The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Radio Make and Model: COBRA 139 XLR
PCB Number:
PLL Number: UPD858
Test gear available: YOU NAME IT, I GOT IT.
Symptoms: Hi everybody! Here's one that has me scratching my head, (and I've been working on these since 1977!) I have a Cobra 139 XLR that is working great in every way, until it is hooked up to an external amp. Just the radio to an antenna or load is running 4-8 watts (adjustable) dead key AM, and 10-30 watts (adjustable) SSB. The radio is not set up for the absolute bleeding edge for power output by any means, it is capable of 30watts SSB, but is not currently set for that type of power output. When I hook it up to an old Penetrater HB-1000, it seems to work on AM. Power outputs seem normal. When I switch to SSB, immediately, the final output transistor tanks! I tested all bias voltages on the final and driver and they are correct. I turned the SSB varicap down to just under 10 watts to drive the amp, and it blew another final on the radio. (I'm starting to run out of finals!) I tried a different "high drive" amp, thinking that even 10 watts was too much for the old Penetrater (which I know it isn't, but gotta try). Same thing. Both amps work fine with another radio, even when I drive them upwards of 20 watts PEP! The only thing different about the final in this radio, is that I no longer have any 2SC1969 finals laying around. I had to switch to using the CB-20w replacement. I believe that is a direct, pin for pin, drop in replacement. I did notice before using the CB-20w, that when I check it with a transistor checker, it is showing as a PNP rather than a NPN like the 2SC1969, but like I said, when it is dropped into the radio alone, it works even better than the 2SC1969 did for output. The final sticking point is this. It is not just this Cobra, but two other radios with the same chassis (Washington, Realistic) that are doing the same thing! Help! Not sure if it is the CB-20W replacement transistor that is the problem, or something I am overlooking.

What is working: EVERYTHING UNTIL HOOKED TO ANY AMPLIFIER

What has been tried: BIAS ADJUSTED, POWER FOR AM AND SSB TURNED TO BARE MINIMUMS, ENSURED PROPER IMPEDANCE MATCHING BETWEEN RADIO AND AMP, AS WELL AS AMP AND LOAD OR ANTENNA.

Did it suddenly happen ?: AS SOON AS IT IS HOOKED TO ANY AMPLIFIER
Anything else that may be of help?:

Hi, dont know how much use this will be, but I recently had to put a CB-20W output transistor in my Ham Jumbo. The CB-20W transistor seemed ok, but as soon as I hooked up my power meter/frequency counter/dummyload it fried itself.

It seems that these CB-20W transistors work well when put through an antenna, but are extra sensitive to external load and seem to blow very easy. - just my thoughts...

Ive seen this with the 858 and other old chassis that have been "broadbanded" by changing the taps on the 2-in-1 transformer can following the mixer.
This "mod" makes it rather sensitive to stray RF - antenna too close to radio or poor grounding on the radio/amp/antenna and the output stage in the radio oscillates somewhere in the 20 to 30 meg range, causing lots of heat in the radios "finals" and killing one or both if you keep it up.

It will just misbehave on AM as there is only a 6 volt supply to the collectors (from the modulator) but on SSB you have the full 13.8V on them... ouch!

PA oscillation is pretty easy to spot - after the first word of audio, the needle showing TX power doesn't return to zero like SSB should. A counter will show the "carrier" is not on the correct frequency, but is instead somewhere near the CB band. The ALC (trimmer cap in this chassis) adjustment operates before the output stage, so that will have minimal or no effect on this issue.

PS: If your meter is showing 30W out of one of these, it's time to get it calibrated or replaced