I'm trying to find a photo of the solder side, and the inside if possible of a Cobra 29 PC board. Mostly because of this( see photo). Also in a last ditch effort to figure out what the heck's wrong with this thing.
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What the hell is this?
- Thread starter ClaimJumper
- Start date
My friend, that is a smd diode. Several manufacturers do that very same thing.
Hmm. Just to the rear of the PLL chip. Are we sure it's not a varactor diode? RCI has been doing this for years since varactors with wire leads became harder to come by in large quantities.
73
73
No audio. I mean, at all. Meter doesn’t move beyond original key up. No swing at all. Replaced original final due to short to the heat sink. Never was audio out of the 4 watt carrier. If I yell or blow into the mic I can hear that thru another radio. No words at all! I have audio out of the PA. BC Coyote and NomadRadio have helped me with this for a while on another thread “Cobra 29LTD parts list”. V4 does nothing so I removed D11 it tests good. Removed the nearest cap, tests good. Double checked 2 mics. I intend to remove and check tr17 and tr18 just because they are in this circuit. I’ve examined every inch of this thing, nothing obvious visually.
I'm obviously WAY too old.
How are you supposed to know "what part is what" when multiple SMD parts have the same marking??? I like old radios!
How are you supposed to know "what part is what" when multiple SMD parts have the same marking??? I like old radios!
It gets worse in this case as there are multiple zener diodes marked T2, not just a 10v zener and the varactor I mentioned. There are also switching diodes marked T2;I'm obviously WAY too old.
How are you supposed to know "what part is what" when multiple SMD parts have the same marking??? I like old radios!
My first step is an image search, because there, you will see the most common ones. With the most likely suspects in mind, I then go to the SMD code databases and double check, comparing package sizes etc.
This next suggestion is more applicable to transistors than diodes, but may still apply. It is common for a radio to use the same parts more than once in the circuit, so look around for another one with the same marking. It is unlikely that they will use two different parts sharing the same marking, so finding another gives you one to test in the event the one in question is smoked. In the case of zeners or varactors, finding another is unlikely, but worth a look anyhow as switching diodes would likely exist in multiple places. Follow the hints the radio provides.
After that, it really does come down to looking at the schematic, or if you dont have one, looking at the nearby circuitry and maybe measuring voltages or signals to get an idea which of the remaining suspects it could be.
It is still... in my humble opinion... goofy to have multiple part types with the same number. Do bear in mind..... I said I was getting WAY too old...
As for the diode, there is a 1SV214 (D16) and a 1SV215 (D13) in the parts list, both varactors. I assume one is VCO tuning and the other is clarifier. As nomad said, if it is right behind the PLL, good chance it belongs to the VCO in the PLL loop. The symptoms you describe don't line up with a bad varactor. But out of curiosity, I would look at the other side of the board at all the diode silk screen labels to see which D# is not there, the one not silkscreened is likely the T2 diode on the back and the parts list will tell you what it is.
I just read through the old thread, but I am at work so I will have to look at the schematic when I get home and see if anything jumps out at me. around V4. I'm almost on overtime so I had better get off the computer lol.
I just read through the old thread, but I am at work so I will have to look at the schematic when I get home and see if anything jumps out at me. around V4. I'm almost on overtime so I had better get off the computer lol.
Taking a quick glance before going to bed and it looks like the mic audio can do two things. It can go to the audio IC chip directly, or it can be ran through the compander chip first. There is a switching IC that selects between the two.
I do not have experience working on this particular radio, but my first suspects would be the switch IC802, the audio bypass caps around the compander IC801, or the compander itself. I would imagine that the compander is used during TX and the audio is sent to the audio amp uncompressed when in PA mode.
I'll have to keep thinking on this, but I think thats the area you need to look. If it is the switch IC802 (or a control signal to it), shorting pin 3 and 4 and getting modulation means the 3-4 compander audio select switch is bad. Shorting pins 1 and 2 of the switch IC and getting modulation means the compander or its associated caps are bad.
I do not have experience working on this particular radio, but my first suspects would be the switch IC802, the audio bypass caps around the compander IC801, or the compander itself. I would imagine that the compander is used during TX and the audio is sent to the audio amp uncompressed when in PA mode.
I'll have to keep thinking on this, but I think thats the area you need to look. If it is the switch IC802 (or a control signal to it), shorting pin 3 and 4 and getting modulation means the 3-4 compander audio select switch is bad. Shorting pins 1 and 2 of the switch IC and getting modulation means the compander or its associated caps are bad.
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