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cobra 2000 receiving and tx but channel not changing.

W

wildwinds

Active Member
Feb 11, 2020
34
41
28
67
Ireland west
Hello first to say what a great forum youn have here, i have been reading the articles trying to fix a cobra 2000 i bought new in the eighties. My father and i used it a lot in them golden dx days. Anyway it was put away in a room where it stayed for many years untill recently when i thought i would give it a try again.
the radio had mods done to it way back and the pll was changed to mb8719 it all worked fine.
I have a fair understanding of the workings and electronics has been a hobby here for many years, but i am by no means professional.
It worked for a while but then died so i replaced the electrolic capacitors but not the tantalum.

So the radio is receiving and transmitting on one frequency but there is no channel selection, I put the radio on 27.405 and checked the signal on a close by sdr, and it was way off at 27.580 or around there.
would i be right in thinking the vco pll is not locking?

So I have checked tp 10 but there no rf there also there is 4.6 volts dc at the tp 10
PIN 17 of the mb8719 has 3.6 volt
PIN 6 has .02 volts
Tp 9 is at 6.7 volts and not changing with the channel selector.
So is there any way i can get it up and running, and ideas where to look for the trouble.
Once again thank you for the great forum
Best regards
jim
 
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Well, try this and get back with us...

This is to check to see if the Rotary channel selector is not causing this fault...

Turn on radio, look at channel display - note the channel..

Turn off radio and unplug it, to discharge the power supply and memory (if any for clock)

Turn the channel knob a few clicks left or right, don't matter...

Plug radio back in, turn on radio - does channel still show previous or is it a new channel?

Check frequency...TX - all that jazz.. A new channel - new frequency?

IF this causes the channel and frequency to change, and you have to do this in order tom make the channel change that means there is a problem with the BCD rotary knob in the channel selector and display. The PLL looks to see if a small switch in the rotary knob assembly lifts and then makes contact again, with an internal detent (the click and engagement you feel and hear when you turn the channel selector) this is supposed to do this whenever you change the channel with the power on. The Detent changes it's state telling the PLL to hold and then whenever the Detent finds another resting spot, the new programming of the Binary Coded Decimal (the BCD programming to program the PLL to the new frequency) tells it to got to this new frequency and lock in.

By what you describe, this sounds like the most likely problem, a frozen rotary assembly or dirty detent switch inside the channel selector can do this - forcing you to turn off power then make the change to new channel by your count then repowering the radio - it can also mean some of the previous work done to the radio is causing this problem - in other words if it's no longer OEM - and the chips been worked on, then this fault may be due to wiring onto other problems that require you to remove the work done to it so you can rebuild it back to it's stock condition and restore normal operation.
 
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hi Andy thanks for the quick reply,
i have tried what you said, ithe display shows the new channel.
I tried a short tx on channel 20 and i can receive it audio and all on the sdr but the frequency is 27 405 regardless of the channel number displayed.
THanks again
 
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ok so i have made a discovery:sneaky:
I was checking out the capacitors and i found that c 135 was not connecting properly, i have re soldered it and now i find
1 that the rx audio is much lower....i can hear stations
2 its not switching to tx no output
if i unsolder the c135 again it all works except there is no frequency changing

also i have checked the binary voltages are changing with the channel selector

thanks j
 
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@wildwinds - if you can get to that site, grab both the 2000 GTL as well as 148GTL MOBILE radio info too.

Why? They pretty much share similar boards and since you mentioned the 80's - the number of parts you may have to replace in the Electrolytic caps alone is quite a bunch.

Having a direction to go and information to work with on similarly made boards can help you in finding out stuff a single service manual will not always tell - or show, you in what to do or where to go in locating parts let alone what they are.

Since you mentioned the AGE of the radio - it was purchased back in the 80's - that usually means the radios' internal parts like those Electrolytic caps - can dry out and corrode internally from age and lack of use (your storage no matter the effort - this corrosion event will occur) these Electrolytic caps contain a weak acidic solution for the purpose they're designed for - polarized capacitors. Ordinary humidity and air contains moisture that the caps can "Breathe in" and release gasses that accelerate this process in time. This is really a normal process, and the longer the radios is USED the caps can stabilize and last a long time. It's when they are discharged and in a relaxed state does the Electrolytic action turn into a GALVANIC reaction and the plates corrode faster due to their internal resistance and their locations on lines that supply power within circuits.

You may be experiencing this now and it simply requires a simple unsolder cap, swap out and solder in new ones to make the radio work well again.
 
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Good morning, bluster day here in Ireland thank you all for the comments.
Yes i have changed all the capacitors except the tantalum ones.
As regards c135 when i remove it the radio transmits a and receives but does not change frequency when i turn the channel selector.
Now i have replaced c 135 there is a noticable lowering of the received audio and the transmit is not working
i have checked the voltage regulator voltages
these are
1 8.37
2 13.5
3 8.37
4 00.0
5 13.5
6 8.3
7 00
8 8.5
no change when i press the transmit key

THe pll voltages
1 6.9 10 8.3
2 3.5 11 .03
3 .06 12 8.31
4 8.3 13 8.31
5 8.3 14 0.02
6 .18 15 .03
7 4.3 16 8.2
8 4.3 17 3.5
9 8.6 18 0.12

Thanks again for the help.
I have download that schematic as suggested and checking out posts on the 148 also
there appears to be stuff on cbtricks but the sites not working.
 
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