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Cobra 2000 Can’t adjust L19
- Thread starter Brian G
- Start date
Ok well Long. story. There is voltage now. Problem I have now is that the frequency counter on radio shows same frequency no matter what I channel I change to on AM or SSB. I can only “adjust” it by turning L19-which I can get wild swings of frequency by turning that which I would think means the voltage is changing but it’s not showing that change on the volt meter. .the voltage reading at TP9 is about 1 volt and again doesn’t change when I turn L19. Ideas on that and or why the frequency doesn’t change when changing channel? Thanks.
You need to 'scope TP10. The signal here is what feeds into the PLL. L21 gets peaked for max signal here, usually between 1 and 2 Volts peak-to-peak. Until you see this, the PLL has nothing to work with. The VCO tuning voltage at TP9 only has meaning when the PLL is getting enough input signal to work with. The tuning voltage at TP9 is the 8719 chip's output. Until it has sufficient input at pin 17/TP10 the chip's output won't be functional.
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Plenty of voltage at TP10. I can tune it all the way to 3.5 volts p to p. But still no change at TP9 still showing 1.1 volts no matter what I do the L19. Although again if I turn L19, the frequency definitely changes but no change in the volt meter which I don’t understand. And no change when I change the channel selector. Whatever the frequency counter shows, it’s just stay there regardless. And I replaced L19 recently. It was working for a time and now back to the same issue (hence the title of this very long thread!)
Pin 6 is your lock pin, when it's low, your PLL is not in lock.
Pins 1 to 2, and 3 to 4 - runs a type of "Gate" oscillator, one is 1/2 the speed of the other. One is fast, the other is slow - for a reason...
This type of oscillator on Pin 1 and 2 and 3 and 4 , uses the PLL clock to look at the outside oscillator - as if it were a clock - and it tries to figure out if its' too fast or too slow to that other clock.
So the Gate and where the two oscillators "meet" is how it determines to speed up or slow down.
Remember they are two "Gated" pulse trains, one 1/2 the size (length) of the other.
For the PLL - it needs the input from the Oscillator it can send to (thru Pin 5) and uses a circuit to compare the clock it controls - to the one L19 controls - your UHIC 007 SIPP chip...It sees a comparison thru TR20 at the PLL's Pin 17
It uses the two Gated oscillators at Pins 1 and 2 - and 3 and 4 paired - it looks at another pin on the opposite side of the PLL - Pin 17 - it needs to "beat" or offer a Heterodyne signal that is the DIFFERENCE between IC2 and L21 - if they aren't "in SYNC" then the PLL is out of LOCK until it can get it under control - it does that by looking at how the frequency on Pin 17 is AHEAD or BEHIND the two "Gates" - if it's too fast, the Charge pump pin 5 drops voltages trying to speed up the Varactor at TP9.
TP 10 is the "comparator" an output signal that goes to PLL Pin 17 - it's the MIXED stuff that L21 and L19 (Varactor) make - then goes to Pin 17 at the PLL - this is the "loop" Pin 1, 2 and 3, 4 along with Pin 5 - affect TP9 and IC2 the one it controls - Pin 17 looks at TP 10 - Comparator and it's heterodyne on L19 and L21 together - to determine too fast or too slow and alters Pin 5 by tiny amounts to make the Varactor in IC2 "resonate" with the divider of the inputs at the Channel Selector - Per the Pin 17 frequency (The heterodyned result, compared to the divided the Channel Selector sets for the divider to pare down this Pin 17 Frequency by to compare)
IF the TP10 isn't running (Per your L19) then the "loop" to check the speed is not working the PLL goes "out of lock" and sent Pin 6 to SINK current (Goes low) so it turns OFF the MB3756's ability to go into TX mode and prevents unauthorized operation. (Hey that's the FCC - not me telling you this)
Pins 1 to 2, and 3 to 4 - runs a type of "Gate" oscillator, one is 1/2 the speed of the other. One is fast, the other is slow - for a reason...
This type of oscillator on Pin 1 and 2 and 3 and 4 , uses the PLL clock to look at the outside oscillator - as if it were a clock - and it tries to figure out if its' too fast or too slow to that other clock.
So the Gate and where the two oscillators "meet" is how it determines to speed up or slow down.
Remember they are two "Gated" pulse trains, one 1/2 the size (length) of the other.
