magnum 257 mods info request

[space cowboy]

I was looking HERE and was thinking of performing these mod(s).
first off, has anyone tried the 100k resistor off the RF gain/mic pot. cbradiomagazine says he hasn't and thinks it isn't needed, but I'm wondering if anyone can comment on it.

I was wondering what needs to be done after doing the 1N914 diode mod to an amber faced magnum 257.
I have an oscilloscope, but don't know where to connect the probe, anyone have that info handy? it's been a long while since I've used one, but I'm hoping it all comes back...
I don't have a tone generator, but I can make and play back a 1 khz tone on the computer. good enough?

thanks.

 

[9C1Driver]

I guess you could try it if your radio is the old dual 1969 final version as pictured in that link. The blue display versions have the CP-1 audio compressor, all they need is a correct tune/alignment and your golden.

 

[cbradiomagazine.com]

On the older magnum 257, do the diode resistor mod, set your AM deadkey and SSB output power where you want them and don't forget to take apart the microphone and turn up the pot inside the microphone!

Don't mess with the 100k resistor, it's not needed. That radio will sound great and swing well with just the NPC mod. It will do about 25 watts and have loud clean audio.

Like I said, always got great reports with that radio. When I lived in Hawaii I had that radio paired with a small 2 pill amp in the mobile and I would talk all over the place.

 

[space cowboy]

I "lost" the the original mic almost immediately cb, too small to be comfortable and too short of a cord. I run US made road devils.
even if the second mod is not necessary, I'd still be interested in hearing about it. is it supposed to be to increase the mod?
I'm going to look at the schematic and see if I can figure it out, but I don't have high hopes.

 

[space cowboy]

ok, so I looked at the schematic, and I'm thinking the second mod puts the resistor in parallel with the mic gain VR to reduce it's resistance.

what I'm wondering about now is the notation on the schematic...
what's with the 5KA? the volume pot is labeled KA too. I was thinking it stood for adjustable, but the squelch and clarified are labeled KB.

 

[space cowboy]

well this didn't work out well. no AM mod, and while ssb works, there's noise that sounds like a chirping bird that disappears when I modulate.
maybe it has something to do with the capacitor that fell out of the board?
I went with the assumption it was c318 since I soldered the diode to one side, without messing with the other leg, no idea why it fell out. I'm no professional, but I'm not that bad.
anyway, according to the schematic the values are right 16v 33 pF, there's no contact between the legs, positive/negative match the other caps I can see underneath...

I undid the mod, but still have the problem
any thoughts, suggestions?

 

[ExitThirteen]

C318 is a 33 microfarad capacitor. Replace C318 with a 33uf, 16V electrolytic capacitor and all should be well.

When you get pots that are labeled "KA" and "KB", this means that they are a double pot, or two variables on the same control.


Hope this info helps.


~Cheers~

 

[space cowboy]

thanks for the info ExitThirteen.


as far as the cap goes, I gave you some bad info on the size. I just re-installed the one that fell out. 20 year old beginner's electronic education obviously hasn't held up too well.

I checked the negative side and have a good connection to ground, but wasn't able to find any connection between the positive side and the board. I'm going to remove as much of the solder from that side and look at the board. I'm hoping I haven't messed up the traces. I had some trouble trying to remove the solder after the cap fell out, copped braid doesn't work too well removing it from holes on the board.

I'm hoping the alligator clip on the other leg will keep it from falling out again.