Cobra 148GTL with intermittent bad final transistor.

[@Willam_J]

Mike - This video reminded me of a recent repair that I performed. I was working on a Cobra 18 Plus which had channel display segments missing. I’m sure that you’ve probably worked on one of these and know that there is a row of solder joints connecting the main board to the display/control board. A few looked sketchy, so I removed all of the solder and then resoldered all of the connections. No luck. I checked the PLL/display driver chip. No luck. I traced out the rest of the display circuit and, for some weird reason, I kept losing digits as I went. I refused to believe that the display had gone bad. It just HAD to be something else!!! Out of despair, I finally removed and tested the display. Sure enough, it was bad. It had started to make me think that I was losing my mind! :-) I replaced the display and the radio is working great now. I learned, long ago, to never assume anything, but I sometimes have to keep reminding myself. If you ever need one of these displays, the Kingbright DA56-11GWA is a good replacement. You can get it from Digi-Key.

[@miguelsalami]

EXCELLENT ANALYSIS❗👍

[@stevesteve6506]

Could you please show me where i need to place my leads inside of the radio to check the final’s bias voltage.

[@billb6029]

This guy's long gone.. There's a video all about checking and setting the final / driver bias on youtube, that may help you if you're still interested

[@TheRadioShop]

Now that was very interesting Mike. I have seen similar things happen to older capacitors where the circuit becomes noise. Man be interesting to see the inside of that transistor. Take some solvent to melt away the silicon.

[@mikesradiorepair]

LOL, could probably chuck it up in the milling machine and shave down through it.

[@KB5PQL]

The exact same problem happened with my Kenwood TS-520se audio transistor. The volume control kept scratching no matter how much deoxit and cleaning I did. I pushed the transistor slightly and the scratching went away. Once I replaced the transistor all was good!

[@jorgemaciel9290]

tengo uno igual y nomas tiene .2 no llega ni a 1 watts lo monitoreo y se escucha muy bien y le meti nuevo el c 2166 el 1969 el d1135 el tr 39 1973 y sigue igual ya le hice los ajustes alas bobinas y no sale mas los vr también ya es un dolor de cabeza el radio

[@11busyboy]

Nice! I learn something every day from video. Glad I suscribed. Great video!

[@RC-Heli835]

Hey Mike where in the world are you man? I learn more about actually hands on diagnosing and fixing electronics watching your videos than any other channels. You got that new machine and got lost in it. Said good bye to the youtube crowd and left us here to die.

[@buckaroobonsi555]

Yes does not matter if it is a machine, electronics or the body anything intermittent can be hard to track down. Often you would tell someone " Well I could not get your car to do what you described if it get's worse bring it back!"...You go to the Doctor and they say " You seem fine no infection I can spot prob. viral if it get's worse come back!" "My heart will skip a beat and when it does sometimes it takes a while to start beating again!" Doctor listens does a an 3 second EKG for a couple seconds in office says your fine next week the paramedics are called because your purple and your heart keeps stopping etc...We hear these stories all the time from family and friends or they happen to us. Electronics are even worse than a mechanical machine because you can not often hear the problem with ears or see it with your eye's or any of your other sense's. You have to have some test gear hooked up and have it fail as your inspecting it to know something is wrong some times. It takes so little too because we are talking about harnessing the flow of electrons not the flow of water or sand or something else you can see with your eye's! I am suprised you do not see even more weird stuff given your current occupation Mike. Let's face it if people send you something their is a good chance it is not working correctly or it would not be in your shop right?!?!?! LOL

[@injoelsgarage3934]

Mike, once again nice trouble shooting, I always learn from your videos. Thanks Again! Joel

[@jeffreylucas3421]

Thanks to your videos you make lite of everything I learned in the technical end of electronics but just still very mysterious to me.

[@mechadrake]

never heard of intermittent bond wire failure before :) cool find

[@orange70383]

Great video,I'm very glad you made this video because I've had 2 of my radios do this exact same thing and I did end up fixing them by replacing the finals but it never made any sense to me as to what the heck the actual problem was. I mean it was a throw my hands up mystery.

[@mikesradiorepair]

LOL, at least I'm not the only one. :) It's probably a bad bond wire internally.

[@Chuck-U-Farlie]

a 1969 in a front mic 148??? i thought all the front mic 148's had the FETs in them. what do you use to replace the 1969 final when you come across a bad one? my guess is that you have a super secret stock of them in a locked drawer near the attack cat.

[@mikesradiorepair]

I have a drawer full of new old stock ones. I buy huge quantities of surplus parts from closed shops and parts distributors every year. Have more parts than common sense. :)

[@LeapFrog_Radio]

Another great video Mike, thank you for sharing your troubleshooting process, this must be a rare fault!

[@mikesradiorepair]

Yeah usually when their bad their just bad. Broken internal connection that is intermittent is rare.

[@DonDegidio]

Mike, How long did it take to finally determine the final was at fault? I have ten minutes left to celebrate my birthday. :-)

[@LeapFrog_Radio]

Roberto, 73 is "shorthand radio speak" for "best regards", the other digits are likely the gentleman's amateur radio callsign or station identifier.

[@mikesradiorepair]

Happy birthday!!! Probably took 30 minutes. Spent some time inspecting for bad solder joints under the microscope. That's exactly what it acted like, bad solder connection. Once I probed the final with the scope it was pretty clear that's where the problem was. Once I unscrewed it from the chassis and found just touching it made it work that confirmed it.

[@LeapFrog_Radio]

Happy B-Day also!

[@LeapFrog_Radio]

+MikesRadioRepair , nice!

[@theradiomechanic9625]

Years ago my cousin brought me a TV set to fix. Had been to three other shops and they told her they couldn't fix it. Would work until it warmed up. Remove the cover and it would work again all day. Put the cover on and in an hour or so dead. Tracked that one for two days. Finally found a hidden resistor that would go open when heated with an iron. Would have told her to junk it if she hadn't been family.

[@mikesradiorepair]

Yeah intermittent problems can be a pain to track down.

[@kevinrea7820]

Hi Mike, I was wondering what you do if you need to replace the driver and final bias diodes at the back of the radio? Do you make up something of your own or is there a source for them? Kevin Rea K6REA