radio's carrier power. Simple reason is that an amplifier will exaggerate how much it multiplies the carrier, compared to the peak modulated power.
The simple answer is to set the radio's carrier at a level that permits the amplifier to show the same relative peak-to-carrier ratio as a barefoot radio.
On a good day. Just the same, here's the procedure we have used for this model the last few decades. Sure, there are other ways, but we settled on this method. It's what people asked for over and over, mostly after hearing it on the air.
Bill of materials:
1) 10k 1/4W resistor
5) 1N4148 or similar silicon signal diode.
1) 470uf or close 25V radial electrolytic cap
1) 2.2k 1/4w resistor
1) 5.1k or 6.8k 1/4w resistor. These two values are not carved in stone, but you want a resistance ratio of two or three to one between the larger and the smaller of the two. These will take the place of the SWR cal control in the SWR meter circuit. And if you just don't care about the radio's internal SWR meter, you can skip these two.
We'll start with TR45, a 2SA733. No, there is no hint in the white screen-print legends, the factory scabbed this transistor onto the board after it was designed. Here's where we clipped the three leads. No good reason to unsolder it. We just need it gone.
I could have used a wider shot to get you oriented, but the upper-right corner shows small rectangular slots where the PA and Ext speaker jacks would have gone if this board were installed in a mobile radio. It's the right-rear corner. No good reason to start with Q45, but it will cause a short "BOOP" sound in the AM transmit audio when you key the mike once the modulation limit is opened up. Feel free to leave it in.
Here you see TR33, a 2SC945 that senses the negative modulation peaks in AM transmit. Removing it opens up the AM transmit audio without screwing up sideband. Sure, you can leave it in, but this will cripple the modulated power as the carrier is reduced. Feel free.
Here we see the underside of VR6, the 5k trimpot that sets AM carrier level. At the center of the pic is a cut in the foil trace connected to the trimpot's center lug, the wiper. VR6 will be used to set the max carrier with the front-panel knob turned full up.
And if VR6 has been removed altogether, you need to put one back in there. This setup requires it.
Next we replace C92. It's originally a 47uf 10 Volt cap. A much-bigger capacitance value is needed to prevent feedback squeal issues at low carrier settings. Yes, its bigger than the original part, but it will stand up on its lead wires just fine, nestled among the surrounding caps.
Here are the foil traces under C92.
Next task is to take four of the 1N4148 diodes and solder them into a series string. With forward-bias voltage on this string of diodes, it will act like a 2.4-Volt zener diode, more or less. A length of sleeve to insulate it is probably a good idea.
This string now gets the anode (not striped) end soldered to the outboard pin of VR6.
The cathode (banded) end gets soldered to a handy ground foil. Yeah, it's a base station. You could probably skip the sleeve, not gonna get jostled like a mobile radio.
Now find the three wires connected to the SWR Cal control, and take each one loose at its far end. The clockwise and wiper wires are soldered to the inboard-front corner of the toggle- switch circuit board. The counterclockwise ground wire is most likely black with a white stripe, soldered to the main pc board near the front-right corner, to the right of the mike-amp circuits. Probably easier to just cut it short to the pc board than to bother unsoldering the end.
Now is a good time to solder one end of the 10k resistor to the foil trace to the rear of the cut separating it from VR6. The ground foil alongside is where the other end goes. Leave some of the "hot" side lead sticking out. It will make connecting to this trace easier a bit down the line.
The black/white wire from the pot's CCW lug gets a black wire lap-spliced to reach VR6. It goes to the outboard-side lug of VR6, where the anode end of the diode string is soldered.
The wiper wire of the SWR cal pot is blue with a white stripe on this radio. Didn't have that color on hand, so a white wire got lap-soldered to it.
The other end of the white wire gets the anode (not banded) end of our remaining 1N4148 diode lap-soldered to it. The cathode end of the diode goes to the hot side of the 10k resistor.
What remains is for the SWR Cal control's clockwise wire to get extended. That wire is green with a white stripe in this radio, but I used orange to extend it to reach VR6.
The other end of the orange wire goes to the now-isolated wiper (center) lug of VR6.
The basic hookup is now complete, but you'll have people griping that the peak transmit power is higher on sideband than it is on AM.
Yeah.
Here's the fix. First, set the bias for both final and driver to 60 mA. Never hurts to put a drop of cleaner on each trimpot and crank it end-to-end a few times, just in case it's getting "scratchy" from age.
In this radio, the final wire is green, and the driver is violet. Plug the violet wire back onto it's header pin on the circuit board. The green wire now gets connected to a full-time source of 13.8 Volts. More than one way to do this. I tin this jumper, I think it's "JP6", but it's obscured in this pic. The end of the green bias-test wire gets stripped, tinned and lapped to this jumper. Or connect it somewhere else if you like.
I just don't like to disable a feature from a radio for no good reason if a pair of cheap resistors will keep it working. These two resistors will take the place of the SWR Cal control, and the SWR meter will still work. The black wire on the 5.1k resistor goes to ground. You can find a foil trace to ground this wire without a pic, right?
And you may decide that the internal SWR meter isn't worth even this much bother. YMMV. What you should see is that the "CAL" function sets the needle to the mark around 3 Watts.
What remains is to set VR6 to the max carrier you want the radio to show. No good reason to set it any higher than 4 Watts.
The minimum carrier will now be a stable one or two tenths of a Watt. The series string of four diodes takes the place of a separate "minimum" trimpot. You'll find that the audio does not get raspy at low carrier settings, all the way down to a tenth of a Watt.
No reason on earth to run it that way, but feel free to do any part of this setup differently.