For the PLL - it needs the input from the Oscillator it can send to (thru Pin 5) and uses a circuit to compare the clock it controls - to the one L19 controls - your UHIC 007 SIPP chip...It sees a comparison thru TR20 at the PLL's Pin 17
It uses the two Gated oscillators at Pins 1 and 2 - and 3 and 4 paired - it looks at another pin on the opposite side of the PLL - Pin 17 - it needs to "beat" or offer a Heterodyne signal that is the DIFFERENCE between IC2 and L21 - if they aren't "in SYNC" then the PLL is out of LOCK until it can get it under control - it does that by looking at how the frequency on Pin 17 is AHEAD or BEHIND the two "Gates" - if it's too fast, the Charge pump pin 5 drops voltages trying to speed up the Varactor at TP9.
A crude example,
but it's the best way to explain how
the Gates and Pin 5 work to form a signal that you see at TP9
but it's the best way to explain how
the Gates and Pin 5 work to form a signal that you see at TP9
TP 10 is the "comparator" an output signal that goes to PLL Pin 17 - it's the MIXED stuff that L21 and L19 (Varactor) make - then goes to Pin 17 at the PLL - this is the "loop" Pin 1, 2 and 3, 4 along with Pin 5 - affect TP9 and IC2 the one it controls - Pin 17 looks at TP 10 - Comparator and it's heterodyne on L19 and L21 together - to determine too fast or too slow and alters Pin 5 by tiny amounts to make the Varactor in IC2 "resonate" with the divider of the inputs at the Channel Selector - Per the Pin 17 frequency (The heterodyned result, compared to the divided the Channel Selector sets for the divider to pare down this Pin 17 Frequency by to compare)
IF the TP10 isn't running (Per your L19) then the "loop" to check the speed is not working the PLL goes "out of lock" and sent Pin 6 to SINK current (Goes low) so it turns OFF the MB3756's ability to go into TX mode and prevents unauthorized operation. (Hey that's the FCC - not me telling you this)
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ok checked around a bit more. now the following pins are off vs spec. Not sure why I keep getting different readings.
Is it normal for voltage readings to change in pll when I change channel? Pins 11-16 change depending on channel. I noticed those are the 6 connected back to the channel selector. So I assume that’s normal? Voltage varies form 0 to 8.
Pins 4 -6 showing ok now.
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Actually, you're doing great!
We just have to figure out why Pin 17 isn't seemnig to get a signal from TR20.
Ok, to tell you more about this PLL loop thingy...
TR20 acts like a mixer, but in doing so, it produces two range of frequencies...
One set very high above and not needed but its' there nonetheless.
The other, is a SUBTRACTION - one frequency from IC2 and it's L19, the other from L21 and its Tripler.
3 times the frequencies you need, but this is what gives it it's accuracy - they could have just done 2 (a doubler) and been done - but the PLL would not be fully able to track all the channel frequencies accurately - it would have "missed" a lot and so it would have drifted or warbled off at times - FCC didn't want that. (it does have SSB so you need the accuracy)
So Cobra was forced to make a Tripler - in doing so, increased the pressure on a set of parts to make this work...Tr29, L21 and R158 and C111 - although nothing serious, it does put the part into a lot of strong noise from this and L21 kinda sorts it down to a range that TR20 can use to "count" against what is from IC2.
So that mess of all those frequencies is then filtered down by being put into a Lo-Pass filter L16, (Tp10) L17, C87 and C86 - a set of coils and caps designed to find and filter out the low-end of the output of TR20 so Pin 17 can utilize it. You can think of Pin 17, like PLINKO - going against all those Channel Selector "pins" - dividing down as it trickles thru to find Pin 5 - meanwhile another part of the PLL is looking as all of this like a watchdog - making sure the PLL is doing this correctly - if anything is amiss, then Pin 6 goes Low and WHAM - no TX.
So when things are good and a signal is at Pin 17 that the PLL can SEE and use, then its' parsed/divided down and a pulse or result is this "spike" on Pin 5 - the why the results is made, is by dividing - counting the number - then it's a carry - onto Pin 5 - then the Pin17 is counted again - another carry...and so it goes...
