Cobra 148-GTL issues

For discussions on CB Radio Repair and Troubleshooting, do not post modification requests here, use the Modifications forum !
Post Reply
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Cobra 148-GTL issues

Post by 878ROB »

Radio Make and Model: Cobra 148-GTL
PCB Number: PC - 412
PLL Number: MB8719
Test gear available: None

Ok guys my radio is one from New Zealand so it uses 26Mhz.

How do i change this to run a factory 27Mhz?

Regards: 878ROB
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
User avatar
The Defpom
Site Admin
Site Admin
Posts: 436
Joined: Friday 20th Aug 2004, 18:06
Location: New Zealand
Contact:

Re: Cobra 148-GTL issues

Post by The Defpom »

That is funny, I get people giving me Cobra 148's to convert from 27 to 26MHz, it almost seems a waste to convert one back !

Basically the NZ mod involves changing a crystal (X3) to drop the radio down by 635KHz (from 11.325MHz to 11.1125MHz) , however, once that crystal has been changed, all you need to do to get 27MHz is pull pin 10 of the PLL low (to 0V), that will shift the radio up by 640KHz (5KHz high from the AU band).

If you want a permanent conversion (as I suspect you would) retune the radios Loop Oscillator adjustments down by 5KHz (L22, L23, L59), if they do not stretch that far, then you may need to solder a small ceramic capacitor across the X3 crystal, probably about 5pF or 10pF (the smaller the better) in order to drop the crystal frequency.

That is unless you happen to have a 11.325MHz crystal laying around, so you can swap it back to original (would still need retuning).
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Re: Cobra 148-GTL issues

Post by 878ROB »

as the radio is there is a link from the top of the X3 crystal running to the top of the L22 tuning pot.where would i obtain 1 of these crystals?
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Re: Cobra 148-GTL issues

Post by 878ROB »

Complicated lol
IMG_0086.JPG
(1.29 MiB) Downloaded 182 times
IMG_0085.JPG
IMG_0085.JPG (394.55 KiB) Viewed 4089 times
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
User avatar
The Defpom
Site Admin
Site Admin
Posts: 436
Joined: Friday 20th Aug 2004, 18:06
Location: New Zealand
Contact:

Re: Cobra 148-GTL issues

Post by The Defpom »

Hi,
You would still need to align the radio after the conversion to AU, not only the frequencies, but you would also need to align the TX and RX sections as well, as they will likely be tuned for best performance on NZ.

You would need a 7 digit frequency counter to correctly align the frequencies, if you do not have one you should not attempt it yourself, as you need to correctly align them with a precision of at least 20Hz (for centre clarifier on SSB), personally I tune to within 5Hz on my bench (the accuracy changes with temperature, so it is a little subjective, in a vehicle it could end up being slightly higher or lower in frequency)

The wire on X3 is there to just help the frequency stability, as the casing is then at 0V instead of floating, it does effect the frequency slightly, and also helps to reject interference form other RF sources form getting into the oscillator.

You should probably find a good radio tech, I am sure that there are some good ones around over there, ideally you should use one that is recommended by someone else, don't just go by the often "self important" garbage that some people say on CB making out that they know what they are doing, often that is not the case at all, that is why I say find someone that is recommended to reduce the chance of making a mistake.

I have some 11.325MHz crystals here if you get really stuck (would happily swap for the one in your radio now, at least I could use the 11.1125MHz)
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Re: Cobra 148-GTL issues

Post by 878ROB »

I have no worries with that, i will print off this page and take it to my local tech and see what he says, as for the X3 crystal i will happily send it over to you. I will have no use for it so i will send it to you.
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Re: Cobra 148-GTL issues

Post by 878ROB »

infact ill take the crystal out now and then i will post it monday for you. send me your address in a pm and ill send you mine back so we can make the swap over.
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
User avatar
878ROB
Dead Key
Dead Key
Posts: 12
Joined: Saturday 5th Jun 2010, 17:51
First Name: Robbie
Location: Brisbane
Contact:

Re: Cobra 148-GTL issues

Post by 878ROB »

Our conversation will be printed out and i will take the radio to my local tech once the new crystal has arrived. Thanks Scott greatly appreciate your kind help.

Regards Rob
Regards: 878ROB

President Washington 18ch with DIGI-SCAN Frequency selector.
Power Desk Mic
Station Master Antenna

Cobra 148-GTL

878 Romeo Oscar Bravo bleeding through a TV near you!
Post Reply