The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Radio Make and Model: Cobra 148 GTL DX
PCB Number: 010AA
PLL Number:
Test gear available: Digital VOM, Stand Alone Counter,

Symptoms: Audio on SSB is distorted (like I'm gargling with gravel)

What is working: Everything else is fine.

What has been tried: Checked biasing, checked offset adjustments, checked ALC setting.

Did it suddenly happen ?: Sometime's it's okay for a few minutes then it all goes Pete Tong.

Anything else that may be of help?: PSU delivers adequate ampage, power leads have been uprated, only using standard dynamic mic.

Any assistance would be greatly appreciated.

Cheers

Steve

Check the screws that hold the main board to the chassis to make sure they are tight.
Dave.

Checked screws - all tight & solder tabs all intact.

Noticed that the problem seems to occur on Hi Band setting only (especially noticable when the 'bleep' goes off - it is distinctly 'bubbly' and not a stable tone) but I'll need to get this confirmed by another station listening to the audio on Mid band.

Steve

Check the base pins of the driver and final to make sure they are not over 0.7v with a dead key on SSB and the mike gain off.

Dave

Checked driver & final base (mike gain fully anticlockwise on SSB) & I get 0.73v & 0.69v on TX

Steve

perhaps you might want to "juggle" the vco can. Slightly tune it on the band that you're having problems with.

0.69v is fine but the .73 needs correcting it's too high.
When you have corrected it check the voltage at the VCO's test point TP2, (that's the lead of R126 behind pin 7 of the PLL) on the highest channel and band, as Ray said above, you might have to slightly adjust the VCO.

Dave/Ray

I checked TP2 voltage on Hi band (Ch40 AM mode) & it was 7.4v

I adjusted L18 for 5.4v (Hi Band CH20 AM Mode) & base voltage has dropped to 7.1v

Re-checked final & driver bias (both within spec at 51ma)

Hope I'm going in the right direction???

Sounds okay on my other receiver (but I've had a Newcastle Brown!)

I'll try to get a radio report from another station tomorrow & post the results.

Cheers

Steve