Cobra 148 GTL Taiwan alignment problem / question

[Low_Boy]

I have raised another Cobra 148 GTL from the dead, but I am now having a alignment problem. The radio has no clarifier mods that I can see and all wires appear to be in the right places. L23, L59 and L22 do not have enough adjustment and the cores are bottomed out.

AM, L23 is supposed to be 34.7650 MHz and I can only get 34.7612 MHz

USB is supposed to be 34.7665 MHz and I can only get 34.7614 MHz.

LSB is supposed to be 34.7635 MHz and I can only get 34.7610 MHz.

I have not checked many parts yet and not sure if it may be a common part doing this. R146 tested good.
I do not think that L24, 25, and L26 could have all went bad at once but that is the next place I am going to check. Does anyone have any ideas of what to look for on this kind of problem?

 

[Robb]

What are the voltages at out of the regulator? Stable? What about VCO voltage?

Did you remove all old glue from components yet?

 

[Low_Boy]

What are the voltages at out of the regulator? Stable? What about VCO voltage?

Did you remove all old glue from components yet?

Robb pretty much done for today. 4AM comes pretty quick. VCO voltage was set to Spec. 3.2V

I believe. I will check all regulators tomorrow. Puts out good power AM and SSB. And does receive well. I now just have to get it to transmit and receive on the right frequency. LOL.
I will keep you posted thanks.
Glue removal was going to happen last.

 

[Dmans]

Low_Boy,
I would check values of R174 and R175. They are tied to the clarifier circuit and can present the symptoms you are experiencing if someone has lowered the values in an attempt to expand clarifier range. Some clarifier modifications "floating around" suggest removing these resistors and installing a jumper wire in their place.

And as Robb said, 8v regulator issues will look the same to the L22, L23 and L59.

73's
David

 

[kopcicle]

I know you won't see this for a bit but...
I'm not a fan of wires hanging out of the radio during final tuneup.
Loading the oscillator and or mixer/buffer is way to easy to accomplish.
With a known good receiver into headphones, put a 1000Hz tone into USB or LSB into the radio under ttest and count up or down accordingly. I wouldn't trust a probe to NOT load the circuit. For AM, fsckit, get close

PM if you still can't get there.

 

[Low_Boy]

Low_Boy,
I would check values of R174 and R175. They are tied to the clarifier circuit and can present the symptoms you are experiencing if someone has lowered the values in an attempt to expand clarifier range. Some clarifier modifications "floating around" suggest removing these resistors and installing a jumper wire in their place.

And as Robb said, 8v regulator issues will look the same to the L22, L23 and L59.

73's
David

Thanks. I did notice someone changed the value of R144. I do not know if it was a attempt to fix this problem. It must have been changed years ago because I have not seen a new resistor with leads that thick.Thanks I will post my findings.

 

[Handy Andy]

Another way to look at this - is if they tried to REVERSE what they did but got it wrong.

There may be a problem or two with the TX side - you may have RX ok, but TX pot - if its' not powered, but they put a bad diode or soldered a blob across one shorting it - can cause "Clarifier" to work "low" - because the voltage gets pulled down from the Clarifier pot, and the TX trim pot -AT THE SAME TIME if they forgot to hook this thing back up to stock.

Why I say that? 3.2V is a bit low - should be between 4.5 to 5.5 volts off the clarifier to the tuning stuff.

Now, if they tried to tune it out using the tank coil cans to offset the effort - well - you do have some wok on your hands.

 

[Low_Boy]

I just went back and double checked. My Sams says the VCO is set at 3.2V. I can always raise it and see what happens.

 

[Dmans]

Low_Boy,
Changing the VCO voltage won't get you any closer to aligning the local oscillator. It will have a dramatic affect on the PLL's ability to lock in a channel since the VCO voltage is used in a massive calculation to find/define the "N code" used to generate the frequency which must be split/divided/tripled/added to/subtracted from to arrive at 26.965 when on channel 1.

I believe Handy Andy was referencing clarifier voltage in his above post.

(Disclaimer. I use the term "massive calculation" when it requires me to remove my shoes and use my little piggies to count that high )

73's
David

 

[Handy Andy]

While you're at it...

Go back and verify the wiring of the Pot is correct.

And be careful, I've attached a PDF from Euro Radio from the CB Tricks website.



The CLARIFIER (Voicelock) SPLITS and drives two sections both have a varactor - so if one failed, or if the pot is not wired right - can be your symptom.



The pot can get accidently wired where the leads show variable - but the voltage never truly goes thru the range of adjustment - an assembly issue.

 

[Handy Andy]

Low_Boy,
I believe Handy Andy was referencing clarifier voltage in his above post.

(Disclaimer. I use the term "massive calculation" when it requires me to remove my shoes and use my little piggies to count that high )

73's
David

SIGH - thank you are correct - CLARIFIER voltage can skew the results - I got caught at that before...

Thank you

 

[Robalo]

remove and check x3 (11.325 mhz )

 

[Low_Boy]

Hopefully I get home in time to do more than eat and go to bed. If so I will start checking tonight.
Thanks All.

 

[loosecannon]

im with Robalo.

try a different crystal and see what changes.

you can also experiment with different aftermarket varactor diodes like the mv2109 or the NTE612 etc...

different varactors will raise or lower the frequency a bit and can help you out of a jam.

When i am trying to get a clarifier to work in these chassis where there is the same amount of slide down as there is up, these methods help.

by messing with R174, R175, the 11mhz crystal, and the varactor, you can make these clarifiers do what you want.

this is for those radios that aren't fortunate enough to have the big 17khz slide crystal in them.
LC

 

[Low_Boy]

Thanks LC. I think this may have to wait until the weekend. Just seems my days are getting longer and I keep getting home later. Never enough time to get back to this during the present week.
Thanks