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29 WX NW ST No TX and No RX; Need Help Please
- Thread starter BuckSkin
- Start date
I'm back --- sorry to have taken so long to get back with this, but I have been down with some sort of killer flu virus allergy infection sore-throat mess for the last several days and just couldn't summon the strength to think straight; although I am far from fully recuperated, I can sort of stagger around now.
Here are the results of my earlier test of W23 while everything was still connected:
B: no mike = 8.60 --- mike keyed = 8.59
C: no mike = 11.42 --- mike keyed = 10.64 (0.78 drop)
E: no mike = 7.96 --- mike keyed = 7.93
and here are the results after I de-soldered and lifted the legs on the driver and final:
B: no mike = 8.61 --- mike keyed = 8.60
C: no mike = 11.56 --- mike keyed = 10.76 (0.80 drop)
E: no mike = 7.99 --- mike keyed = 7.96
Just to be clear, the driver and final are those two flat-backed 3-pronged gizmos that are screw-attached to that multi-angled aluminum "fence" located in the rear corner area; one is a wee bit wider than the other and they look like smaller versions of the W23; do I have this right ?
And, if so, which one is the driver and which is the final ?
Thanks so much !
The driver will be the smaller transistor behind the heatsink in the back left corner if looking from the front of the radio; the final output transistor will be slightly larger and more towards the right, the lettering on it will be facing the front of the radio. Yes they both mount to the aluminum heatsink.
On my Cobra 29 LTD that was messing up, the channel RX/TX LED died on me, and a botched modification was causing strange feed-back issues and a problem in the frequency generating or mixing area of the radio, all mods removed and it is now running A-okay.
My apologies for being a week late, I can look at a voltage chart for your radio and compare your measurements to what "should be present", at a glance it looks ok though, take that with a grain of salt please.
On my Cobra 29 LTD that was messing up, the channel RX/TX LED died on me, and a botched modification was causing strange feed-back issues and a problem in the frequency generating or mixing area of the radio, all mods removed and it is now running A-okay.
My apologies for being a week late, I can look at a voltage chart for your radio and compare your measurements to what "should be present", at a glance it looks ok though, take that with a grain of salt please.
Okay, the smaller "driver" does not appear to have the thin flat metal mount bracket and the screw passes through the thick portion instead (I hope my explanation is making sense); thus, I cannot tell for certain which pins are B, C, and E.
On the driver, is B closer to the outside wall of the radio, or is it closer to the center ?
EDIT: I think I got it figured out; my readings were exact opposites, so the driver is oriented opposite to what I first thought.
While I had the pins de-soldered, I have been doing the test as explained here:
https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07
EDIT: My meter does not have the type readings as explained in the article; instead of "OL", I have plain old "1".
On both the driver and the final, my readings were the same.
Instructions Step 1. I don't get a negative number; I get 699 on both units.
Instructions Step 2. I get 697
Steps 3, 4, 5, and 5-reversed, I get 1
Thanks.
Last edited:
If a driver or final transistor fails short-circuit this can cause the radio to pull too much current, and blow the fuse (if one was used).
I have had a Cobra 25 on the bench that had no receive audio until I replaced the blown final. With the driver and final unsoldered you should hopefully have receive audio. If the radio does not receive or transmit it could point to voltage problem in the frequency mixing or generating sections in the radio, your driver/final may be ok.
I got a little busy over here, but have look at the PDF I have attached (voltage chart), it is for the 29 LTD model, but it will have all of the same "heart" as your radio; and so the voltages will be about the same, minus some missing for the extra features your model has.
The diode check mode is needed to check junctions on the transistor, with only a continuity test you won't know as much about what is going on inside the part.
Edit: here is a sloppy picture to help.
The driver transistor is "backwards" compared to the final, note also the mounting is backwards the "front" or labeled end of the driver faces the antenna connector.
