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cobra 2000 receiving and tx but channel not changing.
- Thread starter wildwinds
- Start date
Its also good to mention that when attempting to power up these 40-50 year old radios and amps you should use a variac and slowly bring the unit up to full voltage. Its a cheap tool if you play with old radios and amps that have been sitting or packed away for many years.
Unless you're referring to the RCI8719, the common mode thinking is, they've never had one...
A 385 board...
@wildwinds - I'm going to throw a lot at you...just look this stuff over and see if any of my notes can help you figure this out...
One user to look up is @Robalo - to look thru his threads as he too is trying to get a similar board working.
Now, in your instance, what he came across - shows some interesting problems...
L21 itself may have failed - but then that would rely on knowing what you find on the BOTTOM of the coil when you unsolder it from the board.
Note his "flat pac"
His UHIC is a SMD board not coated or dipped...
Ok, with that being said... let's go on...
I got this screenshot to help out Robalo - I posted something similar earlier but if you got this schematic - then it's able to give you more info on what you should get in voltages - on the pins...
Again, note the Low-pass filter.
Now, in my thoughts here, you're L21 OR L19 has failed - the "Varactor output" is pretty high, you're running Pin 10 of the VCO thru into a voltage divider into TR20 thru R102 and R103 - this already puts that part into CLASS A operation, so to see 6 or more volts indicates that L21 is not turning - it's not vibrating nor is any ringing event occurring to make the oscillator even "go" - it's steady state. It's dead. Your DC output (not mixed with AC) is pumping into TR20's base...L19 is supposed to use the Varactors inherited capacitance traits from that Charge Pump and correction voltage to even start a ringing loop feedback to get the oscillator to even generate a frequency. This forces Pin 5 voltage to "vary" like an AC waveform imposed on a DC bias - as you'd have with R102 and R103 - being Class A biasing - on TR20 - to even see the waveform and run that loop. BUT R102 and R103 are OUTSIDE of the loop and only to make TR20 sensitive enough to SENSE the small changes and amplify them.
Pin 5? That's the PLL interacting with the Varactor to change it's inherited capacitance traits and keep the oscillation steady. Pin 10 goes to TR20 as a "slow beat sensor" that takes the 33 to 35MHz signal ringing going on and amplify it for Pin 17 of the PLL You don't, it's +6 steady state DC - volts so something isn't right.
Take a look here, if you have Rick Jacksons' Schemo, you'd have this too...
Pin 10 and 5 go to the MB8719 as part of the loop, and when dealing with oscillating circuits, to obtain and MAINTAIN any ringing effect or oscillation to keep a frequency - you can't "load it" with too much current.
Look above, Both Pin 10 and 5 are BUFFERED by some sort of capacitance to keep the oscillator working and just tap off AC vibration (your frequency) off the energy from the ringing event. So, it's not supposed to ADD voltages to your TR20 base - ONLY add an AC component to it that makes this section pulse with energy from the oscillator running inside...
Here's a later 148 model the 148F-GTL
Just some stuff to think about...
Ok, you're getting a frequency, and its on L21, but your TR20 is not sending any information to the PLL thru Pin 17 - or there is something else going on.
So when you mentioned capacitors - yes, just "tack on" a cap below - on the solder pad, foil side across the caps leads of each one SEPERATELY to see if the AC signal can pass thru - else remove the cap and put in (swap) with another to see if they shorted out...
Now for CAUTION - if the caps shorted out, the voltage on Pin 17 would be HIGHER than shown above...if the cap is blown or drifted off - they would force Pin 17 to SINK power and show lower than stated...
One user to look up is @Robalo - to look thru his threads as he too is trying to get a similar board working.
Now, in your instance, what he came across - shows some interesting problems...
L21 itself may have failed - but then that would rely on knowing what you find on the BOTTOM of the coil when you unsolder it from the board.
Note his "flat pac"
His UHIC is a SMD board not coated or dipped...
