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Cobra 85 Dynascan

Nov 28, 2020
68
10
18
70
Muncie, Indiana
I picked one of these Cobra 85's the other day. It had no receive or transmit. Went through the radio and got both transmit and receive now. The problem I am having now is it is transmitting for example on channel 10 and receiving on channel one. This a crystal radio. I have gone through this radio and checked all the transistors with a Multi-Function Tester-TC1. I am just getting started trying to fix radios Here is a list of how it is receiving
Channel 1 Transmit No receive on any channel
Channel 2 " " " " " "
Channel 3 " " " " " "
Channel 4 " " " " " "
Channel 5 " " " " " "
Channel 6 " " " " " "
Channel 7 " " " " " "
Channel 8 " " " " " "
Channel 9 " " " " " "
Channel 10 Transmit Receive on channel 1
Channel 11 " " " " 1
Channel 12 " " " " 2
Channel 13 " " " " 3
Channel 14 " " " " 3
Channel 15 " " " " 5
Channel 16 " " " " 4 & 6
Channel 17 " " " " 4 & 7
Channel 18 " " " " 7 & 9
Channel 19 " " " " 9
Channel 20 " " " " 8 & 10
Channel 21 " " " " 8 7 11
Channel 22 " " " " 11 & 13
Channel 23 " " " " 12 &14
Any help would be appreciated
Thanks Dale
 

First clean the channel selector switch with contact cleaner. If that doesn’t help, study the attached crystal mixing chart to establish any off frequency or dead crystals. A frequency counter is an essential part of any repair bench. I suspect the 7.345 crystal or the connections to that crystal. It's the only crystal that is common with receive only.
Cobra 85 mixing chart.jpg
 
Also check for any orange or dark brown looking glue around the crystal or any other parts. That glue becomes conductive and corrosive with age. It needs to be removed. With that particular glue involved, it’s not uncommon to see the leads corroded off. Corrosive glue removal is discussed on Youtube and elsewhere.
 
Sunbulls I have cleaned the channel selector with contact cleaner. Didn’t change anything. So I started to look around the circuit at the 7.345 Crystal. Still everything looked OK voltages were close around that circuit. I also have a Multi meter that will check frequencies I get nothing. So I decided to pull the Crystal and check for the glue no glue there. So I decided to check my receive with my chart and everything stayed the same with the crystal out. So I am guessing maybe I have a bad 7.345 Crystal I’m looking for one of them now. Thanks for all the help I will get back with you when I find a crystal.
 
Finding another 7.345mhz crystal may prove to be difficult or expensive at best. I have a ton of mixing crystals, but not that one. Your radio's mixing scheme is somewhat rare. The only other radios that I know of that use that particular crystal are the Browning Brownie, and the Tram XL. If you ever see one of those three radios on the cheap, buy it for the parts.
Bomar, Jan crystals and a few others will make one for you, but they’re expensive. If you go that route, be sure to tell them about your radio so they can look up the capacitance load, size and other crystal specs.

Check out Ken’s electronics. He no longer has one listed on his web site, but it wouldn’t hurt to ask. Maybe someone on this forum has a spare one.

In the future, another thing you may want to invest in is a crystal tester. They come in both kit and pre-assembled form. Some of them have a frequency counter built in, but none of them are very accurate. They’re just a cheap and a handy tester that checks for activity and general fundamental frequency.
 
No, use the chart as a reference. Adding that frequency into the mix would make channel 1 receive on 26.5085mhz. Of course all the other channels would also be the same amount lower in frequency.

On loan, that crystal could be useful in testing the corresponding oscillator circuit and others along the receive chain. If it does work, that’s a proof positive indication the 7.345mhz crystal is in fact dead.
 
No, use the chart as a reference. Adding that frequency into the mix would make channel 1 receive on 26.5085mhz. Of course all the other channels would also be the same amount lower in frequency.

On loan, that crystal could be useful in testing the corresponding oscillator circuit and others along the receive chain. If it does work, that’s a proof positive indication the 7.345mhz crystal is in fact dead.
I definitely see how the chart works now thank you very much I did put the Crystal in and it does receive on channel 10 if you’re transmitting on 10 but as you said it sounds like they are off frequency so I am going to keep trying to find a 7.345 Crystal and maybe that will take care of the problem I will let you know I sure appreciate your help thank you
 

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