Cobra 2000 No TX power then whole radio died

[Handy Andy]

15 volts? What is your power supply set for?

Might want to bypass the 120VAC power supply and run a regulated DC 13.8 thru the Uniden plug on the backside.

The Regulation at the Power supply and the supply caps could be cause of the "Fall"

Might want to investigate, and clean up the power supply.
If it blows up, the rest of the radio becomes scrap...​

 

[Brian G]

Ok located the issue of only 6.8v at IC4 and elsewhere. All set. Now will investigate higher voltage on the power supply side.

 

[Brian G]

15 volts? What is your power supply set for?

Might want to bypass the 120VAC power supply and run a regulated DC 13.8 thru the Uniden plug on the backside.

The Regulation at the Power supply and the supply caps could be cause of the "Fall"

View attachment 43395

Might want to investigate, and clean up the power supply.
If it blows up, the rest of the radio becomes scrap...​

For now I am running off my dc power supply. So I think I have found the issue with wattage fading as the radio stays keyed up. The final starts out fine voltage wise but quickly fades down to basically zero. The driver seems to stay ok.

so I replaced the final. I’m not getting anything at the emitter. It should be about 5 volts in AM TX but there’s nothing. So there is no power output obviously. What the heck could be keeping that from putting anything out. There is .about 0.6 volts coming in as spec so why would there be nothing at the emitter?

 

[nomadradio]

so I replaced the final. I’m not getting anything at the emitter.

Schematic shows the emitter lead of the final transistor is grounded. Won't get a voltage reading there.

Are we really talking about the collector? The center lead of the final and driver?

Checking what comes out of the emitter at TR41 would be next. Center lead is TR41 collector, should have full power-supply DC voltage there. The lead towards the front is the emitter, and should have the DC voltage set by the carrier trimpot there. The base lead, towards the rear should have that voltage plus about 6/10 of a Volt. The difference between base and emitter should remain around that 0.6 Volt no matter the base voltage.

And if you do have a normal six Volts plus or minus on the emitter of TR41, the mode selector and/or wires leading to/from it are all that remain to blame for no voltage at the final and driver collector chokes.

73

 

[Brian G]

Schematic shows the emitter lead of the final transistor is grounded. Won't get a voltage reading there.

Are we really talking about the collector? The center lead of the final and driver?

Checking what comes out of the emitter at TR41 would be next. Center lead is TR41 collector, should have full power-supply DC voltage there. The lead towards the front is the emitter, and should have the DC voltage set by the carrier trimpot there. The base lead, towards the rear should have that voltage plus about 6/10 of a Volt. The difference between base and emitter should remain around that 0.6 Volt no matter the base voltage.

And if you do have a normal six Volts plus or minus on the emitter of TR41, the mode selector and/or wires leading to/from it are all that remain to blame for no voltage at the final and driver collector chokes.

73

Yes sorry wrong identification. Center lead. Getting inconsistent voltage readings at both. Are voltage readings at these 2 the simplest way to know if they are good or not? I will check TR41.

 

[Handy Andy]

IF you've reset all your TP wires to their TP Stems
All of the points shown above will be a dead short - goes thru MODE Switch
IF you have issues with the Power Flow, the Mode switch can go bad.


​

 

[Brian G]

Schematic shows the emitter lead of the final transistor is grounded. Won't get a voltage reading there.

Are we really talking about the collector? The center lead of the final and driver?

Checking what comes out of the emitter at TR41 would be next. Center lead is TR41 collector, should have full power-supply DC voltage there. The lead towards the front is the emitter, and should have the DC voltage set by the carrier trimpot there. The base lead, towards the rear should have that voltage plus about 6/10 of a Volt. The difference between base and emitter should remain around that 0.6 Volt no matter the base voltage.

And if you do have a normal six Volts plus or minus on the emitter of TR41, the mode selector and/or wires leading to/from it are all that remain to blame for no voltage at the final and driver collector chokes.

