Cobra 2000 No TX power then whole radio died

[Handy Andy]

thanks Nomad. I checked the 2 bias posts. Same full voltage as at the front lead of TR41. When you key radio it drops to 6.2 at TR41 and at the two bias posts but still no power out. So not the mode selector likely? What is it then?? Mystery.

Did you do a continuity check from those posts thru their wires to the rear collectors?

Switching to SSB mode, they get full rail power supply voltage - you said something to the effect of 15V which is what I was concerned that could take out the radio..- you'd see the power supply rails' full voltage...

If all they did was replace the caps, then the previous owner didn't know that the power supply failure can take out not just the regulator, but the Final and Driver - it might help us to know the history of these radios.

 

[Brian G]

Did you do a continuity check from those posts thru their wires to the rear collectors?

Switching to SSB mode, they get full rail power supply voltage - you said something to the effect of 15V which is what I was concerned that could take out the radio..- you'd see the power supply rails' full voltage...

If all they did was replace the caps, then the previous owner didn't know that the power supply failure can take out not just the regulator, but the Final and Driver - it might help us to know the history of these radios.

. Maybe be the mode switch after all?
At TR41 and both test posts i could adjust the voltage at the outer leg of 41 while transmitting on AM by adjusting vr10 and get a volt reading from less than 1 volt up to about 7 but then sometimes it will show 13 volts at that leg and I can’t adjust it at all. The inner leg consistently shows 6.5-6.8 and the center 13. So it’s intermittent. Is that the mode switch issue you alluded to? But it was showing normal voltages at 41 at times but still there was no output. Shouldn’t there have been when it was reading correctly? Any other way to test it? And if it is bad where the heck would I find a new one?

 

[nomadradio]

Center leg of the driver should show the same 6.2 Volts DC as the center leg of the final. Bad driver bias-test wire? Blown driver collector choke? Those usually split the heat-shrink sleeve before getting hot enough to be damaged.

73

 

[Brian G]

Center leg of the driver should show the same 6.2 Volts DC as the center leg of the final. Bad driver bias-test wire? Blown driver collector choke? Those usually split the heat-shrink sleeve before getting hot enough to be damaged.

73

Hi yes the driver collector choke looks to be smoked. What does that suggest though? I don’t want to install a new one to just have that burn up too. And this just happened yesterday and I’ve been working on this for a few days prior.

 

[Brian G]

Center leg of the driver should show the same 6.2 Volts DC as the center leg of the final. Bad driver bias-test wire? Blown driver collector choke? Those usually split the heat-shrink sleeve before getting hot enough to be damaged.

73

I cannot get a bias reading at all for the driver but there is 13 volts at the test point. Final is ok with bias. How can I have voltage but no current at that test point? That indicate Tr41 is toast? Voltage on front leg of tr41 is 13 volts which I know is not right. Rear leg varies based on Vr 10 adjustment.

I replaced L42. Took it out of an old cobra 140 at the driver.

 

[Handy Andy]

I cannot get a bias reading at all for the driver. Final is ok. And voltage in front leg of tr41 is 13. Rear leg varies based on Vr 10 adjustment. I replaced L42. Took it out of an old cobra 140 at the driver.

More than likely the Driver bias pot is shot,

Have you replaced C124 Electrolytic 10V47uF? Is it even in there? (If so - it's why I'm asking - the 10V blues - take it out - you can sub in another LESSER value of 25V rating - if they haven't already...

• - what is the value of it's feeder resistor? R182?
  • It - should be 220ohm
  • - several versions of the schematics of that area show different values
  • - nothing truly nailed down...1306 versus 2166 reasoning - from the ohmic sense,
  • VR9 (Driver Bias) should never show open, but it does when the Bias Feeder Resistor is a low value where the Bias pot is taxed from the load of the Diode and Base requesting current
    • - can blow the Diode then the Driver
    • - these things used a 1306 - but if you found a 2166 - check your Bias pot make sure it shows ohmic adjustable range (500 ohm) and not blown open
    • - L43 can even pop open like a fuse from the draw the Driver can ask for.

