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Cobra 29 wx st mod

Ocean cruiser

Active Member
May 20, 2019
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Hey gang, so I just got this cobra 29 wx st and noticed that it’s been peaked. Looking in the radio I seen this:
D11-cut
L14 slug- removed
R55-cut
R100- cut and a diode added.

Does anyone know what this mod is called and if there’s other parts That may have been change, added or removed. Thanks
 

It's probably an audio swing mod and or a power mod. The Cobra soundcrapper radio's were not the best. If you know a local tech that has a scope that will tell you all you need to know.
 
It's probably an audio swing mod and or a power mod. The Cobra soundcrapper radio's were not the best. If you know a local tech that has a scope that will tell you all you need to know.
9C1Driver, thanks for the reply. Yup it is a type of power mod/swing kit but I’m trying to see which one it is. Also what other parts they may have messed with so I can return it to stock. It looks terrible on the scope too
 
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I honestly do not understand why these so called technicians keep cutting R55, a 1k at 1/2 watt resistor, that is the resistor used to bleed off the static that the antenna accumulates when driving down the road. There is no other path to ground in that circuit to bleed off the static build up from the antenna, every other part of the circuit near there . If anything, change it to a 10k at 1/2 watt.

Reconnect D11, this will clean up the audio on the scope pattern.

A diode in series with cut R100, by the TX Switch, I have no clue why they did this.
 
Last edited:
I honestly do not understand why these so called technicians keep cutting R55, a 1k at 1/2 watt resistor, that is the resistor used to bleed off the static that the antenna accumulates when driving down the road. There is no other path to ground in that circuit to bleed off the static build up from the antenna, every other part of the circuit near there . If anything, change it to a 10k at 1/2 watt.

Reconnect D11, this will clean up the audio on the scope pattern.

A diode in series with cut R100, by the TX Switch, I have no clue why they did this.
Thank you. I will reconnect D11. R55 also.
 
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I may have an answer to that R100 (or R101) question...

First, it may help to know if that radios' Delta Tune tracks on TX too - that takes a frequency counter or another radio on SSB mode (USB or LSB - don't matter) just key that 29 into a dummy load and tune the other radio to hear the channel your TX-ing on in a SSB mode - tune to zero beat the carrier on the RX side of the LISTENING radio - then turn the Delta tune on the 29' - does it change what you're hearing on the LISTENING radio? Then that diode is shooting power to the Delta Tune circuit - be sure of the parts identification when you do this - fo you can really goof up that radio if you undo the wrong parts...

QuikSnapTXSwitchTR21a.jpg

The TX side, that TR21 - the 2SA733 (PNP) uses a Resistor of 2.2 to 1.0 ohm R101 above) - as a BUFFER to SEND power the Mic Amp's Collector leg when you're NOT transmitting.

The Keying circuit Grounds to Pin 3 - which turns on TR21 to power the TX mixer and the TX side and tell the PLL we're in TX mode Thru D15 - USING GROUND of Pin 3.

So when the radio is in idle - RX mode, poking along - D15 is reverse biased, Pin 3 on the Mic Jack has a small charge on it - which then appears on the Collector Leg of the Mic amp.

Why? To keep the Mic amp quiet so any noise it may have doesn't get injected, summed - into the Audio Amp chip thru R49 (RX audio on ST), R48 mess that takes the RX side or TX Mic Amp audio into the Audio amp.

How does it keep it quiet? Potential difference. A transistor is always on if it's forward biased. In this radio - it pretty much is all the time (CB PA switch) - so to pinch it off, or to quiet it, you can apply a smaller trickle charge on the opposite leg of the amp in question - reducing it's gain - or turning it off and keep it's noise from adding to the ambient the receiver is trying to get outside thru the antenna.

The Diode bypasses a cap delay - the TX circuit uses a capacitor to help in the transition of off state to on state and back off again (TX-To-RX) to keep "chirps" and other noises from appearing as you key up - it is for the effort to allow the radios' PLL to shift gears and apply 455kHz rise in the PLL frequency to mix with the 10.240 at the TX mixer when you TX (pin 3 to ground) and time to turn off, PLL shifts back down 455kHz for RX (Pin 4 grounds Speaker) and R101/R100 are for stability of the switching circuit and TR21 (buffers Base lead).

IF you put a diode in there, you force the line low from TR21 quicker - so you key up faster (relative term) but if the diode prevents the trickle charge to return and turn off that Audio line from your microphone - the Mic amp may inject noise from itself and any ambient noise it can get on the AMC side (limiter or not - it's still there acting like an antenna) or even from the Mic Jack and Mic line itself from the handset - even shielded - it can pick up and send thru noise from your RFI noisy cab or compartment you're using the radio in.