I have had a Cobra 25 on the bench that had no receive audio until I replaced the blown final. With the driver and final unsoldered you should hopefully have receive audio. If the radio does not receive or transmit it could point to voltage problem in the frequency mixing or generating sections in the radio, your driver/final may be ok.
I got a little busy over here, but have look at the PDF I have attached (voltage chart), it is for the 29 LTD model, but it will have all of the same "heart" as your radio; and so the voltages will be about the same, minus some missing for the extra features your model has.
The diode check mode is needed to check junctions on the transistor, with only a continuity test you won't know as much about what is going on inside the part.
Edit: here is a sloppy picture to help.
The driver transistor is "backwards" compared to the final, note also the mounting is backwards the "front" or labeled end of the driver faces the antenna connector.
Last edited:
buckskin, you have the right transistors. your voltmeter reading "1" most likely means the same thing as the "OL" or "open line". this can be confirmed by touching the leads of the meter together and noting what that reading is. (should be something like 000 or 0.00) indicating a dead short.
those transistors are basically a set of two diodes together, so what you are measuring is the one way conduction of each of them. if the readings are .699 with the leads one way, and "1" with them the other way (switching positive and negative leads around) then at this point i am willing to say that they are ok, and you can re-solder them in place.
let's try this another way. using your desoldering braid, remove all the solder from the lead of w23 that showed the voltage drop. you need to make sure that you get ALL the solder off, and that you can see a small gap in between the actual lead of the transistor and the foil pad around it. the idea is to isolate the component lead from the trace.
once this is done, and you can physically move the lead around in its small hole, take another voltage measurement on the component lead (not the trace).
did it still show the same voltage drop? my initial guess is that the lead will show the correct voltage when it is disconnected from its trace.
LC
those transistors are basically a set of two diodes together, so what you are measuring is the one way conduction of each of them. if the readings are .699 with the leads one way, and "1" with them the other way (switching positive and negative leads around) then at this point i am willing to say that they are ok, and you can re-solder them in place.
let's try this another way. using your desoldering braid, remove all the solder from the lead of w23 that showed the voltage drop. you need to make sure that you get ALL the solder off, and that you can see a small gap in between the actual lead of the transistor and the foil pad around it. the idea is to isolate the component lead from the trace.
once this is done, and you can physically move the lead around in its small hole, take another voltage measurement on the component lead (not the trace).
did it still show the same voltage drop? my initial guess is that the lead will show the correct voltage when it is disconnected from its trace.
LC
Before I bought a de-solder pump, I used wooden tooth-picks.
A quick but careful jab into the molten solder helped to separate the component lead from the trace. I would push the solder away into the "green area" or solder mask and it cooled about that same time, it is tricky but this method helped me get the lead unstuck from the pcb trace/hole in the beginning.
It was not too difficult to order solder braid or a hand powered solder sucker, but it was difficult for me to wait on it to come in the mail!
A quick but careful jab into the molten solder helped to separate the component lead from the trace. I would push the solder away into the "green area" or solder mask and it cooled about that same time, it is tricky but this method helped me get the lead unstuck from the pcb trace/hole in the beginning.
It was not too difficult to order solder braid or a hand powered solder sucker, but it was difficult for me to wait on it to come in the mail!
With the C pin de-soldered and free of it's connection, no mike = 5.54v and mike keyed = 2.29v
Whether it makes any difference or not, the driver and final were still de-soldered during this test; should I have re-soldered them ?
Thanks.
well i have to apologize, as i had you do the test the wrong way.
not sure what i was thinking, but i meant to have you take the measurement on the TRACE, not the LEAD of W23 with the collector unsoldered from the board.
no, it won't matter whether or not the driver and final are soldered in for this test, as we have already eliminated them as culprits of your voltage drop.
please try the test again and tell me if the voltage on the collector TRACE drops the voltage on TX with the pin unsoldered.
more to come...
LC
not sure what i was thinking, but i meant to have you take the measurement on the TRACE, not the LEAD of W23 with the collector unsoldered from the board.
no, it won't matter whether or not the driver and final are soldered in for this test, as we have already eliminated them as culprits of your voltage drop.
please try the test again and tell me if the voltage on the collector TRACE drops the voltage on TX with the pin unsoldered.
more to come...
