Cobra 148 GTL production date Mar 1989 Phillipines

[LeapFrog]

What does a "super diode" look like?

The "super diode" can be an NTE 612 (s-ecg 612), NTE 614, or a black glass diode with no markings other than a white dot or stripe. It may even resemble the original part! Alternatively some add an inductor in series to D35.


I would examine the solder side of D35 and see if the solder looks original/untouched.

This thread should help you look to see what may have been done to your radio, so you can reverse the modification when you're ready.
https://www.worldwidedx.com/threads/10-turn-pot-clarifier-mod.43152/

 

[Handy Andy]

Built my dummy load to 50ohm 25 watt max. On the klein it reads 50.4 ohms.......This .4 enough to cause issues or I can do the tx alignment?

Use it for comparison on your radios own built in SWR meter - and adjust SWR CAL for max - should be pretty flat on the FWD setting of the switch of the radios own internal.

This would also help you in determining the condition of the SWR metering system.

Try this. put your dummy load on a known good SWR/Power meter it on its' Antenna jack on the external meter on and set up that radio to key into it...using a coax jumper to the Power meters XMTR or RADIO connector...

Tune to Channel 20 - so you don't piss off locals using Channel 19...

Switch 148's S/RF/SWR to CAL - locate the SWR CAL knob - turn it fully Counter Clock wise (down) then set radio to AM mode - and key radio...

TX light should turn red...

Meters' needle should show some deflection (swings to the right) if not, adjust SWR CAL knob until (while its keyed up into that dummy load) the needle is all the way over to the mark that say CAL on the meter face.

Unkey a moment - let things cool down a minute...Might be a good idea to see how much you had to turn the SWR CAL knob, for is it's nearly all the way over - you have a wattage issue too...Use a separate Power meter AND THE DUMMY LOAD ON THE ANTENNA side of that external meters' FWD or CAL setting to help aid you in the setting and tuning process...Right now you should be seeing the same results on both your external SWR meter and the internal one...

Switch S/RF/SWR to SWR - then key radio

TX light will turn red - and the needle should not move AT ALL - if it does - then you may need to do some more work around the SWR bridge - the meter is a rough estimate and is not terribly accurate - it's made to let you check the SWR to make sure it's ok to use if you encounter reception problems. And can be used to get you into the ballpark so you don't damage the radio from a poor SWR match.

So the above - if the meter SWR indication still shows something - then you may have a harmonic or poor connection to even some bad solder joints along the chain from the Final to the coax connector on the back panel - including a poorly corroded or loose coax connector - needing to be tightened.

The age of the radio can affect the condition or how well the radios own internal soldering can be...be ready to do some touch up reheating work on that radio to provide the OEM quality it once had when it came off the factory line...

But as always - use a dummy load and even if it's rated 10 watts is more than enough for work you have to do.

 

[Handy Andy]

I'd also look at VR10 - by the Power Choke - pic attached...

By there too is the Audio Amp chip - and towards front of the radio is also the MB3756 Power Regulator and TX/RX switch...

Look for "fresh work" on the foil side as a means to help you locate the areas the previous owner did up some work to it...

The schematic is of the area you should recheck for values - hope it helps...

 

[loosecannon]

JLsenior,

it sounds to me like you need to adjust the TX meter trimmer resistor.

EDIT: please be sure you have the SWR/CAL/SRF knob in the SRF position. if you have it in the CAL position, and the SWR CAL knob is turned to the right, it will slam the needle in the meter to the right like you describe. If this is not the case, continue with my initial instructions about the TX meter trimmer.

if you say that the radio deadkeys 1.5 watts, and the meter slams to the right at that power level, try adjusting VR6. it is located just to the right of VR8 and VR9 (component side up, knobs facing you)
turn the mic gain all the way down, key the mic, and adjust VR6 so that the meter moves over to the left. try setting it so that it sits near the "1" on the SWR scale.

this is not an accurate reading of your wattage, but at least will be a baseline for you.

now, you say that the radio only deadkeys .5 watts.

leaving the mic gain down, key the mic and adjust VR10 to read 3 to 4 watts on your external wattmeter, or where ever you want it set.

now, turn the mic gain all the way up, key the mic and note the S meter reading, now whistle into the mic and it should move to the right a bit.

re-solder R129, but leave R44 and D52 cut.
they are your clarifier mod, and you might want that to stay the way it is.
most of us like them unlocked.

let us know how that all goes and we will bias the driver and final next.
LC

 

[JLsenior]

took the radio to a professional cb shop in Indiana and had the radio realigned. They replaced r129 for free and let me watch the process and explained the scope meter signal readings. Also replaced the meter in the radio. This Cobra is back to kickin ***. Meter swings correct again also ....