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Uniden pc78ltd dead key adjustment?

711

Well-Known Member
Mar 22, 2008
329
368
73
I think they are good rigs. I am impressed with the receiver sensitivity. Not sure how it is in a mobile environment. I have the first IF in mine tapped to use an sdrplay for the rx. I can hear good wideband AM, use it as a panadapter to see the whole 11 meter band and even listen to SSB.

I was hesitant about the surface mount stuff but the 2016 cobra 29 ltd classic I bought was much lower quality. Even after the same alignment I give my other radios the rx was poor.

I've been wanting to try the string of diodes in series with d8 on a cobra 29 to lower the carrier. That with the keep alive circuit toll free posted here years ago is supposed to make a much cleaner waveform. Looks like its based on the 3 diode ultra modulation circuit from the 1950s.
543,
Interested in your IF tap and what kind of performance you were getting. I been wanting to build me a circuit to interface my dongle. Didn't know if they needed a buffer and/or LNA. Following your progress on the 3 diode TF mod.
 

543_Dallas

Sr. Member
Jul 25, 2011
2,854
4,055
273
543,
Interested in your IF tap and what kind of performance you were getting. I been wanting to build me a circuit to interface my dongle. Didn't know if they needed a buffer and/or LNA. Following your progress on the 3 diode TF mod.

The performance is great. Good sensitivity and plenty of bandwidth. I can be on ch40, see a transmission on 1. If I click on it I can listen, run multiple reveivers with the sdr and listen to several channels simultaneously. I can receive ssb just can't transmit it.

All I've done is tap L7 with a low value cap. 5pf is all you need. I ran mini coax to the bnc I installed on the back.Tune the sdr to 10.695.

I instainstalled a relay in the radio to break the connection to the IF and short the coax going to the sdr when I transmit. This makes the noise floor on the sdr drop, with proper squelch settings the rx drops out on transmit.

No plans of doing the 3 diode circuit to this rig. It is modulated with external gear. More info is on the MD forum...unless thats a different 711 over there.
 
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711

Well-Known Member
Mar 22, 2008
329
368
73
The performance is great. Good sensitivity and plenty of bandwidth. I can be on ch40, see a transmission on 1. If I click on it I can listen, run multiple reveivers with the sdr and listen to several channels simultaneously. I can receive ssb just can't transmit it.

All I've done is tap L7 with a low value cap. 5pf is all you need. I ran mini coax to the bnc I installed on the back.Tune the sdr to 10.695.

I instainstalled a relay in the radio to break the connection to the IF and short the coax going to the sdr when I transmit. This makes the noise floor on the sdr drop, with proper squelch settings the rx drops out on transmit.

No plans of doing the 3 diode circuit to this rig. It is modulated with external gear. More info is on the MD forum...unless thats a different 711 over there.
I am the same, however I rarely stop by, lots of non technical stuff going on over there , guess i need to start looking closer. I've got an 880 that I'm trying to figure out at the moment. That 78 looks closer to the 980 though
 

Clint Murphy

New Member
Jul 28, 2020
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0
1
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Peak out the coil in front of the final just like the old cobra 29 or uniden 78.

To lower the deadkey remove the large diode to the left of the modulation transformer. Add more diodes in series until you get the deadkey down to where you need it. I have not used the string of diodes but it should work fine. I vary the carrier of mine by varying voltage from the modulator. Basically what the string of diodes will do.

This is my radio. Ignore the big ugly choke attached to the orange wire. The diode will be where I soldered the choke to the board. Circled in blue. The coil I circled in red will reduce the carrier. Peak it out and lower the carrier with the diodes. Detuning the circuit is not the right way.View attachment 20213
Just out of curiosity, why are you using that choke in place of the original diode? Thanks and have a good day.
 

543_Dallas

Sr. Member
Jul 25, 2011
2,854
4,055
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Just out of curiosity, why are you using that choke in place of the original diode? Thanks and have a good day.

If memory serves me correctly I had some fuzz on the modulation peaks that I thought was caused by rf getting into the modulator. I had some of those chokes and it solved the problem.

I originally got into this with a mauldulator and cobra 29. After that I started using the choke on every one I installed. The wire in that picture is coming from an asymod board.
 

Clint Murphy

New Member
Jul 28, 2020
5
0
1
45
If memory serves me correctly I had some fuzz on the modulation peaks that I thought was caused by rf getting into the modulator. I had some of those chokes and it solved the problem.

I originally got into this with a mauldulator and cobra 29. After that I started using the choke on every one I installed. The wire in that picture is coming from an asymod board.
Thanks buddy!! Any more tune up/mod info floating around on these radios? This thread is the only thing I've found online as well as a YT vid with a cap and resistor mod done on the 68 LTX radio.
 

543_Dallas

Sr. Member
Jul 25, 2011
2,854
4,055
273
Thanks buddy!! Any more tune up/mod info floating around on these radios? This thread is the only thing I've found online as well as a YT vid with a cap and resistor mod done on the 68 LTX radio.

I'm not sure. I haven't played with many of the new rigs and rarely work on stuff for other people.
 

Clint Murphy

New Member
Jul 28, 2020
5
0
1
45
I'm not sure. I haven't played with many of the new rigs and rarely work on stuff for other people.
I see guys getting 30-40 watts PEP from these radios, I cant get mine to do that. 15-16 PEP is all I see from mine. Are their meters happy or is my meter just sad?
 

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