Uniden PC78 Elite Troubleshooting

[RokGoblin]

I'm looking at the following schematics for the Cobra 29 LTD CLassic since I can;'t find any specific to my Uniden PC78 Elite. Early 2000s model. https://cbtricks.org/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm.pdf

https://cbtricks.org/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm_sch.pdf

My friend gave it to me after he fried the final(?) by keying up without an antena hooked up. In this scenario, is the primary Q502, and the final Q501? Which one typically burns out and how can I test with my multimeter before tearing into it?

TR14 and TR15 from the schematic both seem to be the same as the Q501 and Q502 positions on my Uniden, but there are no Transistors in the expected location on the diagram (from cobra_29ltd_sm_sch.pdf) on the heatsink. The Q501 seems to be a standard TO-220 type, but the Q502 is different and I can't find a data sheet for it.

Looking at Q501(TR14) it's already an IRF-520 from the factory, but I am not able to read the markings on the Q502(TR15) transistor (yet), but it seems to be a 1957 by the center hole. My friend said it was never repaired or modded, so why the IRF-520 instead of the 2078(?) from the schematic? Did they add the IRF-520 mod themselves in the 2000's?

I'm not trying to upgrade anything and was more hoping for a direct solder in replacement. I am not opposed to the upgrade, I just don't have the test equipment other than the multimeter.

I need a dummy load to test with since I don't yet have a CB antenna, but any other tips are appreciated.

 

[doffo]

For what its worth, today I was messing with my Uniden PC78LTW that was from 1994. That one has a 2166 final in it on the back. You can usually grab a multi meter and probe any of the 3 legs to see if there is a direct short between the legs. (I believe its best to desolder the legs off the board before testing.)

The driver usually never goes out, but most likely the final. Maybe get a dummy load and a watt meter first and hook it up and double check what its doing. I bought a parts RM Italy 203P recently and the person said it stopped outputting power. To my surprise, the fuse holder inside was loosely grabbing the fuse. (This was after removing the mosfets to put others in, but once I replaced them and it acted the same, I then realized the loose fuse holder. DOH!)

Neat radio you got. I was on the lookout awhile back for one, but I will be happy with this old PC78LTW.

 

[RokGoblin]

With my version of logic, the schematic is telling me the signal flows from TR15, to TR14, or in my case Q502 to Q501, so the final would be Q501. I seem to be quite lucky in that this one has an IRF-520 in that position.
So it seems a direct solder in place might be possible without additional mods. Is this correct? I will borrow my friends antenna to test things out. I gotta find a local elecrronics supplier in North Denver now.
It's really hard to tell, but here's the final (see photo). Maybe they did start making it with that part in the early 2000s based on availability and the existing mods.

 

[RokGoblin]

Yup!! IRF-520. I'll try find one locally here in Denver, and drop it in.

 

[DTB Radio]

I am about 99.999999% sure that the IRF520 was never used in that model from the factory. This is most likely after-market, either a botched repair attempt or power upgrade, but if your friend had it from new and never had any mods done, it does raise an eyebrow a bit. As for your current issue, any one of several parts SHOULD drop in. 2SC2029, 2SC2078, 2SC2166, 2SC2075, and 2SC1969. The 2SC2312 can drop in, but would not give as much output as it requires more drive. As for testing, since what you have is a MOSFET, you would need to build a circuit to test it, or use a component tester that can ID MOSFETS. Easiest way in this case is just drop in one of the above transistors and see if it fixes the problem. As for the driver, it CAN blow, but much lower chance of that. 2SC1957 and 2SC2314 are the two usually used there, with the 2314 by far being the most common.

 

[RokGoblin]

I am about 99.999999% sure that the IRF520 was never used in that model from the factory. This is most likely after-market, either a botched repair attempt or power upgrade, but if your friend had it from new and never had any mods done, it does raise an eyebrow a bit. As for your current issue, any one of several parts SHOULD drop in. 2SC2029, 2SC2078, 2SC2166, 2SC2075, and 2SC1969. The 2SC2312 can drop in, but would not give as much output as it requires more drive. As for testing, since what you have is a MOSFET, you would need to build a circuit to test it, or use a component tester that can ID MOSFETS. Easiest way in this case is just drop in one of the above transistors and see if it fixes the problem. As for the driver, it CAN blow, but much lower chance of that. 2SC1957 and 2SC2314 are the two usually used there, with the 2314 by far being the most common.

Thanks, my friend said maybe he bought the "Upgraded" model on Amazon, it was a while ago, LOL. He said it was new in box.