Cobra 2000 No TX power then whole radio died

[Brian G]

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

View attachment 43444​

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,

Makes sense. How would I mount the new version if needed?

 

[nomadradio]

Ideally you'd like the body of the diode laying up against the plastic body of the transistor.

Just one problem. The old part had really long leads. Extending them to reach the holes in the circuit board is a chore, but doable.

The end of the MV1Y with the tiny color dot is the cathode, same as the end marked with a band on a glass diode.

73

 

[Handy Andy]

Got a TRC-485 anywhere?

​

They Glued their MV-1Y clones to the BODY of the part, then soldered the wires underneath to the Bias feeds.

No, I said that right - they soldered the Diodes UNBANDED side - to the Bias supply voltage - thru a Resistor/Bead for the DRIVER on the Left, and fed the FINAL thru a Green bodied Choke, with a ferrite bead choke (2X the choke) on the FINAL - on the right.

The BANDED side went to RF ground so they followed the Emitter leg down and soldered to it's pad on the main PCB.

Each transistor had an individual lead to the Bias voltage - not thru the Base lead - just met there thru the jumpers and the Chokes, the Diodes never get close to the Base lead or it's input signal..

So they buffered and choked off RF from returning back into the Bias diodes and altering the Class - but they Glued the Bias Diodes directly to the body of the Transistor.

You were forced to install new diodes every time...

In another thread, the above heat sink mounting hardware insulator - note the PVC and the "Curl" in one - this is a dangerous moment - for as these thing heat up, they expand and those insulators try to transfer the heat away but as that happens the expansion and contraction of all the components involved - it's gets toasted - Radio Shack - we got dinged for that every time one was returned.

 

[Brian G]

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

View attachment 43444​

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,

What is the purpose of L44 in my radio. It’s a little black ring/circle with 3 leads tiny leads into the board but it looks like at least one lead is broke. Not sure how critical it is or how Ling it’s been like that. It’s so small don’t know if I can even do anything with it. Also do you happen to have a pic of the solder side of this board at L44? Thanks.

 

[nomadradio]

L44 is a "transmission-line" transformer. The doughnut-shaped core is called a toroid, and it's made from a ferrite material. Serves to match impedance feeding out of the predriver transistor TR39 into the driver transistor TR38. If one leg breaks off, you should not get much power at all, if any.

73

 

[ExitThirteen]

Replace C152, and C157. These two are disc capacitors located in the driver/final section of the radio. C157 is .0047uF, and C152 is 470pf. Replace those, and let us know how you get on. I'm suspecting one of those capacitors have failed short, which is causing extra voltage to "leak" into the bases of the driver and final transistors.


~Cheers~

 

[Shockwave]

Got a TRC-485 anywhere?

View attachment 43447​

They Glued their MV-1Y clones to the BODY of the part, then soldered the wires underneath to the Bias feeds.

No, I said that right - they soldered the Diodes UNBANDED side - to the Bias supply voltage - thru a Resistor/Bead for the DRIVER on the Left, and fed the FINAL thru a Green bodied Choke, with a ferrite bead choke (2X the choke) on the FINAL - on the right.

The BANDED side went to RF ground so they followed the Emitter leg down and soldered to it's pad on the main PCB.

Each transistor had an individual lead to the Bias voltage - not thru the Base lead - just met there thru the jumpers and the Chokes, the Diodes never get close to the Base lead or it's input signal..

So they buffered and choked off RF from returning back into the Bias diodes and altering the Class - but they Glued the Bias Diodes directly to the body of the Transistor.

You were forced to install new diodes every time...

At least they were silkscreened onto the board, as a reminder to reinstall them after a new final or driver. Do you know if mounting the thermal tracking diode on the transistors case, made any difference or benefit in its ability to track faster? With all of that metal tab exposed out of the top of the TO-220 case, I'd have been tempted to find an "eye connector", that would fit snuggly around the insulator bushing and make direct contact to the tab. Physically attach the diode to that.

 

[Brian G]

L44 is a "transmission-line" transformer. The doughnut-shaped core is called a toroid, and it's made from a ferrite material. Serves to match impedance feeding out of the predriver transistor TR39 into the driver transistor TR38. If one leg breaks off, you should not get much power at all, if any.

