Cobra 2000 No voltage at Test points 7 and 8

[nomadradio]

Be sure to download the diagram they show in the fleabay listing for the switch. Shows which pins are common, and which are left, middle and right for each section.

The numbers stamped on the back may or may not match the numbering on the rear of the original switch. If they do, you're in like Flynn.

If not, some "interpretation" will be called for to get it hooked up working.

73

 

[Brian G]

Ok well I am back!! Got the new mode switch in. Voltages seems to all be restored at the switch and TP 7 and 8. But I still have 2 problems:

1 there is 13 volts at the collectors of both the final and driver on SSB and AM transmit. It should only be about 6-6.5 on AM transmit. The base of both are ok and obviously the emitters are zero at ground. And there is still no RF out

2. And there is no bias current to either! Zero. How can that be?? There is about 0.7 volts at the driver bias diode and 1.2 at the final diode. I believe from what Nomad had said before that sounds about right so I dont think those are bad.But nothing happens when I turn either variable resistor

 

[nomadradio]

Until the driver and final begin to draw current, the voltage on AM will rise higher than the normal reading. Once the final and driver are drawing current, it pulls this voltage back down to what the trimpot has set it for.

Have you checked the DC voltage at the base of the driver and final? Should be just over 6/10 of a Volt, or 600 mV to obtain a bias-current reading.

73

 

[Handy Andy]

TR35 - handles the switch of the MB3756, if you get a "Green" dash light for RX, does the light turn to RED when in TX?

 

[Brian G]

TR35 - handles the switch of the MB3756, if you get a "Green" dash light for RX, does the light turn to RED when in TX?

Yes, the Transmit light does come on when I key the mike.

 

[Brian G]

Until the driver and final begin to draw current, the voltage on AM will rise higher than the normal reading. Once the final and driver are drawing current, it pulls this voltage back down to what the trimpot has set it for.

Have you checked the DC voltage at the base of the driver and final? Should be just over 6/10 of a Volt, or 600 mV to obtain a bias-current reading.

73

Yep, there is about. 0 .7 at both bases.and yes that is what I was thinking too The fact that there is no bias current was why the voltage is staying high all the time regardless of mode. I can hear a weak carrier on my other radio but no wattage reading at all.

 

[Brian G]

Until the driver and final begin to draw current, the voltage on AM will rise higher than the normal reading. Once the final and driver are drawing current, it pulls this voltage back down to what the trimpot has set it for.

Have you checked the DC voltage at the base of the driver and final? Should be just over 6/10 of a Volt, or 600 mV to obtain a bias-current reading.

73

Another point not sure what this means. The red wire coming off the AM regulator to the switch says it should be 6.8 or so. But it is showing 13.0. When I lift the emitter leg off the pad there, it shows 6.3 which is fairly close but the pad shows 13 which of course is why the red is showing 13 because that’s where it solders in. The 13 is coming from the other side (13.8 there) of R193 that straddles the emitter and collector. So how do you get only 6.8 down the red wire if there is 13 volts at R193 feeding it? Guess I don’t get that circuit. Or if that has anything to do with the main issue.

 

[guitar_199]

With the driver/final NOT being biased "on" they are not drawing current so, in a way, the transistor serving as the AM regulator has "no load on it". Nothing drawing current to pull it down... so it is going to go to the max value.

I hope this idea gets across.....

Think of a circuit where you have a 9 volt battery in series with two 1K resistors.... lets call them Ra and Rb. With it all hooked up you will measure nominal 9v when you measure across the battery...... half of that across Ra and half of that across Rb (roughly 4.5 v each!).

Now, reach in and pull Rb out of the circuit.

All of a sudden.... no current is flowing in the circuit.

Voltage across Ra will be Ra x current, but current = 0... so there will be 0 volts across Ra. Now stick the probes on the two contacts where Rb USED to be.... you will measure ALL nominal 9volts!

In a closed loop fed by a source the sum of the voltages IN the loop MUST = the supply.

With one resistor missing (open) ALL of the 9v supply will show up there AT THE OPEN.

Back to your circuit.......

The driver/final are not biased on.... so they are not pulling any current...so THEY are acting like an OPEN in the loop. So.... you would measure ALL of the 13.8 vot supply....because NO CURRENT is being pulled through the regulator......to cause it to regulate.

When you find out what is causing the driver/final not to ""turn on" this problem will most likely fix itself.

 

[Brian G]

With the driver/final NOT being biased "on" they are not drawing current so, in a way, the transistor serving as the AM regulator has "no load on it". Nothing drawing current to pull it down... so it is going to go to the max value.

I hope this idea gets across.....

