Cobra 2000 gtl issue

[kaos513]

Thanks Andy I will check tonight and report back

 

[Handy Andy]

Damn Handy Andy, thats specific instructions right there. Im absolutely confident he will get it fixed with your help.

This may not be over.

But as a warning for others that own these radios that have not been recapped yet, this specific repair may cost you a pretty penny trying to fix it after this condition.

I've seen this happen when people first try to turn on a radio that has been sitting for years on a shelf.

If it takes more than a second - or two - it may be too late and you can't stop it from the failed boot fast enough for the power supply caps will dump their charge into the mess and let all the magic smoke out even if you manage to turn off the power.

It deals with old caps - when they dry out - and then power is applied, they will "gass" and even deform - causing the plates to shift - if ever so slightly because of the increased pressure - the package may survive the surge but the bulge, if it's excessive - will destroy the plate foil wrap from deformation - shorting them out.

This can force a shorting condition and VR10 - if set anywhere but mid-point - can send power INTO that cap that is now shorting out - and VR10 being directly fed by the 8 volt MB3756 CONSTANT via pin 1 - will dump at least an amp into the pot and potentially pop the pot open like a fuse and render the circuit dead

That's' why I said - if it takes a second or two to even "turn on" - even to light any panel lights, that delay, it may be too late.

• - all because the cap (any or all subsequent after) pulled all the current thru the substrate and due to the wattage rating the pot is (about 1/4W - 1/6th on later boards) it can blow open and you get no AM power - no way to turn on TR42 except for audio drive.

So if one cap blew, it can even take out the Audio amp from the apparent "dead short" condition or prevent the power from passing thru. CB/PA switch is also affected - the audio amp is capable of pushing several amps into the switch - the caps are to prevent this from happening.

It can even affect the PLL and prevent a "lock" condition takes too much current to keep the line "logic level High" enough to keep TX working - caps on the line used to keep the PLL from "false triggering" (a filter cushion) will just hold it low and no TX can occur.

So if the power switch is on, but the lights won't come up - shut it off and hope no wisps of smoke emanate from the vents - just make a plan to spend time gently resuscitating the chassis by recapping.

Cheap insurance.

He's' getting SSB, just worried that TR42 or VR10 are goofy...or the Mode Switch pole that the finals feed from - popped open from a poor "jumpered final" mod. RWOB.

• RWOB? Another story for later - it will take some popcorn to help offset the duration of time the story takes to tell...

​

Here's the "logic" as to why I posted this...

​

 

[bevis]

Handy Andy, do you know a place to source a recap kit for a Johnson 250 and a Robyn T123B ? It would be easier to just get good recap kits rather than trying to source each cap individually. I have 3 Johnson 250's to do and 2 Robyn T123bs to do before i power them up. One of the johnsons is new in the box and never used BUT the damn things are 40+ years old and will certainly all need new caps.
Not trying to hijack the thread, just thought id ask before the OP responds with his results.

 

[Handy Andy]

Check with @Klondike Mike - he's here of the forums - give him a "poke" - send him a PM - he just might have what you need.

 

[bevis]

Check with @Klondike Mike - he's here of the forums - give him a "poke" - send him a PM - he just might have what you need.

TY

 

[kaos513]

Here's why I ask...

Cobra 2000 and the Cobra 148 are very similar - even in function.

IF the AM Regulator seems ok - why don't you get power?

Requires the AM Regulator to even have something to turn on with.

So TR41 can show good.
So can TR42...

So Switch to AM mode - if no voltage...turn off the power on the radio.

Locate Collector Leg of TR38 - or use TP8 WIRE (not post)...attach one lead of DVM here.

Locate MB3756 Pin 2 - or the Radio Power Switch.

Switch MODE to SSB - USB and LSB - to test should show SHORT (nearly 0 ohms) using other Test lead of DVM sent to 1X.

Now, switch radio to AM mode - Test the same way, only in X10 setting...

Does it show 200 to 230 ohms? (May need to reverse leads to help offset polarity of caps along the line)

If so - remove test leads - replace TP8 back to post.

Restore power to radio and locate VR10 - make sure VR10 is sending out an adjustable voltage to the UNBANDED side of D63 - if none to very low voltage - locate C173 10uF 16V - replace and try again.