Sure don't want to discourage anyone from experimenting.
73
The simple answer is to set the radio's carrier at a level that permits the amplifier to show the same relative peak-to-carrier ratio as a barefoot radio.
On a good day. Just the same, here's the procedure we have used for this model the last few decades. Sure, there are other ways, but we settled on this method. It's what people asked for over and over, mostly after hearing it on the air.
Bill of materials:
1) 10k 1/4W resistor
5) 1N4148 or similar silicon signal diode.
1) 470uf or close 25V radial electrolytic cap
1) 2.2k 1/4w resistor
1) 5.1k or 6.8k 1/4w resistor. These two values are not carved in stone, but you want a resistance ratio of two or three to one between the larger and the smaller of the two. These will take the place of the SWR cal control in the SWR meter circuit. And if you just don't care about the radio's internal SWR meter, you can skip these two.
We'll start with TR45, a 2SA733. No, there is no hint in the white screen-print legends, the factory scabbed this transistor onto the board after it was designed. Here's where we clipped the three leads. No good reason to unsolder it. We just need it gone.
I could have used a wider shot to get you oriented, but the upper-right corner shows small rectangular slots where the PA and Ext speaker jacks would have gone if this board were installed in a mobile radio. It's the right-rear corner. No good reason to start with Q45, but it will cause a short "BOOP" sound in the AM transmit audio when you key the mike once the modulation limit is opened up. Feel free to leave it in.
Here you see TR33, a 2SC945 that senses the negative modulation peaks in AM transmit. Removing it opens up the AM transmit audio without screwing up sideband. Sure, you can leave it in, but this will cripple the modulated power as the carrier is reduced. Feel free.
Here we see the underside of VR6, the 5k trimpot that sets AM carrier level. At the center of the pic is a cut in the foil trace connected to the trimpot's center lug, the wiper. VR6 will be used to set the max carrier with the front-panel knob turned full up.
And if VR6 has been removed altogether, you need to put one back in there. This setup requires it.
Next we replace C92. It's originally a 47uf 10 Volt cap. A much-bigger capacitance value is needed to prevent feedback squeal issues at low carrier settings. Yes, its bigger than the original part, but it will stand up on its lead wires just fine, nestled among the surrounding caps.
Here are the foil traces under C92.
Next task is to take four of the 1N4148 diodes and solder them into a series string. With forward-bias voltage on this string of diodes, it will act like a 2.4-Volt zener diode, more or less. A length of sleeve to insulate it is probably a good idea.
This string now gets the anode (not striped) end soldered to the outboard pin of VR6.
The cathode (banded) end gets soldered to a handy ground foil. Yeah, it's a base station. You could probably skip the sleeve, not gonna get jostled like a mobile radio.
Now find the three wires connected to the SWR Cal control, and take each one loose at its far end. The clockwise and wiper wires are soldered to the inboard-front corner of the toggle- switch circuit board. The counterclockwise ground wire is most likely black with a white stripe, soldered to the main pc board near the front-right corner, to the right of the mike-amp circuits. Probably easier to just cut it short to the pc board than to bother unsoldering the end.
Now is a good time to solder one end of the 10k resistor to the foil trace to the rear of the cut separating it from VR6. The ground foil alongside is where the other end goes. Leave some of the "hot" side lead sticking out. It will make connecting to this trace easier a bit down the line.
The black/white wire from the pot's CCW lug gets a black wire lap-spliced to reach VR6. It goes to the outboard-side lug of VR6, where the anode end of the diode string is soldered.
The wiper wire of the SWR cal pot is blue with a white stripe on this radio. Didn't have that color on hand, so a white wire got lap-soldered to it.
The other end of the white wire gets the anode (not banded) end of our remaining 1N4148 diode lap-soldered to it. The cathode end of the diode goes to the hot side of the 10k resistor.
What remains is for the SWR Cal control's clockwise wire to get extended. That wire is green with a white stripe in this radio, but I used orange to extend it to reach VR6.
The other end of the orange wire goes to the now-isolated wiper (center) lug of VR6.
The basic hookup is now complete, but you'll have people griping that the peak transmit power is higher on sideband than it is on AM.
Yeah.
Here's the fix. First, set the bias for both final and driver to 60 mA. Never hurts to put a drop of cleaner on each trimpot and crank it end-to-end a few times, just in case it's getting "scratchy" from age.
In this radio, the final wire is green, and the driver is violet. Plug the violet wire back onto it's header pin on the circuit board. The green wire now gets connected to a full-time source of 13.8 Volts. More than one way to do this. I tin this jumper, I think it's "JP6", but it's obscured in this pic. The end of the green bias-test wire gets stripped, tinned and lapped to this jumper. Or connect it somewhere else if you like.
I just don't like to disable a feature from a radio for no good reason if a pair of cheap resistors will keep it working. These two resistors will take the place of the SWR Cal control, and the SWR meter will still work. The black wire on the 5.1k resistor goes to ground. You can find a foil trace to ground this wire without a pic, right?
And you may decide that the internal SWR meter isn't worth even this much bother. YMMV. What you should see is that the "CAL" function sets the needle to the mark around 3 Watts.
What remains is to set VR6 to the max carrier you want the radio to show. No good reason to set it any higher than 4 Watts.
The minimum carrier will now be a stable one or two tenths of a Watt. The series string of four diodes takes the place of a separate "minimum" trimpot. You'll find that the audio does not get raspy at low carrier settings, all the way down to a tenth of a Watt.
No reason on earth to run it that way, but feel free to do any part of this setup differently.
Sure don't want to discourage anyone from experimenting.
73
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