Cobra 89 XLR, low modulation.

[LeapFrog]

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Any ideas, tips or suggestions would be appreciated.
PA works okay, ferrite in L16 is noticeably cracked as is the primary in the double can by TR2 (unmarked)
Would the cracked ferrites cause this, i don't think so but I've never encountered a busted ferrite until I got inside this radio.

TR16 doesn't show much gain (i think) (c458)
TR15 shows gain. (A844)

Also the collector on TR14 was clipped, I soldered a leg on it but I don't know if the c458 is still any good.

I can probe around anywhere with the scope, i'm just not experienced enough yet to know how to diagnose this radio.

I have added 56 ohms to the base of TR20, and replaced D14 with a 1N4148.
Power mics make no difference!
VR5 is dead in the water also, meaning it makes no difference.

 

[Robb]

Well; I guess only Xit13, unit 399, or Shockwave can answer, since you asked for techs only . . .

 

[LeapFrog]

Well; I guess only Xit13, unit 399, or Shockwave can answer, since you asked for techs only . . .

I count you in the group also my friend, I said that meaning only people who can do more than just modifications. I on other hand am so wet behind the ears, most of my time spent has been on crap mods instead of learning proper diagnosis & buying gear.

I am calling myself a tweaknition and not putting down on anyone.
Someone can post in here and tell me i'm a jackass, and I'd be okay with that.
I understand just how pretentious I have been with you all, I apologize for being such a charlatan.

 

[Robb]

Q: why did you bypass the mic gain control?
Q: What is the dead key set at; you didn't say?
Q: is there an AMC control on this board?
Q: is there a tantalum in the mic circuit?

 

[LeapFrog]

Q: why did you bypass the mic gain control?
Q: What is the dead key set at; you didn't say?
Q: is there an AMC control on this board?
Q: is there a tantalum in the mic circuit?

A:Incase of failed potentiometer (odd chance in hell I know, I cleaned it also)
with it inline, i noticed it had an effect halfway through its range

A: around 3.8 watts maybe less
A:VR5, adjusting does nothing
A:Yes looks like four or five near the audio chain

 

[Robb]

Well, if vr5 does nothing, sounds like the area needs to be explored better - eh? It should change. So if it doesn't; then something in that circuit is amiss. Bad diode, tantalum, or transistor. Like I said - haven't looked at the schematic.

 

[LeapFrog]

Well, if vr5 does nothing, sounds like the area needs to be explore better - eh?

Agreed, going to change tants, I don't see anything clipped anywhere, minus the leg of TR14 which I solder-bandaged..

VR5 not working tells me signal "falls off" before it ever reaches AMC.
EDIT: I think that's the case, i don't know for sure.

 

[LeapFrog]

Well PA works so I'm thinking a pre-driver issue but I have not a clue yet.

 

[sonoma]

do you have another VR5 to make sure it is good. or even jump it to see if audio increases. even the resister feeding it.

 

[loosecannon]

check the voltages on IC2 to make sure they are correct. (schematic on cbtricks shows voltages)
check TR14 and VR5 itself. it may be worn out.

just a couple of guesses.
LC

 

[Robb]

Agreed, going to change tants, I don't see anything clipped anywhere, minus the leg of TR14 which I solder-bandaged..

VR5 not working tells me signal "falls off" before it ever reaches AMC.
EDIT: I think that's the case, i don't know for sure.

Well; not exactly. If the AMC control isn't working; then it has been disabled. Check the AM Limiter? If the AMC control on the board isn't working, it is usually an indicator that the limiter has been pulled.

 

[sonoma]

if the VR is bad it will not work as he stated. also I read on one of his post it would start to work then quit. I have seen so many of the 858 PLL radios that some of the vr's are dead. so put new ones in and all is good. hard to keep up with 2 people posting about the same radio in the same post to keep them straight. glad leap frog started his own now.

 

[loosecannon]

so i just watched the video, and the first thing that i saw was that you chose a very bad place to ground your scope probe. never ground your probe to one of those aluminum walls inside a CB radio.

there is heat sink compound underneath that wall that insulates it from the chassis.
yes, there are screws there, but they have the grease on them too and should not be trusted.

i realize you are measuring AC and grounding to the chassis is acceptable, but i would advise against using those heat sinks.
use the chassis itself or a tuning can just to make sure,
LC

 

[sonoma]

that will give you a lot of wrong readings as LC just told you. have not looked at your video as of yet.

 

[LeapFrog]

[photo="medium"]4465[/photo] first digit is 2v pin1
one pin shou have a voltage present and its not


pin7 is high and pin 3 is missing voltage
GOING TO STUDY SHEMATIC