Cobra 148GTL (Twn) diy Tune-up (newbie)?

[Roachman]

Peaking a radio is not done by twiddling the variable resistors. It is done by using a nylon tuning tool on the TX cans in USB mode with a dummy load and a steady hand with a decent watt meter. For AM, the AM Modulation variable resistor is tweaked with a O'scope and a watt meter. Unless you have a MOSFET final in that radio, 6w is too much to set the dead key at for AM . . .

Things are beginning to take shape. Thank you for the info Robb. I'm getting a better idea of what I can and can't do. What I don't want is the channel 6 crap so this is great.

 

[unit_399]

Peaking a radio is not done by twiddling the variable resistors. It is done by using a nylon tuning tool on the TX cans in USB mode with a dummy load

I can understand that the carrier balance and the driver/final bias would be set in the USB mode. But I don't understand how to peak the transmit "cans" in USB. Please explain your technique. Thanks.

- 399

 

[Robb]

Yep; first adjust the driver bias and the final bias in USB mode with the mic gain set at zero.
Set bias to factory specs 35mA for the driver and 50mA for the final.

Have the radio hooked up to a watt meter and a dummy load.

Use a 1khz tone into a Silver Eagle mic about a foot from the speaker producing that tone, but I first turn the mic gain down to zero first. Then I start the 1khz tone, key the mic, and turn the mic gain knob up on the radio until I see the watt meter read just 10 watts. Radio is still in USB mode on ch 20.

Start with the first TX (L47) can and turn it until I see it rise or fall; then turn it until it goes as high as it can. Unkey the mic and let the radio cool for 30 seconds or so after tuning each coil. Next can (L48) is done the same way. Do the next two cans the same way (L46 & L45) as well. Then tune L38 lastly - done!

You should see about 12-14 watts on the meter from the 10 watts you started with. It is important to remember to let the radio cool off between each TX can being peaked because it will stress the final and driver.

DO not adjust L36 - leave it alone!!!
That coil requires a Spectrum Analyzer to adjust; so forget about it (unless you have $3k to buy one). You will only untune the 54mhz trap circuit, and you won't want to do that. It will only mess up the transmit harmonics for the worse - if you do . . .

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf

 

[unit_399]

Gracias Amigo

 

[Roachman]

Yep; first adjust the driver bias and the final bias in USB mode with the mic gain set at zero.
Set bias to factory specs 35mA for the driver and 50mA for the final.

Have the radio hooked up to a watt meter and a dummy load.

Use a 1khz tone into a Silver Eagle mic about a foot from the speaker producing that tone, but I first turn the mic gain down to zero first. Then I start the 1khz tone, key the mic, and turn the mic gain knob up on the radio until I see the watt meter read just 10 watts. Radio is still in USB mode on ch 20.

Start with the first TX (L47) can and turn it until I see it rise or fall; then turn it until it goes as high as it can. Unkey the mic and let the radio cool for 30 seconds or so after tuning each coil. Next can (L48) is done the same way. Do the next two cans the same way (L46 & L45) as well. Then tune L38 lastly - done!

You should see about 12-14 watts on the meter from the 10 watts you started with. It is important to remember to let the radio cool off between each TX can being peaked because it will stress the final and driver.

DO not adjust L36 - leave it alone!!!
That coil requires a Spectrum Analyzer to adjust; so forget about it (unless you have $3k to buy one). You will only untune the 54mhz trap circuit, and you won't want to do that. It will only mess up the transmit harmonics for the worse - if you do . . .

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf

Likewise Robb...thank you. That helped fill the minor gaps in the manual's specs regarding procedure and details like Mic Gain/USB. Thank you!!!! B/w the tone generator, manual, your rundown, and everyone's advice/guidance, I hope to do this tonight. With a lot better understanding than I had a few days ago.

 

[Robb]

Gracias Amigo

De nada.

 

[Shorebird69]

Everybody has a Smart phone these days go to the Market and get a free App( https://play.google.com/store/apps/details?id=de.guentherkrauss.audio.audiotesttonegenerator) for a Tone generator and use that to adj the dead key. That whistlein AUUUUDIOOO BS is what you get from a TS cb hack Golden screwdriver shop is inaccurate. as is Transmitting Spurious emissions while Whistelin in the wind looking for your AUUUDIIII OOOOOO. To Properly tune a Radio you need a Dummy Load.

/
yep, another newbie question...would that be fed into the antenna jack? and a db meter from the speaker or ext spkr?

 

[loosecannon]

hi shorebird,

the answer is no, the post you quoted is referring to a tone generator app that you can install on your phone or on your home computer.

you would then use this app to generate a 1000hz tone, and then you would hold the microphone of the radio near the speaker of the phone or computer.

this is not the "professional" way to do it, but it will allow the radio's meter to sit steady long enough to notice what difference the adjustment is making, whether its raising or lowering your power output.

you NEVER adjust those coils with just a deadkey, or you would end up with no headroom for audio.
your radio would have a large deadkey and sound super quiet on the air.

i mean this in the most encouraging way, but your question sort of implies your experience level with this type of thing, so im just going to say be careful inside your radio, don't use metal tools to adjust things, and don't try to work on other peoples' radios.

best of luck, we all have to start somewhere!
LC