Cobra 2000 GTL Very LOW RF output problem

[Unit 75]

I purchased this Cobra 2000 GTL on eBay as a parts radio. I know...I know...lol. It is made in Taiwan and the board number is PC-497AB. Initially the radio would only transmit very low on both AM and SSB. That is how it arrived. The radio has since been totally re-capped, electrolytic, ceramic and tantalum caps. All voltages on IC5 (mixer) are good. Pre driver TR39 (C1973) is good I installed a new driver and final as well as a new bias regulator TR37 (C496) Everything looks good up to and including the driver TR38 (C1306)
The driver voltages are:
Base .455volts rx AM .740volts tx AM
Collector 12.95volts rx AM 6.6volts tx AM
Emitter tied to ground.

Something is not right with the bias regulator voltages, yet I have the driver set at 30mA and the final bias at 60mA.

Bias Regulator
Emitter .489volts in AM rx mode (which is good) and 1.277v in AM tx mode (which is high)
Collector 3volts in AM tx mode and rx mode (should be 2.66v tx AM and 7.28v tx SSB)
Base 1.25v in AM tx mode

I have .489 volts rx into R180 and 8.31volts tx into R180
.489 volts rx at anode of D76
3.00 volts rx at cathode of D76

Final transistor (C1969) TR36
Emitter tied to ground
Collector 12.95 volts rx 6.6 volts tx
Base 175mv rx .944 volts tx (should be about .4volts in rx AM mode)

Radio is putting out .8 watts (carrier) and goes up to 2 watts on a whistle
SSB does 3.5 watts max.
The radio has been totally re-aligned. Synthesizer/PLL, Receive and transmitter peaking coils are new (L45, L46,L47,L48)
So, something is not right with the bias regulator voltages ( bias diodes are good)
Base voltage on final is low in rx mode.

Any ideas anyone??

 

[Unit 75]

I purchased this Cobra 2000 GTL on eBay as a parts radio. I know...I know...lol. It is made in Taiwan and the board number is PC-497AB. Initially the radio would only transmit very low on both AM and SSB. That is how it arrived. The radio has since been totally re-capped, electrolytic, ceramic and tantalum caps. All voltages on IC5 (mixer) are good. Pre driver TR39 (C1973) is good I installed a new driver and final as well as a new bias regulator TR37 (C496) Everything looks good up to and including the driver TR38 (C1306)
The driver voltages are:
Base .455volts rx AM .740volts tx AM
Collector 12.95volts rx AM 6.6volts tx AM
Emitter tied to ground.

Something is not right with the bias regulator voltages, yet I have the driver set at 30mA and the final bias at 60mA.

Bias Regulator
Emitter .489volts in AM rx mode (which is good) and 1.277v in AM tx mode (which is high)
Collector 3volts in AM tx mode and rx mode (should be 2.66v tx AM and 7.28v tx SSB)
Base 1.25v in AM tx mode

I have .489 volts rx into R180 and 8.31volts tx into R180
.489 volts rx at anode of D76
3.00 volts rx at cathode of D76

Final transistor (C1969) TR36
Emitter tied to ground
Collector 12.95 volts rx 6.6 volts tx
Base 175mv rx .944 volts tx (should be about .4volts in rx AM mode)

Radio is putting out .8 watts (carrier) and goes up to 2 watts on a whistle
SSB does 3.5 watts max.
The radio has been totally re-aligned. Synthesizer/PLL, Receive and transmitter peaking coils are new (L45, L46,L47,L48)
So, something is not right with the bias regulator voltages ( bias diodes are good)
Base voltage on final is low in rx mode.

Any ideas anyone??

FIXED
It ended up that this radio never had C154 (.047ufd) installed. I installed it and re-tuned the RF section. Keys at 5w and swings to 8w on AM an I'm getting approx. 18-20w PEP on SSB.
Thanks all.
-75

 

[S&W357]

Good! Nice to keep the 2000’s in action.

 

[Kemosabe55]

So what exactly does C154 do if it may or may not have been installed in the radio? If your radio didn’t have it how did it work before? I have 2 Cobra 2000 and one has it and one doesn’t

thanks

 

[Handy Andy]

Let's take a look



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.

 

[357magnum]

" Handy Andy " to the rescue ! Wasn't there a song like that ? Nope that was ' Jim Dandy " ! Thanks Andy ! You know as Iv'e stated before as a 2K owner I save all this info ! You never know when I might need it ! Stay Healthy & Safe ! 73 & God Bless & Thank You for Andy ! Leo

 

[Dragon13662]

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.

Are there suppose to be diodes attached to the driver and final? Previous owner of this Cobra 2000gtl I have he said had them. driver and final were replaced and they were not put back on the board. By any chance do you have a pic of the solder side of a Cobra 2000gtl board showing the position of these diodes?

 

[HavaV10]

my 1985 Cobra 2000 is missing that C154 and it's been working fine. Don't understand why it's not in there.