Cobra 148GTL This issue has me stumped

[Handy Andy]

At 1:49 in your latest vid - did you notice the "offset" voltage change on the scope screen - you had to re-center the wave. What are the details of this - did you have to go back and adjust - why? Was it a centering error or did I just see a bias "glitch".

Go back to the 0:38 second mark of the vid for your 27.965 read, AM mode - I see your wavform out of lock - but I also see a "sag" motion with this. Not quite sure how to interpret it, but I still think you have a voltage "sag" issue in and around the two modes of AM and USB - as if there is a "Fault" caused by poor contacts.

Ok, the one area you had the "offset" power issues is fed directly by the 8 volt constant from the MB3756.

The Coils associated with the tripler L21 output - are off of the S403 switch all tied into that area.

Again, a power supply difference...

I'll review more - right now I've got some chores to do...
:+> Andy <+:

 

[Robalo]

Look what i just found. Next to R238(10k) there is R85(560k) and R78 (10k) i hope this is it. Happy Father day !!!!!!!!!

 

[Robalo]

thank you Handy Andy for the help on the cobra 2000 gtl with PC-497AB the radio never works on upper 40/40 or from 24.455 to 28.045 after too many hours of work, parts cleaning alignment don't get me wrong but the radio rocks on normal 40 channel and one of my local from channel 17 offered me 40.00 dollars for it and i sold it. thanks Andy i learn my lesson on broadbanding issues and also want to said sorry to Tolteca for jack his treat. happy dx 73's

 

[Low_Boy]

Hijacked, derailed, I'm confused.

 

[Handy Andy]

He worked on an expansion trick - sorry to see it didn't work.

Not sure of the kit used or what was done - but at least he found a failed cap that affects the detected audio - from the conversion process for SSB.

Just confused as to what was done to "expand" the channels - it appeared to work but something else wasn't right so until I know more - can't really say if it was a hijack or not...

But this goes back to another thread dealing with the "pump" charger on the 8719 that makes the PLL accelerate "frequency rises" or decelerate "frequency slows" and R95 a 1.5K that worked on one of the two inverters that are essentially designed to be free running "clockers" and between these two the radio determined if it had to go up in frequency (add a pulse "a charge - hence the term CHARGE PUMP" from internal clock - making the Varactor respond in one way) or slow down (Remove or sink some current from that loop) to make the Varactor respond the opposite. (from IC2) and with all the stuff going on I got confused too - one problem (here) then two and I had to sort it all out - whew...

Anyways...
When you remove the R95, you take away power from the clocks - those inverters - so when the PLL senses it needs to change the Varactors' response - that lack of power didn't help and puts the PLL in out of lock mode.

It needed two "clocks" running at a different speed so it can look to see if the one clock was off - which helped determine the way the PLL had to go to make the clocks "phase" in step again - by looking at not just that, it was off, but by phase detecting the status of the inverters - it can see if the loop was too fast or too slow and "step" accordingly.

Whew...again...
:+> Andy <+:

 

[nomadradio]

Quick test. Do any of the three 'clarifier center' coils have an effect on more than just one mode? Each one should change the channel frequency ONLY on one mode, and NOT the other two. A slug that causes frequency change when turned in more than just one mode says shorted switching diodes. D32, D33 and D34 would be worth testing first when this fault arises.

Not the only possible root cause, but the first ones to check.

Oh, and the schematic shows a 11.3258 crystal in this model. Finding the 11.1125 crystal in the radio suggests the "six-position switch" channel mod has been done. Not my favorite setup. Found it to be sensitive to temperature changes.

73