Cobra 2000 GTL Volting?

[nomadradio]

This is not a symmetrical thing.

If you do this the other way, and 'volt' the DRIVER'S collector from straight DC, NOT the final, there is a problem.

Reading all I could about AM modulator circuits when I was young and impressionable the ARRL books pointed out a big difference between modulating a tube final and a transistor final.

The problem is that the bipolar transistor has a small amount of carrier "feedthrough". Simply forcing the collector voltage to zero won't force the carrier level coming out of the final to zero. Can't achieve 100% negative modulation peaks.

A tube, on the other hand, will happily reduce it's output to zero when the anode voltage falls to zero on your negative audio peaks.

But the driver transistor has very little 'leak through' carrier. Not enough to leak into the final and pass through. This is the reason you see both driver and final powered by the high-level modulated B+ voltage, rather than the final alone.

But the driver's 'leakage' carrier is low enough that you'll have no trouble achieving 100% negative modulation peaks whether the final is also modulated or if it simply serves as a linear stage after the modulated driver transistor. That lower 'leakage' level is why modulating the driver alone won't create this problem of getting 100% negative peaks.

We started doing it when customers wanted to see the same peak output on AM that they saw on sideband. The Uniden 40-channel SSB CB designs of the 80s (1979, really) modulate the final B+ through a NPN power transistor. It will always drop a volt or so between the B+ supply and the final collector, reducing the peak modulated voltage below the straight DC-voltage feed that the final/driver are fed in sideband mode. A difference of a volt and a half or two reduces those AM modulation peaks just enough for wattmeter nuts to notice and be offended.

Volting the final was the only way to get them to shut up about it.

Radios built by RCI are different, and have the same PNP-transistor current path feeding the final/driver in both AM and sideband modes, so you won't see this difference. Peak modulated AM voltage coming out of the modulator is the same as the steady DC voltage in sideband mode.

Volting that kind of radio is a can of worms I'll let someone else open.

But about the capacitor trick. Some number of 23-channel AM CB radios would have a 10-ohm resistor in line with the driver transistor's collector circuit. This was clearly meant to reduce the driver's output level a little. Found that putting a 10 or 22uf electrolytic across this resistor would boost the positive modulation peaks by two or three Watts. This was around 1976 or so, but I won't stake any claims of being "first". No point to it.

Again, it wouldn't boost the radio's actual transmit range enough to spit on. But the wattmeter worshippers loved it.

73

 

[Shockwave]

I wonder what would happen if we volted that 1973 pre-driver and the 1969 final while reducing the DC carrier voltage setting to the 2166 driver to prevent overdrive?

 

[Handy Andy]

I wonder what would happen if we volted that 1973 pre-driver and the 1969 final while reducing the DC carrier voltage setting to the 2166 driver to prevent overdrive?

Then keep the NFB circuit you'd find - if any - in your radio...

You'll need it to protect the 2166 which is getting to be unobtainium now...

Too much audio drive into that part will destroy it without the NFB filter to reduce it's peaks - the Carrier Leakthu or driver level for envelope is certainly lower, but if you can linearly drive the Final better with it, then you're all set.

You can see that in this graphic...

It's parts Numbers R208/9 and C200...



Even HR2510's did this too...

Now to address the SSB 1973, there are several options including a power protection method that provides a mean to "zener" the peaks if you are worried...

The upgrade consists of taking out that EMITTER resistor in there and swapping it to a higher value - like more of 22 to as high as 47 ohms, This takes SOME OF THE EFFORT OF POWER OFF THAT PART AND REDUCES HEAT PRODUCTION - IT IS MORE IN CLASS A THAN AB. It also affect the admittance - so you know - you may need to up some caps and lower others during this re-tune to find the right balance - each model of board is different let alone in the year of the board you do this mod to.

There are always options, some require more effort in parts but then, if you want to provide the level of support the part needs then you as a responsible tech needs to understand the mod I do...

If any of you remember CB Tricks asymmetry mod thread...
You will recognize this mod as in that thread...
​

This may look to be overkill, and in a way, it is, but it is drawn this way to show functional blocks ad-nauseam sections and parts to address as individual sections of a bigger picture of effect.

Again it's not written in stone - just remember to keep the Pre-driver flowing forward biased so you'll need to address how much of that 8 volt TX stuff you'll need to keep this going.

If you wish to add a relay, that is possible yes, but this instance, the Resistor divider circuit RVD1 and RVD2 are installed within and between the three diodes. This keeps the Pre-driver "on" but not cooking all the time you're in SSB mode - you drop some voltage to save the part in SSB but only affects TX voltages' when in AM where the supply is only 1/2 the supply rail and swings from there - it's those "swings" you need to be aware of...

• DP - Protection Diode
• CF - Capacitor Filter - smoothing effect if and when clipping were to occur.
• DT - TX switch power feeder Diode (steering)
• Ds - AM Power feeder (steering)

Two R Buffer resistors are offered locations I've tried with good results to provide a means to keep the Pre-driver from overheating.

