looking to drop the carrier down. for the amp. how is the best way to go about this? i see a couple of methods mainly in certain forums but unsure about which one is best
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cobra 140gtl NPC-rc mod?
- Thread starter Uncle Ronnie 336
- Start date
comparing off the washington/142gtl method i saw on another forum with my own notes:
1. Remove TR32 [ACTUALLY DONT DO THAT. TRY CHANGING R104 to 22K TO OPEN UP THE THRESHOLD CLEANLY]
2. jumper r123
3. Add a 10 uF 25 or higher volt electrolytic cap to
the following
spots:
Attach the positive leg to pin 9 of IC4,
and attach the
negative leg to the junction of R121/D62/R204. [THIS JUCTION DOESNT EXIST IN THE 140.a small trace gets a precision cut between R121 AND VR6 560ohm and 4148 diode will do. the pcb trace cut is between the diode legs in the pic]
4. Set the driver bias to 50 mA.
5. Set the final bias to 100 mA.
6. Solder the end of the final rail wire that is
furthest from
the final transistor to the cathode (banded) end of
D60. this takes a lot of load off of the regulator and increases the PEP output by a couple watts
7. Set the dead-key power to 1-3W using VR6.
8. Tune the RF amp chain coils (L36 and L26 through
L29) for
maximum peak (modulated) output power.
1. Remove TR32 [ACTUALLY DONT DO THAT. TRY CHANGING R104 to 22K TO OPEN UP THE THRESHOLD CLEANLY]
2. jumper r123
3. Add a 10 uF 25 or higher volt electrolytic cap to
the following
spots:
Attach the positive leg to pin 9 of IC4,
and attach the
negative leg to the junction of R121/D62/R204. [THIS JUCTION DOESNT EXIST IN THE 140.a small trace gets a precision cut between R121 AND VR6 560ohm and 4148 diode will do. the pcb trace cut is between the diode legs in the pic]
4. Set the driver bias to 50 mA.
5. Set the final bias to 100 mA.
6. Solder the end of the final rail wire that is
furthest from
the final transistor to the cathode (banded) end of
D60. this takes a lot of load off of the regulator and increases the PEP output by a couple watts
7. Set the dead-key power to 1-3W using VR6.
8. Tune the RF amp chain coils (L36 and L26 through
L29) for
maximum peak (modulated) output power.
Attachments
don't bother with that mod.
The NPC part will work but the cap is not the ideal value.
the volting the final part, while it may give you a higher PEP (won't be noticed by anyone but you) you will lose some of the punch of your modulation.
here is a much better mod, but know that you have to have an oscilloscope to do any NPC mod properly. without a scope you're basically doing the same thing as clipping the modulation limiter.
Remove R104 (AMC limiter resistor), and replace with a 20k ohm trimmer pot, you'll have to solder this on the solder side of the PCB.
Remove R204 (560 ohm), and replace with a 1.2k ohm resistor.
Install a 1uf, non-polarized capacitor between pin 9 of IC4 (TA7222AP audio IC) and the junction of R204/D62/R121. The capacitor can be mylar, metal film, polyester, mica dipped or ceramic.
Remove the 2SC1419 at TR34, and replace with a TIP41.
Set your deadkey to 2W at VR6.
Using a 1khz tone, and a scope, set your modulation on AM with the 20k trimmer pot you soldered in at R104. Adjust this trimmer until your NEGATIVE peaks are approximately 90%.
At this point you should have 180~200% positive peaks with no flat topping.
remember that NPC mods only really work when you lower the deadkey to 1 or 2 watts. otherwise the radio really doesn't have the headroom to produce the big modulation peaks.
LC
The NPC part will work but the cap is not the ideal value.
the volting the final part, while it may give you a higher PEP (won't be noticed by anyone but you) you will lose some of the punch of your modulation.
here is a much better mod, but know that you have to have an oscilloscope to do any NPC mod properly. without a scope you're basically doing the same thing as clipping the modulation limiter.
