Cobra 29 Constant Key Up

[Chris Lawrence]

my apologies for the statement "everything looks like a shorted transistor".

I should have looked at the voltage chart before guessing, as i would have seen that your voltages look correct for TX, and the problem is that the collector of TR21 is not dropping to 0 volts on RX.

here is a voltage chart that includes IC3, your TX mixer chip.
please check the voltages on all pins of IC3 in both RX and TX and post them here.

I don’t know how to simulate RX mode when the whole problem is that it is stuck on tx. Let me know and I can report back.

TX Mode Voltages: 2.6/1.9./1.1/2.6/0.0/7.6/2.1/5.0/7.7

Note: When I lift the legs to L21 - which are on the same trace as ic3 pin 6 - pin 6 goes to 0.6v it will unkey, however, the red ptt lights stays on.

Does that help?

 

[NZ8N]

Something is "pulling down" the trigger side (Base) of TR21 to put it into TX mode, which also then directly feeds the resistor in series with the RED LED, turning it on. Just enough.
Shorted pin 3 in the Mic socket?


Follow the Red lines, these are directly to the keyed side of the TX Switch. If any of these are shorted, there it is. You want to look for the lower resistances (under 1k ??), not the higher ones. Higher resistances won't have enough "pull" to pull down the TX trigger side of the TX Switch.
Are you sure you put TR21 back in the correct way?

#1 - Check C121 along the right front of the board, this is where the PTT line goes from the Mic socket to the board.
#2 - Check the pins of the Delta Tune control potentiometer in the face. Make sure the one side terminal is not shorted to case somehow. This is fed +8v RX from the TX Switch (when unkeyed, feeds half of the RX circuits)
#3 - Double check C117, also check C118 in front of it (disc cap). You could try taking C117 out, it won't hurt just for test.
---- Continuing -----
#4 - Pull one leg of R15 (feeds RF Preamp)
#5 - Pull one leg of R18 (feeds FET1 1st RX mixer)
#6 - Pull one leg of R32 (feeds Base of 2nd IF TR10)
#7 - Pull one leg of R65 by Mic preamp

I doubt it is this, but D15 goes from the Trigger side of TR21 to the PLL to put it into it's TX mode.

 

[Chris Lawrence]

Something is "pulling down" the trigger side (Base) of TR21 to put it into TX mode, which also then directly feeds the resistor in series with the RED LED, turning it on. Just enough.
Shorted pin 3 in the Mic socket?

View attachment 59179
Follow the Red lines, these are directly to the keyed side of the TX Switch. If any of these are shorted, there it is. You want to look for the lower resistances (under 1k ??)

I’ll follow through with your suggestions. Last week I pulled the leg of every resistor in the red lines after your previous suggestions. I’ll get back to work on it thanks for the response.

 

[Chris Lawrence]

C121

 

[NZ8N]

See? It was something simple. Glad you found it. Just needed to trace it back, sometimes 1 piece by 1 piece. It could have been any of the above, even the shorted mic socket or the Delta Tune terminal.

 

[Chris Lawrence]

See? It was something simple. Glad you found it. Just needed to trace it back, sometimes 1 piece by 1 piece. It could have been any of the above, even the shorted mic socket or the Delta Tune terminal.

Yeah! Thanks for the help. I appreciate it.

 

[loosecannon]

Awesome!

i have to admit that a shorted ceramic disc cap was not high on my list of suspects.

LC

 

[nomadradio]

Congratulations for sticking with it!

73

 

[Chris Lawrence]

Congratulations for sticking with it!

73

Thanks for the help

 

[Eldorado828]

Congratulations, that's good stuff!