Cobra 29 channel 19 on 18 sometimes

[Robb]

Possibly the traces/solder joints need to be re-done in that entire circuit. Tried replacing the pll? Just because the ch sw was cleaned doesn't mean it was effective; could be just worn out/shaft wobble. Lot of variables here, and I am not a tech - just a hobbyist.

Exithirteen might give some insight, he has a lot more experience. You did use a voltage chart on the radio; so what is left? If this is an intermittant problem; then there has to be a part that is failing due to thermal issues or just a cold/bad solder joint. . . .

 

[1jonnyz]

Hi Robb, I don't think it's the pll because I'm getting the same frequency coming out of IC2 pin 6 as I'm getting at pin 22 of IC1. I will have to look further at solder joints etc.

 

[loosecannon]

sorry about disappearing on you like that.

i hate to start guessing, but without the radio in front of me, it's about all i've got left.

try replacing the PLL and/or the VCO chip. yes, that is a complete shotgun approach, but with all your voltages and freqs showing correct, except for one section of the PLL, im not left with too many places to turn.

most of the time with a problem like this, that seems apparent, but the radio checks out, the problem turns out to be part of the user's installation or something like that.

just to confirm, you can make this problem happen at your shop right?

if the radio were here, i would be trying to figure out which specs change when the problem happens.

PLL chips do go bad, and in weird ways, it's just not very common.
I just replaced an MB8719 that failed in a way ive never seen before.
(repeating series of channels and not due to a wiring error)

best of luck with it.
LC

 

[ExitThirteen]

Replace C100 and C101, and see if that restores operation. These two capacitors are bypass caps that are tied from a couple of the programming pins to ground. I have seen these caps go short and thus will change the programming of the PLL. Also wouldn't hurt to replace C94 as well. Replace those caps and let us know how you get on.

~Cheers~

 

[1jonnyz]

Sorry everyone, I had MAJOR problems at my house due to the blizzard here in PA!! I finally got around to working on this piece. I changed the 3 caps Exit Thirteen and no change. I had another 7310P that I switched out at IC2 and it made no difference. I still get about 2 Mhz in RX and goes to 27.185 in TX on channel 19 at pin 22 of IC1. I rechecked the voltages on IC1 and the only one that is off is pin 10 which should be 1.9 but is coming in at .9 Pin 9 is showing 5.12 mhz which is spot on. Scratching my head hear!! BTW, LC, I can't get it to drift here at the shop!

 

[loosecannon]

1JZ, use this schematic as a reference for what im suggesting:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29_ltd_classic_sch.pdf

coming out of pin 10 of the PLL, we connect to C94, then to L22, then to C87, then to pin 4 of the VCO chip.

notice that pin 4 of the VCO chip is an input to a mixer internal to the IC, and the output goes to pin 6 of the VCO chip.

now follow pin 6 out of the VCO chip, and where does it connect to? pin 22 of the PLL chip.

something tells me that there is a problem with L22, as it is the most likely culprit in the chain, aside from the VCO chip itself, which could be bad, but i wouldn't start there.

does L22 seem to peak with the slug dead even with the top of the can?
this would be a false peak because it is the maximum inductance value of the can.
when this happens, it is the internal capacitor of the tuning can that has gone bad, and the tuning can needs to be replaced.

if you haven't messed with L22 yet, peak it per the service manual using your oscilloscope, or failing that, an RF probe on your DC voltmeter.

if you can't find any problems with L22, and it seems to peak properly, then i would start with the little disc caps in the chain. these are: C94, C87, C163, C86, and C93.

also check R94 to make sure it is at 100 ohms.

also make sure to re-heat and re-apply all the solder connections in this area, especially the PLL and VCO chips.
coils also!

if none of this yields any clues or solutions, replace the VCO chip. (can't remember if you did that once already or not)

right now, L22 seems like a very good possibility.

does the channel 9 function work correctly?
LC

 

[1jonnyz]

LC, that is the schematic I've been using. I had no trouble getting a peak reading out of L22 on my scope, the core is about 1/3 of the way down the can.

I did change out C94, 87, and 163 previously. I get 5.12Mhz coming out of pin 10 and I get 15.36 Mhz going into pin 4 of IC2, so that looks good.

I did change out R94, and C86 and 93 previously too, but I still get the funky downmix out of pin 6 of IC2 and at pin 22.

Channel 9 works fine, and I have tried 2 different IC2 chips and the same thing.

 

[Robb]

You previously stated that this problem occurs from time to time/occasionally.
If that is the case, I would still suspect a part that has a thermal issue or a bad solder joint.

 

[1jonnyz]

I had another Cobra sitting here from 1995 that had a 2816C in it, and I am still getting the same thing going on at pin 22. I have changed everything from pin 10 of IC1 to pin 4 of IC2, except for L22. I have changed IC2 and no change. I have changed everything from IC 2 pin 6 to pin 22 and same thing. It might just be time to admit defeat!

 

[Radio Tech]

Quite a few years ago I had a 29 doing about the same thing as the one you have is doing. Someone had done a lot of mods to this radio and it ended up at my shop as a parts radio. One day I grabbed it off the shelf and decided to do a bit of trouble shooting.

I sat down on a friday morning and worked on the radio 10 hours straight. Seems there was one problem after another. Replace pll, regulators, VCO can, everything. I then decided to replace the channel selector, varactor diode, and removed all the wax that was around the pll.

Fired the radio up and did an alignment and it worked perfect. I still have it on the bench as a test radio. But what part solved the problem?

Channel selector had several solder blobs on it.
Varactor diode was cracked.
The wax was a dark brown like someone had ran a soldering iron through it and had metal particals present.

Could had been anyone of the problems. This was the radio that inspired me to start the thread over at CBT called:
"Understnading how cb radio works"