Cobra 2000 GTL Mic Gain Issues

[Raccoon]

I have an old Cobra 2000 GTL that means a lot to me . It was once my grandmothers , and I inherited it after she passed a few years ago . It sat for a few years before she passed , and then it sat for a few more years before I inherited it . I did run it for awhile without any issues , but then all of a sudden it started acting up , so I sent it to the local "Radio Doctor" and he took care of it's issues . But now it's mic gain is acting up . At around the 9 or 10 o'clock position it seems to be fine , but my audio is obviously low ... so when I start taking it up higher to around the 11 or 12 position , it gets garbled sounding , and not understandable . Which mostly happens on the words that have a bit more punch to them when pronounced . I tried researching it online , but I haven't been able to find anything anywhere with anyone that might have had some sort of a similar issue .

Maybe it's a dirty , or bad pot ... it may even possibly be the mic . It's a new mic , but it isn't a higher end mic like an Astatic or a Turner . Could it possibly be something a little more involved ? I can generally work on my own equipment . But if it's something more involved that requires test equipment ... than it's back to the radio doctor for it .... lol

I'm just wondering if anyone might have any ideas as to what might be going on .

 

[nomadradio]

The meter lights aren't pulsating in time with your transmit audio, are they?

This radio sounds as if it's 40 or more years old. Electrolytic capacitors begin to go bad from age alone around the 20 or 30 year mark.

Mileage affects how many and how soon these parts go bad. Sounds as if this radio has a lot of miles on it.

Replacing one or two at a time as they break down and cause trouble is what we call "electronic whack-a-mole". If your tech replaced only a few of these that were creating trouble, this is probably what's going on. One or more additional caps has failed.

Replacing all of them at once sounds good, but each part creates the opportunity for error. A tiny bridge of unwanted solder across two foil pads that should be separate, or a capacitor inserted backwards are among the 'introduced' faults that can emerge after "re-capping" a radio. It's best done one part at a time, making sure to note which way the part was oriented before removal. Probably a slower process that way, but less prone to error.

These capacitors serve not only to filter hum and noise from the power that branches off and feeds all through the radio, they serve to prevent signals in one circuit from 'leaking' into another one where that signal doesn't belong. This is called "bypassing" or "decoupling". The bulk of all those electrolytic capacitors have that job. Wacky feedback or noise troubles just sound like one or more failed electrolytic caps.

73

 

[Crawdad]

I have an old Cobra 2000 GTL that means a lot to me . It was once my grandmothers

How cool is that ?

 

[Klondike Mike]

This capacitor kit will get you started in the right direction;

https://klondikemikescapkits.com/products/cobra-2000-gtl-electrolytic-capacitor-kit-deluxe

 

[Raccoon]

This capacitor kit will get you started in the right direction;

https://klondikemikescapkits.com/products/cobra-2000-gtl-electrolytic-capacitor-kit-deluxe

I've already got that kit Mike . In fact I ordered it from you .

 

[Handy Andy]

I have several 'theories" but require you to measure the pots' resistance thru it's range.

It's when you said you go up past 11 o'clock it gets garbled and not understandable.

So here' goes what my thoughts are...

Firstly - the Mic (Dynamike) potentiometer works in a specific way so it "buffers" the input - Pin 1 is VR405's "Audio In" wire - whatever color yours may have - goes to one of the "constants" that Dynamike uses as the resistive element.

The Wiper side of this - heads off to the Main board by C108 (Right side Opposite side away from PLL - towards front)

The wire from the MIc jack to the POT is NOT shielded, but the Wire from the MIC Jack FROM the Pot is. (That is if it's OEM.)

The way it's wired, uses COMMON shield - Pin 2 - so if you're running a 5-wire to a 4-wire converter, You can damage the pot from power shooting thru it to ground.

OR, of at any time during it's life had ever run 5 - pin to 4 - pin conversions - the POT - being a 1K - can blow OPEN condition like a fuse. if they used Pin 4 to do everything - tied to ground. OR if you had an internal "echo or noise toy" - this can blow the pot too - trying to send power and audio thru the line into the board at C108.

You can DVM Pin 1 and Pin 2 of the MIC jack and rotate the Dynamike knob

​

- Since you seem to have variable - up to a point

• - turn the Dynamike all the way down
• - test Pin 1 to Pin 2 of the Mic jack thru your DVM - should show a nearly dead short at the DVM's 1X setting
• - then turn it up to your last known "good" setting - again measuring from Pin 1 to Pin 2 on the Jack
  • - you should have an ohmic reading
• - then when you turn past that 11 o'clock position - may go open (O.L.) or flashing 1
  • - turn DVM to 10X setting and re-measure - still O.L or 1's? The pots bad just replace it.