73

I am getting basically full power (13.8) out of the front and center legs of TR41. Normal (about 7 volts) out of the rear leg. But this is the AM regulator. So would that affect SSB too? There is no power out on SSB either. Just trying to understand. But either way I assume this need to be replaced.

 

[Brian G]

15 volts? What is your power supply set for?

Might want to bypass the 120VAC power supply and run a regulated DC 13.8 thru the Uniden plug on the backside.

The Regulation at the Power supply and the supply caps could be cause of the "Fall"

View attachment 43395

Might want to investigate, and clean up the power supply.
If it blows up, the rest of the radio becomes scrap...​

Interesting looks like some work was done in this power supply. C302 has been replaced by a 220 UF and C410 is a 6800. Are those ok? I actually read somewhere about increasing C410 even more. or should I put them back to stock sizes?

 

[Handy Andy]

I am getting basically full power (13.8) out of the front and center legs of TR41. Normal (about 7 volts) out of the rear leg. But this is the AM regulator. So would that affect SSB too? There is no power out on SSB either. Just trying to understand. But either way I assume this need to be replaced.

AM Regulator voltage sounds about right - there is a leeway because the output of the main pass transistor TR41 has a 220ohm resistor across it's Collector/Emitter legs - takes some of the power Oomph out of the power swing - might want to leave it in there -cheap insurance.

In SSB mode - the output leg "goes dead" - not switched in - it will have a small voltage on it though.

The Mode switch takes over the power feed from the AM Regulator - AM mode only gets TR41 - SSB mode is thru the MODE switch the full battery supply voltage - there is no fuse, and it can handle several amps, but not made for dead shorts - 5 ~ 7 amps Maximum is what the switch can pull thru itself.

Interesting looks like some work was done in this power supply. C302 has been replaced by a 220 UF and C410 is a 6800. Are those ok? I actually read somewhere about increasing C410 even more. or should I put them back to stock sizes?

The larger values do help, but the size and quality of the drop in replacement is the main concern. Try for now, to stick with OEM - you can add a capacitor in there if needed - tightly - but it can go in there.

Some caps can cause more noise than they are filtering - so if you don't know how old the caps are in there - time to look into replacing the Electrolytic ones..

Many in this thread and helping you, in other ones started by you, pretty much say - suggesting - the same thing - it's experience with these boards - they are age related problems - so replacing the Electrolytic caps - at least several larger valued types - can go a long way in the recovery of the Radio

In fact - they can be the sole reason for the many problems you're having with voltages thru the board.
Which is why I worry your issue with Mode and power supply falling problems - can be related to someone trying to run more power thru the mode switch than they should - or had an catastrophically epic fail in the Driver and Final area

 

[Brian G]

AM Regulator voltage sounds about right - there is a leeway because the output of the main pass transistor TR41 has a 220ohm resistor across it's Collector/Emitter legs - takes some of the power Oomph out of the power swing - might want to leave it in there -cheap insurance.

In SSB mode - the output leg "goes dead" - not switched in - it will have a small voltage on it though.

The Mode switch takes over the power feed from the AM Regulator - AM mode only gets TR41 - SSB mode is thru the MODE switch the full battery supply voltage - there is no fuse, and it can handle several amps, but not made for dead shorts - 5 ~ 7 amps Maximum is what the switch can pull thru itself.



The larger values do help, but the size and quality of the drop in replacement is the main concern. Try for now, to stick with OEM - you can add a capacitor in there if needed - tightly - but it can go in there.

Some caps can cause more noise than they are filtering - so if you don't know how old the caps are in there - time to look into replacing the Electrolytic ones..

