cobra 146 gtl,no tx audio

[ramblingman]

cobra 146gtl,with no tx audio,on am or ssb.the rx audio is fine.mic checks good on mic tester.parts changed so far,audio chip,balanced modulator chip,mic amp transistors,am regulator,final,driver,checked for loose connections found none.also changed tr42,tr43,tr44,tr45,tr46,tr28,tr29,c122,c123,d43.i have tinkered on cbs for 7 years and have fixed quite a few with all kinda different problems.but i am at a standstill on this one.thanks for any advice,voltage test -point pinouts ect. jimmy/pathfinder/mississippi/u.s.a...

[Rick]

Check TR25, 26 and 28, also check you have volts at the base of TR25 and check for audio at the base of TR26, Also check the Mic Gain control.

Rick.

[ramblingman]

so far i have checked tr25 tr26 and tr29 all three transistors checked good.the voltages at these three transsitors are as follows,,tr25-emitter-1.69 vdc.,base-2.38 vdc.,collector-1.70 vdc., tr26-emitter-1.67 vdc.,base-1.67 vdc.,collector-1.70 vdc., tr29-emitter-0.94 vdc.,base-1.35 vdc.,collector-3.10 vdc.,,,these voltage measurements were taking with the transistors in circuit.i did not measure the area with the transsitors out of circuit.should have been the same though.i also traced the audio line from the mic connector audio pin to the variable vr5 with is the amc variable,and it has contituity.still no tx modulation.also i forget too mention on last post.when i first checked the radio it had a little bit of modulation on ssb,but nothing on am.but it would swing the needle about a watt on ssb.and like i said the carrier(dead key)is fine on am about 3 watts just no forward on modulation.the rx audio is fine also as i stated before.any other ideas would be appreciated ..thanks for the help so far........ jimmy

[Rick]

Question, you say that the base voltage of TR25 is 2.38vdc and the collect of TR26 is 1.70vdc, if this is so you have a OS. Check circuit diagram and you will see what I am getting at. Do you have audio at the base of TR26?. is TR27 short?.

Rick.

[ramblingman]

i removed tr27 and checked it.it checked good.,and the voltages are tr25-e-1.70--c-1.70---b-2.35,,,,,tr26-e-1.70--c-1.70---b-0.0,,,,,tr27--e-0.0--c-0.0--b-0.80---tr28,tr29,tr37...has no voltage on any leg..also tryed the radio with tr27 out of circuit..still no tx modulation.also traced the audio line from vr-5 amc variable,to the audio pin of mic jack and it has some continuatity.i loaded up a circuit diagram from the net,it is not to plain,and can not see it too good.but it looks like tr27-the af-antt./mic shunt// is in line with ic-3 the an612 pin 1,that is the balanced modulator.i wonder if the ic could be opened interally..any other thoughts or ideas are appreciated...........thanks jimmy

[Rick]

Well if as you say there no volts at TR28, 29 and 37 it looks like you have lost the B+ line or the feed to transistors check R152(470R) you should have about 7.5 to 7.8 VDC at the hot end of the R152 and the transistor end should be about 6.8 to 7.2VDC. What is volts at Pin 6 of IC3 (AN612)? I will try to ask the question again do you have any audio at TR26 (2SC1312)? Is the PA working?

Rick.

[ramblingman]

o.k....the voltages are as follows..on one end of r152 the voltage is 8.34 vdc,,,on the other end of r152 the voltage is 4.50vdc...and the voltage on pin 6 of ic-3 is 7.40 vdc...there is no audio at tr26,,,but if you plug your ext.speaker in the p.a. jack you have audio,but it does not seem loud enough unless you turn the volume control up.i didnt mention this first but i guess i should have,,when i got the radio someone had jumped out d-43,,dont know if that caused the problem,but i replaced d-43 anyway.the voltage at this point d-43 is very low.dont know if it suppose to be low at this point or not.but the voltages at tr 28 and tr 29 and tr37 and diffently at zero....i am checking these points with the radio powered up but i am not checking it in tx mode.

[Rick]

One think I do not understand is you say there is 8.34VDC at the hot end of R152 and 4.50VDC and the other end which is connected to TR28 and then you say (but the voltages at tr 28 and tr 29 and tr37 and diffently at zero) this is not possible has the Emitter of TR28 is soldered to R152 (Check your diagram and you will see what I am getting at). When you replaced D43 you did put it in the right way round? And you have used the right diode?

To perform a PA test, just plug an extension speaker in to the PA socket and turn the Mike Gain up full and the volume set to about mid range and switch to the PA mode key the mike and you should hear your self in the extension speaker, you may get some feed back this is normal due you being close to the extension speaker.