IF the count is SLOWER - the counting takes LONGER - so the RESULT takes longer to appear at Pin 5 - so the 1 and 2, 3 and 4 still work, but the firing of Pin 5 takes longer - so the control output of those 1 - 5 into R207 is lower, this makes the Varactor capacitance change to a LOWER value, so it then makes L19 ring to a higher frequency and this is what I meant when I said it Speeds up - lower voltage - faster oscillator at TP9 - when the L21 and L19 are close - the count races up and down at the same rate - so the frequency doesn't change (Varactors) means it's in lock. Pin 6 goes HIGH and your able to TX.- Too fast at TR20 or L21 then the counter divides faster, the spikes appear sooner, and then Pin 5 pulses are appearing faster and out of a proper proportion to 1 and 2, 3 and 4 - RAISING the output to R207 (TP9) The Varactor capacitance increases and so L19 Ringing slows down to a lower slower frequency. Makes those Pin 5 hits take a little longer - and the PLL slows down its oscillator i this fashion.
So what you need to locate, is that output of TR20 that goes thru that Lo-pass filter and into Pin 17 - gotta' find that signal!
TR20 can go bad, so you might want to look for a spare to swap in. See if it can attain and run those frequencies thru it without blowing up.
We just have to figure out why Pin 17 isn't seemnig to get a signal from TR20.
Ok, to tell you more about this PLL loop thingy...
TR20 acts like a mixer, but in doing so, it produces two range of frequencies...
One set very high above and not needed but its' there nonetheless.
The other, is a SUBTRACTION - one frequency from IC2 and it's L19, the other from L21 and its Tripler.
3 times the frequencies you need, but this is what gives it it's accuracy - they could have just done 2 (a doubler) and been done - but the PLL would not be fully able to track all the channel frequencies accurately - it would have "missed" a lot and so it would have drifted or warbled off at times - FCC didn't want that. (it does have SSB so you need the accuracy)
So Cobra was forced to make a Tripler - in doing so, increased the pressure on a set of parts to make this work...Tr29, L21 and R158 and C111 - although nothing serious, it does put the part into a lot of strong noise from this and L21 kinda sorts it down to a range that TR20 can use to "count" against what is from IC2.
So when things are good and a signal is at Pin 17 that the PLL can SEE and use, then its' parsed/divided down and a pulse or result is this "spike" on Pin 5 - the why the results is made, is by dividing - counting the number - then it's a carry - onto Pin 5 - then the Pin17 is counted again - another carry...and so it goes...
IF the count is SLOWER - the counting takes LONGER - so the RESULT takes longer to appear at Pin 5 - so the 1 and 2, 3 and 4 still work, but the firing of Pin 5 takes longer - so the control output of those 1 - 5 into R207 is lower, this makes the Varactor capacitance change to a LOWER value, so it then makes L19 ring to a higher frequency and this is what I meant when I said it Speeds up - lower voltage - faster oscillator at TP9 - when the L21 and L19 are close - the count races up and down at the same rate - so the frequency doesn't change (Varactors) means it's in lock. Pin 6 goes HIGH and your able to TX.- Too fast at TR20 or L21 then the counter divides faster, the spikes appear sooner, and then Pin 5 pulses are appearing faster and out of a proper proportion to 1 and 2, 3 and 4 - RAISING the output to R207 (TP9) The Varactor capacitance increases and so L19 Ringing slows down to a lower slower frequency. Makes those Pin 5 hits take a little longer - and the PLL slows down its oscillator i this fashion.
So what you need to locate, is that output of TR20 that goes thru that Lo-pass filter and into Pin 17 - gotta' find that signal!
TR20 can go bad, so you might want to look for a spare to swap in. See if it can attain and run those frequencies thru it without blowing up.
I checked Tr20 again. All 3 readings looks good. I get 3.3, 8.2, and 4.0. I also get 3.7 at pin 17-all of which are normal right?
I get a max of 1.7 volts at L19 TP9 but it “flickers” around a lot while turning the slug. . I checked the VCO again. The one odd out there is Pin5. Says it should be 3.4 volts but I’m only getting 0.5.
Also just for info, I can peak out L21 on scope to 5.4 volts p to p (Tp 10) and L20 to 470 mV p to p (Tp 1)
could there be a loose or bad solder joint somewhere in there? I’ve looked up super close with magnifier a d don’t really see one but who knows. Or could that VCO pin 5 be an issue?
thanks for all the help!
other weird shit.