Again, note the Low-pass filter.
Now, in my thoughts here, you're L21 OR L19 has failed - the "Varactor output" is pretty high, you're running Pin 10 of the VCO thru into a voltage divider into TR20 thru R102 and R103 - this already puts that part into CLASS A operation, so to see 6 or more volts indicates that L21 is not turning - it's not vibrating nor is any ringing event occurring to make the oscillator even "go" - it's steady state. It's dead. Your DC output (not mixed with AC) is pumping into TR20's base...L19 is supposed to use the Varactors inherited capacitance traits from that Charge Pump and correction voltage to even start a ringing loop feedback to get the oscillator to even generate a frequency. This forces Pin 5 voltage to "vary" like an AC waveform imposed on a DC bias - as you'd have with R102 and R103 - being Class A biasing - on TR20 - to even see the waveform and run that loop. BUT R102 and R103 are OUTSIDE of the loop and only to make TR20 sensitive enough to SENSE the small changes and amplify them.
Pin 5? That's the PLL interacting with the Varactor to change it's inherited capacitance traits and keep the oscillation steady. Pin 10 goes to TR20 as a "slow beat sensor" that takes the 33 to 35MHz signal ringing going on and amplify it for Pin 17 of the PLL You don't, it's +6 steady state DC - volts so something isn't right.
Take a look here, if you have Rick Jacksons' Schemo, you'd have this too...
Pin 10 and 5 go to the MB8719 as part of the loop, and when dealing with oscillating circuits, to obtain and MAINTAIN any ringing effect or oscillation to keep a frequency - you can't "load it" with too much current.
Look above, Both Pin 10 and 5 are BUFFERED by some sort of capacitance to keep the oscillator working and just tap off AC vibration (your frequency) off the energy from the ringing event. So, it's not supposed to ADD voltages to your TR20 base - ONLY add an AC component to it that makes this section pulse with energy from the oscillator running inside...
Here's a later 148 model the 148F-GTL
Just some stuff to think about...
Ok, you're getting a frequency, and its on L21, but your TR20 is not sending any information to the PLL thru Pin 17 - or there is something else going on.
So when you mentioned capacitors - yes, just "tack on" a cap below - on the solder pad, foil side across the caps leads of each one SEPERATELY to see if the AC signal can pass thru - else remove the cap and put in (swap) with another to see if they shorted out...
Now for CAUTION - if the caps shorted out, the voltage on Pin 17 would be HIGHER than shown above...if the cap is blown or drifted off - they would force Pin 17 to SINK power and show lower than stated...
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Good Morning all
i must say ye have great patience to stick with this problem, thank you .
Its very good of you to take the time and explain it so well. The diagrams are fantastic.
I started exploring the low pass filter stage, checking capacitors, no change but then i checked out L17 it read ok, but when i replaced it back to the board the voltages on the collector of TR20 is now 8.2 v.
base is low at 2.01 and emitter 4.6.
there may have been a bad connection on the L17.
I still not seeing any stable signal on pin 17 of pll perhaps just something very small and unstable
What rf voltage should that signal be approx at pin 17? Should it be 1.360 at this stage.
TP1 ....100Mv at 35.335Mhz
TP9.... 6.8v
TP10....8.2V
TR29....C ...33.975MHz.... E....11.325MHz
At least its progress.
thanks again
i must say ye have great patience to stick with this problem, thank you .
Its very good of you to take the time and explain it so well. The diagrams are fantastic.
I started exploring the low pass filter stage, checking capacitors, no change but then i checked out L17 it read ok, but when i replaced it back to the board the voltages on the collector of TR20 is now 8.2 v.
base is low at 2.01 and emitter 4.6.
there may have been a bad connection on the L17.
I still not seeing any stable signal on pin 17 of pll perhaps just something very small and unstable
What rf voltage should that signal be approx at pin 17? Should it be 1.360 at this stage.
