Cobra 2000 No voltage at Test points 7 and 8

[Dmans]

The one that connects over to C157 is the one that is wrong.

So perhaps something weird there.

I would make sure that your "borrowed/re-purposed" L44 has the enamel scraped off the magnet wire where it is soldered to the board. It may have been previously(before you repurposed it) but could be stuck further through the circuit board in this chassis.

Just a wild guess-but worth looking at.

73
David

 

[Handy Andy]

Here's some help...

L44 - IF you're going to have any success at all...needs to be wired right...

Taken from PC-122
(everything else uses that Glue)​

So:

RED - Goes to Pre-Driver Collector
GOLD - Goes to POWER (8 V TX)
TWIST - Goes to Input Cap that heads to Driver​

That has been the "standard" Uniden follows for pretty much all their line.

Easier to think of RED for RF line, From Pre-Driver then winds (winds) across to the Driver - Gold mines the Power.

As far as Capacitance used, hate to tell you but this did vary amongst the models years of production.

The basic was the Power (GOLD) used a 103 Disc, which is ok to a point - when it got weird was that they chose other smaller caps to offset the output of the Predriver and the Input Cap to the Driver from this little part.

Go back to your Donor radio - see what they used for these ....

​

Look for the STARRED components - why? Not all L44's are created equal - they put these together in these radios but seems that each brand and model had to be "tweaked" - so to help you with that - you'll need to go back to the radio L44 came from and locate and use the values those parts are.

 

[Brian G]

I would make sure that your "borrowed/re-purposed" L44 has the enamel scraped off the magnet wire where it is soldered to the board. It may have been previously(before you repurposed it) but could be stuck further through the circuit board in this chassis.

Just a wild guess-but worth looking at.

73
David

Yeah there still is yellow glue like stuff still stuck to it. Was afraid to pick it off in fear of damaging it. But I will attempt it.

 

[Brian G]

Here's some help...

L44 - IF you're going to have any success at all...needs to be wired right...

View attachment 44327
Taken from PC-122
(everything else uses that Glue)​

So:

RED - Goes to Pre-Driver Collector
GOLD - Goes to POWER (8 V TX)
TWIST - Goes to Input Cap that heads to Driver​

That has been the "standard" Uniden follows for pretty much all their line.

Easier to think of RED for RF line, From Pre-Driver then winds (winds) across to the Driver - Gold mines the Power.

As far as Capacitance used, hate to tell you but this did vary amongst the models years of production.

The basic was the Power (GOLD) used a 103 Disc, which is ok to a point - when it got weird was that they chose other smaller caps to offset the output of the Predriver and the Input Cap to the Driver from this little part.

Go back to your Donor radio - see what they used for these ....

View attachment 44328​

Look for the STARRED components - why? Not all L44's are created equal - they put these together in these radios but seems that each brand and model had to be "tweaked" - so to help you with that - you'll need to go back to the radio L44 came from and locate and use the values those parts are.

Yeah problem is I got that replacement from someone on here so I don’t have the donor radio to look at those other parts. In my radio, I don’t think C203 was even present. Is that possible? I saw the holes for it but was curious why it looked like nothing was in there. I stuck a 33 Pf in there but that didn’t make any difference with it without it. C157 is a .0047 in mine already. Not sure which part that third star is referring to.

In addition there is a small cap soldered on the bottom side of the board between 2 legs of L44. Not in front of it now such can’t tell you the value but I think that is factory as well because I’ve seen views of other Cobra 2000 boards and I see that cap there in most views. Not sure what that does.

I will confirm the connections of l44 from your pic.

 

[Handy Andy]

Ok, more than likely it is "stock" and can be a DIFFERENT value than state in the schematics - it's what I wanted to advise you on, UNIDEN set up their coupler one way, others didn't always follow - and if they did, may have oriented the leads of L44 differently.

In basic, remember that 0.0047uF (472 Disc) is the next RANGE of Lower values used by Uniden and Cobra DOWN (Smaller) to reduce RF shunting - meaning they allowed for more RF power to transfer from one side (Pre Driver) across to the Other (Driver) the coupling cap to the Driver used in these Cobra boards was a 0.0047uF or 472 - while UNIDEN used even smaller values of coupling to the Driver - usually between 0.0018uF (182 Disc) to as low as 0.001uF (102 Disc) - so the 0.0047 used is pretty small - but if the coil is not soldered in right, that value is too high and you lost more RF in the shunt it is, than can make across to the Coupling Driver cutting off the output.

