Cobra 1000 warble sound on transmit @ full power

[Lkaskel]

Hi Everyone,
I have a Cobra 1000 on the bench that has a warble sound when transmitting at full power. During the alignment, if I back the power down to under 2 watts the carrier is clear and looks perfect on the scope. When I adjust L13 to spec power the carrier gets very fast warble/slight hum that increases as I get to 4 watts. I have the mic gain at 0 and the AMC (VR6 at minimum so I do not believe that it coming from audio feedback. The driver and final are a 2028 and 2029 (as shown in schematic) but the heat sink compound is a mess so I assume that someone had them out at some point. Any thoughts? Is it possible that the final is breaking down?

Thanks!!!

 

[1iwilly]

slight hum that increases as I get to 4 watts gives me the impression power supply have you tried on an external power supply ??

 

[nomadradio]

slight hum that increases as I get to 4 watts gives me the impression power supply have you tried on an external power supply ??

It's the first thing I would try.

If it still does this on the external supply, it's time to start replacing 40-plus year old electrolytic caps. This is the kind of symptom they'll cause when they finally get old and croak.

73

 

[Lkaskel]

Ok, so, I missed a few details.
- Aligning on an external supply. I tried on both a switching and linear. The internal supply will only go up to 12.7 vdc. I need to replace the regulator in it.
- All caps replaced with Nichicon.

 

[nomadradio]

If it's visible on a 'scope, it's a feedback problem of some kind.

Loose ground screw at the edge of the pc board?

And if you can't see it on a 'scope, use a radio that has FM to listen for it. If it's louder on FM than AM receive this would point to the PLL section.

73

 

[Handy Andy]

The worse case scenario?

Bad power supply, but warble usually indicates a heavy current draw, so if the Cobra 1000 is using a simple Regulator for it's PLL, so does the PLL vary too?

Check TR18 - a simple DVM DC and AC check (AC for that hum will show as a ripple voltage) then Zeners for the PLL also (PIN7/11 UPD2816) - but DO NOT RULE OUT a "stupid mod" done to the DELTA TUNE pot - which can be the reason for this whole mess in the first place...

 

[Lkaskel]

Hi Everyone,
Well, another one done. This one was interesting!!! The issue came from the dreaded brown glue. There was a resistor and a coil next to L16 that had the glue all over them and they were conducting. Some clean up and a replacement resistor and it's working.

Regarding the power supply, the 220 ohm resistor on the PS board was cooked. It was way out of tolerance. I replaced that resistor as well and the PS is working perfectly. I bet that the previous owner had this cranked up and stressed out the PS.

Oh Well,
On to the next one.