TP1 ....100Mv at 35.335Mhz
TP9.... 6.8v
TP10....8.2V
TR29....C ...33.975MHz.... E....11.325MHz
At least its progress.
thanks again
Unfortunately Thanks to Wild for starting this thread . I hope you get it fixed soon , Special Thanks to Andy , Nomad & any others that have contributed 
Being a Cobra 2K owner I am absorbing as much of this as I can ( not that I have this problem . Also have printed the info out & saved it ! I think I mentioned it in the beginning Wild " Help will be on it's way ! " We have some smart op's on this forum ! 73 & God Bless , Leo
Being a Cobra 2K owner I am absorbing as much of this as I can ( not that I have this problem . Also have printed the info out & saved it ! I think I mentioned it in the beginning Wild " Help will be on it's way ! " We have some smart op's on this forum ! 73 & God Bless , Leo
Well, the PLL chip itself as with any Linear device - you can't apply voltages that exceed it's sources. So if you're running 8 volts into it - the P -2- P (Peak to Peak) should not exceed that - so you should see a signal about 5 to 6 volts peak max on your scope - it floats at 1/2 supply rail - or the ~3.8V reference seen on the Datasheets. So it don't swing much...
That's what I mean by sinking or supplying power into the TR20 Low-Pass mess. you don't need a lot of signal to upset it's apple cart.
Ok, to get back to what we see.
Tr20 looks good, but if the Hfe in it, has gone south (as in a bad transistor that got too hot) you get great DC voltages but lousy Frequency response past Audio frequencies. So keep options open with that one...
Lets look further back, into the PLL itself - ok look at pins 1 and 2 as a set, and Pins 3 and 4 as a SEPARATE set. Also Pin 5 and where it is in a loop caught up in those pins. There is also a Pin 6 with a Diode designed to FORCE or Pull power from a line. Follow that line it's goes to a lot of places including the MB3756 Regulator...But come back because we have to see why this, nearly entire half of the chip, isn't oscillating too. Because R163 is a 22K - its'; for the use of Pull-up to keep marginal locks working unless there is a catastrophic failure of the system. Marginal locks? Yes, thermal is one, voltage dropouts from Rx/Tx switching issues - are another, otherwise a slow cap that takes too long to charge onto something pulling down Regulated voltages the PLL looks to - and causing it's own on-board regulator to shift drivers - yes a PLL needs an on-board regulator too so you know...
Notice where and how TP9 works and where it is, it's using the PLL free-running loop pins 1-2 and 3-4
These two pairs form a free running "gated" oscillator - one high speed, another low speed. Between the two - is where Pin 17 is "compared" and if it misses - it sets a count - and too many misses generate a output on Pin 5 - Pin 5 is your correction to the Varactors' ability to make L21, L19 and L21 work...
You mentioned L17 - I think you meant L21 or even L20 - L17 is where? on your board? (it's a coil so it would show a short) Maybe you can scope that one side at a time - and you'd have MHz (Read a LOT OF HERTZ) of frequency on one side but not so many MHz on the other (attenuated) - it's a low-frequency choke - like power supplies use...take a higher frequency ripple and smooth's it out - its is not perfect nor is it used in that fashion - just acts as a high impedance doorway - to keep high-frequency out (stays with the VCO side) and let low-frequency (the Clock Errors - your BEAT Frequency differences) pass into Pin 17 to cycle with the other half f the chip.
Now, if you are getting 33 MHz then it looks like you now have the UHIC chip running, you're just not getting the PLL gating event to compare to Pin 17 - so scope the 1-2 pin and 3-4 pin with a high-impedance probe looking for about 10MHz for starters to see if you can see "clocking" going on.
Why the 1-2 and 3-4 pins? Well, Pin 5 is needed so that the PLL thinks it's ok to compare Pin 17's frequency input and send any data out - thru Pin 5 - so it looks and uses 1-2 and 3-4 mess that cycles - to generate either a pulse or hold low - to correct the Varactor. So if 1-2 and 3-4 are not "oscillating" - Pin 5 won't work and so the PLL thinks something's wrong and sets Pin 6 to lock out everybody from using the signal present.
