Cobra 29 channel 19 on 18 sometimes

[1jonnyz]

I am working on my buddy's 29, he's having problems with it drifting channels. Sometimes he will lose people on 19 flip to 18 and there they are. I have checked the voltages on the 2816 PLL and they are good. Pin 10 is perfect at 5.12 mhz, pins 12 and 13 are dead on at 10.24. Pin 22 is the one that has me stymied. It reads about 3.89 mhz on ch. 1 RX, and 26.965 TX. I know that it should be .91 and 1.365. BTW, IC2 is good as far as voltages. Also, someone put an echo in and used the delta tune for the controls. D17 does have an 8.2K in series with the diode. Any ideas on where to look? I am no expert, still learning here!

 

[loosecannon]

sounds like a VCO problem.

first thing to do is make sure that L19 is set to 3.2 vdc on channel 40, and should be somewhere around 2 vdc on channel 1.

connect the voltmeter to the bare (non painted) lead of R88.

(this is for the PLL chip IC1) check to make sure that pins 20 and 21 are both 0 volts (grounded) on both RX and TX.
make sure that pin 22 is at about 2.4 volts on both RX and TX.

My initial guess is that the channel switch itself needs cleaning or replacing, if you don't find any faults in the PLL section.

one test to do is when you "lose" people on a certain channel, try keying the mic before doing anything else. does the RX/TX light change from green to red when in this "condition"?

what service info are you using for this?
LC

 

[1jonnyz]

Hi LC, first off, I am using cbtricks for the service info.

Yep, I have 3,2 volts on 40 and about 2.2 on 1. I have 0 volts on pins 20 and 21. and I have 2.4 on pin 22. I asked the guy about the red light and he has no clue.

I switched out ic2 with another I had sitting around and the mhz on pin 22 were like 2.5 rx and 4 tx as opposed to what I posted above. I put the old one back in. NOW, I have no modulation, good carrier but no modulation. I checked TR17 and have 1.3 volts on tx, should be 7.7, and on rx I have 1.9 and it should be 4.8?? I don't know what is going on. I also have no PA. I get a lot of noise when I touch IC4.

 

[Robb]

I would have thought that the ref xtal is faulty.
Channel selector is internally dirty and/or worn if the channel is still a problem; as LC said..
Modulation is another/separate issue.

Do you have a bench freq counter?

 

[1jonnyz]

Yep, I sure do Robb. X1 is fine, putting out a perfect 10.24

 

[loosecannon]

well the good news is that you do have some things working that help us know where to look.
audio amp is good, as you have receive audio.
VCO seems good since you can key the radio.

the PLL chip itself is probably good as your voltages are within spec.

im leaning heavily on replacing or cleaning the channel switch.
if you choose to clean it, you have to take the whole switch apart in order to do it.
there is no point in spraying contact cleaner in the back and turning the switch.

i have written up the procedure here on the forum and in other places, so do a search here for channel switch cleaning or something like that.
the secret is to not cut off the little white plastic nubs on the back, and to keep everything in order and oriented correctly.
in your case, be sure to very carefully bend all the little prongs out so they will make contact with their respective discs.

do you have the correct VCO freqs at TP3?
ch. 1 = 16.270
ch. 40 = 16.710

i was looking at the voltages for TR17, and it's collector is fed from the main 8 volt regulator, TR23.
check the voltages on each leg.
you should have supply voltage (13.8v or so) on the collector, and 8 volts on the emitter leg.
if the 8 volts is not there, this will definitely cause the mic amp to not turn on, and will also wreak havoc all over the radio, as there are many branch circuits fed from TR23.
This could be your PLL problem also!

if you don't see the 8 volts on the emitter of TR23, remove it from the chassis completely, and test it using your DMM.
if it checks good, start looking at D22 (9.1v zener) and D21, and then all the associated components of TR23.

after typing all of this up, i would check TR23 first, then verify the freqs at TP3, and then give the channel selector a good cleaning if no clues popped up in the other tests.

