Cobra 139xlr low power issue.

[dxing.bruce]

Hey: Gang.

I'm really stuped with this Cobra 139xrl I'm working on.

I recapped the entire radio including the tantalum & electrolytic capacitors. I also replaced the four 2sc1419 regulators with new ones.
The radio will only put out 1 w on AM & 3 w on SSB.

I replaced the 2sc1306 driver & 2sc1307 final with new old stock ones I had, same issue still.

I even replaced the pre driver 2sc1973, issue still excists.

I even installed a new relay, did not solve the issue.

I'm totally stumped.

Any suggestions guys as I'm pulling my hair out with this radio.

Thanks in advance.

 

[TM86]

I had one 139xlr that got weird after I swapped out the 2sc1419's with the NTE equivalent. Swapped them again and it started working like it should. It wasn't RF power out on that one, though, it was over in the area that supplies power to the channel display. Which really was a nice visual clue. It ended up being two or three that were bad from that one batch.

So my only suggestion is to check that your new 2sc1419's are actually good. As I found out the hard way, new doesn't always mean good by default.

 

[Handy Andy]

What wattage were you getting before you did the work?

If it was low then, and still low now. Throwing new parts in it might help in the replacement of older worn outs but this is more solid state than a simple old car needing a valve job being a mechanical worn down sort of thing.

Speaking of the "Worn down" - a lot of times the older radios get zapper accumulation arthritis due to the long life they've already had and now they need some more work along the lines of cosmetic surgery, but of the orthoscopic kind, not just a simple face lift.

So you're ok to replace parts, but I hope you kept the older ones for they may offer the clues as to it's original problematic condition. You may have to put them back in.

Murphy Repair Services ™ always said...

It may be necessary to replace the parts you put in...in order to recreate the original condition it had. Then we can fix that...

It is my fear that you may have more of a problem with the output network and some failed disc caps in that section.

So you may have to put the OEM parts back, not the Electrolytic's and the Tant's, that work is power feed side which if you investigate and check with a voltmeter - will help verify your work. But, what I am referring to, is the Semis you took out. Need to investigate the problem by scoping and even swapping out parts - to locate the problem area. (my thoughts? check the output network)

In my experiences, the older Disc tuning caps in the output PI-filter are failing as well as the possibility to a shorted "PIN diode" section - usually caused the low-power condition - and it's usually from someone running a "relay'd" amp on a relay'd" radio that, between the two switching on and off - a transient spike arced a cap to fail internally.

Even IF the radios is pristine - the substrate used in-between the separation of the cap plates, like you may have discovered in those Electrolytic's you swapped out - do age and corrode from the long term storage not just from abuse - but also from age and the reactivity of the medium used can fail when you need it most and simple power up and SWR spike can cause this.

 

[nomadradio]

What wattage were you getting before you did the work?

You beat me to it, Andy! If the radio was broke before the recap, that part is still broke.

And if it showed normal wattage before, that's a big difference.

Has a big influence on the strategy to adopt next. More than one way to make a transmitter weak.

73

 

[dxing.bruce]

What wattage were you getting before you did the work?

If it was low then, and still low now. Throwing new parts in it might help in the replacement of older worn outs but this is more solid state than a simple old car needing a valve job being a mechanical worn down sort of thing.

Speaking of the "Worn down" - a lot of times the older radios get zapper accumulation arthritis due to the long life they've already had and now they need some more work along the lines of cosmetic surgery, but of the orthoscopic kind, not just a simple face lift.

So you're ok to replace parts, but I hope you kept the older ones for they may offer the clues as to it's original problematic condition. You may have to put them back in.

Murphy Repair Services ™ always said...


It is my fear that you may have more of a problem with the output network and some failed disc caps in that section.

So you may have to put the OEM parts back, not the Electrolytic's and the Tant's, that work is power feed side which if you investigate and check with a voltmeter - will help verify your work. But, what I am referring to, is the Semis you took out. Need to investigate the problem by scoping and even swapping out parts - to locate the problem area. (my thoughts? check the output network)

In my experiences, the older Disc tuning caps in the output PI-filter are failing as well as the possibility to a shorted "PIN diode" section - usually caused the low-power condition - and it's usually from someone running a "relay'd" amp on a relay'd" radio that, between the two switching on and off - a transient spike arced a cap to fail internally.

Even IF the radios is pristine - the substrate used in-between the separation of the cap plates, like you may have discovered in those Electrolytic's you swapped out - do age and corrode from the long term storage not just from abuse - but also from age and the reactivity of the medium used can fail when you need it most and simple power up and SWR spike can cause this.

Hey: Guys. Got it fixed. This radio looked virgin inside as all the wax was still in the pots & there were no mods but it turns out someone monkeyed with the transmit alignment pots in the section before the pre driver. Got it working correctly now. Thanks for all the help guys.

 

[Handy Andy]

Glad you found the problem!

And for the reminder for us all, that Pristine and "untouched" do not mean it's left alone and not messed with - obviously someone did and it got messed up and you tweaked it back into service!

Nice job!

 

[nomadradio]

Good to hear you figured it out. Congratulations on a job well done. Re-capping a radio is not for the faint of heart, or the easily bored.

73