73[/QUOTE great! Where am I going to find something to replace that with if is damaged? That could ruin the whole repair. Seems like such an odd part. My initial search turned up nothing. ]

 

[Handy Andy]

great! Where am I going to find something to replace that with if is damaged? That could ruin the whole repair. Seems like such an odd part. My initial search turned up nothing.

If you're referring to this...

Per @nomadradio - that is the part L44 is a Bifilar wound toroid.​

Sorry this looks like hell but...it is from a Documentation project from the X


​

As a word of caution:

Because this brought up quite a debate in a former site...

The "TWIST" can be found in various orientations - some are tied at the foil pad that feeds the Collector of the Predriver, another may use the Power Feed side arriving from the 8V TX - in this instance the TWIST is on the output leg towards the cap that feeds the DRIVER - C157.

In the above its from a Cobra 148.

 

[Brian G]

Yeah I get all that about this part. The issue again is that it appears to be damaged. I need to know where to find another one. I don’t think this is a part I can buy like other components.

 

[Brian G]

If you're referring to this...

View attachment 43459

Per @nomadradio - that is the part L44 is a Bifilar wound toroid.​

View attachment 43460

Sorry this looks like hell but...it is from a Documentation project from the X

View attachment 43461
​

As a word of caution:

Because this brought up quite a debate in a former site...

The "TWIST" can be found in various orientations - some are tied at the foil pad that feeds the Collector of the Predriver, another may use the Power Feed side arriving from the 8V TX - in this instance the TWIST is on the output leg towards the cap that feeds the DRIVER - C157.

In the above its from a Cobra 148.

Yeah I get all that about this part. The issue again is that it appears to be damaged. I need to know where to find another one. I don’t think this is a part I can buy like other components

 

[Brian G]

Got a TRC-485 anywhere?

View attachment 43447​

They Glued their MV-1Y clones to the BODY of the part, then soldered the wires underneath to the Bias feeds.

No, I said that right - they soldered the Diodes UNBANDED side - to the Bias supply voltage - thru a Resistor/Bead for the DRIVER on the Left, and fed the FINAL thru a Green bodied Choke, with a ferrite bead choke (2X the choke) on the FINAL - on the right.

The BANDED side went to RF ground so they followed the Emitter leg down and soldered to it's pad on the main PCB.

Each transistor had an individual lead to the Bias voltage - not thru the Base lead - just met there thru the jumpers and the Chokes, the Diodes never get close to the Base lead or it's input signal..

So they buffered and choked off RF from returning back into the Bias diodes and altering the Class - but they Glued the Bias Diodes directly to the body of the Transistor.

You were forced to install new diodes every time...

In another thread, the above heat sink mounting hardware insulator - note the PVC and the "Curl" in one - this is a dangerous moment - for as these thing heat up, they expand and those insulators try to transfer the heat away but as that happens the expansion and contraction of all the components involved - it's gets toasted - Radio Shack - we got dinged for that every time one was returned.

No I don’t have a TRC radio. Still unclear how to mount these. I’m hoping the ones there are not bad in the first place. I did switch them out from a Cobra 140 that I know was working prior. So I’m not going to touch those for now. What is the green and white dots on them? I assume that is differentiate them but is that a universal marking? Want to make sure I put the replacements in correctly.

My bigger worry now is that L44.

 

[Brian G]

Replace C152, and C157. These two are disc capacitors located in the driver/final section of the radio. C157 is .0047uF, and C152 is 470pf. Replace those, and let us know how you get on. I'm suspecting one of those capacitors have failed short, which is causing extra voltage to "leak" into the bases of the driver and final transistors.


~Cheers~

Done.

 

[Shockwave]

My bigger worry now is that L44.

The good news is that part was used in many different radios that had the MB8719 PLL chip. We have lots of people on this forum that have worked on that board and probably more than one with a parts donor. If you're stuck, I would just ask if anyone has an L44 that you can obtain. If the core in not cracked, the original part could be rewrapped with the same size magnet wire, as another alternative.