Think of a circuit where you have a 9 volt battery in series with two 1K resistors.... lets call them Ra and Rb. With it all hooked up you will measure nominal 9v when you measure across the battery...... half of that across Ra and half of that across Rb (roughly 4.5 v each!).

Now, reach in and pull Rb out of the circuit.

All of a sudden.... no current is flowing in the circuit.

Voltage across Ra will be Ra x current, but current = 0... so there will be 0 volts across Ra. Now stick the probes on the two contacts where Rb USED to be.... you will measure ALL nominal 9volts!

In a closed loop fed by a source the sum of the voltages IN the loop MUST = the supply.

With one resistor missing (open) ALL of the 9v supply will show up there AT THE OPEN.

Back to your circuit.......

The driver/final are not biased on.... so they are not pulling any current...so THEY are acting like an OPEN in the loop. So.... you would measure ALL of the 13.8 vot supply....because NO CURRENT is being pulled through the regulator......to cause it to regulate.

When you find out what is causing the driver/final not to ""turn on" this problem will most likely fix itself.

Thank you, very good explanation and I totally get it. Just have no idea why/where that driver/final turn on isn't happening.

 

[guitar_199]

I just took another look at the SAMS schematic in the area of the driver and final.

First, AGAIN, let me warn you...... while I have education in electronics.....I have been in software since around 1984 as a vocation. Now, closer to retirement, I am trying to get closer to my "electronics" as a hobby/for fun. What I am telling you is.... I don't have a wealth of EXPERIENCE (hands on) but the stuff just kind of "makes sense" in a theoretical point of view. WITH THAT IN MIND..... I not only INVITE....I BEG anyone else who reads what I post......If I am wrong.... PLEASE educate me!!!! I am not sensitive.... I don't whine.....I don't think I know it all! I am just trying to help where I can. Sometimes an entry of mine.....might spark an entry from someone who actually KNOWS what they are doing!!!!

Now........

I have done some reading on the transmit strip in the Lou Franklin big book on "Understanding and Repairing CB Radios". From what I understand..... drivers and finals are generally biased RIGHT ON THE HAIRY EDGE of "turning on"! IN AM only I think they are Class C meaning actually sort of "under biased" ..... SSB is generally Class B meaning they are biased just RIGHT AT the turn on point....but NOT QUITE.

Further it is my understanding that it takes RF coming from earlier in the TX strip. Meaning.....where the mixers first combine frequencies to CREATE the TX frequency...through any TX RF amplifier stages.... but you HAVE to get RF to the base of the driver to even START turning the driver and THEN the final on.

With that in mind...I go back to HA's last post......

make sure that all of the voltages the switch ON IN XMIT are actually coming ON when you press the mic button. Those voltages serve to turn some of those earlier stages on.

Look at the driver final area of the SAMS schematic.......and slide left....looking below the circuitry. You are looking for CKT TRACE #12. As you slide back towards the left....most of the circuitry is powered by #12......both the driver and final in the bias network......but also TR39 the pre-driver, IC5 the transmit mixer..... all of that is powered by #12 which is the 8.15V XMT supply. With the button pushed that would HAVE to be there and would be a very basic thing to check.

QUESTION HERE! Do you have a scope? Or a frequency counter? Possibly BOTH?

If that #12 supply is good...... we are going to have to determine whether the XMT Mixer is happy (meaning putting out a carrier) to go over to TR39. If not why? If so.... is it getting TO TR39 and through it to a proper level.

This is going to have us chasing backwards for a bit.

Note that IC5 seems to mix 7.8 MHz with 34 MHz (for the difference!) to put out 27 MHz. That goes to the pre-driver, and on to the driver which in turn feeds the driver.

We will need to make sure that "strip" of circuitry is doing it's job!

BUT FIRST..... like HA said.... let's check the switched power supply sources. That is ALWAYS "low hanging fruit".


This is kind of the direction I am thinking.

 

[Brian G]

I just took another look at the SAMS schematic in the area of the driver and final.

First, AGAIN, let me warn you...... while I have education in electronics.....I have been in software since around 1984 as a vocation. Now, closer to retirement, I am trying to get closer to my "electronics" as a hobby/for fun. What I am telling you is.... I don't have a wealth of EXPERIENCE (hands on) but the stuff just kind of "makes sense" in a theoretical point of view. WITH THAT IN MIND..... I not only INVITE....I BEG anyone else who reads what I post......If I am wrong.... PLEASE educate me!!!! I am not sensitive.... I don't whine.....I don't think I know it all! I am just trying to help where I can. Sometimes an entry of mine.....might spark an entry from someone who actually KNOWS what they are doing!!!!

Now........