​

In UNIDEN's way of thinking, they wanted to get rid of the 2nd harmonics and other noise so they used higher values of capacitance when it comes to power supply RF Bypass - so they used a 0.01uF (103 Disc) being their mainstay of value to take RF of the power lines at places like where L44 works. Cobra used 0.0047 at C158 - Grant XL had used values from 0.0047uF (472 - like Cobra) onto as high as 0.047uF (473) on later models. (It can explain why Uniden never produced a lot of Schematics in later years, too many revisions along the years a radio was produced - makes for a TON of paperwork others don't need to have on file)

So, while you work on this - look at my photos - your line that goes to the "GOLD" and the Ferrite (if you have it) that line is the 8V TX power line - the cap used is 0.0047 (472 Disc), might want to use a lower, smaller - value or - remove it for now to help you find signal to send over.

When you work this section I hope you're "tweaking" the Pre-drivers input coil to see if any RF peaking has been affected by the change of capacitance in this L44 area - for it can (does) change the performance of the Pre Driver.

This L44 coil also acts like a tuning trap - so it is "frequency sensitive" - until you get the radio stable and able to key up - keep the 0.0047uF as is or use a lower, smaller value to help improve the amount of RF that can transfer over - then as you're able to stabilize, this cap can be returned to ORIGINAL 0.0047 (472 Disc) as you retune the Pre Drivers Input tank circuit and peak out this section...L45 - for as you retune the section will respond strongly to specific ranges of tune L45 produces and these "peaks" you can then tweak even better by changing the caps values (Especially C158) to help peak out the section.

Now you said something that scares me, it's about C157, you're seeing 45MHz or thereabouts, that's a free running resonance you don't want - that is what I call a Parasitic - it can kill parts upstream, because it's from the "ringing" they can produce in oscillation - their main peak resonance frequency.

Be careful and keep you're power being applied times short - VERY SHORT - just to verify - until you get a handle on that 45MHz signal and tame it.

 

[Brian G]

Ok, more than likely it is "stock" and can be a DIFFERENT value than state in the schematics - it's what I wanted to advise you on, UNIDEN set up their coupler one way, others didn't always follow - and if they did, may have oriented the leads of L44 differently.

In basic, remember that 0.0047uF (472 Disc) is the next RANGE of Lower values used by Uniden and Cobra DOWN (Smaller) to reduce RF shunting - meaning they allowed for more RF power to transfer from one side (Pre Driver) across to the Other (Driver) the coupling cap to the Driver used in these Cobra boards was a 0.0047uF or 472 - while UNIDEN used even smaller values of coupling to the Driver - usually between 0.0018uF (182 Disc) to as low as 0.001uF (102 Disc) - so the 0.0047 used is pretty small - but if the coil is not soldered in right, that value is too high and you lost more RF in the shunt it is, than can make across to the Coupling Driver cutting off the output.

View attachment 44344​

In UNIDEN's way of thinking, they wanted to get rid of the 2nd harmonics and other noise so they used higher values of capacitance when it comes to power supply RF Bypass - so they used a 0.01uF (103 Disc) being their mainstay of value to take RF of the power lines at places like where L44 works. Cobra used 0.0047 at C158 - Grant XL had used values from 0.0047uF (472 - like Cobra) onto as high as 0.047uF (473) on later models. (It can explain why Uniden never produced a lot of Schematics in later years, too many revisions along the years a radio was produced - makes for a TON of paperwork others don't need to have on file)

So, while you work on this - look at my photos - your line that goes to the "GOLD" and the Ferrite (if you have it) that line is the 8V TX power line - the cap used is 0.0047 (472 Disc), might want to use a lower, smaller - value or - remove it for now to help you find signal to send over.

When you work this section I hope you're "tweaking" the Pre-drivers input coil to see if any RF peaking has been affected by the change of capacitance in this L44 area - for it can (does) change the performance of the Pre Driver.

This L44 coil also acts like a tuning trap - so it is "frequency sensitive" - until you get the radio stable and able to key up - keep the 0.0047uF as is or use a lower, smaller value to help improve the amount of RF that can transfer over - then as you're able to stabilize, this cap can be returned to ORIGINAL 0.0047 (472 Disc) as you retune the Pre Drivers Input tank circuit and peak out this section...L45 - for as you retune the section will respond strongly to specific ranges of tune L45 produces and these "peaks" you can then tweak even better by changing the caps values (Especially C158) to help peak out the section.