LC
No need to apologize; I would have never known the difference anyway.
Voltage on trace: no mike = 12.40v and mike keyed = 12.31v
Thanks.
just saw your last post. re-solder everything before taking the measurements below.
also, just as a lark, you might try removing w23 and testing it like you did the driver and final.
please understand that i am shotgunning here, and just trying to find anomalies that could point us toward the problem.
the only thing i think i see so far is the voltage drop on the 13.8v line in TX, and to be honest, i don't know how much difference a .8 volt drop here would really matter.
Someone else might know, but i don't.
one of my suspicions is that one of your mixer signals is at too low a level or even missing which might cause your strange symptoms.
problem is, the way to test for that is with a freq counter and an oscilloscope.
trying to test for mixer signals being present with only a voltmeter is not something i am well versed in. again, someone else might be, but not me.
I can have you make an RF probe for your voltmeter so you can trace RF in to the radio, but that can really only tell you if RF is present or not, and it won't quantify the level of it.
honestly, with no experience using an RF probe, you could misinterpret the reading because it's bouncing around and assume RF is present where it's not.
so, what i would like to do is have you start going through the radio, testing all the active devices, as these are the most likely to fail.
Right now, i want to know the RX and TX numbers on IC1, IC2, IC3, FET1, FET2, TR6, and TR7. also TR21 just for the heck of it. (TR is a standard abbreviation for transistor, but in your radio they may all be "W" markings for the transistors)
use this chart to compare your readings with, and post anything that looks off from what the chart shows: http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf
both TR6 and TR7 should be E C B when looking at the flat part of the transistor and leads facing down.
FET1 will be D S G, and FET2 will be G S D when viewing the same way.
hopefully these readings will lead us somewhere, because if not, im afraid you're going to need to send it somewhere with the right equipment.
LC
also, just as a lark, you might try removing w23 and testing it like you did the driver and final.
please understand that i am shotgunning here, and just trying to find anomalies that could point us toward the problem.
the only thing i think i see so far is the voltage drop on the 13.8v line in TX, and to be honest, i don't know how much difference a .8 volt drop here would really matter.
Someone else might know, but i don't.
one of my suspicions is that one of your mixer signals is at too low a level or even missing which might cause your strange symptoms.
problem is, the way to test for that is with a freq counter and an oscilloscope.
trying to test for mixer signals being present with only a voltmeter is not something i am well versed in. again, someone else might be, but not me.
I can have you make an RF probe for your voltmeter so you can trace RF in to the radio, but that can really only tell you if RF is present or not, and it won't quantify the level of it.
honestly, with no experience using an RF probe, you could misinterpret the reading because it's bouncing around and assume RF is present where it's not.
so, what i would like to do is have you start going through the radio, testing all the active devices, as these are the most likely to fail.
Right now, i want to know the RX and TX numbers on IC1, IC2, IC3, FET1, FET2, TR6, and TR7. also TR21 just for the heck of it. (TR is a standard abbreviation for transistor, but in your radio they may all be "W" markings for the transistors)
use this chart to compare your readings with, and post anything that looks off from what the chart shows: http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf
both TR6 and TR7 should be E C B when looking at the flat part of the transistor and leads facing down.
FET1 will be D S G, and FET2 will be G S D when viewing the same way.
hopefully these readings will lead us somewhere, because if not, im afraid you're going to need to send it somewhere with the right equipment.
LC
I hope you guys haven't been thinking that I had abandoned this; I have just been first = miserable with a flu-like bug and then overwhelmed in catching up on all the other chores that I had let slide while I was feeling puny.
As soon as I get opportunity, I will do the next tests on our list.
I appreciate you guys and your patience with a complete beginner.
As soon as I get opportunity, I will do the next tests on our list.
I appreciate you guys and your patience with a complete beginner.
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