I have done some reading on the transmit strip in the Lou Franklin big book on "Understanding and Repairing CB Radios". From what I understand..... drivers and finals are generally biased RIGHT ON THE HAIRY EDGE of "turning on"! IN AM only I think they are Class C meaning actually sort of "under biased" ..... SSB is generally Class B meaning they are biased just RIGHT AT the turn on point....but NOT QUITE.

Further it is my understanding that it takes RF coming from earlier in the TX strip. Meaning.....where the mixers first combine frequencies to CREATE the TX frequency...through any TX RF amplifier stages.... but you HAVE to get RF to the base of the driver to even START turning the driver and THEN the final on.

With that in mind...I go back to HA's last post......

make sure that all of the voltages the switch ON IN XMIT are actually coming ON when you press the mic button. Those voltages serve to turn some of those earlier stages on.

Look at the driver final area of the SAMS schematic.......and slide left....looking below the circuitry. You are looking for CKT TRACE #12. As you slide back towards the left....most of the circuitry is powered by #12......both the driver and final in the bias network......but also TR39 the pre-driver, IC5 the transmit mixer..... all of that is powered by #12 which is the 8.15V XMT supply. With the button pushed that would HAVE to be there and would be a very basic thing to check.

QUESTION HERE! Do you have a scope? Or a frequency counter? Possibly BOTH?

If that #12 supply is good...... we are going to have to determine whether the XMT Mixer is happy (meaning putting out a carrier) to go over to TR39. If not why? If so.... is it getting TO TR39 and through it to a proper level.

This is going to have us chasing backwards for a bit.

Note that IC5 seems to mix 7.8 MHz with 34 MHz (for the difference!) to put out 27 MHz. That goes to the pre-driver, and on to the driver which in turn feeds the driver.

We will need to make sure that "strip" of circuitry is doing it's job!

BUT FIRST..... like HA said.... let's check the switched power supply sources. That is ALWAYS "low hanging fruit".


This is kind of the direction I am thinking.

All sounds reasonable thank you. Will explore that area a bit. I do have a scope. Curiously I had an issue with L44 previously so perhaps there’s still some kind of problem in that area.

 

[guitar_199]

Interesting thought. That is the tapped coil that couples the pre-driver to the driver. If there was a problem there you might not get the carrier from the pre-driver to the driver...and that COULDN't be good!!!

Curiously I had an issue with L44 previously so perhaps there’s still some kind of problem in that area.

 

[Brian G]

Interesting thought. That is the tapped coil that couples the pre-driver to the driver. If there was a problem there you might not get the carrier from the pre-driver to the driver...and that COULDN't be good!!!

Ok how do I confirm that? I used the scope and it does look like the carrier is going through the circuit. I probed at the predriver collector to the “out” of that coupling coil at C157 to the base of the driver. Seems to be a signal all through there.

 

[guitar_199]

From looking at that area in the SAMS schematic...... as you can see.... L44 is a coil in parallel with a cap. At the bottom of our coil is our friend CKT TRACE #12. It feeds THRU the coil and the top connects to the collector of TR39 (pre-driver). The tap on that coil heads to the right to CKT TRACE #198 which appears to be the cap C157 which then feeds the base of the driver.

A dirt simple check would be..... key the microphone.... see if you have that 8.08 v on the TR39 collector.........AND.....see that you have somewhere between 8.08 and 8.05 on the side of that cap C157. You may be just able to look at the pins of L44 itself and see what you need....but BOTH of those points should see the DC voltage on the bottom pin...that come in on Ol' #12. Does that make sense?

Note ..... this ONLY tells you CONTACT/CONNECTION is made. It is (I suppose) possible that something wonky is wrong with the tap or the tuning of L44.

But at least THIS test tells you if the tapped coil is physically "connected through" and passing the DC. That would be a FIRST check.

 

[Brian G]

From looking at that area in the SAMS schematic...... as you can see.... L44 is a coil in parallel with a cap. At the bottom of our coil is our friend CKT TRACE #12. It feeds THRU the coil and the top connects to the collector of TR39 (pre-driver). The tap on that coil heads to the right to CKT TRACE #198 which appears to be the cap C157 which then feeds the base of the driver.

A dirt simple check would be..... key the microphone.... see if you have that 8.08 v on the TR39 collector.........AND.....see that you have somewhere between 8.08 and 8.05 on the side of that cap C157. You may be just able to look at the pins of L44 itself and see what you need....but BOTH of those points should see the DC voltage on the bottom pin...that come in on Ol' #12. Does that make sense?

Note ..... this ONLY tells you CONTACT/CONNECTION is made. It is (I suppose) possible that something wonky is wrong with the tap or the tuning of L44.

But at least THIS test tells you if the tapped coil is physically "connected through" and passing the DC. That would be a FIRST check.

Yes there is definitely 8 volts all through those points.