Cobra 25 WX NW ST,, Audio Amp TA7222AP,, 2 Black short wires ???

[baconater357]

Hello, on the Cobra 25 WN,NW,ST on the Audio Chip Amp,,where it is soldered, there are 2 short Black wires added to the solder, and the wires go to the Audio Chip connections, does anyone have a pictire or can you explain where these wire go to and from. They got remove on a re-solder, and unknown where they go, cannot find a schematic for the life of me. Can any one post a pic if possible, and or explain where to and from the wires go. Thanks you in advance...If no idea on this , the radio will be used for parts, or the land fill. Thanks,,,

 

[HighPlainsDrifter$]

Maybe this can help ... only 1 black tho

 

[sunbulls]

Those wires were included to permit the shortest path possible for better ground isolation. When you look at the printed circuit, notice those solder points in the photo are already grounded without the wires. Those wires were added in later additions. Parts radio, possibly, but not with that particular problem. “Landfill” is a word that is considered sacrilege in the repairman’s dictionary. Think recycle or another man’s treasure.

 

[Handy Andy]

Find the parts that the jumpers go to on the top, you'll see one is a large filter capacitor. This is for the problem with older charging systems - the alternator whine - they figured they needed this to help reduce the ripple when working around the 7222P chip - which is very sensitive to ripple.

Later revisions even soldered a simple disc cap from the ground to the rear tab to reduce that ripple and other mods from the factory include soldering the tap of the 7222 chip with a cap to the side panel chassis as a means to reduce noise.

 

[HighPlainsDrifter$]

You guys got my curiosity up ... This one has been sittin around waiting for me to finish the truck that's going to carry it. The first pictures are a 1998 25 WX ST recently acquired here to ride in the tractor. these are from a 2013 25 WX NW ST (obviously) for the farm truck.Quite a difference from one to another as you've stated.

 

[Handy Andy]

It is a good question and shows a limitation in RF design.

One main reason being ground loops.

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The black wire - CLOSES the loop using a STRANDED wire - more of an inductive loop - follow the route the larger foil trace that ORIGINATES back at the main power feed and power choke - note the larger land of solder applied to the power choke area then typical green enameled traces flowing off from it.

Follow that trace from the front of the radio - backwards, it will arrive to the larger power choke area AS A SINGLE TRACE line.

IF the loop was closed (folded-routed) back onto itself - it can generate a SECONDARY element of instability and noise because of a shielding event (Emanations) and the ripple currents trying to "jump" from one side over the trace to the other -affecting not just DC values but the communication between sections too.

• To help keep this simple: The radios' own Ground buss, also is an antenna for noise.
• They use the Wire as a means to kick off RF from forming in the loop and force the RF to stay within the trace to be then harnessed off into the Filter caps like the 473's 103's and even Electrolytic's that are soldered to it along this traces route.
• Follow that trace you'll see where the largest filter value caps are located by the most sensitive areas of the receive - Audio Chip and Power choke.

IF you have an older Cobra 25 or even a Uniden PC66/68 - you'll see where a foil trace is LIFTED to the top component side of the board allowed to continue and then rerouted down to a similar larger foil trace at the front of of the main PCB (Display side) as a ground buss feed and provides the errant shielding in the 455kHz IF section within the Cobra 25/PC-66 series.

PC-66/PC-68

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This is nothing new, just the way RF noise and shielding is used to keep the radio from squealing like a pig from poor RF current taming and improper lead dress. They're trying to find ways to make the radios' receive and Transmit more effective in todays noisier vehicles.

 

[Mark A Bradley]

HEY,THANKS A LOT,,made my day, just happen to work on the radio and check this question, and you guys totally blew my mind. Thanks, no I can put these srtupid little wires back on, next time I am going to take a picture, I have the camera out right now. OK,again, I totally appreciate all you have done, take care and GREAQT JOB on the answer to this one. Awesome work,,talk soon, Mark,,73's KB9GTB