The Voltage divier circuit values are of your choice - usually you want to keep the input "high side" lower than the mid-set point to ground side - as a ratio of about 1:1.5 ~ 2 - with the RVD2 being the higher of the two. You want to dissipate power but not swamp the circuit and lose the effects.

The "extra" parts involved are for a mixture or both AM SSB chassis and AM Only - FM - although possible - is your choice in using these - you don't really need to, due to the nature of the Frequency versus envelope peaking

In a way, it cleans it up, but in others you lose your ability to "peak" out quicker for SSB non-linear result. You have more of a marshmallow effect but it saves the part - you'll have to talk to your customers about preservation versus the upgrade - if they'd rather keep it stock - then you answer is to just raise the Emiiter resistor to the lower range of resistance listed and use the "diode steering" but then too, allow the 8V feed in there too. So buffer resistor with ~68 to 100 ohms both sides going in - to lessen the current - so that when and if peaks SWING blow a cutoff, you won't kill the Predriver - this also cushions any peaking event but again you save the part...

Otherwise if you just keep a level head and the operator is willing to work with you in limits they will put these parts in - then you can reduce the parts count considerably and not even have to use the 8V feeder from the TX side to help offset - you just change the Emitter resistor to a value you and your customer can accept and run with it like previously discussed...

 

[NZ8N]

Put a 47 ohm resistor to the source modulated voltage (mode switch), and an 8v zener diode on the end of the resistor to L44, problem solved. The negative peaks (troughs) will then modulate from 0v or whatever the zero carrier voltage is, to max at 8v.

It may flat top the predriver voltage when it hits 8v, though.

Just a thought. Voltage divider, to reduce the 13v from the mode switch to 8v instead?

 

[Handy Andy]

.Just a thought. Voltage divider, to reduce the 13v from the mode switch to 8v instead?

I have found that if you don't do something to keep the Pre-driver forward biased, the results could go poof...

So with some radios I've seen using heavy modulation and low carrier - mostly from AM-Regulator types, the Pre-driver can go flat and wind up dying for the cause - simply because they wanted to use low-power high swing modulation.

The divider tends to take some of the oomph off the line - as said earlier. It simply provides a steady DC voltage you can pre-set using the divider to keep the Pre-driver going. That's where that ratio - comes in. You don't want to stiffen the Audio to a DC level, but you also don't want the reverse bias condition the audio can present to the Pre-driver - destroying it...

• However - if you feel you can run with fewer parts, you can.. Just remember, you have to have an understanding with your customer on it, else you're covering a "you did it - so fix it" warranty problem. Save yourself the headache and use what works as you see fit - I just provided a method to keep the Driver from reverse bias conditions where there is more power on the Base than the Collector or Emitter, if you had not kept the 8V TX in place.
• You can also look at this "addition" as a voltage power summation. You don't want clipping nor saturation, do this mod in a fashion that you can keep enough DC to satisfy the conditions it will be used in, and you're fine.
• When in the TX mode, that extra power is keeping the modulation from swinging too far NEGATIVE - when you stop TX-ing, the divider simply drains off the residual power levels to a point where that Pre-Driver is not placed in a truly Class A condition. It's allowed to cool - that is how you develop the Divider as a means to take off power the Audio Bias would present either in AM only or in AM-Regulator types - this divider will need different custom values for the install it is used in. PITA - yes, but possible.
• In the "summation" between both sides, you are also keeping wider swings from potentially (SIC) placing the Pre-Driver in a condition that exceeds it's voltage ratings or worse - being left switched on in all modes TX or RX or otherwise.
• When using this in an AM-Only Transformer types, you will achieve swings in excess of 24 volts or 2X the supply rail - it's possible especially when "Volted" or otherwise the Limiter is bypassed. So you need to think of protecting the parts further down the chain as well. See NFB earlier.
• What you cannot predict is how the end-user will abuse the mod or radio and use it the wrong way.

The steering diodes simply keep the power from crossing barriers - note the PHASE 2 side, that one will allow 8V TX power thru - so a limiter or buffer resistors are used (Suggested) and if you wish - the ENTIRE Line can be reduced before this - hence the extra Buffer resistor - but it is not necessary because the 8V is far less than the 12V arriving from the other side. You just need to understand the provisions - keep the Predriver in a Off state when it should, or provide for the Predriver to be current limited once you exceed a set value of power so you are not hurting the Driver.

Because - and I only post this due to the nature of the conditions I've seen radios hot-rodded by people that you would only had hoped they Worked for Radio Shack - instead they ran equipment designed to create Chaos - and I do not say that lightly either.

You can see this come across your desk...



Try it yourself - I call these diode additions "cap killers" because that is exactly what they can do - too much input voltage spike across the coil, generates a killed cap from excessive ratings exceeded and the cap in it's power feed from this. Unfortunately - has to be replaced - it's a simply time to $$$ effort.

So if there are people that will remove the LIMITER and SPIKE diode, well then you can always offer this...


Like I said earlier - you and your customer have to have an understanding...

DISCLAIMER...
These circuits are for EDUCATONAL PURPOSES ONLY!