Hey guys,
I have an interesting one for you. I've never gotten a radio with this fault before and frankly it's driving me nuts. I have a Cobra 148 that after aligning everything, side band is always still 1.5khz off frequency. When I put it on LSB I have to turn the clarifier about 15% low and when I have it on USB I have to turn it about 15% high. I measured the 34Mhz oscillator to see how much I was compensating and it is indeeda bout 1.5khz.
It's almost as if the shift in the 7.8Mhz oscillator isn't getting out. I can see the 7.8Mhz oscillator changing when I switch to side band and I can also hear the side band change when I tweak the 7.8Mhz while receiving a signal, so it's not like the oscillator isn't working or it's signal isn't going anywhere. It definately has an effect on the radio and also properly shifts. Regardless the amount I'm off frequency on side band corresponds exactly with the amount of shift that this oscillator is supposed to provide when it changes from 7.8Mhz to 7.985 to 7.8010.
This radio had a high frequency mod where PIN 10 was being sent about 4V through a 3.3k resistor sourced from L21, this mod is unfamiliar to me and it seemed to cause a lot of drift so I undid it. Instead I replaced the mixer crystal with an 11.325 and did the usual logic switch on pin 10 and 8. This fixed the stability but not the bizarre issue with the shift.
Sideband reception also seems to be low as I was not able to properly calibrate the side band receive meter.
Any ideas? Can't seem to figure this one out.
I have an interesting one for you. I've never gotten a radio with this fault before and frankly it's driving me nuts. I have a Cobra 148 that after aligning everything, side band is always still 1.5khz off frequency. When I put it on LSB I have to turn the clarifier about 15% low and when I have it on USB I have to turn it about 15% high. I measured the 34Mhz oscillator to see how much I was compensating and it is indeeda bout 1.5khz.
It's almost as if the shift in the 7.8Mhz oscillator isn't getting out. I can see the 7.8Mhz oscillator changing when I switch to side band and I can also hear the side band change when I tweak the 7.8Mhz while receiving a signal, so it's not like the oscillator isn't working or it's signal isn't going anywhere. It definately has an effect on the radio and also properly shifts. Regardless the amount I'm off frequency on side band corresponds exactly with the amount of shift that this oscillator is supposed to provide when it changes from 7.8Mhz to 7.985 to 7.8010.
This radio had a high frequency mod where PIN 10 was being sent about 4V through a 3.3k resistor sourced from L21, this mod is unfamiliar to me and it seemed to cause a lot of drift so I undid it. Instead I replaced the mixer crystal with an 11.325 and did the usual logic switch on pin 10 and 8. This fixed the stability but not the bizarre issue with the shift.
Sideband reception also seems to be low as I was not able to properly calibrate the side band receive meter.
Any ideas? Can't seem to figure this one out.