New final for Cobra 25...FQP13N10?

[kraftwerk1976]

Hello, everybody. I had forgotten I had an account here. I had asked one question way back in 2006. Looking at the forum, there seems to be a ton of good info.! Looks like a nice one. Anyway, I have two radios I want to change out the finals on. One is a Cobra 25LTD Classic, and the other is a President AR-44, which is basically the same as the Cobra 25 GTL. I turned on the President AR-44 the other day and noticed the S-Meter was going back and forth, for some reason (when another radio I had just removed did NOT have any noise) with the antenna unplugged. I then noticed the ammeter on my power supply varying from 4 to 4 and a half amps...WAYYY high for just being in the receive mode. After about ten seconds, maybe, I unplugged the President and the heat sink near the final transistor was very, very, hot, and it smelled like some electronic part was burning, maybe. After thinking about the problem, could it be that one of the three legs of the final is touching a component is shouldn't? That might explain the excessive current draw when only in RX. The final is a 2SC2075.

--With the above said, I will probably be needing to put a new final transistor in there. I looked around the house and found some FQP13N10 MOSFETs for another project. I had read that the FQP13N10 transistors were similar to the ERF2030s...which in turn were replacements for 2SC1969s...which are similar to the 2SC2078, and 2SC2075, which is the final in the President I need replaced. Before I give my President AR-44 and FQP13N10 to the tech., will the FQP13N10 transistor indeed be a suitable transistor for a radio that used a 2SC2075? I only spent a few bucks on the FQP13N10s, so no loss if one can't be used in the radio. I bought the President AR-44 off of Ebay maybe ten years ago, NOS. I plugged it up one time, and it did the same thing, with the current draw problem and S-Meter fluctuating weirdly. So, any idea on how to mod. this radio with the new FQP13N10, assuming it can be done? If it was just a simple, de-solder the old transistor, drop-in and solder the new transistor type job, it would be no problem, but not knowing much about transistors in the first place, and having the problem of the excessive current draw on RX, it sounds like there is more to the problem than I could handle.

--Forgot to add, but assuming the FQP13N10 can work with the above President AR-44, would it work with a Cobra 25LTD Classic? That radio has a 2SC2078, different from the President AR-44. They are similar radios, but not sure just HOW different. Thanks for any help.

[Deleted User 14541]

The 13n10 and 2030 are mosfets. The 1969 and 2078 are bipolar transistors. Other mods have to be made to install a mosfet final in a radio with a bipolar. I have dropped a 13n10 into a radio set up for a 2030 and it made my watt meter move more. Refer to spec sheets and make sure the gate voltage is right. Mosfet finals aren't my cup of tea but I have experimented a bit. The radios end up putting out a dirtier signal, cause more RFI and make amplifiers do wierd things. All that trash makes a watt meter swing though.

[kraftwerk1976]

Thanks for the info. I noticed there is a kit for the ERF-2030 which has the transistor, one capacitor, and some polarized part. What is that polarized part, exactly? I saw instructions for the 2030 kit, but don't know what that part is. Thanks.

[Deleted User 14541]

The polarized part is often referred to as a companion part. It consists of a diode and a couple of resistors. It provides bias current to the mosfet. You can't just drop the mosfet in because the stock bipolar final in the cobra 25 was ran in class C.

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[kraftwerk1976]

Thank you, 543. I actually did go to that website, but somehow overlooked the page that had those pictures of the "insides" of the "companion" part. Thanks, a lot. I will show this page to the local tech. here, after I gather all needed resistors and diode, and make a companion part. Should be a fun little assembly.

[TheCBDoctor]

Thank you, 543. I actually did go to that website, but somehow overlooked the page that had those pictures of the "insides" of the "companion" part. Thanks, a lot. I will show this page to the local tech. here, after I gather all needed resistors and diode, and make a companion part. Should be a fun little assembly.

Good morning kraftwerk1976,

Before you put in any new parts may I suggest that you remove the Final and measure the current. If it is still high the problem is not the Final. One method I use to find the short in a circuit is to unsolder some components or jumpers leading to the different circuits. When the current drops you have located the circuit that has an issue.

Smell the radio; a bad component will give off an odor. Shorted transistors have a different odor than a shorted resistor. Capacitors generally don't give off an odor unless they are split open, in which case, the problem is obvious.

One final note on MOSFETs: They are being used because they are cheaper to use, not because they are better. If you can replace the Final with a Bi-polar transistor, such as, the 2SC2075 or 2SC2078 you should use that method. The 2SC-1969 today is not the same as the original Toshiba 2SC-1969. The new ones can't handle the current on SSB (Not an issue with the Cobra 25 LTD). If you do use a Toshiba 2SC1969 you will notice a drop in the peak to peak power; they do not have the gain of the 2SC-2078.

Respectfully as always,

Rick

The CB Doctor (Retired)

[kraftwerk1976]

Thanks, Rick. Good info. If I can beg, borrow, or find a 2075 or 2078, I might just end up using that. I am tempted to just troubleshoot the radio myself and look at various components for burn marks, burnt traces, whatever.

[Deleted User 14541]

Try a cb shop for the 2078. They push the mosfet conversion because they make a killer profit on it. A lot of good finals have been taken out of radios. There is probably bin full of 2078 finals at most shops.

[MDYoungblood]

I agree with both CBDoctor and 543, first make sure that is the problem and then do the easy replacement. Watch out for the ones on that auction site, a lot of fakes out there. The last resort should be modding and the ERF2030 mod does the best.

3's

Greg