So in a way, the Diode "dumps" charge to allow the radio to key up without delay - but in doing do, puts the RX side in a potential noise loop from it's own internal noise and birdies than can form because the Mic amp never truly turned off - its' ready to amplify any noise present.

You also may find the Mic amp PULSES from the initial switchover - the cap in the circuit needs to stabilize so it acts like a dead short attempting to obtain it - R100/!01 the trickle charge it provides keeps the cap topped off and you don't wind up with a surging audio POP you get when you use that diode.

This is why some older cobras squeal in RX when volume gets turned up too high. The Collector and it's support circuitry has failed in some way, usually the cap - and even the routing of the 4.7K resistor used to hold the charge in the cap is not enough to keep the amp off - so it takes in all the extra Audio power that is supposed to stay out of it - from the AMC Limiter "feeder" line and generates the squeal from that simple loop onto the Mic wire in the handset.

This may have been more than you need to know, but I've spent a lot of time on the 29's and PC-76-78 units and see this a lot - doesn't hurt the radio - but it can make you sound like crap if things start to go south as age or power supply jolts take their toll on these radios.

So it pays to know how that mod works, or what it does, in the radio so you can reconnect the parts right.
 
I may have an answer to that R100 (or R101) question...

First, it may help to know if that radios' Delta Tune tracks on TX too - that takes a frequency counter or another radio on SSB mode (USB or LSB - don't matter) just key that 29 into a dummy load and tune the other radio to hear the channel your TX-ing on in a SSB mode - tune to zero beat the carrier on the RX side of the LISTENING radio - then turn the Delta tune on the 29' - does it change what you're hearing on the LISTENING radio? Then that diode is shooting power to the Delta Tune circuit - be sure of the parts identification when you do this - fo you can really goof up that radio if you undo the wrong parts...


The TX side, that TR21 - the 2SA733 (PNP) uses a Resistor of 2.2 to 1.0 ohm R101 above) - as a BUFFER to SEND power the Mic Amp's Collector leg when you're NOT transmitting.

The Keying circuit Grounds to Pin 3 - which turns on TR21 to power the TX mixer and the TX side and tell the PLL we're in TX mode Thru D15 - USING GROUND of Pin 3.

So when the radio is in idle - RX mode, poking along - D15 is reverse biased, Pin 3 on the Mic Jack has a small charge on it - which then appears on the Collector Leg of the Mic amp.

Why? To keep the Mic amp quiet so any noise it may have doesn't get injected, summed - into the Audio Amp chip thru R49 (RX audio on ST), R48 mess that takes the RX side or TX Mic Amp audio into the Audio amp.

How does it keep it quiet? Potential difference. A transistor is always on if it's forward biased. In this radio - it pretty much is all the time (CB PA switch) - so to pinch it off, or to quiet it, you can apply a smaller trickle charge on the opposite leg of the amp in question - reducing it's gain - or turning it off and keep it's noise from adding to the ambient the receiver is trying to get outside thru the antenna.

The Diode bypasses a cap delay - the TX circuit uses a capacitor to help in the transition of off state to on state and back off again (TX-To-RX) to keep "chirps" and other noises from appearing as you key up - it is for the effort to allow the radios' PLL to shift gears and apply 455kHz rise in the PLL frequency to mix with the 10.240 at the TX mixer when you TX (pin 3 to ground) and time to turn off, PLL shifts back down 455kHz for RX (Pin 4 grounds Speaker) and R101/R100 are for stability of the switching circuit and TR21 (buffers Base lead).

IF you put a diode in there, you force the line low from TR21 quicker - so you key up faster (relative term) but if the diode prevents the trickle charge to return and turn off that Audio line from your microphone - the Mic amp may inject noise from itself and any ambient noise it can get on the AMC side (limiter or not - it's still there acting like an antenna) or even from the Mic Jack and Mic line itself from the handset - even shielded - it can pick up and send thru noise from your RFI noisy cab or compartment you're using the radio in.

So in a way, the Diode "dumps" charge to allow the radio to key up without delay - but in doing do, puts the RX side in a potential noise loop from it's own internal noise and birdies than can form because the Mic amp never truly turned off - its' ready to amplify any noise present.

You also may find the Mic amp PULSES from the initial switchover - the cap in the circuit needs to stabilize so it acts like a dead short attempting to obtain it - R100/!01 the trickle charge it provides keeps the cap topped off and you don't wind up with a surging audio POP you get when you use that diode.

This is why some older cobras squeal in RX when volume gets turned up too high. The Collector and it's support circuitry has failed in some way, usually the cap - and even the routing of the 4.7K resistor used to hold the charge in the cap is not enough to keep the amp off - so it takes in all the extra Audio power that is supposed to stay out of it - from the AMC Limiter "feeder" line and generates the squeal from that simple loop onto the Mic wire in the handset.

