i have a couple of cobra 148gtl (side mic) radios and i need to put a variable (carrier) power control in them. i have searched for hours and cannot find one online for some reason. thanks for the help
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I need a proper cobra 148gtl variable power mod
- Thread starter groundwire
- Start date
We have been doing it so long I don't think we have a written record of the procedure. The parts you'll need are a 10k resistor, a 1N4148 generic small diode and 3 wires that will reach from the front-panel control to VR10 near the rear of the pc board. Best policy is to use the radio's SWR Cal control. Take the wires from the control and solder the wiper wire to the clockwise-lug wire. A 4.7k resistor now gets one side added to the two wires you just joined, the other end to the ground wire that came off the control's counterclockwise lug. This makes the SWR-meter circuit think the control is turned full clockwise. You may not care, but this keeps the SWR meter active. You just calibrate your SWR reading by setting the carrier level once the rest of the job is done.
Easiest way to set minimum carrier without adding a trimpot is to hijack the two tiny double-junction diodes D52 and D75 from alongside the 11.325 crystal. If the radio's clarifier has already been unlocked for sideband use, they aren't in the circuit anymore anyway. Remove these two diodes and lap-solder them in series. Now the end of these two with the paint dot (band) on the end gets soldered to a ground foil near to the carrier trimpot VR10. The unbanded end of the end-to-end pair goes to the outboard solder pad of VR10, the foil pad nearest the side rail.
The foil pad at the center pin (wiper) of VR10 gets severed. A wire from the VR10's center pin goes to the clockwise lug of the SWR cal control. A wire from the counterclockwise lug of the SWR cal goes to the outboard lug of VR10, where the diode got soldered. A wire from the wiper lug of the SWR Cal control long enough to reach VR10 is next. The far end of this wire gets lap soldered to the anode (not banded) end of the 1N4148. The banded end of this diode goes to the far end of the cut on the foil trace at VR10. The 10k resistor gets one end soldered to this same spot, the other end to a nearby ground foil.
The two diodes you soldered end-to-end and placed on one end of VR10 serve to make the minimum setting of the SWR Cal control a fairly stable 1/10 of a Watt more or less, but without adding a trimpot. If those two diodes were removed at some time in the past, a series string of four (yes four) generic 1N4148 diodes will do the same job, with only a little more effort. The original two teensy diodes were in fact "doubles" with two diodes in series inside each part.
Somebody (else) should probably put this up on a YT vid.
73
Easiest way to set minimum carrier without adding a trimpot is to hijack the two tiny double-junction diodes D52 and D75 from alongside the 11.325 crystal. If the radio's clarifier has already been unlocked for sideband use, they aren't in the circuit anymore anyway. Remove these two diodes and lap-solder them in series. Now the end of these two with the paint dot (band) on the end gets soldered to a ground foil near to the carrier trimpot VR10. The unbanded end of the end-to-end pair goes to the outboard solder pad of VR10, the foil pad nearest the side rail.
The foil pad at the center pin (wiper) of VR10 gets severed. A wire from the VR10's center pin goes to the clockwise lug of the SWR cal control. A wire from the counterclockwise lug of the SWR cal goes to the outboard lug of VR10, where the diode got soldered. A wire from the wiper lug of the SWR Cal control long enough to reach VR10 is next. The far end of this wire gets lap soldered to the anode (not banded) end of the 1N4148. The banded end of this diode goes to the far end of the cut on the foil trace at VR10. The 10k resistor gets one end soldered to this same spot, the other end to a nearby ground foil.
The two diodes you soldered end-to-end and placed on one end of VR10 serve to make the minimum setting of the SWR Cal control a fairly stable 1/10 of a Watt more or less, but without adding a trimpot. If those two diodes were removed at some time in the past, a series string of four (yes four) generic 1N4148 diodes will do the same job, with only a little more effort. The original two teensy diodes were in fact "doubles" with two diodes in series inside each part.
Somebody (else) should probably put this up on a YT vid.
73
What board marking, or link me schematic to that Cobra to cbtricks.
Mike
Mike
I just removed the potentiometer from the board and ran wires to a control of the same value when I did it and it worked just fine.
groundwire,
Just so you know:
That schematic that you posted by Rick Jackson, has many problems with items that are either missing or labeled wrong. Just one example would be the "Transmit-Receive LED" it's not there. One other problem is that you can't add any "Sticky Notes" to it once you find a problem.
I have kept a personal list of all the mistakes I've found over the years, just so I can go back to my list when I find them again, or add a new one to my list.
One would be better off using both Rick's and the original schematic 148 GTL schematic. Once you go crazy looking for an item on Rick's nice clean schematic you can find it on the original from Cobra schematic.
That schematic alone could start an entirely new post on things that people have found wrong on that schematic.
I don't think he's available like he was in the past to let him know about the problems, but he was very good at cleaning up the old schematics so they were easier to read.
To those of you looking at this schematic, don't complain when you find a problem, I've dealt with it for a long time.
73
Just so you know:
That schematic that you posted by Rick Jackson, has many problems with items that are either missing or labeled wrong. Just one example would be the "Transmit-Receive LED" it's not there. One other problem is that you can't add any "Sticky Notes" to it once you find a problem.
I have kept a personal list of all the mistakes I've found over the years, just so I can go back to my list when I find them again, or add a new one to my list.
One would be better off using both Rick's and the original schematic 148 GTL schematic. Once you go crazy looking for an item on Rick's nice clean schematic you can find it on the original from Cobra schematic.
That schematic alone could start an entirely new post on things that people have found wrong on that schematic.
I don't think he's available like he was in the past to let him know about the problems, but he was very good at cleaning up the old schematics so they were easier to read.
To those of you looking at this schematic, don't complain when you find a problem, I've dealt with it for a long time.
73
What about putting something like this between the RF buffer and RF driver? Ive used this setup as an an audio switch and a HF AM modulator. It will require some RF chokes or wire wound resistors to work right, but a 2N3904 should give about 30dB attenuation with no bias and no attenuation with with hard bias. Seems fairly linear in between too, so it would make a great variable attenuator. All Electronics Channel on youtube has a video about using a BJT as an attenuator.
You do not want to attenuate the carrier feeding the driver stage. This will reduce peak modulated power. Not what you were seeking, right?
The method of just wiring an external control in place of the trimpot works well enough.
But the added details of the setup I described serve to allow the same peak modulated power no matter how low the carrier is set. That may or may not matter to everyone.
73
The method of just wiring an external control in place of the trimpot works well enough.
But the added details of the setup I described serve to allow the same peak modulated power no matter how low the carrier is set. That may or may not matter to everyone.
73
C
Again.
73
Jeff
The Cb tricks web site is not available....Account Suspended.
Again.
73
Jeff
I just made the link a sticky at the top of the CB section
73
Jeff
73
Jeff
Nomad I have a question. Are the double diodes used in most of the older sideband radios on the clarifier section? If so when they are missing what would the factory part number be for the double diode? And what would the outcome be if they were replaced with a 1n4148? and are these diodes still made under a different part number? I know the 1S????, cant recall the number off the top of my head can not be found that easy.