When switching to ssb, the offsets almost are backwards. Another words when I go to LSB, the frequency goes higher. When I go to USB it goes lower. And if I place L19 “just right” the channel selector will work but only down to channel 12. Below that it doesn’t change. Grrr!! I’m sure it’s all related.
Just relax, Rome wasn't built in a Day...
Ok, tells me there is more, but with PLL Pin 5 you're getting a low voltage - so that indicates a loop there isn't right...
Let's go a little further in...
You'll need to put an oscilloscope on Pin 17 of the PLL. You need to know if you're getting anything as RF in there.
Needs to be within 2MHz or less, so you don't need any elaborate setups - but you'll need to scope it to find any signal - that would prove TR20 is doing it's job.
I'm leaving this here as a scratchpad for us to work off of...
The PLL is the MB8734, but will work for 8719 also (Electrically similar)
I would like to know of the condition of the Channel Selector - I've known of Radios that got "face planted" on the channel knob, breaking the wafers and rotary switch assembly getting bent - causing it to lob and become eccentric, so the switch didn't work 100% - some channels work because of a specific set of two pins off the Selector that operate the LOCK line...lined up to work that channel. Your symptoms remind me to ask you - because this symptom may be more of a mechanical than a Electrical condition.
Looking at front LEFT Corner by PLL and Ribbon cable from Channel selector
Ok, tells me there is more, but with PLL Pin 5 you're getting a low voltage - so that indicates a loop there isn't right...
Let's go a little further in...
You'll need to put an oscilloscope on Pin 17 of the PLL. You need to know if you're getting anything as RF in there.
Needs to be within 2MHz or less, so you don't need any elaborate setups - but you'll need to scope it to find any signal - that would prove TR20 is doing it's job.
I'm leaving this here as a scratchpad for us to work off of...
The PLL is the MB8734, but will work for 8719 also (Electrically similar)
Looking at front LEFT Corner by PLL and Ribbon cable from Channel selector
- When the knob isn't lined up, the switch GROUNDS the LOCK line (holds it low) which keeps the PLL for even seeking any input - Pin 6 works BOTH ways - it's the Watchdog of the chip - things ok - it will let LOCK work, if something isn't right outside - the LOCK pin wont allow the PLL to even work or try to align up a signal to tune thru the BCD programming pins.
- Verify that Pin 6 LOCK line is being "released" to float at a voltage, even though it is not in lock - it should be about 0.3 to as much as 1.2 volts floating - if the Channel Selector switch is working right.
- THERE IS A JUMPER you can remove it to help you troubleshoot the Channel Selector - just remove the jumper to defeat the interlock on the channel selector...
- in doing this, you may need to MANUALLY turn off power and turn it back on to reset the pins to load the programming - the Jumper works with Pin 6 to accomplish the "auto-addressing" load feature when Pin 6 goes low to change the channel - the PLL stops, When you are finished changing the channel - the Pin 6 goes back to "float" voltage - this allows the PLL to "see" new channel data and load the setting to get the new divider result - this is the INTERLOCK the channel selector uses.
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View attachment 43228
Channel selector knob is in excellent condition from the look and feel of it.
I don’t see a ribbon cable or jumper that you reference. There are 7 different colored wires soldered directly into the board near the pll that I assume come from the channel selector. See attached pic. There are a couple of small jumpers in front of that but not sure which one to remove. See pic.
like I mentioned the channel selector changes the frequency normally until you get down to channel 12. The voltage reading as you change the channels steps down nicely. But when you get to channel 12 it goes from 1.2 volts to about 235 mv and stays there for other channels below that.
getting about 8.2 volts at pin 6 peak to peak after turning radio off then on again.getting about 1.0 MHz at pin 17.
Channel selector knob is in excellent condition from the look and feel of it.
I don’t see a ribbon cable or jumper that you reference. There are 7 different colored wires soldered directly into the board near the pll that I assume come from the channel selector. See attached pic. There are a couple of small jumpers in front of that but not sure which one to remove. See pic.
like I mentioned the channel selector changes the frequency normally until you get down to channel 12. The voltage reading as you change the channels steps down nicely. But when you get to channel 12 it goes from 1.2 volts to about 235 mv and stays there for other channels below that.
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