This may have been more than you need to know, but I've spent a lot of time on the 29's and PC-76-78 units and see this a lot - doesn't hurt the radio - but it can make you sound like crap if things start to go south as age or power supply jolts take their toll on these radios.

So it pays to know how that mod works, or what it does, in the radio so you can reconnect the parts right.
Andy, once again, thank you so much for sharing your knowledge. The more knowledge the better. I will go through the radio this evening. Thanks again.
 
  • Like
Reactions: 357magnum
I may have an answer to that R100 (or R101) question...

First, it may help to know if that radios' Delta Tune tracks on TX too - that takes a frequency counter or another radio on SSB mode (USB or LSB - don't matter) just key that 29 into a dummy load and tune the other radio to hear the channel your TX-ing on in a SSB mode - tune to zero beat the carrier on the RX side of the LISTENING radio - then turn the Delta tune on the 29' - does it change what you're hearing on the LISTENING radio? Then that diode is shooting power to the Delta Tune circuit - be sure of the parts identification when you do this - fo you can really goof up that radio if you undo the wrong parts...


The TX side, that TR21 - the 2SA733 (PNP) uses a Resistor of 2.2 to 1.0 ohm R101 above) - as a BUFFER to SEND power the Mic Amp's Collector leg when you're NOT transmitting.

The Keying circuit Grounds to Pin 3 - which turns on TR21 to power the TX mixer and the TX side and tell the PLL we're in TX mode Thru D15 - USING GROUND of Pin 3.

So when the radio is in idle - RX mode, poking along - D15 is reverse biased, Pin 3 on the Mic Jack has a small charge on it - which then appears on the Collector Leg of the Mic amp.

Why? To keep the Mic amp quiet so any noise it may have doesn't get injected, summed - into the Audio Amp chip thru R49 (RX audio on ST), R48 mess that takes the RX side or TX Mic Amp audio into the Audio amp.

How does it keep it quiet? Potential difference. A transistor is always on if it's forward biased. In this radio - it pretty much is all the time (CB PA switch) - so to pinch it off, or to quiet it, you can apply a smaller trickle charge on the opposite leg of the amp in question - reducing it's gain - or turning it off and keep it's noise from adding to the ambient the receiver is trying to get outside thru the antenna.

The Diode bypasses a cap delay - the TX circuit uses a capacitor to help in the transition of off state to on state and back off again (TX-To-RX) to keep "chirps" and other noises from appearing as you key up - it is for the effort to allow the radios' PLL to shift gears and apply 455kHz rise in the PLL frequency to mix with the 10.240 at the TX mixer when you TX (pin 3 to ground) and time to turn off, PLL shifts back down 455kHz for RX (Pin 4 grounds Speaker) and R101/R100 are for stability of the switching circuit and TR21 (buffers Base lead).

IF you put a diode in there, you force the line low from TR21 quicker - so you key up faster (relative term) but if the diode prevents the trickle charge to return and turn off that Audio line from your microphone - the Mic amp may inject noise from itself and any ambient noise it can get on the AMC side (limiter or not - it's still there acting like an antenna) or even from the Mic Jack and Mic line itself from the handset - even shielded - it can pick up and send thru noise from your RFI noisy cab or compartment you're using the radio in.

So in a way, the Diode "dumps" charge to allow the radio to key up without delay - but in doing do, puts the RX side in a potential noise loop from it's own internal noise and birdies than can form because the Mic amp never truly turned off - its' ready to amplify any noise present.

You also may find the Mic amp PULSES from the initial switchover - the cap in the circuit needs to stabilize so it acts like a dead short attempting to obtain it - R100/!01 the trickle charge it provides keeps the cap topped off and you don't wind up with a surging audio POP you get when you use that diode.

This is why some older cobras squeal in RX when volume gets turned up too high. The Collector and it's support circuitry has failed in some way, usually the cap - and even the routing of the 4.7K resistor used to hold the charge in the cap is not enough to keep the amp off - so it takes in all the extra Audio power that is supposed to stay out of it - from the AMC Limiter "feeder" line and generates the squeal from that simple loop onto the Mic wire in the handset.

This may have been more than you need to know, but I've spent a lot of time on the 29's and PC-76-78 units and see this a lot - doesn't hurt the radio - but it can make you sound like crap if things start to go south as age or power supply jolts take their toll on these radios.

So it pays to know how that mod works, or what it does, in the radio so you can reconnect the parts right.
Hey Andy, I just thought I’d post some pics of the radio before I start replacing the parts.
Andy, once again, thank you so much for sharing your knowledge. The more knowledge the better. I will go through the radio this evening. Thanks again.
 

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