 

[Brian G]

More than likely the Driver bias pot is shot,

Have you replaced C124 Electrolytic 10V47uF? Is it even in there? (If so - it's why I'm asking - the 10V blues - take it out - you can sub in another LESSER value of 25V rating - if they haven't already...

• - what is the value of it's feeder resistor? R182?
  • It - should be 220ohm
  • - several versions of the schematics of that area show different values
  • - nothing truly nailed down...1306 versus 2166 reasoning - from the ohmic sense,
  • VR9 (Driver Bias) should never show open, but it does when the Bias Feeder Resistor is a low value where the Bias pot is taxed from the load of the Diode and Base requesting current
    • - can blow the Diode then the Driver
    • - these things used a 1306 - but if you found a 2166 - check your Bias pot make sure it shows ohmic adjustable range (500 ohm) and not blown open
    • - L43 can even pop open like a fuse from the draw the Driver can ask for.

Looks like R182 is a 210 from best I can tell ( brown red brown) . Did you mean C214? I had replaced it previously when I recapped it but I did again. The driver is a 1306. I replaced VR9 as well. I cannot find L43. I see it on the schematic but don’t see it on board. Can you point it out? But either way replacing those 2 didn’t make any difference.

 

[nomadradio]

The smoked driver collector choke L42 generally indicates a surge big enough that the driver transistor did not survive. When the driver quits, it tends to shoot collector voltage out of its base terminal back into the bias pot and bias diode D50. If D50 fails as an open circuit, the new driver transistor will draw enough current to smoke L42, and itself as well, most likely. Never replace a driver or final without being certain the bias diode and trimpot are okay.

73

 

[Handy Andy]

Looks like R182 is a 210 from best I can tell ( brown red brown) . Did you mean C214? I had replaced it previously when I recapped it but I did again. The driver is a 1306. I replaced VR9 as well. I cannot find L43. I see it on the schematic but don’t see it on board. Can you point it out? But either way replacing those 2 didn’t make any difference.

You don't see these?

My GOD man!
What the h*ll are you up against?​

If you need help, better supply more pictures because your typing is not giving us enough info on what you're doing...

 

[Brian G]

You don't see these?

View attachment 43431

View attachment 43432

My GOD man!
What the h*ll are you up against?​

If you need help, better supply more pictures because your typing is not giving us enough info on what you're doing...

Thanks. There was some black shit in the board near L43 covering the board label so I couldn’t see it and the Sam’s facts pics didn’t show it. But either way it look fine, doesn’t look popped that I can tell. I actually had pulled d49 and 50 out of my parts radio before so I don’t think they are the problem.
But to answer your question about what I’m up against? I have no fuckin idea and I’m almost done with it. Because it probably isn’t anything huge. I just can’t see it. 2 months and more working on this radio in virtually every part of it. And now all this.
But anyways pretty good bet the driver is toast. Check out this pic of it. Hard to see but the plastic casing is completely separating from the metal back. Problem is why did that happen and I’m afraid the replace my may burn up too.

I was able to find an NTE equivalent for D50 if I need it. But having a hard time finding one for D49 (MV 13 HY). NTE cross reference brings up 2 when I put in just MV-13 but not sure if either is ok? NTE 116 and 177. Either one a match? Not sure I need it but just in case.

 

[Dmans]

I just can’t see it. 2 months and more working on this radio in virtually every part of it. And now all this.

Brian G.,
Don't feel too bad.

I have been working on a friends radio since September of last year. (The Friend does have plenty of other radios) It's not my full time job and I am only working on it in my spare time. Regular job is taking 65-70 hours a week. I don't charge for my repairs except for the parts. I have taken one week off from this Kenwood to gather my thoughts (and a few more parts). But I have learned plenty. Mostly what is sticking in my mind is: "A Kenwood 940S will never be on my bench again!" But I'm a sucker for proving those wrong that say "It can't be done!"