That's what I mean by sinking or supplying power into the TR20 Low-Pass mess. you don't need a lot of signal to upset it's apple cart.
Ok, to get back to what we see.
Tr20 looks good, but if the Hfe in it, has gone south (as in a bad transistor that got too hot) you get great DC voltages but lousy Frequency response past Audio frequencies. So keep options open with that one...
Lets look further back, into the PLL itself - ok look at pins 1 and 2 as a set, and Pins 3 and 4 as a SEPARATE set. Also Pin 5 and where it is in a loop caught up in those pins. There is also a Pin 6 with a Diode designed to FORCE or Pull power from a line. Follow that line it's goes to a lot of places including the MB3756 Regulator...But come back because we have to see why this, nearly entire half of the chip, isn't oscillating too. Because R163 is a 22K - its'; for the use of Pull-up to keep marginal locks working unless there is a catastrophic failure of the system. Marginal locks? Yes, thermal is one, voltage dropouts from Rx/Tx switching issues - are another, otherwise a slow cap that takes too long to charge onto something pulling down Regulated voltages the PLL looks to - and causing it's own on-board regulator to shift drivers - yes a PLL needs an on-board regulator too so you know...
Notice where and how TP9 works and where it is, it's using the PLL free-running loop pins 1-2 and 3-4
These two pairs form a free running "gated" oscillator - one high speed, another low speed. Between the two - is where Pin 17 is "compared" and if it misses - it sets a count - and too many misses generate a output on Pin 5 - Pin 5 is your correction to the Varactors' ability to make L21, L19 and L21 work...
You mentioned L17 - I think you meant L21 or even L20 - L17 is where? on your board? (it's a coil so it would show a short) Maybe you can scope that one side at a time - and you'd have MHz (Read a LOT OF HERTZ) of frequency on one side but not so many MHz on the other (attenuated) - it's a low-frequency choke - like power supplies use...take a higher frequency ripple and smooth's it out - its is not perfect nor is it used in that fashion - just acts as a high impedance doorway - to keep high-frequency out (stays with the VCO side) and let low-frequency (the Clock Errors - your BEAT Frequency differences) pass into Pin 17 to cycle with the other half f the chip.
Now, if you are getting 33 MHz then it looks like you now have the UHIC chip running, you're just not getting the PLL gating event to compare to Pin 17 - so scope the 1-2 pin and 3-4 pin with a high-impedance probe looking for about 10MHz for starters to see if you can see "clocking" going on.
Why the 1-2 and 3-4 pins? Well, Pin 5 is needed so that the PLL thinks it's ok to compare Pin 17's frequency input and send any data out - thru Pin 5 - so it looks and uses 1-2 and 3-4 mess that cycles - to generate either a pulse or hold low - to correct the Varactor. So if 1-2 and 3-4 are not "oscillating" - Pin 5 won't work and so the PLL thinks something's wrong and sets Pin 6 to lock out everybody from using the signal present.
- What can throw you for a loop here (SIC) is the fact that the TP9 has both DC and a weak AC component to it on the same line at the same time. If C90 (R104 / C90) is bad or shorted, you'll have too low of power to even get the oscillator to start. so look for and check values on that TP9 line...
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Thank you. It's just such a beautiful radio, and the fact it brought many contacts from USA and Europe. In fact my father made several friends in USA and developed into writing letters.
Back in those late seventies early eighties, there were no mobile phones set etc, so for us it was amazing to hear skip and make contacts.
We had a moonraker 4 attached and on a few occasions we contacted Australia...memories
Back in those late seventies early eighties, there were no mobile phones set etc, so for us it was amazing to hear skip and make contacts.
We had a moonraker 4 attached and on a few occasions we contacted Australia...memories
Ok theres a good deal of info there to check .
