Cobra 148 GTL Oldy frequency drift

Willo · Post by **Willo** » Tuesday 13th Mar 2007, 18:42

Hi there I have a Cobra 148 GTL old model with the MB8719 chip.My problem with this radio is when she is cold it Tx's on frequency but as it warms up the TX slowly slides off freq.It only about 1kc but its really bugging me. The radio is stock and has a standard clarifier. Could someone help me in make it stable.It's only TX,RX is fine doesn't move.
Thanks Glenn

Willo · Post by **Willo** » Wednesday 14th Mar 2007, 13:55

Just an update the RX is slowly drifting now....It like this: Turn the radio on when its cold and when I TX to another radio I'm slightly off frequency(about 11 o'clock on the receive radio) after a while it drifts back to 12 o'clock on the receive radio. The RX seems fine but ofter a while it too drifts off centre clarifier to about 1 o'clock ( so when cold 12 o'clock centre I have everyone clarified.After some time I have to put on at about 1 o'clock to have everyone clarified.) Hope you follow.

Willo · Post by **Willo** » Wednesday 14th Mar 2007, 23:05

It is a PC-412 board cant find any other number on it.

zodiac · Post by **zodiac** » Thursday 15th Mar 2007, 5:00

Glen,
Has the radio always been like that or is it that you've only just noticed it.
All radios need to warm up before you TX on them and more so on SSB, AM and FM doesn't matter all that much as you'll not notice the difference as much. If the radio stays on frequency once it's warmed up then it sounds like it could do with a re alignment but not until it's fully warmed up.
Dave.

Willo · Post by **Willo** » Thursday 15th Mar 2007, 14:38

Thanks for the info Dave, It hasn't always been like this.It's been in storage for about 10 years.I pull it out and fired it up and it seems to have this problem. When I first used it after storage It TX'ed on freq and slowly went off,so I dragged it off freq a little so now at cold start it is off and slowly comes back on freq.Hope I havn't confused you...appriciate the help very much. I'm only a hack but absolutely love electronics and CB's are my favourite as I have played around with them since I was a lad.But I have enough knowledge to do repairs but limited in fault finding.

Many Thanks Glenn

Uncle Ray · Post by **Uncle Ray** » Thursday 15th Mar 2007, 15:24

Hello there..
First up I would be checking the xtals for "dry joints" then the caps around the vco area. I have had problems with drift BUT after trial and error found a couple of cheap electrolitics leaking.
Try re-soldering the xtals for a start.
Goodluck , I appreciate how anoying it is.
P.S , if no luck , open the clarifier....
Regards. Uncle Ray.

Willo · Post by **Willo** » Thursday 15th Mar 2007, 21:33

Thanks Uncle Ray did that but its still the same.RE: unlocking the clarifier I,m stuck on IC MB3756 cant seem to locate it.It,s says to solder the orange from clarifier to pin #3 on IC MB3756.......Just cant seem to locate th IC . There is a couple that I cant read becaue they are crampt in...Any idea were MB3756 is...Thanks Glenn

RedEye · Post by **RedEye** » Saturday 17th Mar 2007, 14:10

Here's an easy clarifier mod.... Directions are very clear and it even shows different versions (Clarifier wire color codes) It states that it's for ST models but it lists all 148's
http://www.cbtricks.com/members/cblinco ... racker.gif

Unless you have a freq counter attached, I also suggest robbing a "self centering" voicelock knob from a cobra 29... It's makes it so much easier to find "center" Remember to align the radio with the switch in center position.
I never understood why cobra used those switches on the AM model 29 and not the 148....
Go figure!

Most 148's do drift when going cold to warm, Just align after a 15~20 minute warm-up

Hope some of this helps!
73's

Willo · Post by **Willo** » Saturday 17th Mar 2007, 17:56

Thanks Redeye I appriciate your feedback very much....and diagram

Rick · Post by **Rick** » Friday 23rd Mar 2007, 10:14

If it is a PC-412 PCB Check caps C212, C213 and C231 all known to give this fault.

Rick.

Willo · Post by **Willo** » Saturday 31st Mar 2007, 23:20

Thanks Rick been away for a few days,will try replacing them,Thanks very much for your help.

Willo · Post by **Willo** » Thursday 12th Apr 2007, 14:21

FIXEDYou bloke are tops....The old girl is stable....BUT...She is a little off Frequency on AM, can someone tell me which pot will bring me back on freq ??Its only slightly off, just anough to have distorted modulation around the edges...

Thanks to all you guys who helped my.

Rick · Post by **Rick** » Friday 13th Apr 2007, 8:17

L23 is the AM coil.

Rick.

Willo · Post by **Willo** » Saturday 14th Apr 2007, 14:43

Thanks Rick......and thanks for walking me through my problem.....It's hard to find help for CB in Australia as it has phased out a bit over here but there still are a heap of us that love the old radios.......Cheers buddy...

Thanks: Rick Redeye Ray Dave.

Glenn

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Cobra 148 GTL Oldy frequency drift

Cobra 148 GTL Oldy frequency drift

Re: Cobra 148 GTL Oldy frequency drift

Re: Cobra 148 GTL Oldy frequency drift