Now you said something that scares me, it's about C157, you're seeing 45MHz or thereabouts, that's a free running resonance you don't want - that is what I call a Parasitic - it can kill parts upstream, because it's from the "ringing" they can produce in oscillation - their main peak resonance frequency.

Be careful and keep you're power being applied times short - VERY SHORT - just to verify - until you get a handle on that 45MHz signal and tame it.

ok will check it out.
What is that cap soldered to the solder side of the board at L44?
Is it possible that C203 was not even installed in my radio? If so, should I leave it out or out one in (as I did)?
Do you think the 45 MHz rogue signal is being caused by something at the L44 area?

 

[Handy Andy]

Look at the pads that cap solders to, could be C203 as the cap, It should be the RED pad for the Pre-Drivers Collector leg, the other GOLD to the Power Feed

Refer back to my post for those colors.

The 45MHz rouge signal MORE THAN LIKELY - is a parasitic oscillation you don't want.
C158 is important for this...

The area you're working in, uses L44 and those caps - as a resonant tank circuit - much like the coupling tank circuit of the Final to the output network. It's a similar process - power goes in and to make it work, needs to make the power flow and resonant for RF - Exist at the same time

 

[Brian G]

Look at the pads that cap solders to, could be C203 as the cap, It should be the RED pad for the Pre-Drivers Collector leg, the other GOLD to the Power Feed

Refer back to my post for those colors.

The 45MHz rouge signal MORE THAN LIKELY - is a parasitic oscillation you don't want.
C158 is important for this...

The area you're working in, uses L44 and those caps - as a resonant tank circuit - much like the coupling tank circuit of the Final to the output network. It's a similar process - power goes in and to make it work, needs to make the power flow and resonant for RF - Exist at the same time

Ok so with my frequency counter I am getting 27.189 (which is what the radio channel is set on and what the radios counter is showing) all the way from the base of the pre driver to the “out” of L44 to C157 to the base of driver. That’s as it should be, correct??

But still there is no RF power, no bias current at all on driver or final.


I am getting 8 volts all the way through from the predriver/L44 to the leg of C157 that connects with L44. On the other side I get 0.7 volts all the way to base of the driver.

There is no change when I adjust L45 or 46. Something is obviously still keeping the bias current from kicking in.

 

[Handy Andy]

Whew, this was a big hurdle, now you just have to fix the line from L44 to the Driver - look for open lines and bad soldering causing shorts and do continuity checks across the L44 TWISTY output thru to C159 - check to make sure no broken traces - and past C159 - into the Bias circuit - which can have a short - lots of curved pads and close quarters back there.

But still there is no RF power, no bias current at all on driver or final, and I am getting 8 volts all the way through from the predriver/L44 to the base of driver. That should only be about .0.6 at the driver but maybe because there is no bias current that’s why?

IF there is no Bias, you may still have signal - just weak and swings negative.

However in this case, you may have a short or a bad cap - causing the current to get sucked away - and takes the RF with it - C214 comes to mind...D50 if it's not shorted, check to make sure it's not shorting to the rear panel - the insulator may be MICA, but it's not puncture proof, it can still pinch and perforate into the back panel - double check your work!

​

Keep going, you're almost there!

I hope you're using the right heat sink goo - the white - not Grey - else just wipe off with a alcohol wipe to get back to base metal and start over.

IF you are using the physical diode - make sure you are soldering the ANODE (unbanded leg) to the correct pad - it's not the base lead of the 2166 - a Ferrite bead jumper goes there from this ANODE spot.

​

 

[Brian G]

Whew, this was a big hurdle, now you just have to fix the line from L44 to the Driver - look for open lines and bad soldering causing shorts and do continuity checks across the L44 TWISTY output thru to C159 - check to make sure no broken traces - and past C159 - into the Bias circuit - which can have a short - lots of curved pads and close quarters back there.



IF there is no Bias, you may still have signal - just weak and swings negative.

