13.8v green final bias wire on D-44?/Cobra 148GTL/NPC mod?

[Robb]

D-44?/Cobra 148GTL/NPC mod?

Got this Cobra 148GTL that has the green bias wire clipped from the bias voltage supply junction and soldered to the cathode end of D-44.

Never seen the volting mod done like this on a 148GTL. The AM Limiter (TR24) has not been clipped out; nor has it been disabled from what I can see thus far. There is no 10mfd electrolytic cap on pin 9 of IC6 as one would do with the NPC mod either. Just doesn't make any sense.

The PEP output of the radio is about 20 watts. Trying to get it back down below stock output so that a local can put a 150 watt AB class amp on this thing. Even with the carrier turned down to 1 1/4 watts, it is still swinging to full output.

Anyone seen this mod before? is there another part of the board I should look at to see if there is any other part that may have been modified along with the D-44 mod? According to the schematic, the cathode of D-44/1N4003 attaches to the positive 13.8v input rail and the anode is attached to ground. Some kind of protection diode for the voltage regulator MB3756 I guess? Thing is, this isn't a 1N4003 diode in the D-44 spot. It looks like a 1N4148, or a 1N914, or something similar.

I want to put a 1N4003 diode back in D-44's place and put the green wire back on the bias junction so I can set the final bias, lower the PEP, and send this radio back on its way. But I don't want to overlook the un-obvious attempt at the NPC mod. Just looks like someone volted the final and nothing more. Anyone seen work like this before? Is there a sneaky freeky-deeky way to do an NPC mod without the usual spots used? Me thinks 'no'; but the radio says 'yes'.

J.J., ExitThirteen, AudioShockwav, Loosecannon?

This is an older 148 w/the 5 pin side mike.


The green wire that has been clipped from the bias junction (in the center of this picture just above/right of the horizontal yellow wire) is attached to the cathode of D-44

 

[Robb]

Disconnected the green lead and put it back on the bias junction. Re-adjusted the driver bias to 25mA and the final bias to 50mA. Still swinging up to 20 watts PEP on AM and SSB. I cannot seem to find any place on the board that had a trace cut around TR24. Still looking.

I can turn the ALC all the way down on the board and it has no effect at all.
Same with the AM Modulation pot.
My eyes are getting tired and its late; I'll give it another whack tomorrow in the morning . . .

 

[ExitThirteen]

You may have to remove and replace D44. Since there was a bit of current going thru it with the load of the final going thru that green wire, the diode may have gotten damaged. Also, take a closer look at TR24 on the component side of the board, sometimes people will snip 1 lead of the transistor (usually the emitter leg) and then twist the transistor back into place so it doesn't look like it's been touched. If you're planning on running this on SSB, check to make sure TR34 is in place (C1675) and R159 is in place (8.2-10k). These are part of the ALC circuit. If using on AM, make sure TR26 (C945) and R131 (10k, personally I use 47-100k to bring up the audio a bit, as this is the AMC) are in place. Check all that out and let us know how you get on.


Almost forgot... Check R228, it's near the C1419 AM pass regulator transistor, and see if there's a 1N4148 diode inline with it. It's a 560 ohm resistor. It won't affect SSB output, but will affect AM modulation.


~Cheers~

 

[unit_399]

Robb -

Did you ever get this one figured out ??? I would be interested to know what was the problem.

- 399

 

[Robb]

Never could find any problem with the legs or traces of TR24. It was intact and all of the other things that ExitThirteen asked me to check were also OK. Still puzzled. Shelfed it until I have some time to look at it some more; gave it back to the guy and told him to run it w/o the amp for now. Beginning to think that TR24 is open; so I may have to replace it. It is a 2sc945 IIRC. Need to dig through some stuff and get/find one out and try it.

Any other thoughts?