Hi folks, need help on this please. I have a 2000girl on the bench that displays the correct freq on rx but changes to 35 mhz when keyed. I have a sencore sc3100 scope with built in freq counter which shows that the radio is in fact dead on freq when keyed so it seems to be a display only problem. Also it displays in the 35 mhz on usb and lsb in rx and tx mode both. Any ideas?
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Cobra 2000 freq reads 35 mhz on key up
- Thread starter Red eagle
- Start date
Someone who actually knows something will chime in soon enough, but are the FET's in the counter actually working? Sounds like you're missing the 7.8 MHZ IF signal.
To start, I'm assuming that the 35 MHz reading moves up and down in frequency as the channel is changed. That's an important detail.
If the mode-selector switch is working okay, you should show the 27 MHz channel frequency in AM receive only, but not transmit.
There are two input frequencies to this counter. First, the 35 MHz from the PLL. Second, is the 7.8 MHz carrier frequency.
In AM receive, the second 7.8 MHz carrier frequency is shut off, and used for the transmit carrier only. The counter substitutes 7.8000 MHz into the display's internal arithmetic. That should get subtracted from the 35 MHz reading to show the 27 MHz channel frequency. But only if the section of the mode selector commanding the counter works okay. That's how the counter knows what mode you select, and how it should treat the 7.8 MHz input. It gets confused when that input is wrong.
In sideband (all the time) and AM transmit-only, that carrier gets counted and subtracted from the 35 MHz coming into the other input from the PLL.
Never have figured out how to troubleshoot one of these without a 'scope. Even takes a 'scope to check the input to the counter from the mode selector. Pins 12, 13 and14 on J410 indicate LSB, USB and AM in that order. Only one of those three will have a pulse of 8 Volts DC for that mode only. If all three read zero, there's a mode-selector issue.
If that checks okay, I would follow the 7.8 MHz SSB receive carrier from the counter's input socket downstream.
FET502 will show a lot more signal level on the drain than on the gate. TP502 should show a peak when L502's slug is tuned.
The 7.8 MHz branch of the counter's signal chain can fail in more than one step of the circuit.
We have found the two FETs in the counter module to be most vulnerable when someone unplugs, or plugs back in the signal connector. The one with the two gray shielded leads on the module's side.
The only time it's safe to molest that plug is with the AC cord unplugged. The clock/counter module is always powered, whether the radio's power switch is on or off. Powering the radio off with only the power switch, and pulling that plug creates the hazard to the FET inputs. Creates the risk of reversed DC polarity between the powered-off radio circuits and the FETs, still under power from the clock feature. Pulling the AC cord is the only way to avoid losing this roll of the dice.
If this radio is old enough, it could have a bad section in the mode selector and a broken 7.8 MHz input channel in the counter, both. The older a radio gets, the more risky it is to assume that "just one" thing is wrong.
73
If the mode-selector switch is working okay, you should show the 27 MHz channel frequency in AM receive only, but not transmit.
There are two input frequencies to this counter. First, the 35 MHz from the PLL. Second, is the 7.8 MHz carrier frequency.
In AM receive, the second 7.8 MHz carrier frequency is shut off, and used for the transmit carrier only. The counter substitutes 7.8000 MHz into the display's internal arithmetic. That should get subtracted from the 35 MHz reading to show the 27 MHz channel frequency. But only if the section of the mode selector commanding the counter works okay. That's how the counter knows what mode you select, and how it should treat the 7.8 MHz input. It gets confused when that input is wrong.
In sideband (all the time) and AM transmit-only, that carrier gets counted and subtracted from the 35 MHz coming into the other input from the PLL.
Never have figured out how to troubleshoot one of these without a 'scope. Even takes a 'scope to check the input to the counter from the mode selector. Pins 12, 13 and14 on J410 indicate LSB, USB and AM in that order. Only one of those three will have a pulse of 8 Volts DC for that mode only. If all three read zero, there's a mode-selector issue.
If that checks okay, I would follow the 7.8 MHz SSB receive carrier from the counter's input socket downstream.
FET502 will show a lot more signal level on the drain than on the gate. TP502 should show a peak when L502's slug is tuned.
The 7.8 MHz branch of the counter's signal chain can fail in more than one step of the circuit.
We have found the two FETs in the counter module to be most vulnerable when someone unplugs, or plugs back in the signal connector. The one with the two gray shielded leads on the module's side.
The only time it's safe to molest that plug is with the AC cord unplugged. The clock/counter module is always powered, whether the radio's power switch is on or off. Powering the radio off with only the power switch, and pulling that plug creates the hazard to the FET inputs. Creates the risk of reversed DC polarity between the powered-off radio circuits and the FETs, still under power from the clock feature. Pulling the AC cord is the only way to avoid losing this roll of the dice.
If this radio is old enough, it could have a bad section in the mode selector and a broken 7.8 MHz input channel in the counter, both. The older a radio gets, the more risky it is to assume that "just one" thing is wrong.
73
Last edited:
I'm 2nd-ing' Nomads - but only to add that you mentioned Re-capping.
I can't help but wonder if that and the reassembly are the issue (it gets swamped by the tripler overloading) and per Nomad's comment - that "35MHz" frequency would change - you did say that it is on frequency even when the counter it's NOT saying the same thing.
So if any foil or aluminized shields were removed and replaced or simply not put back in the right way, that creates a small antenna to pick up and swamp out even the best shielded equipment because they all share the same common ground - RF ground is different from the Electrical ground.
So you may have to re-trace your assembly steps and look into burnishing the ground plane "cabinet" mounting as well as shield cover to ground - because of age alone you can have a lot of oxides and tar nicotine and dust and dirt from places you don't really want to know about - can get in there....affecting the ability to sort out the signal present on the signal wire that is shielded - versus what the shield has on it and the noisy ground plane that it thinks is correct.
I've used steel wool and alcohol onto carb cleaner and sea-foam to cut thru to the bare metal it once had to clear up a noise buzz from the polling and hum from the clock and counter assembly - just the case itself has a type of plating that you have to get thru to base metal to restore the original contacts and places the RF has used in past to go ... you don't always get that back on re-assembly.
I can't help but wonder if that and the reassembly are the issue (it gets swamped by the tripler overloading) and per Nomad's comment - that "35MHz" frequency would change - you did say that it is on frequency even when the counter it's NOT saying the same thing.
So if any foil or aluminized shields were removed and replaced or simply not put back in the right way, that creates a small antenna to pick up and swamp out even the best shielded equipment because they all share the same common ground - RF ground is different from the Electrical ground.
So you may have to re-trace your assembly steps and look into burnishing the ground plane "cabinet" mounting as well as shield cover to ground - because of age alone you can have a lot of oxides and tar nicotine and dust and dirt from places you don't really want to know about - can get in there....affecting the ability to sort out the signal present on the signal wire that is shielded - versus what the shield has on it and the noisy ground plane that it thinks is correct.
I've used steel wool and alcohol onto carb cleaner and sea-foam to cut thru to the bare metal it once had to clear up a noise buzz from the polling and hum from the clock and counter assembly - just the case itself has a type of plating that you have to get thru to base metal to restore the original contacts and places the RF has used in past to go ... you don't always get that back on re-assembly.
- No one is chatting at the moment.
-
-
dxBot:63Sprint has left the room.
-
dxBot:kennyjames 0151 has left the room.
-
-