2 differant power levels on ssb

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kc9ogb
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2 differant power levels on ssb

Post by kc9ogb » Thursday 10th Sep 2009, 11:31

Radio Make and Model: cobra 139 xlr
PCB Number: PC-176AA
PLL Number: UPD858
Test gear available: freq counter ,multi meter, test meter ( pwr, mod, swr), iron etc.
Symptoms: first prob is 2 diff pwr levels on ssb, usb modulates 12-15 w but lsb will only swing on a whistle , reg mod only pruduces about 2.5-5 watts

What is working:

What has been tried: moved the regulator beside the xtals to a remote loc.

Did it suddenly happen ?: not sure, but i dont believe so
Anything else that may be of help?: radio has been sitting for nearly a year, just not sure where to start, never seen any thing like this before.

hotshot
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Re: 2 differant power levels on ssb

Post by hotshot » Sunday 20th Sep 2009, 23:25

sounds like mic gain trouble check audio side of the rig
you will always miss the shot you don't take.

kc9ogb
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Re: 2 differant power levels on ssb

Post by kc9ogb » Thursday 24th Sep 2009, 10:38

Ive gone through the audio circuitry including the dynamic pot, power is signicantly differant, lsb wil mod about8-9 with an ahhhhhh, where as without any other changes except the mode to usb, usb mod about 25...20w meter tight to the end. Realisticly after the osc. the circuit for usb and lsb are the same circuit, not understanding where the power level could change....really got my brains rattled.

kevin

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The Defpom
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Re: 2 differant power levels on ssb

Post by The Defpom » Thursday 24th Sep 2009, 12:51

The problem is probably the crystal filter, it may be filtering out the LSB sideband, I have come across this myself before on the same chassis, I had to retune the LSB offset and I.F. to be closer to the centre channel freq to get more of it into the passband of the filter.

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