Many in this thread and helping you, in other ones started by you, pretty much say - suggesting - the same thing - it's experience with these boards - they are age related problems - so replacing the Electrolytic caps - at least several larger valued types - can go a long way in the recovery of the Radio

In fact - they can be the sole reason for the many problems you're having with voltages thru the board.
Which is why I worry your issue with Mode and power supply falling problems - can be related to someone trying to run more power thru the mode switch than they should - or had an catastrophically epic fail in the Driver and Final area

so you think TR41 is ok then? Nomad was saying that front leg should be about 6.8 volts which is what the schematic says too. Full voltage seemed to much? But I suppose that could be ok because if you’re saying TR41 goes dead for SSB and there’s no output on SSB either, then likely the problem is somewhere else?

As far as the power supply: Those caps could be causing the higher voltage? Still trying to understand why the voltage was so high out of the power supply board. The main board voltage returned to normal once I switched through a DC power supply instead of the AC so I figure the problem must be in that power supply board? I have a 10uf cap I can sub in for the 220. But the zener and the big cap underneath I have to order. Also it looks like the bridge rectifier is not the original. Can’t really tell the number on it but if that’s not the right one that could be causing the higher voltage I assume?

I will put the 10uf stock back on the power supply board. All the electrolytics in this radio have been replaced.

Back to no power out: This original issue with no power started with bad SWR. There were key ups for only a couple of seconds here and there trouble shooting it but maybe that caused all this. But I replaced the final and driver and still nothing.

 

[Handy Andy]

Hmmm...where did I post that power supply schematic?

Check Post #16, there is a pass transistor Q401 - handles about 3 amps - look into this pass transistor going bad...

If it has had work done - look for the Power supply board to have been fiddled with when you have voltages like that - Post #16 is the Cobra 2000's version - it is a typical Regulator circuit found even in the Cobra 139XLR - only this has an "extra protection" circuit that uses an FET to help with ripple or line noise before it can damage the pass transistor - it's not short circuit protection. It's noise protection.

Spikes are handled thru the Zener crowbar effect Too long of a duration of short it's supposed to pop a fuse, but it can take itself out if the operators were trying to get more out of the supply leaving you - the next operator in line - vulnerable.

Considering the age, and how you're seeing things go wrong in there - might want to fix the power supply before it can kill the radio from it's ability to shoot over 20 volts AC into the radio from it.

In post #16 the Zener regulation used, uses a 1K and 6.2 V 1W Zener to "regulate" so it allows for some variability but if they played with the Resistor of this, then All the bets are off.

In the supply sense R301, R302 works as a divider to tell the regulator the condition of supply and demand - so there will be sags in voltages, but not SURGES beyond 14 volts - if the supply is working right. R303 limits current thru the Zener, but the Base of the follower transistor and the 1K resistor - makes the upper side of the regulation rise more sensitive and makes the regulator drop quicker than to recover and if voltage spikes, the Zener can get damaged by that spike so they cushion it with a 1K resistor so it can "crowbar" and reset as you use the radio - say in - heavy TX modes of SSB. The R302, R3102 set the mid-point - R303 adds a level of sensitivity for regulation in loading.

 

[Brian G]

Hmmm...where did I post that power supply schematic?

Check Post #16, there is a pass transistor Q401 - handles about 3 amps - look into this pass transistor going bad...

If it has had work done - look for the Power supply board to have been fiddled with when you have voltages like that - Post #16 is the Cobra 2000's version - it is a typical Regulator circuit found even in the Cobra 139XLR - only this has an "extra protection" circuit that uses an FET to help with ripple or line noise before it can damage the pass transistor - it's not short circuit protection. It's noise protection.

Spikes are handled thru the Zener crowbar effect Too long of a duration of short it's supposed to pop a fuse, but it can take itself out if the operators were trying to get more out of the supply leaving you - the next operator in line - vulnerable.

Considering the age, and how you're seeing things go wrong in there - might want to fix the power supply before it can kill the radio from it's ability to shoot over 20 volts AC into the radio from it.

