Cobra 148 GTL variable deadkey mod

[Dark Star]

Hi all,
I couldn't find a direct answer to this anywhere so I apologize in advance if this topic is in another thread that I may have overlooked. I need to get a variable deadkey on my 148 GTL. I would like to tie it into my rf gain pot if possible. Can this be done to a 148 or no? I have it on my 29 LTD and it's been a lifesaver for dialing in my amp. Any help would be greatly appreciated. 73s

 

[nomadradio]

We use the SWR Cal control. Unhook all 3 wires that lead to the control so you can splice a longer wire and sleeve the lap splices to each.

You first cut the trace between the wiper of the original carrier-level trimpot and the circuitry to the rear of it. A wire now goes from the now-separated wiper of the trimpot to the clockwise lug of the front-panel control.

A 10k 1/4W resistor now goes to the foil trace to the rear of where it was severed from the trimpot's center (wiper) lug. The other end of this resistor goes to the handy ground foil adjacent. Leave a quarter inch or so of resistor lead sticking up from the trace that was severed from the trimpot. A wire from the center (wiper) lug of the SWR Cal control goes to the rear of the radio next to the original carrier trimpot. Lap-splice the anode (no band) end of a generic 1N4148 type diode to the end of this wire. The cathode (band) end of this diode goes to the trace we severed from the carrier trimpot's center lug. If you left a small piece of the 10k resistor's lead sticking out, lap-solder the band end of this diode to it.

The counterclockwise lug of the SWR Cal control gets a wire leading back to the original carrier trimpot. Where you connect this wire will determine what the minimum carrier level will be at the left-most travel of the SWR Cal knob. Simply grounding this wire is the easy answer. A 1k trimpot between the wire and ground will let you adjust the minimum carrier setting. The original carrier trimpot now sets the max carrier level.

It's popular to "power boost" the final stage. The bias-test wire that leads to the final transistor first gets used to set the final transistor's idle current on sideband/no audio to 60 mA. This wire gets the end cut off, stripped and soldered to the main power supply voltage, usually the center pin of the AM modulator transistor. This permits the PEP in AM mode to equal that seen in sideband.

Gotta shoot pics of this setup someday. Been doing it so long we never needed a record of it to train a new tech.

One thing we learned to check first is R228, a 560-ohm 1/4W resistor just to the rear of the carrier trimpot. It gets clipped as a "swing" mod. Makes an average-reading wattmeter happier, but reduces the audio at the other end. Putting it back always improves the audio. And if R228 looks undisturbed, you're good to go.

73

 

[Dark Star]

Thanks for the info! This radio is untouched still in the box so I don't want to hack the crap out of it but I do need a variable output to tune it to my amp. You've given clear and detailed instructions so I should be able to get through the mod but if you ever do get around to taking pics of the mod, it would be helpful to newbies such as myself. Thanks again!!!

 

[9C1Driver]

Much better choice then loosing the RF gain feature in my opinion. You could also add a variable pot to the rear of the radio if you didn't want to change any of the controls on the faceplate.

 

[Dark Star]

I live in an area where there really aren't any locals for which I would have need for the rf gain feature so I was actually considering using that pot instead. That's how I have my 29 LTD set up. That being said, I always considered in radio swr features to be all but useless so, as you say, that is probably the best option.

 

[Squints]

Can the super audio swing mod thats popular with cobra 29,s also be in stalled in a cobra 148 ?.
A jumper is removed near the final thats cut then added a capacitor and resistor type mod can this be done on the cobra 148 or grant xl ? If so which jumper do i remove ?

 

[w9cll]

Can the super audio swing mod thats popular with cobra 29,s also be in stalled in a cobra 148 ?.
A jumper is removed near the final thats cut then added a capacitor and resistor type mod can this be done on the cobra 148 or grant xl ? If so which jumper do i remove ?

I am not sure about the mod but I do know that most super audio mods done to multi mode radios screw up the SSB side of things.

 

[nomadradio]

mod thats popular with cobra 29,s also be in stalled in a cobra 148 ?.

Not much point. The Cobra 29 mod adds missing parts to the radio so the carrier can be controlled.

The 148 radios have that stuff already built in. Only takes a few parts to turn the built-in carrier control to a swing mod. Much less complicated.

73

 

[Cable Guy]

Would that be the resistor/diode mod, popular with the exports, assuming it is a later mk2 model?

 

[nomadradio]

mk2 model?

Huh? Not sure which page we're on with that. The export radios use a totally-different AM modulator circuit that has a big PNP transistor modulating the driver and final. The Uniden SSB CB radios used a different circuit with a NPN transistor.

Different circuit, different mod.

73

 

[Cable Guy]

Huh? Not sure which page we're on with that. The export radios use a totally-different AM modulator circuit that has a big PNP transistor modulating the driver and final. The Uniden SSB CB radios used a different circuit with a NPN transistor.

Different circuit, different mod.

73

Guess not, I thought the mk2 148 was the original export model copied by others. Sorry. I have a 148 with the large pnp pass regulator. Maybe it was a mistake.

 

[Cable Guy]

I have only had the privilege of peeking inside one 148, and it's near (not exactly) identical to the exports I seen. It does have the large pass regulator. I assumed it would take the diode peak swing mod as it is so similar. Maybe the large transistor has been changed, but it's still a larger pnp regardless.

 

[Cable Guy]

I opened it up to verify, it does indeed have a large 2sb817 PNP pass regulator. HOWEVER, the schematic calls for a 2sa1012 PNP which is smaller. I suppose someone replaced it along the line, it is an old radio, but it IS a 148 with a PB010 board and, so far as I have gathered from that, the MK2 model. That's why I asked if that may be the mod he was talking about. If it was like this one, the diode swing mod should work. Interestingly, there are many versions of this radio. The version being talked about must be a different animal than what I have observed. My understanding is that the MK2 is a gtl dx model, but this faceplate must have been changed because it just says 148gtl, not dx. I'm confused thoroughly about this model now.

 

[sp5it]

PB010 just NPC/RC it.
Mike

 

[unit_399]

CG -
I'm confused. What radio do you have: A standard 148GTL, or a 148GTL-DX (PB-010 board) ?? If its the 010 board, and it has a 2SB817 in the TR51 position, you need to check that TR53 is a 2SA1012 (or equiv), and TR52 is a 2SD471 (or equiv). Then check that R295 is a 47 ohm/1 watt resistor. This is the complete PB-010 modulator upgrade, and doubles the modulator output. If the transistors and the resistor haven't been changed the 2SB817 upgrade is useless. BTW, R295 should be mounted above the board for cooling, and small clip-on heat sinks should be installed on TR52 and 53. Good Luck. 73.

- 399