Power restored?

(... I'll wait here for you to catch up... )

Damn Handy Andy, thats specific instructions right there. Im absolutely confident he will get it fixed with your help.

Set to ssb Removed Tp8 purple wire attached one lead from volt meter to purple wire then other lead to pin 2 on voltage regulator results are 22.8 m ohms, d63 I get 6vdc

 

[Handy Andy]

Ok, no short? Or are you saying it's a short - switch to AM - then test again.

22.8MEG or 22.8 ohms? Gotta know what you're referencing...

Be sure to check the CENTER LEG (Collector) of TR41 in this test too...continuity.

I'm thinking the power feed line from the Audio Amp to TR41/TR42 may have opened up - it's why you're doing the continuity check...

IF the Cobra 2000 is open - locate and TRACE back to the front panel switch (the one that also sources the lamps) board (PC-478) has a branch that loops back to the radios Audio Amp - yes, it's #1 feeder but a branch heads back from here over by the Audio amp - if they made any light upgrades - you may need to check the power feeds that branch off back to the radio from here...you lost power - Pin 2 from the MB3756 is the MAIN power input - so when you switch modes - you're open. You're getting power - but from another spot for SSB - think RWOB mod or method.

 

[Handy Andy]

I'll post this separately...

PC478 board -

Locate the the color code of the wires that loop back to display board and to main PCB.

TP8 when in SSB should short to any 13.8V wire test point even at the power switch (when switch is depressed. turned on but unplugged)​

 

[kaos513]

My meter shows M then ohm sign

 

[kaos513]

22.23 M ohms

 

[1iwilly]

is there any voltage to tp-7 tp-8 ???

 

[Handy Andy]

He gets SSB to work, but how well for AM?

Now he's stuck trying to figure out ohmic readings on his multi-meter - meter

I'm thinking the power feed line from the Audio Amp to TR41/TR42 may have opened up - i

 

[kaos513]

Tr 42 center reads 18 ohms
Tr 41 reads 20 ohms

 

[Handy Andy]

Ok, thats better - the test passes - switch to AM and review post #14 - using graphic on #17 as a guide.

So you're up to speed...

You seem to be ok with power branches, so were now onto finding out what went wrong at TR42 and TR41.

I'm making several presumptions...

You checked and ok'd (passed) TR41 and TR42 they're good.

VR10 - we need to know if you get a voltage on D63 - should about 4~6 volts ADJUSTABLE at VR10.

C173 - still ok?

We now need to do a continuity check from TP8 to Collector of TR41 - IN AM MODE - should be 0 or thereabouts.

Then go to PIN 2 for MB3756 - it's voltage input feed - SHARES same line that FEEDS Audio amp and TR41/TR42 feeder in AM mode.

Place Meter so you can see R193's value should be 220ohms when you measure from TP8 to PIN2 of MB3756 (power feeder line - would show continuity to MODE SWITCH in AM and then at Pin 2 of the MB3756 it would see 220 ohm resistor)

What I think he is seeing...
​

 

[Handy Andy]

Just so you know, 22 ohms is a bit high, might indicate switch issues - don't be surprised if the switch "drags down" your power levels you should have at Driver and Final.

Added:

If the LAST test, from TP8 to Pin 2 of MB3756 shows OPEN - suspect the trace line to Collectors of TR42 and TR41 to be blown open or AM Mode in S403 - may be open.

To Isolate - Check Pin 2 of MB3756 to PIN 1 of Audio AMP - should be a dead short.

then Pin 1 of Audio Amp to TR41 and TR42 (Collectors) - Show open? Check Trace - else suspect S403...

To verify - locate Wire from Collector of TR41 that goes to MODE SWITCH - remove wire at MODE Switch...
(EDIT BY Poster H.A.)
This should read:

To verify - locate Wire from Collector of TR41 to MODE switch - remove wire at COLLECTOR by TR41 and tap this to Pin 2 of MB3756 in TX Mode
- get Carrier? Problems is at TR41/TR42​

Place radio in TX mode - touch Collector wire from TR41 to PIN 2 of MB3756 8 volt - get carrier? There's your answer.