Cobra 148gtl Hi fi Tx/Rx mod?

[Robb]

hifi audio on a cb,

Might seem silly George. But it does improve the fidelity pretty well. I've tried it on an old Uniden Grant that you helped me fix; the one that I repaired on the forum here awhile back if you remember. Worked outstanding. Works within the confines of the filter; broader and with a much flatter response.

You aren't boosting the bass and highs as much as much as you are just allowing them to pass without being restricted by the presence of the choice of cap values chosen by the mfr - as I'm sure you know well enough. Try it sometimes on one of your radios; this will also improve the RX. Which has proved to me to be a huge plus - all on its own. Try it; think you will like it. Allows even a stock mic to not sound like a right wanker when TXing as well.

 

[Klondike Mike]

Sounds like a topic for some engineer to jump in and prove/deny that this mod works.

 

[Robb]

Sounds like a topic for some engineer to jump in and prove/deny that this mod works.

Imagine one way to prove is to do the mod to a radio, run the output to a PEP wattmeter, use a sound gen into a mic and sweep the freq between 50 and 3k cps and see if the output stays fairly consistent. Before modding it, do the same test first. That should be definitive enough. So long as the mic has a flat response as does the generator.

Prolly do the same thing with a RF sig gen for the RX.

The overall width is still being determined by the filter.
However, a better freq distribution should be noticed before the filter kicks in and notches both ends off.

 

[dave457]

I've done both, the caps mod and direct injecting, and there is a big differance in tx audio quaility when run with audio prossesing

 

[Abikerlad]

So did it add more lows ?

So has anyone performed the above mods ? Did it ads more lows ?

 

[Klondike Mike]

So has anyone performed the above mods ? Did it ads more lows ?

I finally got around to doing this mod on my Cobra 2000 and recommend it. The RX audio is greatly improved but you have to use a better speaker than the crappy stock one. Someday I've have to mod the speaker box to compliment the radio.

The TX is improved a well as shown on the snapshot below of a transmission monitored from the AUX port of the radio. I, unfortunately did not get a baseline view before I did the mod. I've have another stock 2000 that I will have to someday get that baseline view. As well, you'll need a microphone with a broader response to get all that juicy audio into the radio. And lastly, as Robb mentioned, there still is the internal audio bandwidth limiting filter. Overall, I get good audio reports without having all the rack gear, xlr connectors and a foot pedal.

 

[Robb]

It does work. Not a wive's tale. The Uniden President Grant I'd modified that same way gets the best receive/return compliments from a legal output radio that I have. Using a modified D-104/M6 mic that had the 270k resistor clipped for extended freq response - BTW. Never tried it with a studio mic yet; but I may in the future. It might not be too easy to find the 2.2uf film capacitors for those who want to do this mod, but the rest of the parts are readily available everywhere.

With your SpectrumView program, you can see that the mic's freq reponse spikes in a few different locations along the graph. This will happen with mics being used that do not have a flat freq response - which makes up most of the favored mics normally used for CB radios. The flatter the response from a mic; then the more even, natural the voice can sound in TX mode given the limited response a radio has. But using a studio mic and modding the radio still can make an appreciable difference - IMO . . .

*EDIT*
Even though there is a filter to limit the response range, it doesn't shut off completely at first. So it will still have some added freq range slightly beyond above/below the filter cutoff points and then drops down some 6-12db's. If your 148 has a 4k filter in it, it will probably get 4.6khz audio bandwidth @ 0db and -12db on the edges. Not shabby at all if you boost the EQ before the upper and lower cutoff points; but better than stock even if you don't.

 

[red97k1500]

Sorry to bump such an old thread but I did the rx part of this mod. But it brought my receive down on both lsb and usb.... I have to have the volume turned all the way up now just to hear any body on side band. The receive on am works just fine. I used non polarized electrolytics were it called for 2.2uf

 

[Robalo]

The receive on am works just fine. yes sure AM mod you got it.

 

[red97k1500]

Yeah I knew it was an am mod.... but I didn’t know it would make receive on side band unusable.

Edite: maybe it’s because I used electrolytics? Hopefully someone who has done this mod will chime in.... the gold finger version says you can use non polarized electrolytics. And the motormouth maull version specifically states do not use electrolytics. If worse comes to worse I will just put the factory caps back in.