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49-er D882 Output Transistor Smoked
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Evening all.? Some troubling news... but I want to bring it up to the group in the hopes that the same problem can be avoided for others building the radio. I've had my DDS controlled 49-er on the air now for the past week and have been making dozens of contacts with it. Fired it up this evening and called CQ a few times. Suddenly, a sharp "pop" came from the board and the D882 started smoking like crazy. Powered the radio down, and sure enough, the D882 is bulged out in a tiny area and looks quite charred. Now it's off to find a replacement... but first, a few questions. I noticed last night that the D882 was quite hot to the touch. I was thinking about putting a heat sink on it, but it was late and never got around to it. Would a heat sink be almost a requirement for that transistor - especially if the failure was caused by overheating? When I followed the instructions to adjust R1 on the DDS board to achieve a peak-to-peak voltage of 4 volts, I made the measurement using my VTVM and an RF probe. I connected the probe to J3, and the ground strap of the probe to the ground pin to the left of J3. When I first took a reading, I was getting around 2 volts... so I adjusted R1 until the VTVM was dead on 4 volts. Was adjusting for 4 volts using an RF probe incorrect? Should I have used a scope instead? I'm just wondering if I was actually overdriving the D882 because my measurement was way off. Would like to avoid this happening again for obvious reasons.... especially if we're going to be teaching a class with this project. :-) Aaron - N2HTL |
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Aaron, I think you definitely over drove the PA! Typically, an RF probe is a half-wave rectifier.? If you are trying to measure the peak-to-peak voltage, the best way is with an o'scope.? With the RF probe you would see the RMS value. Of course, you can convert the RMS value to peak-to-peak. So, by my reckoning, if you measured 4 V on your VTVM, your J3 output would have been roughly 11.3 V p-p; a little bit more than required!? So, I'd say you would have been overdriving the D882. 73, -dennis W6DQ Inyokern CA
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Aaron Heverin
Hi Dennis... and thanks for the reply. I actually discovered this sad fact last night. The weather in my neck of the woods has been too nice the past few days to do any experimenting, but I hooked up my scope and checked it. Wow! The reason I didn't use the scope in the first place is that the trace is a little too blurry to be much use and trying to measure voltages on it is not very accurate. I figured the RF probe was a safer bet. Lesson learned. I'm actually glad this happened because it gave me some much needed cautions to put into the manual I'm writing on assembling and testing the radio for our club's class.? I also think it's time to rebuild my scope. :-) I ordered about 24 new D882s just in case we run into this problem again especially with newer, more inexperienced hams in the class who are going to build the radio. Aaron - N2HTL On Sun, Apr 17, 2016 at 10:08 PM, w6dq@... [SoftwareControlledHamRadio] <SoftwareControlledHamRadio@...> wrote:
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Aaron
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