However in this case, you may have a short or a bad cap - causing the current to get sucked away - and takes the RF with it - C214 comes to mind...D50 if it's not shorted, check to make sure it's not shorting to the rear panel - the insulator may be MICA, but it's not puncture proof, it can still pinch and perforate into the back panel - double check your work!

View attachment 44370​

Keep going, you're almost there!

I hope you're using the right heat sink goo - the white - not Grey - else just wipe off with a alcohol wipe to get back to base metal and start over.

IF you are using the physical diode - make sure you are soldering the ANODE (unbanded leg) to the correct pad - it's not the base lead of the 2166 - a Ferrite bead jumper goes there from this ANODE spot.

View attachment 44373​

What should the voltage be between c157 and the base of driver on TX? There is 8 volts into c157 from the twisty of L44 but the other side is showing 0.7 to the jumper and then to base of driver.

 

[1iwilly]

this is what it shows on schematic

What should the voltage be between c157 and the base of driver on TX? There is 8 volts into c157 from the twisty of L44 but the other side is showing 0.7 to the jumper and then to base of driver.

 

[Handy Andy]

What should the voltage be between c157 and the base of driver on TX? There is 8 volts into c157 from the twisty of L44 but the other side is showing 0.7 to the jumper and then to base of driver.

Capacitors Block DC power, - so C157 is doing it's job at the DC level.

Your Bias is correct - as @1iwilly screenshot shows...

There is 8 volts at Twisty, because it has to supply power to the Pre-Driver so it can even produce the RF that also passes across L44 into C157 and Driver section.

What you are looking for is something not in the right place. You have one of two (possibly many) things potentially going wrong...

A broken connection - needs a part replaced or bad solder

Or

A bad component - that part needs to be replaced it's taking away signal from the Driver

Also - driver 2166 needs to work here, so check it's Collector (Center leg) for the same output as your TP 7 and 8 - BACK to your stems - continuity check works to verify the connection is good to the collector but you also need to check for voltage too.

 

[Brian G]

Capacitors Block DC power, - so C157 is doing it's job at the DC level.

Your Bias is correct - as @1iwilly screenshot shows...

There is 8 volts at Twisty, because it has to supply power to the Pre-Driver so it can even produce the RF that also passes across L44 into C157 and Driver section.

What you are looking for is something not in the right place. You have one of two (possibly many) things potentially going wrong...

A broken connection - needs a part replaced or bad solder

Or

A bad component - that part needs to be replaced it's taking away signal from the Driver

Also - driver 2166 needs to work here, so check it's Collector (Center leg) for the same output as your TP 7 and 8 - BACK to your stems - continuity check works to verify the connection is good to the collector but you also need to check for voltage too.

Ok thanks. I am getting 13 volts at TP 7 and 8 as well as at the center leg of the driver. But you are saying that I could get the voltage there but something between the stems and the collector is not allowing the current to flow? If so guess I don't get how the voltage could "get through" but not the current.

By the way I did check/ replace C214 and D50. Didn't help....

So I am still looking for that needle in the haystack I guess....

 

[Handy Andy]

Sigh...

Check for shorts across from one trace to another - your BASE of the driver is not getting enough of an RF signal to turn on - follow the path from Twisty to the Drivers Base - along the way are going to be parts that feed Bias voltage into it - but shouldn't let RF "leak out" - it needs to stay in the path towards the Driver.

That's why I asked about the Continuity as well as to make sure the Diodes are not shorting out to the side panel...

Seems you've done that, now you have to go find that fish - the one that lets RF get away...By your description - either the Driver is blown - or there is an RF leak in your boat - gotta' fix that so the Driver can work again.

 

[Brian G]

Sigh...

Check for shorts across from one trace to another - your BASE of the driver is not getting enough of an RF signal to turn on - follow the path from Twisty to the Drivers Base - along the way are going to be parts that feed Bias voltage into it - but shouldn't let RF "leak out" - it needs to stay in the path towards the Driver.

That's why I asked about the Continuity as well as to make sure the Diodes are not shorting out to the side panel...

Seems you've done that, now you have to go find that fish - the one that lets RF get away...By your description - either the Driver is blown - or there is an RF leak in your boat - gotta' fix that so the Driver can work again.

So frustrating. I don’t think (i hope not) it’s the driver as I’ve replaced it a couple of times thinking it was bad. I don’t see any reason it would be toast again at this point but who knows. guess I will Continue to probe away and see if I stumble on that needle.
thanks.