Cobra 2000 gtl issue

bevis said:

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

Click to expand...

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...

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.

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.

Handy Andy said:

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.

Click to expand...

TY

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.

I'll post this separately...

PC478 board -

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

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

Handy Andy said:

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

Click to expand...

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)

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:

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