Cobra 138XLR Audio Problem ?

[Handy Andy]

@Switch Kit - you realize R90 is a jumper - but on the bottom side of the board?

It is my fear that the pot's output modded (if it is done intentionally by someone else) this way, puts the dynamike control in a precarious position.

Until someone else can produce another photo of their board - the jumper BELOW - and the jumper above - scenario - is a big red flag to me.

 

[TM86]

Hopefully this will suffice.

 

[TM86]

FYI - Also, not all Cobra 4 pins are created equal. According to the OM, the 138XLR uses Relay PTT and the 139XLR uses Electronic PTT.

The 139XLR mic is wired like an electronically switched radio, but the radio itself is definitely relay switched. For more info see the schematic from here:

Cobra 139 XLR Schematic

Cobra 139 XLR Schematic

www.worldwidedx.com

If you look on the radio side of the mike connector you'll see pin 4 is hard wired to pin 1.

No doubt Cobra was either gearing up to the make the switch (no pun intended) to electronic keying, or just wanted to sow as much confusion as possible with very little effort. If the latter, it's working 45 + years later.

 

[Handy Andy]

Ok, I see a jumper in the R90 ABOVE board, while VR6 is soldered to what I believe are the Dynamike wires - so that means that the Dynamike pot is wired wrong - or as another has stated earlier, repurposed for another use.

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But does not explain the "sudden increase" when the pot is turned from down all the way to just off it's stop.

So that then tells me the pot wiper that is supposed to make it variable - has an open spot - not from the composition - it shows 10K across the fixed portion but the spot can be from a "detent" that shows up from lack of turning and use - so the control just has to be used a bit more to see if the indentation formed from being left there in one spot on the composition - can be glossed or burnished over from the carbon composition left on the surface that the wiper can use - in hopes it can transfer some carbon back to the detent.

You can clean the pot and take care of a lot of the "scratchy" effects, but a type of varnish that forms from lack of use, done by the way it was stored and then pulled out and back into service - the composition is old, and many of today's cleaners are way too aggressive in cleaning it off - they can remove the carbon in the composition too.

So you have to remeasure the pot via the wiper arm and then review what the pot is / was rewired for and work accordingly. It simply may be from resoldering the pot onto the wrong pads or the wiper pad is transposed to another pad.

Thanks for and to @TM86 for the help!

 

[TM86]

Thanks for and to @TM86 for the help!

Just doing my part to keep those old radios on the air and out of the landfill.

 

[Dr_DX]

If you look on the radio side of the mike connector you'll see pin 4 is hard wired to pin 1.

No doubt Cobra was either gearing up to the make the switch (no pun intended) to electronic keying, or just wanted to sow as much confusion as possible with very little effort. If the latter, it's working 45 + years later.

Correct. I think we are both right. The 858 SSB chassis uses a mechanical relay to switch RX & TX.

Technically, Electronic switching involves grounding out the TX line where Relay switching involves connecting the RX & TX wires together - ground isn't necessarily involved. It works in this case because in both radios pin 1 & 4 are connected internally.

I just feel sorry for the person that picks up an old 138XLR stock mic and tries to use it on a modern 4 pin Cobra radio...or their Stryker SR955HPC.

 

[Dr_DX]

Hopefully this will suffice.
View attachment 62218

Looky there Earl, it is the same as the schematic....

I bet that someone replaced R90 with that jumper - I know I would. You lose a little linearity but it allows the Dynamike to turn off the mic audio, not just turn it down. It is also bent where all the other "factory" jumpers are straight as an arrow. And we know someone has been in TM86's radio - the diode on the other side of VR6 has been clipped.

 

[Holydvr]

I can take any pics needed of my radio. I put a sticker on the top of it noting the issue. It does not behave like the other four of the same model I have. I had been meaning to set it side by side with two others I have that “work correctly” but plumbing around the house got in the way for a bit. If the pics are still of interest, let me know. Bad part is I will not be home until Sunday morning. May be able to get pics Sunday evening or so.