In post #16 the Zener regulation used, uses a 1K and 6.2 V 1W Zener to "regulate" so it allows for some variability but if they played with the Resistor of this, then All the bets are off.

In the supply sense R301, R302 works as a divider to tell the regulator the condition of supply and demand - so there will be sags in voltages, but not SURGES beyond 14 volts - if the supply is working right. R303 limits current thru the Zener, but the Base of the follower transistor and the 1K resistor - makes the upper side of the regulation rise more sensitive and makes the regulator drop quicker than to recover and if voltage spikes, the Zener can get damaged by that spike so they cushion it with a 1K resistor so it can "crowbar" and reset as you use the radio - say in - heavy TX modes of SSB. The R302, R3102 set the mid-point - R303 adds a level of sensitivity for regulation in loading.

So This kind of complicates the troubleshot because I previously checked Q401 and it checked out ok so don’t think it’s that and Those 3 resistors are stock original values so do t think it’s that. Didn’t touch those 2 smaller transistors.

Like I said I ordered new zener. I had an extra 2sc1419 which is the other bigger transistor on that board so I swapped that out too and I guess I will downgrade that 220 UF back to the 10.

What about the bridge rectifier causing that increase because it’s definitely not the original one?

in the meantime like I said I’ll power it through the power supply instead of the AC line because that restored normal board voltage. But would like to get to bottom of that power supply.

However the no power out is certainly my bigger concern.

I think I have literally had an issue and repair in every area of this radio since December. Modulation strip, receiver, synthesizer, power supply, TX strip. So frustrating.

 

[nomadradio]

Blown-out mode selector.

If you have voltage on the front lead of TR41, the reading there should match the reading at the two bias-test posts on the main circuit board.

If you don't, this narrows it down to the mode selector and/or wires leading to and from it.

The sharp-eyed reader will be asking how the emitter voltage of TR41 can be higher than the base (rear) lead. The answer is R193. It's 220 ohms in parallel with TR41 collector-to-emitter. It will raise the voltage at TR41 emitter when there is no load current feeding into the final/driver. When the final/driver begin to draw current, TR41 emitter voltage will fall to the normal base-voltage-minus 0.6 Volts. Under normal operating conditions, R193 reduces the heat load on TR41 a small bit.

73

 

[Brian G]

Blown-out mode selector.

If you have voltage on the front lead of TR41, the reading there should match the reading at the two bias-test posts on the main circuit board.

If you don't, this narrows it down to the mode selector and/or wires leading to and from it.

The sharp-eyed reader will be asking how the emitter voltage of TR41 can be higher than the base (rear) lead. The answer is R193. It's 220 ohms in parallel with TR41 collector-to-emitter. It will raise the voltage at TR41 emitter when there is no load current feeding into the final/driver. When the final/driver begin to draw current, TR41 emitter voltage will fall to the normal base-voltage-minus 0.6 Volts. Under normal operating conditions, R193 reduces the heat load on TR41 a small bit.

73

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.

voltage at driver: ground, 290 millivolts (AM transmit), 1.0 v
Voltage at final: ground, 6.2v , 0.9.
Almost seems to be the driver? Though I replaced it couple days ago. Maybe I toasted it again? I have a couple more on order.

 

[Handy Andy]

Like I said I ordered new zener. I had an extra 2sc1419 which is the other bigger transistor on that board so I swapped that out too and I guess I will downgrade that 220 UF back to the 10.

You can use a higher value, just you may want to swap out that for a newer one.

One (frustrating) aspect of Capacitors - especially filter ones - like the ones used in main power supply feeds, are their ESR / ESL appearance in the circuits they are used in.

Not so much digital as the analog linear types - for they show the whines, spikes and noises the cheaper caps seem to act poorly upon. So I'm suggesting a good quality cap for both output and input filtering.

10uF seems kinda small I've seen 100uF but remember too, the 2000 does not have a power supply filter system downstream into the power supply branches like the other smaller lesser cousins and brothers which do,