Blew up PA FET by LISTENING?

[Dirkus]

I was sitting at my desk in the shop last night, idly spinning the knob across the bands to listen to whatever was out there, and stopped to listen to the ridiculous amount of howling contacts on the 20m FT8 frequency when my radio made a loud POP. It continued receiving, but I smelled smoke so I shut it down and opened it up.

Now there is a perfect pinhole through the middle of the PA FET that wasn’t there before, and some soot on the main board facing it. The main board looks unhurt, but the PA FET is toast.

IMG_3201.jpeg (131.43 KiB) Viewed 1425 times

Just to be clear, I didn’t touch the PTT, no CAT control hooked up that might have accidentally keyed the transmitter, and I was actively listening to transmissions when this happened. My fingers were no where near the PTT, I was holding the radio in my left hand and spinning the dial only.

My understanding is that the PA FET shouldn’t have been doing anything at all at this point, much less doing anything that would blow it up. The antenna I was using was a EFHW antenna set up for 10m, but again I was NOT transmitting at this point, just listening! When I had been transmitting earlier in the night, it was in the 10m band that matched my antenna, with a SWR of <1.2. However, that was probably an hour before this.

What on earth could have caused this?

Is this related to the other topic on this board about a very weak carrier constantly transmitting? I think it might be, as I was seeing the same issue on my unit. I suspect something faulty on the main board might be holding the PA FET on at some level all the time.

A new FET and main board are already on their way. Hopefully they get here before field day. However, I would really appreciate it if someone could enlighten me on the theory of operation of the transmission portion of the circuit, and why that constant weak carrier frequency transmission is always there. Thank you.

[DL2MAN]

Hi,

(tr)uSDX is a direct mixing receiver, meaning the si5351 generates the frequency, you´re listening to. This also means, if you´re next to the TRX with a second RX, you will permanently hear the oscillator on the Frequency, you´re tuned to as a "carrier".
But this has no effect on the PA, as this is a different path.

Your PA FET can have blown, if Vox was enabled. Any noise would trigger PTT in that case.
Was Vox enabled ? or was there anything connected to Mic/Key socket ?

73 Manuel; DL2MAN

[Dirkus]

Hi,

(tr)uSDX is a direct mixing receiver…

Thanks for the explanation I definitely need to do some more study on RF.

…Was Vox enabled ? or was there anything connected to Mic/Key socket ?…

No sir, the only things connected were 13VDC, the antenna, and audio out to an external speaker. I was listening to transmissions at the time. My experience was that if the unit is transmitting, the receive audio is muted, cut off by the transistor on the main board that’s between the RF pin and the detector. However, I was listening at the time this happened, and continued to listen for a moment before I noticed the smell of a Smoke-Emitting Transistor. I’m sure you can see why I’m confused here!

[w4mqc]

Same problem here. Does anyone know where I can get my LOB (little orange box) repaired? The RX works fine, just no TX after a snap, crackle and pop. Am 79 and vision not great so need send it off I guess?
Alan, W4MQC.

[Dirkus]

Same problem here. Does anyone know where I can get my LOB (little orange box) repaired?…

If you were closer to my QTH I’d offer to help. I may not be very good at understanding RF theory yet, but I’ve been playing with electronics in general long enough to have a pretty fair hand at soldering. And it sounds like I’m about to get some more practice at surface mount component replacement soon!

[Dirkus]

My parts have landed in New York, now it’s up to the USPS to deliver them.

I’m still confused about what I might have done wrong here. Anyone want to toss some ideas out there? There’s no nearby HF transmitters that I’m aware of that could have overloaded it, no thunderstorms that could have gotten it (and I was holding it at the time, so if it was lightning I wouldn’t be making this post!) no power surges, nothing else that I can think of.

I am unburdened by the weight of having a clue here. Someone please give me a hint! Thanks!

[VE3RRD]

Put the transceiver in "practice" mode if you are only listening, then it can't accidentally transmit. Menu 2.8 I think.

[Dirkus]

Put the transceiver in "practice" mode if you are only listening, then it can't accidentally transmit...

I have used that mode before, when trying to practice CW using the radio and the built in morse decoder. However, just to be clear, transmitting was not the issue here. The received audio was never blanked as it would be if an accidental transmission had occurred. The radio was receiving the entire time.

HOWEVER, I think I have found my culprit. My desktop 12V "cigarette lighter outlet" power adapter that I've used with loads of other items in the past has apparently started to experience some kind of voltage drift. Just out of curiosity, I checked the output voltage. It's currently pushing a no-load voltage of 15.44. The truSDX is about as close to a "no-load load" as you're going to get, so I'm willing to bet that it saw a similar input voltage at the time. Normally I'd say "eh, what's a couple volts between friends" but in this case I think the PA FET couldn't handle the additional voltage and blew out. From here forward I'll only be powering it with a portable lithium battery pack (a NOCO GB70) that's putting out a very clean 12.2v.

Now that that mystery is solved, I'm just waiting for my parts.

Same problem here...
...Alan, W4MQC.

Check your power supply, Alan. If it's anywhere over 13v, that might be the source of the issue.

[YS1RS]

Just happened to me!
I just finished assembling my new HI Bands (tr)uSDX.
All went well. Perfect I would say. No issues at all.
I am using a battery pack composed of 18650 batteries.
It was working perfectly but suddenly it turned off.
Slight burned out smell.
No sign of damage visible. Just the smell.
If I remove the RF Board, the main board turns on immediately.
The only component in the RF board that could behave like a cat is that transistor FDT86256.
Tested it as a FET and is not passing the tests performed. Definitively gave up the ghost!

Hope to find a replacement locally.... if not... The I'll have to order replacement from Mouser.
I don't believe I could find those in my little country.

Can I use the old 3 transistors that I have on my LO BAND?
Those transistors seems to be more resilient than the new 86256, cause I've been using the same battery pack with no problems at all.
Yes, I know, the new 86256 is more resistant to SWR but I only use resonant antennas anyway.

Let me know if I just remove the burned out FET and just install 3 of the old transistors. I believe those could be easier to find locally.... Not sure yet.

Will update in the morning with the results.

Thanks.

73
Rob
YS1RS

[YS1RS]

Update: No FDT86256 are locally available. Looks like this is common in EU, at least, more than in the American Continent. Here, you find plenty of BS170.

After reading many posts here and there, came to the conclusion that better order the real part replacement and a few extras too. Seems to be the weak link in this platform.

Learnt the hard way that you work the filters tuning using 5.0 VDC from USB and never use 12 VDC or more to do this task.

Three weeks to one month wait time for the replacement...

73
Rob
YS1RS

[YS1RS]

Parts received from Mouser.
(tr)uSDX FET Replaced with good quality one.
Filters adjusted using 5 VDC.

73
Rob.
YS1RS