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Lafayette SSB-140 throws carrier on SSB PTBM058COX

Aztec

New Member
Dec 2, 2016
22
3
3
33
Leon, GTO. Mexico
Hello everyone,

I have a lafayette SSB-140 base station. The chassis is a PTBM058COX which is also similar to PTBM048. I just got through an alignment with this thing, all went well, except for the fact that when I hook this thing up directly to my antenna, it gives me a carrier on sideband. A fairly big one too. The weird thing is that when I hook up my SWR/Power meter, the issue does not occur. There is no problem with the antenna as far as I can tell, SWR is slightly over 1:1 and I see no problem with the coax, it's almost brand new. The thing just does not like being hooked up directly to the antenna, with SWR meter inline, all is fine. The balanced modulator isn't the issue either, tweaking it doesn't help.

Anyone here have any idea what could be causing this?
 

try this:
connect the SWR meter in between the radio and antenna and set it to the SWR CAL position.
then turn the CAL pot all the way clockwise (maximum meter deflection).

now put the radio in SSB mode (either USB or LSB), mic gain all the way down, and key the radio.

does the SWR meter needle move to the right and stay there at all?

now switch to the other SSB mode, and repeat the test.

compare the SWR meter reading with the one from the other SSB mode and make a determination as to which way you need to need to adjust the "carrier balance" VR inside the radio in order to null it in both modes.

this may or may not have any effect on your situation, but at least you'll know that you have that VR adjusted correctly.
LC
 
If you had an external frequency counter in the coax line with this radio you would be seeing an off-the-wall frequency that's not anywhere near the channel you're on.

Old Cybernet radios would be sensitive to small SWR changes and respond by oscillating on SSB. It will show a SSB carrier on one antenna, but not another. Or on the antenna, but not on the dummy load. When this happens, the frequency of the carrier on your wattmeter is determined by unwanted feedback in the radio's power stages, and NOT by the channel selector. The radio's carrier-balance pot affects only your desired on-channel signal, and won't have any effect on an oscillating final circuit.

Making sure all the ground screws on the circuit board are tight is the first place to start. A stripped thread in the radio's chassis will prevent one from tightening properly. A small machine bolt in the circuit board's ground hole with a nut on the opposite side is the only cure for this.

Setting the final transistor's output coil or the driver coil just below the highest peak-power setting can make it stable again sometimes.

Setting the final and/or driver bias too high can aggravate this fault, too.

Umm, you're not using a Dosy wattmeter that has the ungrounded-coax socket problem, are you?

73
 

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