Cobra 2000 GTL Help Needed

[Probate]

Hi All,

I have here is a eBay special that came with no transmit, wires missing on voice lock, and the list goes on. I’m at a crossroads here, I have receive, audio, however no carrier transmit. I’m missing at IC5 SO42p mixer the 7.8 and 34mhz. Scratching my head trying to find the culprit. Any help from you fellow techs would greatly be appreciated.


Thanks

Tom

 

[nomadradio]

Time to find out if any 27 MHz energy is leaking out of the S042P mixer chip.

If you can put a second radio on the same channel, plug a coax jumper into its antenna jack. Back the shell off the plug at the other end and see if you can "sniff" any signal near that chip. All you're looking for on AM is a carrier, or try SSB and see if you hear modulation with the mike gain turned up.

If you can hear one, the trouble is down stream from there. And if not, there's no point to looking for faults downstream from it. That chip almost never fails, but the two inputs feeding it must work before you have a transmit signal at all.

73

 

[Probate]

Im getting nothing when i tried that, my voltages are ok at there mixer. I should also mention that the receive is weak too. I will look into this further tomorrow. I appreciate the response.

Regards

 

[nomadradio]

DC voltage readings alone don't tell you what the AC circuits are doing. Only tells you that the DC operating-bias voltages are okay.

Half the picture only.

A fault in a circuit that's AC (as in RF)-only circuit or component won't cause a bad DC-voltage reading.

This is where a 'scope pays for itself. Makes it sound as if one of the two input frequencies to the S042 chip is missing. Probing the two sets of input pins on the mixer chip would answer that question.

If you had a receiver that can eavesdrop in the 7.80xx MHz carrier frequency, that might help. But only if that's the input that's missing.

The other input is the 35.xxx frequency that sets the channel coming from the PLL section. If either of those two inputs is missing, you won't even have a weak 27 MHz transmit signal to hear on a monitor receiver. Frequency for channel 1 is 34.765 MHz.

73

 

[Probate]

Hello Sir,

I traced the problem to the VCO IC, I replaced the VCO and most operations are back to normal. The receive and transmit are slightly off not tracking together. I will continue this journey tomorrow, I greatly appreciate your help.

Regards

 

[nomadradio]

The factory-original clarifier circuit makes the front-panel control operate in receive only. Transmit frequency comes from two voltage-divider resistors mounted at the inboard end of the circuit board with the flip switches on it.

If turning the coarse or fine clarifier has no effect on the frequency while the mike is keyed, this is the stock setup.

Modding it to lock the transmit to the receiver frequency is a popular mod.

73

 

[Probate]

Hello,

Just got back to working on the radio, I found that the clarifier circuit was modified to a fault. I do need a 2sc1969, however I'm out. What do you recommend for a drop in replacement or something else with a slight mod with a soldering gun. I know that the 2166,2312,1969 are obsolete. If you can also tell me where to purchase said part.

Thanks!

 

[999]

Hello,

Just got back to working on the radio, I found that the clarifier circuit was modified to a fault. I do need a 2sc1969, however I'm out. What do you recommend for a drop in replacement or something else with a slight mod with a soldering gun. I know that the 2166,2312,1969 are obsolete. If you can also tell me where to purchase said part.

Thanks!

I have a takeout from a 148 if you're interested.

 

[HighPlainsDrifter$]

If he dont want it ... I would go for it. Got a Cobra 29 that might like it ...

 

[Probate]

I can buy new from a local guy here that is a friend, he asking 14 each. He is a retired tech and these are NOS. I was looking for more of a recommendation going forward on where to buy replacements without getting ripped off. He has only 3 left and I wanted to find a source for a better price for future replacements. I appreciate the offer tho.

 

[Shockwave]

I can buy new from a local guy here that is a friend, he asking 14 each. He is a retired tech and these are NOS. I was looking for more of a recommendation going forward on where to buy replacements without getting ripped off. He has only 3 left and I wanted to find a source for a better price for future replacements. I appreciate the offer tho.

You and 50,000 other people would like to find a good source for the old transistors. Notice it took less than an hour for someone to want the working pull offered by 999? The better the price, the more likely they are counterfeit. You could easily spend $12 for a counterfeit on eBay. Give the man the $14 and walk away happy knowing for sure, you got the "Real McCoy". With respect to future replacements for this radio, just make sure the SSB bias is set right, don't use too much AM carrier and keep the input SWR to any amp it may drive, reasonable and won't need more replacements.

 

[HighPlainsDrifter$]

You and 50,000 other people would like to find a good source for the old transistors. Notice it took less than an hour for someone to want the working pull offered by 999? The better the price, the more likely they are counterfeit. You could easily spend $12 for a counterfeit on eBay. Give the man the $14 and walk away happy knowing for sure, you got the "Real McCoy". With respect to future replacements for this radio, just make sure the SSB bias is set right, don't use too much AM carrier and keep the input SWR to any amp it may drive, reasonable and won't need more replacements.

Dude ... Don't get the wrong idea, I would not try to buy anything out from under anybody. But I am looking for 2 of these transistors, that's for sure.....

 

[Shockwave]

Dude ... Don't get the wrong idea, I would not try to buy anything out from under anybody. But I am looking for 2 of these transistors, that's for sure.....

Oh, I noticed you said "If he don't want it". I just think it's cool that people are willing to take the time and see if they have these obsolete parts, to helps others keep rigs on the air. Even better, that others wanting them, don't cut in front of those that asked first and ask for the opportunity, if the first one turns it down. Which, it appears he already has: "I appreciate the offer tho".

 

[HighPlainsDrifter$]

As a driver it seems to be getting harder to make contact with other trucks .... less and less even have a radio in the truck ... Strange thinking to me but it's true. Seems people believe a smart phone is all you need .... LOL ... So I figure reach out further for possible contact, and what the heck I was a prepper long before it was cool.

 

[nomadradio]

Around here they call the CB the "backup radio". It only gets turned on when the traffic backs up. Then you'll hear that it you had it turned on five minutes ago and heard the warnings you could have exited and gone around the backup.

73