Cobra 2000, 148 AM Bias and operating classes/class in various modes

[LeapFrog]

Hello all, I am posing a question here in a new thread in order to not de-rail another recent thread with an off topic discussion.


My question is in regards to this photo:
[photo="medium"]4238[/photo] .
Is the bias supply, connected to the same bus as the RF pre-amp (2sc1973) and the TX mixer? (Looks Like It Is)

And if so what does this mean for AM operation, biased class C or no?

 

[Robb]

Not an engineer NOR a tech; but that red line is the 8v supply rail that feeds the bias circuit down at the left end of the red line. The bias is controlled by the diode which regulates the bias to AB thru the driver or final heat - affecting that changes the bias diode voltage flow. You don't see that regulation in a Cobra 29 driver or final. That is the way I understand it; please correct me if I am wrong.

 

[JoeDirt]

Bias in SSB operation is set to 25ma for the driver and 50ma for the final. (per factory service manual) This should be Class A with 12 volts on the collectors. In AM mode the voltage regulator drops the collector voltage by about half. Assuming that lowers the bias to Class C?

I just know high level modulation schemes are class C or lower. Without getting too technical (on stuff that I really dont know much about) The AM carrier is enough to essentially "turn on" the transistor or tube. With a carrier the waveform can be replicated at a full 360* cycle. So no bias or higher class bias isn't needed, increasing the efficiency of the AM modulator.

I was told by someone who knows a lot more than I do, that high level modulation scheme cannot be higher than class C for the modulated amp.

 

[JoeDirt]

Not an engineer NOR a tech; but that red line is the 8v supply rail that feeds the bias circuit down at the left end of the red line. The bias is controlled by the diode which regulates the bias to AB thru the driver or final heat - affecting that changes the bias diode voltage flow. You don't see that regulation in a Cobra 29 driver or final. That is the way I understand it; please correct me if I am wrong.

You have it right,
Its just two separate bias schemes I think the 148 has better regulation because it has SSB. The 25/29 being fixed and the 148 being regulated with thermal tracking.

 

[543_Dallas]

I don't know why you couldn't modulate transistors biased above class C but can see it being very inefficient. I don't know enough about the subject to back up that statement.

Here is the info I mentioned in the other thread about modifying the bias circuit for class C. Just grounding L40 and L43. I never tried this to see if the modulator would run cooler with the 148.




https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.bargainxchange.com/Asymod/manuals/Asymod%20III%20Installation%20Manual.pdf&ved=0ahUKEwjsxNjQ6-nPAhWEQyYKHTliA9UQFghGMA0&usg=AFQjCNEoNf2ZRkUOg4f6nXXGrzuoihRDAA&sig2=ZlqQb9Xs4XQi6vgR_2D1Cw



I mentioned in the other thread the mauldulator* heatsink would run 170 degrees on the 148 and barely gets warm with a 29. When I switched to the 29 the carrier control on the modulator had to be turned up a significantly more to achieve the same carrier. My thoughts are the final and driver in the 29 being class C need a little more push because there is no bias supply...to be fair it's also a different final and circuit.

 

[LeapFrog]

Thanks Guys for the conversation!

If I "volt" the 148 final (as per the infamous NPC mod) the radio will not change bias voltage between AM & SSB, correct?

Thanks for educating me, and for joining the conversation.
-LeapFrog

 

[543_Dallas]

Thanks Guys for the conversation!

If I "volt" the 148 final (as per the infamous NPC mod) the radio will not change bias voltage between AM & SSB, correct?

Thanks for educating me, and for joining the conversation.
-LeapFrog

I've tried that and didn't like the results. You do get more power out of it but the negative peaks won't reach 100% modulation. The carrier won't pinch but the negative peaks are still squared off.

 

[JoeDirt]

I don't know why you couldn't modulate transistors biased above class C but can see it being very inefficient. I don't know enough about the subject to back up that statement.

Here is the info I mentioned in the other thread about modifying the bias circuit for class C. Just grounding L40 and L43. I never tried this to see if the modulator would run cooler with the 148.


View attachment 19128

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.bargainxchange.com/Asymod/manuals/Asymod%20III%20Installation%20Manual.pdf&ved=0ahUKEwjsxNjQ6-nPAhWEQyYKHTliA9UQFghGMA0&usg=AFQjCNEoNf2ZRkUOg4f6nXXGrzuoihRDAA&sig2=ZlqQb9Xs4XQi6vgR_2D1Cw



I mentioned in the other thread the mauldulator* heatsink would run 170 degrees on the 148 and barely gets warm with a 29. When I switched to the 29 the carrier control on the modulator had to be turned up a significantly more to achieve the same carrier. My thoughts are the final and driver in the 29 being class C need a little more push because there is no bias supply...to be fair it's also a different final and circuit.

Ya I really dont know enough either. The heat could be a tuning miss match on the output pie network. I have observed huge miss matches from the factory. Even with two exact Cobra radios ones good and the other, not so much..... Some wake up after changing a 20 picofarad cap to a 5 or 40 (whatever it takes) and the radio gains 15 watts pep.

 

[JoeDirt]

Thanks Guys for the conversation!

If I "volt" the 148 final (as per the infamous NPC mod) the radio will not change bias voltage between AM & SSB, correct?

Thanks for educating me, and for joining the conversation.
-LeapFrog

Dont volt or do the NPC mod on a 148. The NPC mods work great on Galaxy style radios you get a nice power gain and the audio quality stays the same.

With the NPC on a 148 you dont get that many more watts and a crappy muffled sound with less audio loudness.
Volting the final isnt needed either. On a high level modulation scheme (148 for example) The Darlington pair modulates the driver and final combo. Volting the final turn it into a AB class amplifier. Some say you lose loudness, I never payed much attention because you dont gain enough watts to really matter.

148's swing good with just turning up the mod limiter, I would just do a factory alignment. Phase the microphone correctly (for best asymmetry) turn up the mod limiter. Maybe open up the RX for a nice sounding receiver. If you know where to look I have a good write-up on that.

 

[9C1Driver]

Agreed. The Cobra 148, Grant xl, and Grant Lt were never intended to be heavy swingers. They are low powered radio's that with a proper alignment sound superb and are stable on SSB.

 

[LeapFrog]

I am going to low level modulate the radio, If I learn enough about what it takes.

 

[JoeDirt]

I am going to low level modulate the radio, If I learn enough about what it takes.

Are you going to low level modulate with AM? Its doable but you would need a AM filter for 7.8mhz, there will need to be some other mods also.....

 

[LeapFrog]

Yes AM, the ssb mode is not gong to be a concern for this project.