Decided to restore this old radio that I bought several years back. It was screwdrivered to death. I realigned the PLL and the Receiver. I have all HP Equipment so I can trust the results. After resetting the DC Voltage back to 13.8, yea they turned that up too. The RF Finals are dead. That is another issue. The Receiver comes in at about 80uV. If I drive enough signal in it is ok. Rechecked all the PLL Frequencies and Voltages, dead on. RF gain Ctrl works. I am going to isolate the Coax to the Chassis and inject a signal directly in the Radio. Any history on low RX Sensitivity and what to look for ? Anyone know what CB Radio Mobile they used in that radio ? Thanks in advance. Andy
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DAK IX
- Thread starter ashorn
- Start date
Best of my recollection, the receiver includes a handful of 10mm slug-tuned "can" inductors/transformers. They have a skinny ferrite slug threaded into it.
The position of this slug can reveal a fault we see more and more often. Most of these 'cans' have an internal capacitor that resonates it to the desired frequency. If (when?) that capacitor fails, anyone checking its alignment will find that the slug now appears to peak with the top of the slug dead even with the rim of the hole. That position is the max inductance setting. If it seems to peak at this exact position, it's not really a peak. It's the slug running out of travel before reaching a resonant peak. If any of those cans appears to have peaked at this slug position, it could explain weak performance. Adding back capacitance across the winding where the internal cap has failed can bring it back on peak. If you know the value of the cap inside that can, just use that one. But most of these coils have no hint about that internal cap's capacitance value. Schematics never give this away. We use a trimmer cap.
Had a 1981-vintage Cobra 142GTL come in that reminded me of the "Customer States ...." videos on YouTube. As in "Customer states, brakes are noisy", followed by video of the brake rotor separated into two pieces.
Customer states, "no receive from this radio".
Turns out it had seven cans with failed internal caps. Sure enough, it had no receive until you pumped almost two Volts of RF into the radio.
But this strategy brought back the receiver.
One can at a time.
And brought back the missing half of the transmit wattage. Didn't pay much attention to transmit performance until it would receive like it should. Would only swing to 5 or 6 Watts on sideband. Couldn't get a carrier on AM, only modulation.
I have long suspected that prolonged exposure to high humidity, or condensing dewfall causes the capacitor to fail. This one kinda gives away that part of its history.
The one trimpot that's shiny got replaced about ten years ago.
It's like Indiana Jones said "It's not just the years, it's the miles".
And for the record, seven trimmer caps to bring a radio back to life is not the record-setting count.
73
The position of this slug can reveal a fault we see more and more often. Most of these 'cans' have an internal capacitor that resonates it to the desired frequency. If (when?) that capacitor fails, anyone checking its alignment will find that the slug now appears to peak with the top of the slug dead even with the rim of the hole. That position is the max inductance setting. If it seems to peak at this exact position, it's not really a peak. It's the slug running out of travel before reaching a resonant peak. If any of those cans appears to have peaked at this slug position, it could explain weak performance. Adding back capacitance across the winding where the internal cap has failed can bring it back on peak. If you know the value of the cap inside that can, just use that one. But most of these coils have no hint about that internal cap's capacitance value. Schematics never give this away. We use a trimmer cap.
Had a 1981-vintage Cobra 142GTL come in that reminded me of the "Customer States ...." videos on YouTube. As in "Customer states, brakes are noisy", followed by video of the brake rotor separated into two pieces.
Customer states, "no receive from this radio".
Turns out it had seven cans with failed internal caps. Sure enough, it had no receive until you pumped almost two Volts of RF into the radio.
But this strategy brought back the receiver.
One can at a time.
And brought back the missing half of the transmit wattage. Didn't pay much attention to transmit performance until it would receive like it should. Would only swing to 5 or 6 Watts on sideband. Couldn't get a carrier on AM, only modulation.
I have long suspected that prolonged exposure to high humidity, or condensing dewfall causes the capacitor to fail. This one kinda gives away that part of its history.
The one trimpot that's shiny got replaced about ten years ago.
It's like Indiana Jones said "It's not just the years, it's the miles".
And for the record, seven trimmer caps to bring a radio back to life is not the record-setting count.
73
Best of my recollection, the receiver includes a handful of 10mm slug-tuned "can" inductors/transformers. They have a skinny ferrite slug threaded into it.
The position of this slug can reveal a fault we see more and more often. Most of these 'cans' have an internal capacitor that resonates it to the desired frequency. If (when?) that capacitor fails, anyone checking its alignment will find that the slug now appears to peak with the top of the slug dead even with the rim of the hole. That position is the max inductance setting. If it seems to peak at this exact position, it's not really a peak. It's the slug running out of travel before reaching a resonant peak. If any of those cans appears to have peaked at this slug position, it could explain weak performance. Adding back capacitance across the winding where the internal cap has failed can bring it back on peak. If you know the value of the cap inside that can, just use that one. But most of these coils have no hint about that internal cap's capacitance value. Schematics never give this away. We use a trimmer cap.
Had a 1981-vintage Cobra 142GTL come in that reminded me of the "Customer States ...." videos on YouTube. As in "Customer states, brakes are noisy", followed by video of the brake rotor separated into two pieces.
Customer states, "no receive from this radio".
Turns out it had seven cans with failed internal caps. Sure enough, it had no receive until you pumped almost two Volts of RF into the radio.
But this strategy brought back the receiver.
One can at a time.
And brought back the missing half of the transmit wattage. Didn't pay much attention to transmit performance until it would receive like it should. Would only swing to 5 or 6 Watts on sideband. Couldn't get a carrier on AM, only modulation.
I have long suspected that prolonged exposure to high humidity, or condensing dewfall causes the capacitor to fail. This one kinda gives away that part of its history.
The one trimpot that's shiny got replaced about ten years ago.
It's like Indiana Jones said "It's not just the years, it's the miles".
And for the record, seven trimmer caps to bring a radio back to life is not the record-setting count.
73
So after looking around under the radio and trying to decide where to try the Trimmer Caps I came across a Cap soldered across a can. It was jumping the can and not across anything. I removed the Can, buzzed it out. Open on the one side. I got lucky one of the wires was broken off the post. Had to unwrap it one turn under a microscope and re-solder it back. Put it back in and guess what static. switched from the load to an antenna. Traffic !! Retuned it. .3uV for 10dB. So the person "Repairing" the unit took a quick fix way out. Did find the mixer coil needed a cap across it. to get a peak. It was meant to be because the Cap I removed worked just fine across the mixer coil. You help me repair the radio in a way you did not expect, or did I. Thanks Andy KA3ODJ
On to the TXer. No RF out. Tested the Tube Section just past the 2SC1306 RF Amp, all is ok and even Modulation. Using another CB to drive the Tube Amplifier in the radio, have it padded down to 2Wts input drive. Replaced Pre-driver 2SC388A with NTE229, have gain in that stage. Aligned the 3 "27 MHz" Coils. Only have about half the required drive in to this device. Going to replace the 2SK19GR with a NTE133 and see if that makes things any better. Aligned the PLL Circuits again just to make sure, they were right on except for Q409 Buffer Transistor has low output going to change it out also with an NTE229. Looks like The "Screwdriver Teck" that tried to fix this radio had the 13.8 VDC turned up to 18.9VDC. I think some devices took a hit. I also have drive to the 2SC495 Driver but no amplification. Have voltage there but low drive. Nothing in or out of the 2SC1306. Both were replaced. I often whish my Signal Generator had more drive power. I could start at the 2SC1306 and work back. Any suggestions on anything I missed ? Thanks Andy
