Cobra 2000 GTL Very LOW RF output problem

• Too much bias, and the transistor heats up, you lose power and can destroy the transistor from RF being re-rectified into a current that can feed back into the transistor.
• Another effect is the ability for Caps to "block DC" but also provide a clipping / clamping action to an incoming AC signal using a DC voltage - makes AC and RF signals' outputs across these circuits reduce, attenuate, by forcing DC into stay as a static voltage from the Cap to Cap portion of the circuit forcing the DC value to rise and even can block RF completely (The reduced power effects across the cap - against the DC bias blocking the RF passage)
• - research Ripple Filters to know more about these effects on AC signal passage thru a circuit using caps as DC blockers
• - PIN diodes have similar effects to RF and are switched on and off by a controlling voltage.

Handy Andy said:

Let's take a look

View attachment 38731

In the schematic - C154 provides a means to remove RF from entering into the Bias circuit.

Hence his issues with the Bias.

How come different radios same model but not cap - check the years they were built.

The revision of the board.

There may have been a change in trace routing to reduce a crossfeed issue (as I call it) where RF can be induced in a trace close to another that is producing RF - it can generate a interference pattern and small eddy currents can build into big problems if the RF is not removed effectively.

As you can see in the above, and if you look at how they are making EN369 devices, look to the transistor as part of this - the output of the emitter is a PN junction - means it can rectify. So it can generate a current that if the Ferrite Bead is shifted out of position, wrong permeability or not effective enough in removing RF off the line - this junction will act like a diode detector and redirect RF into a DC bias affecting the Final transistor.

• Too much bias, and the transistor heats up, you lose power and can destroy the transistor from RF being re-rectified into a current that can feed back into the transistor.
• Another effect is the ability for Caps to "block DC" but also provide a clipping / clamping action to an incoming AC signal using a DC voltage - makes AC and RF signals' outputs across these circuits reduce, attenuate, by forcing DC into stay as a static voltage from the Cap to Cap portion of the circuit forcing the DC value to rise and even can block RF completely (The reduced power effects across the cap - against the DC bias blocking the RF passage)
• - research Ripple Filters to know more about these effects on AC signal passage thru a circuit using caps as DC blockers
• - PIN diodes have similar effects to RF and are switched on and off by a controlling voltage.

In other radios using AM regulation as well as SSB - the Admittance window needs to be made - or I should say - corrected, to accept RF as a given level of power from a device generating a wide range of power values - so they use the cap - tied to a cap - like you'd have for a resistor voltage divider, to reduce the RF window but allow enough RF to get thru and continue on being amplified and out on it's way...You can't use a resistor here, but you can use the caps and tweak their values to make the following stages transistor work without clipping - yet have a strong enough signal to work with when it needs it to be for SSB or a very dynamic (swinging) output like SSB is.

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