Remove R104 (AMC limiter resistor), and replace with a 20k ohm trimmer pot, you'll have to solder this on the solder side of the PCB.
Remove R204 (560 ohm), and replace with a 1.2k ohm resistor.
Install a 1uf, non-polarized capacitor between pin 9 of IC4 (TA7222AP audio IC) and the junction of R204/D62/R121. The capacitor can be mylar, metal film, polyester, mica dipped or ceramic.
Remove the 2SC1419 at TR34, and replace with a TIP41.
Set your deadkey to 2W at VR6.
Using a 1khz tone, and a scope, set your modulation on AM with the 20k trimmer pot you soldered in at R104. Adjust this trimmer until your NEGATIVE peaks are approximately 90%.
At this point you should have 180~200% positive peaks with no flat topping.
remember that NPC mods only really work when you lower the deadkey to 1 or 2 watts. otherwise the radio really doesn't have the headroom to produce the big modulation peaks.
LC
i have a 200mhz china scope. everything looks good there. the volted final takes a lot of heat from the regulator and everything runs cooler so im gonna stick with that, but i'll try the diff cap and resistor.
the tip41....its still a to-220 device. i have on hand an NTE270 which is a to-3. IF i were to un-volt the final, my instincts say a bigger package NPN upstream from the driver-final would be better no?
You'd have to drill another mounting hole higher up, and bend the legs of the TO-3P NTE270 to fit the pin spacing and cutout notch in the board for the original TO220. But it should work.
I used to pull the fixed value 1.5k at R105 in MB8719 chassis' 140's, 142's, Washingtons and even Madisons (R130?), and put a 1N4148 diode and a 500 ohm pot in series in place of the 1.5k, to match the Cobra 148's and 2000's for adjustability. Very early 148's had the fixed value too, according to the schematic, but I have only seen the ones with the adjustment.
uncle Ronnie,
Sounds like you have a handle on what you want to do, so i'll just add my 2 cents worth of experience with this stuff and you can take or leave any or all of it.
first, the NTE270 is a darlington transistor and the 140GTL uses two transistors to make the darlington pair, so i can't say for sure that the NTE270 will or won't be a drop in replacement. you may have a few circuit modifications to make.
next as for the package size of the transistor, yes the NTE270 is a larger transistor but it's not going to run any cooler than a TO-220 package.
all it's going to do is get the heat from the front to the back of the transistor faster.
this has all been studied before and the real answer is that unless you increase the size of the heatsink, the transistor is going to average the same temperature.
again, take it or leave it, but im not guessing here.
as for the volting of the final, yes it will make the regulator run cooler since it only has to modulate the driver. the trade-off is that you no longer have a true high level modulated radio since you are only modulating one stage of the RF amplifier.
yes, it will make the difference between 18 watt peaks and 25 watt peaks but there isn't an S-meter in the world that will notice that difference.
the loss is, in my opinion much more significant than the small gain.
the loss is in the "punch" of your audio.
I have done radios both ways more than a few times and i can tell you that the difference is noticeable. once i heard it for myself i went right back to modulating the driver and the final.
Lastly, for the NPC mod to work properly, you will need to have an adjustable AMC and i can't remember whether or not the 140GTL has one. there may be some versions that do and some that don't.
good luck on your modding and please come back and let us know how it all worked out.
LC
Sounds like you have a handle on what you want to do, so i'll just add my 2 cents worth of experience with this stuff and you can take or leave any or all of it.
first, the NTE270 is a darlington transistor and the 140GTL uses two transistors to make the darlington pair, so i can't say for sure that the NTE270 will or won't be a drop in replacement. you may have a few circuit modifications to make.
next as for the package size of the transistor, yes the NTE270 is a larger transistor but it's not going to run any cooler than a TO-220 package.
all it's going to do is get the heat from the front to the back of the transistor faster.
this has all been studied before and the real answer is that unless you increase the size of the heatsink, the transistor is going to average the same temperature.
again, take it or leave it, but im not guessing here.
as for the volting of the final, yes it will make the regulator run cooler since it only has to modulate the driver. the trade-off is that you no longer have a true high level modulated radio since you are only modulating one stage of the RF amplifier.
yes, it will make the difference between 18 watt peaks and 25 watt peaks but there isn't an S-meter in the world that will notice that difference.
the loss is, in my opinion much more significant than the small gain.
the loss is in the "punch" of your audio.
