I would like to achieve lower dead key and 100% modulation or better from an asymmetry mod if possible
Tall order...
{Miss Me Yet?)
Most people want a quick fix to a complex problem - Complex? Well, yes, because you not only have differences between AM Only and SSB types - but the variants of support between them in each type - add to the problems of "here - this is what you do"
So you gave me an AM Only - so I must also let others know that due to the nature of this platform versus the others shown - to work up a simple solution is not as easy as it looks. Nor will the platform this uses, will work on others.
You actually have all the answers from the earlier links I provided - at least to build from. The rest of the application comes from your willingness to experiment - for I don't have your radio in front of me to show you what needs to be done.
So you mentioned you have low-modulation.
Make some choices...
Idealize the Driver and Final - or do we work on the power developed from the Pre-Driver?
Why Power from the Pre-driver?
Pre-driver gives you Carrier - set in stone - the Driver and Final have to work with what they have to amplify and fully modulate it.
In some manufacturing - they purposely degrade the level of modulation that can be applied to the Driver and Final - thru the use of several methods.
Clipping, Volume (ALC/AMC even AVC), adjust Audio Amp level drive (Cap choices to lessen the drive signal and place the tonal emphasis on centering and Narrowing the bandwidth present for the Driver and Final) are several methods seen in the parts and values chosen from one particular radio but in a SIMILAR platform - not one part value is the same. IT may still be the same circuit used, but the parts and the placement (how the divider networks used in it) of values are all different.
- One thing I mention separately - depends upon the variables (values) used in the Driver and Final (Collector coupling) that gives you your "plate modulation" (Class D) approach.
- The Pre-driver is driven hard into Class A (15K to 68 ohm) Closer to "C" but still Class A.
- Note L6 and L10 - denote a coil but also a RESISTIVE value added, associated - with it. This affects the Inductive Reactance (XL) - SWAMPNIG is the effect here - reducing the inductance offsets the amount of modulation that is being applied - thru a secondary resistive element of a low value.
So if the Pre-driver is producing too strong in carrier - why would this affect the Driver and Final - The Modulation effect isn't the true effort, it is what they do to affect that power "vectoring" that affects the level of modulation and the influence it has on the presence of carrier from the BASE of the Driver to amplify.. Too much of one signal at the Base leaves you little that can be done to affect modulation unless you fully affect the Collector input admittance and the output of itself to couple to the subsequent stage.
These things (Circuit blocks) are Reciprocal. You can increase one, but it affects all - you decrease the same one - it affects all.
So if carrier input is stronger than the Audio Input - What choices do you have?
Once you see the function of each stage in the TX strip - it is simply a matter of optimization - then once you have achieved the 100% Modulation effect from what is there - the mods you need to install are simple and straightforward.
Where to start?
Again choices, you can Optimize the Driver and Final by looking, changing and observing - then applying the correction to the Resistor values
- - L6 uses R20 - 220 ohm
- - while L10 is an UNKNOWN value you will have to discover.
- I've seen Coil and Resistor formed as a solid coil
- - only the Resistor forms the BODY for the Coil a physical package with set dimension
- - so you may need to make some pre-wound coil forms of similar winding counts to idealize L10 to accomplish the 100% or close to the XL amount you will need.
Adding this in...
- - Looking thru other radios' how many others use a resistor across the DRIVER Collector coil?
- - Look at other radios from that era, or vintage - you don't see too many EVEN USING Resistors across the coils of any output Driver or Final.
- It's why I bring it up for "optimization" - your whole problem may just be the use of the two resistive elements' to reduce the overall Audio drive and vectors it adds to the power the Collectors use.
Once you've found the right amounts of resistance (if any) and got 100% Modulation done, you can then work on a power divider for the Pre-Driver.
Or just work on the Pre-Driver and it's carrier output - keeping the Driver and Final STOCK - left alone - and you just tweak the output of the Pre-Driver - it's
Carrier - to a level that the Driver and Final - in their state of being - can fully modulate the signal from the carrier sent thru it from the Pre-driver.
The Pre-driver receives fully battery voltage all the time - the 15K resistor just turns it on to accept RF from the Mixer and Band Pass Filter (B.P.F.)
So you can change the voltage and current present to power the Pre-driver (at the Collector) - by adding a dropping resistor (at the Collector) - that would lower the power level but doesn't give you any flexibility to inject a boost in power you can tap from from the Audio signal that is sent to the Driver and Final.
That is why I did this little graphic at that link, just to show what you can do to apply a direct power from the Modulation transformer and yet keep the Negative spikes from damaging the Pre-driver - allow for more complete power to envelope tracking to fully modulate - or whatever percentage of modulation to direct carrier - mixing you need.
Phase 1 is your Diode - to keep the rest of the DC POWER feeder line from reacting to power swings.
Phase 2 is the coupling from Audio and Bias that goes to the Driver and Final - the Ds and Rs are arbitrary and are only used if you BYPASS the clipper diode.
This is all sent to the Collector thru the
point on the power line towards the Coupling coil - that heads off to the Pre-driver Collector.
D
p is the Power Polarity protection diode to prevent the negative swing spikes from damaging the section - it is also a necessary protection for if ever the battery leads are applied wrong - you can't replace the pre-driver OEM - for it isn't made anymore.
To complete this is pretty straight forward.
Design a resistor divider network to take the 13V Straight DC, and LOWER it so you can see the carrier power level drop to compare the now dropped power, to the Driver and Final can now fully modulate the carrier the Pre-driver is producing.
Then apply the Audio Bias side using RBUFFER value to MAINTAIN the fully 100% modulated envelope.
If you chose a more aggressive approach, you can tap from the Mod transformer directly - and inject the audio and bias signal directly but you will need to use both the Spike Diode Ds and a Swamping Resistor - to reduce the Audible distortion effects the Clipping will create - Rs uses a Low value between 220 ohm onto 1K - with 1K allowing for more (audible) clipping distortion.
