Distorted modulation on CB

Good evening all. Just wondering if anyone can point me in the right direction. I have a Cobra 148GTL-DX on a Silver Rod. There seems to be a lot of distortion on modulation and RX isnt much better. I has had the audio chip replaced. Not sure if this is just an old set and tired or there is a real fault, would really love some pointers on whether to find a fix or scrap. Cheers all, 73s

Was there no sound at all till the audio chip was changed.
How long have you had the radio.
Did it just go funny ?.

Hi Michael,

The Cobra 148 must be one of the most 'diddled' with and pontificated about radios in CB history!

You'll find all sorts of nonsense floating about surrounding the circuitry, design and operation of them and usually they've been 'modded' to hell and back or all sorts of unsuccessful repairs performed on them.

In the 'old' days of earlier 19-clickety-click very few had gone through such rigorous 'modding' and most garbled SSB transmission problems stemmed back to poor power supplies or RF coming back down the antenna system basically using it (and you!) as an antenna ground plane. 

This usually came with reports of how it works fine in the car (on a 12 volt battery system that can supply umpteen amperes) but doesn't hooked up to the cheapy, few amps, from a CB shop or 'good' mate cheap, rubbish power supply at home....

The RF 'feedback' problem (which won't always show with just a damn SWR meter) was either never resolved or swapping antennas and some 'decent' coax bunged on for good measure "fixed it"..... usually with lengths of 'multiple half waves' coax as suggested by your friendly neighbourhood CB shop making the coax a better 'groundplane' or counterpoise than it was first time around(!)

(Nowadays there's a lot more understanding regards such matters as well as common mode currents.)

Of course there was the good old 'power mic' problems too.....overdriven audio stages on the CB or the internal electronics of such a microphone 'amplifying' the RF feedback problem to the umpteenth degree.


I could as has been done on many occasions provide you with a total 'stab-in-the-dark' guess solution some of which have gone down in CB mythological history such as the "Yeah mate, needs re-capping that does".......that one never gets 'old' apparently.

On the Cobra these 'suggestions' will often revolve around the electrolytic capacitors surrounding the audio chip. Often cited as the "But they're just 10 volt types" never knowing the basic electronic design of audio DC isolation or frequency feedback so will never, ever see the power supply voltage on them during normal operations. (A rare exception noted further down.)

You say that your audio chip has been 'replaced' and I hope with a decent version rather than the el-cheapo Chinese copies that spew from their semiconductor plants. (Mostly 'Asian' actually with the Philippines, Taiwan and Vietnamese plants often incorrectly 'copying' the semiconductor mask or PN diffusion processes!)

If it was then the many guesses at changing resistors or capacitors in it's surrounding circuitry may actually help its poor operation.

If the original chip blew short taking out the power lead fuse then and only then might it be worth checking those '10 Volt caps'.
(How many times has the "yeah, that'll do" bigger amps fuse or silver foil 'temporary' fuse wrap become a permanent fixture?)

That's why 'myths' persist....with a grain of truth or exceptional circumstances.


My best 'guess' for you (as in not having the radio in front of me) is to check that the soldering of the replacement audio chip doesn't resemble some lead lined robotic/terminator seagull/pigeon poop monstrosity. Or that the PCB copper tracks haven't lifted from using Dad's old 1960's plumbers soldering iron. 

Or probably the old example where one of those PCB chassis screws tumbled under the table, just out of reach, killing my back so we won't bother or 'mysteriously vanished' cases and never replaced. On some radios the chassis provided ground continuity for the main PCB!!


Get it repaired or bung it on eBay with the 'spares or repairs, no returns' nonsense that abound nowadays. I'm still amazed at the silly prices paid for such items.


Probably not a lot of help and I am known to 'waffle' but there you go.

I could have just left it with the "Yeah, needs re-capping......bung it on the bay".


All the best,
Victor

Cheers Victor, lots of info there and just to come back on a few. This one has not been modded or tampered with since new (to my knowledge) the audio chip was replace by a very reputable repair shop in town who has been in business for many years and has all the proper kit. Power supply is a mercury power unit so not a cheapo which I avoid at all times. I have not opened it and even if I di I would not know what I am looking for if I tried. I would guess its at its life end and time to retire (like I did some years back  ) I wont try selling it to some poor sap but will ask around if someone wants it for spares. Thanks for all the info and much food for thought bud.

Not had it long Alan, seems there was no sound at all and chip was then replace by good technician.

No worries there Michael.

The 'copied' chips can be difficult to tell from OEM stuff and even I've been caught out with them before now. (I used to work in the semiconductor industry and you wouldn't believe some of the junk made out there!)

If this radio was working well before but started playing up since it'd be worth checking out your antennas connections in case there's any problems with RF coming back affecting the radio. If it was always like that it may be worth seeing if you can hook it up to someone else's antenna setup or maybe a mobile setup and see if it stills plays up. If it works out OK on another setup then the old 'coaxial choke' might be an option for your Silver Rod.

Otherwise I congratulate you on your ethical stance of not wanting to offload it to some 'poor sap'.
(Others wouldn't be so honest or claim the 'no returns' covers them.)

I wish you well with it all and hope you get sorted out. (Even if you land up buying a shiny new rig. )


All the best to you,
Victor

New pm at the top.

Hello Michael,

There are very few radios worth trying to save when they've come to the end of their lives but the Cobra 148GTL-DX is a very well respected radio and can command some astonishing prices on the auction websites.


If it's in good otherwise good condition and you can rule out stray RF coming back down you coax I would seriously look at having it sorted out.


Best of luck, 


73's Gary.


C.T. Admin.

Cheers Gary.

It may be the RF. Do you have any spare coax in the cable going to the antenna? If so get a 2 litre pop bottle and wrap 5 turns around it to make a coil, doesn't matter if you're using RG213 or RG58, 5 turns for either. That'll act as a choke. If that cures the garbled audio then common mode RFI is the issue. The problem with that is really it needs sorting at the antenna end of the coax, either by creating that same 110mm diameter 5 turn coil out of coax or, better still, fitting a RF choke inline. Don't bother with clip on ferrites, you'll need lots of them unless you get a very big one and wrap the coax through it several times and you'll need a Mix 43 or ideaally 61 one. Different mixes perform better at different frequency ranges.