All voltages checks should be made in the TX mode. In any case it sounds like TR25 and 26 have gone south for the fall and should be change, as you say that TR25 (e-1.70--c-1.70---b-2.35) which tell me that TR25 has a Emitter to Collector short and the same with TR26 or maybe you are checking the transistor incorrectly, this is just to see if you are check them correctly with you test meter (DVM) connect the negative test probe to Ground (E.G. one of the tuning cans or the negative wire from you power supply or battery) and the positive test probe is used to check the component.

Rick.

[ramblingman]

o.k. you were correct on the voltages,i rechecked tr28-and the emitter is 4.50 vdc,,the collector is 3.70 vdc.,,and the base is 1.37 vdc,,tr29 is e-1.37,,c-3.70.,,b-0.95.,,tr37 is e=3.25,c-2.50,,b-2.47..pin 6 of ic-3 is 7.40 vdc.also audio at p.a. jack is good.rechecked d-43 and i have it installed correct.and the voltages at tr25 are e-0,,c-7.80,,b-7.80,,and tr26 are e-0,,c-7.80,,b-0. i have also done some more voltage tracing after tracing i found that the b+ voltage that is at the hot side of r152 is lost at the connection of r42 and tr11 i have not checked tr11 yet or r42,so i dont know if they are bad,i will also have to look at the circuit diagram to see there purpose or if there are even in the audio line.anyway i sure do appreciate all the time you have spend helping me so far rick....i have asked this question on about four other forums with little or no input. then once again thanks very much,,jimmy,,,,

[Rick]

You have a PM.

[ramblingman]

hadnt had a chance to do the headphone test yet.probably can get to it tomorrow.i checked tr25,tr27.and they appear to be open.i can get no contintuity on the v.o.m,,they are both c945q,and tr26 is a c1312f it appears to be good.no shorts,or open conditions.that is as far as ive got so far.i pick back up on it tommorow.......p.s. my email address is jimbo66@netdoor.com thanks rick.........jimmy

[ramblingman]

rick,i cant understand why my email is not working it is jimbo66@netdoor.com but i also have a yahoo address mybe you could try it is tinkerjim84@yahoo.com maybe it will go through.i apprecaite the time u have taken. p.s. still aint tryed the headphone test ,been real busy the last few days but i will do it soon.thanks again for all the time you have taken on this and for your patience........jimmy/mississippi u.s.a.

[captian_radio]

Ramblingman you have the right man helping you with your radio problem.Last year Rick helped me sort out a problem with my TRC 451 that had a problem similar to yours, he was very patient with me and after a while with his great knowledge I was able to get it going.Stay with it and do what he says and i,m sure you will get it sorted out.
Bob

[ramblingman]

hello once again rick,jim here i have been busy with work a lot of o.t.you probably know the feeling.but anyway i finally got back on the old 146 a little while last night.i tryed the headphone test like you told me,and results are there is no audio at that point.i also changed out tr25 and tr27 with no help.the voltages at the points you told me to check are off too.i think you were right i think it has had too much voltage through it.i also changed a few caps in that area around tr25 and tr27 but still no help.any other ideas would be appreciated.thanks jimmy ramblingman259 jackson,ms.

[Rick]

I have sent you an email

Rick.

[ramblingman]

hello rick,i did some more checking tonight.here are the results.on the headphone check,i checked for audio at base of tr27,there is no audio.the voltages are as follows.tr27-e-0.0,c-0.0,b-0.80,both in rx mode as well as in tx mode..tr25.-e-1.70,c-1.70,b-2.40..tr26,e-1.60,c-2.40,b-0.0..tr28,e-3.85,c-3.80,b-3.10.voltages are same in rx mode and tx mode.tr29,e-1.35,c-3.10,b-0.90.voltages are same in rx and tx mode..i changed out tr-27 with no change still no tx audio...i also touched around with end of headphone wire,around area of tr-25,tr-26,tr-27..no audio found........what a duzzie this one is.but i aint a giving up on the old girl yet.....thanks for all the help so far.........jimmy

[15minigrass]

Well if as you say there no volts at TR28, 29 and 37 it looks like you have lost the B+ line or the feed to transistors check R152(470R) you should have about 7.5 to 7.8 VDC at the hot end of the R152 and the transistor end should be about 6.8 to 7.2VDC. What is volts at Pin 6 of IC3 (AN612)? I will try to ask the question again do you have any audio at TR26 (2SC1312)? Is the PA working?

Rick.

Hey rick, I got a carrier on SSB mode on my 146. Could it be IC3??? I tried tuning VR4 but nothing happens. It sometimes doesnt have the carrier but as soon as I modulate it keys up @ 4-5 watts on SSB. I modulates well but only problem is it deadkeys 4-5watts....

Mahalo Jim....