Hi all,
I have a mid-90s 25 LTD classic, with the mic jack on the side, and I'm having trouble tracking down its issue. I got this in a garage sale radio box, so no losses if nothing more comes from it, but I've been getting into radio repair work so these old junkers have made good learning challenges.
Anyway, when I got the radio and first powered it, the power circuit for the PLL had only 0.2 volts, after disconnecting circuits one by one I found that D13, an 8.2 volt zener, had shorted. So I replaced it and now have power to the PLL and related circuits. Second issue I found, it would transmit, at 0.2 watts, intermittently. I found that the PLL wasn't always locking and found a broken solder joint on a capacitor near L15. (one that is in the wax) Fixed that and now the PLL & VCO is working and producing 10.24 and 16.xxx frequencies.
I have cleaned the switches, so they are working. Mic is tested good on another cobra radio.
Now for where it currently sits. When I transmit, it has about a 0.2 watt carrier, and when I add audio, it drops. I used an RF sampler to look at the signal on my scope and I can confirm that it does modulate the signal.
Next issue is the receive, I have a basic RF signal generator, one with a tuning knob, 1KHz tone, and band selector, that produces a close-enough signal to trace at least. When I set radio on CH19 and rf gen close to 27.185, the S meter goes up and will follow the strength of the sig gen output or RF gain control. But there is no audio. I ran my finger across the audio IC and it does create static, so it seems to be ok. So I'm not sure where the signal is getting lost in-between.
Anyone have any thoughts on other things to check?? Thanks!
I have available:
- 100mhz o-scope
- Basic RF signal generator
- digital and analog volt meters
- frequency counter
- 1GHz spectrum analyzer
I have a mid-90s 25 LTD classic, with the mic jack on the side, and I'm having trouble tracking down its issue. I got this in a garage sale radio box, so no losses if nothing more comes from it, but I've been getting into radio repair work so these old junkers have made good learning challenges.
Anyway, when I got the radio and first powered it, the power circuit for the PLL had only 0.2 volts, after disconnecting circuits one by one I found that D13, an 8.2 volt zener, had shorted. So I replaced it and now have power to the PLL and related circuits. Second issue I found, it would transmit, at 0.2 watts, intermittently. I found that the PLL wasn't always locking and found a broken solder joint on a capacitor near L15. (one that is in the wax) Fixed that and now the PLL & VCO is working and producing 10.24 and 16.xxx frequencies.
I have cleaned the switches, so they are working. Mic is tested good on another cobra radio.
Now for where it currently sits. When I transmit, it has about a 0.2 watt carrier, and when I add audio, it drops. I used an RF sampler to look at the signal on my scope and I can confirm that it does modulate the signal.
Next issue is the receive, I have a basic RF signal generator, one with a tuning knob, 1KHz tone, and band selector, that produces a close-enough signal to trace at least. When I set radio on CH19 and rf gen close to 27.185, the S meter goes up and will follow the strength of the sig gen output or RF gain control. But there is no audio. I ran my finger across the audio IC and it does create static, so it seems to be ok. So I'm not sure where the signal is getting lost in-between.
Anyone have any thoughts on other things to check?? Thanks!
I have available:
- 100mhz o-scope
- Basic RF signal generator
- digital and analog volt meters
- frequency counter
- 1GHz spectrum analyzer