I have done radios both ways more than a few times and i can tell you that the difference is noticeable. once i heard it for myself i went right back to modulating the driver and the final.
Lastly, for the NPC mod to work properly, you will need to have an adjustable AMC and i can't remember whether or not the 140GTL has one. there may be some versions that do and some that don't.
good luck on your modding and please come back and let us know how it all worked out.
LC
i haven't messed with the NPC mod yet but after long runs TXing, what i see on scope, and given that i'm driving 9530 mosfets, i have a nice 3<19W, slightly high fwd RMS wattage, nothing is off or raising alarms so i'll just leave well enough alone. I did change R104 to 18k, which is juuuussst right it seems. it can do high peaks but I went for durability here all around and it is tight on channel very neighbor-friendly on the band
You can still modulate a "Volted" final. You just have to couple the final to the audio output yet isolate the audio AC component from the impedance of the power rail/supply.
So feed the final thru a choke suitable for the current draw but with ample audio frequency rejection and now the audio only sees the input of the final.
The heat dissipation of the 2sc1969 in these platforms goes way up compared to the increased output power. This is due to the circuit being tuned to produce rated power at about half the output impedance as say a cobra 29.
Way back when, and way way back when this mod was done full tilt. Finals were 30 cents
I had an older chap on this forum give me the rundown on this, and doing what I do, I realized right away he was right.
This same idea applies to Galaxy and Ranger radios as well. The rigs with the shorting bar that people remove, and tie the driver to the AM regulator and the center pin of the final to 12vdc.
Well. Take your audio signal and couple it with a 1000uf cap to the final and feed the final through an appropriate choke.
Whamo. NPC, high level modulation, output numbers that sell.
So feed the final thru a choke suitable for the current draw but with ample audio frequency rejection and now the audio only sees the input of the final.
The heat dissipation of the 2sc1969 in these platforms goes way up compared to the increased output power. This is due to the circuit being tuned to produce rated power at about half the output impedance as say a cobra 29.
Way back when, and way way back when this mod was done full tilt. Finals were 30 cents
I had an older chap on this forum give me the rundown on this, and doing what I do, I realized right away he was right.
This same idea applies to Galaxy and Ranger radios as well. The rigs with the shorting bar that people remove, and tie the driver to the AM regulator and the center pin of the final to 12vdc.
Well. Take your audio signal and couple it with a 1000uf cap to the final and feed the final through an appropriate choke.
Whamo. NPC, high level modulation, output numbers that sell.
yep been there done that.
used the transformer from a 29LTD to modulate the final in the old MB8719 chassis after being shown by a guru here.
i found that it made the difference between 18-20 watt peaks and 25-28 watt peaks in the radios i experimented with.
i also found that i could make almost 25 watt peaks by changing a few capacitor values in the final and driver circuit without volting the final and going through the hassle of modulating it.
if you have a procedure for modulating the final after volting it you should add it to this thread as it seems to have some pretty good info for future searchers.
LC
used the transformer from a 29LTD to modulate the final in the old MB8719 chassis after being shown by a guru here.
i found that it made the difference between 18-20 watt peaks and 25-28 watt peaks in the radios i experimented with.
i also found that i could make almost 25 watt peaks by changing a few capacitor values in the final and driver circuit without volting the final and going through the hassle of modulating it.
if you have a procedure for modulating the final after volting it you should add it to this thread as it seems to have some pretty good info for future searchers.
LC
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