If TR23 checks out good, and there is 8 volts on the emitter, then pull TR17 out and test it with your DMM.
if it's good, check R64 and C37.

that should give you some places to start looking.
let us know what you find.
LC

 

[midnight special]

just my 2 cents worth,,, has the delta tune been opened,,,(slide),,,,the reason i am asking is because i have seen a cobra 29 do the same thing after a echo board was installed,,,what had happened was the tech(??) pulled the delta tune out and just laid it inside,,,,,installed echo knobs in it place,,,, the mistake done was as follows,,, the tech did not center the delta tune for center freq,,,then he didnt tape or lock it there,,, then didnt insulate it at all no tape or bubble wrap around it just let it lay inside chassis,,,,it was just rolling around in the chassis and bumping around would touch the tabs on pot to cause it to go up and down freq,,,,,,,,just my thoughts

 

[loosecannon]

that's a good point Midnight Special.

definitely make sure the delta tune mod is up to snuff, or just remove it to eliminate it as a possibility.
i hadn't considered the possibility of a slider mod causing instability, but it could definitely be a problem.

now i'm wondering if this delta tune mod was done with one lead of the pot going directly to ground, which could take out the 8 volt regulator if that's what was powering it.

absolutely a good place to look.
LC

 

[1jonnyz]

LC, I have a disc bulging out of my back and can't really do anything today. I can say that I did test TR23 the other day and it was fine. As far as the channel switch, I am thinking that is a problem too. There is a segment out of the LED too.

As far as no modulation... this has an echo board installed and I am thinking something in that is messed up. C37 is removed and the echo is wired in there. I am thinking if I remove the wires and put a 3300pf cap in there then I can see if the problem is in the echo or somewhere else?

I will check the frequencies at TP3 tomorrow and get back to you. Thanks for your help, I appreciate it!!

 

[Robb]

Less than half the time I found that resoldering the LED board corrects the LED segment. If not; then it may need replacing. Beginning to suspect your channel selector switch . . .

 

[Sideways]

Does it do it on any other channels? Or just 19/18?
If not, channel selector gets my vote. Most likely just dirty if its an older radio, or been in a dusty enviroment...
If its all channels, could be just a cold solder joint shorting out near the vco area.
just my .02

 

[loosecannon]

1JZ, i have never heard of anyone installing an echo board that way.
very strange.
if it were me, i'd remove it, and if the customer wanted it, install it using the mic jack wires like pretty much everyone does.
it would seem to me that putting the echo in that way would have a good chance of taking out TR17.

if TR23 is putting out 8 volts, as measured at the emitter leg, but you are only getting 1.3 to 1.9 volts at TR17, then something is causing the voltage to drop out.
check R70, R69, R64, R65, R66, R67, and C66.

the first thing i would do is replace C37 and see if the 8 volts came back.
if not, check to see if TR17 is blown. if not, check the components i listed above.

i think you might have two separate problems. one being the channel switch, and the other being the possible echo board fiasco.

The LED segment could be caused by the channel switch problem, so fix that first, and if it doesnt bring that segment back, then replace the display LED itself.

You definitely have a head scratcher there.
It's hard to troubleshoot something like this over the internet.
best of luck,
LC

 

[1jonnyz]

OK, here's the scoop. As far as the delta tune goes. R&R in Claymont did the original work on this, and apparently just put an 8.2K resistor in series with D17. The owner of this radio did say it will drift on other the channels too.

I did remove the echo from C37 slot and replaced it with a 332 cap and now have modulation!! I am assuming there is a problem with the echo. I rechecked all voltages on IC1, IC2, and IC3 and they are all on the money. I checked the frequency at TP3 on channels 1 and 40 and they are both spot on. IC1 pins 10, 12, and 13 are spot on. The only problem is the frequency on pin 22, it;s about 3mhz on rx and goes to 26.965 on tx on channel 1. I know that is wrong, and am wondering if a bad or dirty channel switch would cause that? Looking at the schematic I wouldn't think it would be the problem.

 

[1jonnyz]

I just took the channel selector apart and cleaned it. It was a filthy bugger! Unfortunately got it all back together and same symptoms... frequency messed up at IC1 pin 22, and LED still out,the LED being no concern. Would the goofy frequencies at pin 22 be the reason it's drifting? Although sitting on the bench powered up for a few hours and no drift at all.

 

[1jonnyz]

Any ideas on the downmix problem LC, Robb, anyone?