I have to take a break away from it now for a few weeks worse luck.
So Is it ok to resume when I get back .
J
I have to take a break away from it now for a few weeks worse luck.
So Is it ok to resume when I get back .
J
Walking away from a knotty problem can gain you a fresh look, later on when you're ready.
I still suggest concentrating on the parts "upstream" from pin 17 of the PLL chip.
We'll leave a light on.
73
I still suggest concentrating on the parts "upstream" from pin 17 of the PLL chip.
We'll leave a light on.
73
Thanks lads, I know what you mean nomad
So I'm am not at the radio but the problem is still in my head.
So tr 2o takes in the tripled crystal freq.
Also the frequency generated by the vco, I still not sure how the vco gets its frequency from,
It them mixes them and sends on the signal to the low pass filter, who just let's the below 2mhz signal carry on to pin 17 of the pll.
L16 l17 and a few capacitors are suspects
Also I think I will change tr20 when I get back to it.
Thanks..j
So I'm am not at the radio but the problem is still in my head.
So tr 2o takes in the tripled crystal freq.
Also the frequency generated by the vco, I still not sure how the vco gets its frequency from,
It them mixes them and sends on the signal to the low pass filter, who just let's the below 2mhz signal carry on to pin 17 of the pll.
L16 l17 and a few capacitors are suspects
Also I think I will change tr20 when I get back to it.
Thanks..j
I'm working with @Robalo too...
You can see more cobra 148 unlock PLL concepts and troubleshooting here...
We too are also looking at L21 mess...that goes to TR20
This is the L21 Tripler mess and its associated...
So TR20 needs something from it's BASE going to the UHIC chip, at 33MHz or so at the same time L21 is producing this signal too - - but applied to the Emitter of TR 20 - the Collector shows, or reflects the output onto that Low-pass filter that goes to Pin 17.
If too much 33MHz+ signal goes to the chip - it won't or it can't sort it out - it has to be a low frequency for it to even "Track" - so if tis' getting a lot of RF from this section at 33MHz or above, it can't do anything with it.
You can see more cobra 148 unlock PLL concepts and troubleshooting here...
We too are also looking at L21 mess...that goes to TR20
This is the L21 Tripler mess and its associated...
So TR20 needs something from it's BASE going to the UHIC chip, at 33MHz or so at the same time L21 is producing this signal too - - but applied to the Emitter of TR 20 - the Collector shows, or reflects the output onto that Low-pass filter that goes to Pin 17.
If too much 33MHz+ signal goes to the chip - it won't or it can't sort it out - it has to be a low frequency for it to even "Track" - so if tis' getting a lot of RF from this section at 33MHz or above, it can't do anything with it.
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Good afternoon all,
so the radio is fixed,
Came back yesterday after our walk refreshed !!!
SO this morning had a look at all the posts and decided to have a another look
I did a check on the back of the board for shorts and or broken tracks ...nothing all good.
So back to in 17 and backwards checked out the capacitors resistors in the plf all good
Then i took out the tr20 checked it it read ok on the tester, but your comment about this suggested i change it, So i had an old mckinley here way past repair and took a c1675 from it and it burst into life.
So the motto take nothing for granted..
I wish to thank Handy Andy and Nomadradio and Robalo for their help.
I hope Robalo you have success with your radio... i know you will.
Best regards
jimmy
so the radio is fixed,
Came back yesterday after our walk refreshed !!!
SO this morning had a look at all the posts and decided to have a another look
I did a check on the back of the board for shorts and or broken tracks ...nothing all good.
So back to in 17 and backwards checked out the capacitors resistors in the plf all good
Then i took out the tr20 checked it it read ok on the tester, but your comment about this suggested i change it, So i had an old mckinley here way past repair and took a c1675 from it and it burst into life.
So the motto take nothing for granted..
I wish to thank Handy Andy and Nomadradio and Robalo for their help.
I hope Robalo you have success with your radio... i know you will.
Best regards
jimmy