Take a break from it for a while. Not worth your sanity!

73
David

 

[Brian G]

Brian G.,
Don't feel too bad.

I have been working on a friends radio since September of last year. (The Friend does have plenty of other radios) It's not my full time job and I am only working on it in my spare time. Regular job is taking 65-70 hours a week. I don't charge for my repairs except for the parts. I have taken one week off from this Kenwood to gather my thoughts (and a few more parts). But I have learned plenty. Mostly what is sticking in my mind is: "A Kenwood 940S will never be on my bench again!" But I'm a sucker for proving those wrong that say "It can't be done!"

Take a break from it for a while. Not worth your sanity!

73
David

thanks man I appreciate it! Good advice. Fortunately I get forced to take a break while I wait days for replacement parts come in. Peace.

 

[Handy Andy]

To help your situation, that parts blown - popped the epoxy lifted from the tab...

Don't give up, it's a labor of love really - and thank you for even trying to work on such a project as this and work with us trying to imagine what's in front of you - this isn't a "Christine" - type of vehicle that repairs itself - just remember what it did to others, in doing damages to it - it took out those it didn't like.

​

Radio isn't a monster - it's a process - we weren't born with a set of encyclopedias to learn from.

Because even if you did, you had to learn how to read then to understand them.

So take a break - too much intense work on such a unyielding project - even I've lost radios on my bench and had to cough up the cost and shame.

But them too, it serves as a trip for memory lane for the next time - "Don't let this happen to you" kind of moment.

It's one of many reasons why we're here, to reduce the suffering others can face.

 

[Brian G]

To help your situation, that parts blown - popped the epoxy lifted from the tab...

Don't give up, it's a labor of love really - and thank you for even trying to work on such a project as this and work with us trying to imagine what's in front of you - this isn't a "Christine" - type of vehicle that repairs itself - just remember what it did to others, in doing damages to it - it took out those it didn't like.

View attachment 43443
​

Radio isn't a monster - it's a process - we weren't born with a set of encyclopedias to learn from.

Because even if you did, you had to learn how to read then to understand them.

So take a break - too much intense work on such a unyielding project - even I've lost radios on my bench and had to cough up the cost and shame.

But them too, it serves as a trip for memory lane for the next time - "Don't let this happen to you" kind of moment.

It's one of many reasons why we're here, to reduce the suffering others can face.

thank you , I truly appreciate the help and patience. Wish I had you guys sitting here next to me-would make things much easier lol!

Weird question- I was trying to find a replacement for D50 online. I think NTE601 is a sub for MV-1Y. But that part just looks like a regular diode/resistor with axial leads. If I should need to use that, how do I mount it because D50 is that weird little button type one that has a screw mount in it? Does it need to be attached to heat sink or no? Thanks.

Probably no more updates until I get some parts. Like I said just worry why that driver blew out in the first place. Hope replacement doesn’t.

 

[Handy Andy]

IT used to be that the MV-1Y - aka Varistor used to bias them, they made for pennies a pound.

​

Nowadays they use a diode - but the rub here is diodes have changed. Their voltage drop is different than what they were from just a decade ago, So we have to accommodate the changes - its' why you see the resistor - it isn't really needed but the diodes Intrinsic - read Ohmic to impedance - character is different. So they don't work as well as their earlier - yet cruder/cruddier - brothers that had a PN junction that more closely matched those of their transistors they were designed to work with - Bias or otherwise

So they add in a resistor that alters the diodes thermal profile - the way it behaves when it's used as a means to bias - when exposed to heat and cold. The Resistor offsets the lesser drop, because most diodes now are Schottky types that only have a P or N junction - not both - the Resistor takes place (loose term) the other missing half and also the intrinsic layer where the two P to N melded together - to make the part more effective in providing protection in Bias for the class the circuit it's used in, needs, as a thermally tracking device as well as a method to regulate the voltage drop properly so the device it's to work with - stays working in it's class it's supposed to,