Tecsun PL-880 AM BC band died sudderly and found out why

[Kage]

Just a tip for anyone who owns this radio and suddenly AM stopped working.
The symptom is kind of odd, FM and shortwave all work fine but going to AM results
in loud static with no stations picking up unless you have a transmitter right next to the radio.

Opened it up and nothing obvious looked wrong so I start injecting a 800kHz signal from my generator
into various areas and found the first stage AM antenna amplifier seems to be weak or dead.
To confirm it I check on the scope at the Drain of Q26 while injecting a signal into its Gate.
The waveform looked fishy and didn't seem to be amplifying as much as it should.

So after much headache I unsoldered the smaller than rice sized SMD component which is a BF510 n-type FET.
Digging around in my part box I found a through hole 2SK161 which at least has the same pinout even if it's 8x
the size of the SMD part!
Matched pin for pin touching the board by hand and the BCB came back alive.

I ended up soldering it in place bent over for now. The sensitivity is not as good as it was with the original
transistor so I need to find a legit replacement but have no idea where to look that won't cost much.
Only other idea is to replace it with a part that has close to the same specs 
With any luck maybe I will find a better part than stock and actually increase original performance but that
would be a crazy hack that would deserve its own post.

As to how the transistor died is a complete mystery. The radio was fine one day, then dead the next. I did not have it
plugged into an outdoor antenna. The gate and source of the FET are in parallel with the loop stick inductor so
how on earth the ferrite antenna could have generated a big enough pulse to fry the FETs gate is beyond me!
If I didn't know better I'd say the front-end got cooked by sitting under a broadcast antenna, but no
it wasn't from me 

I will say one thing that was odd is that this happened while I was camping out by a river with no houses around or stations.
While out there right before the AM died I was hearing some really strange wooshing click sounds which my friend found
strange sounding. It was strong sounding interference. Wonder if something was out there that fried my radio!? 
Probably just a coincidence.

[PXR-5]

Good catch, you are braver than I to solder on these SMD PCBs

Glad you got it working

-Jeff

[redhat]

The world went SMD in the 80's.  It's really not that bad once you have decent tools.  All my new projects are smd.

+-RH

[M R I]

I have a Sony ICF-2010 that had low receive. I replaced two FET’s with cross reference replacements. They worked. The gain was not as good as the originals. I replaced them with originals from Sony. It went back to normal.

[East Troy Don]

Apparently this is not an anomaly with he Sony ICF-2010 - the exact same thing occurred with mine. I never endeavored to repair it as my  2010 was never more than a marginal  performing receiver out of the box.

[M R I]

Mine has various modifications. It needed diodes installed so the pre amp FET does not blow with a external Ant. This was not the FET I replaced for this problem. As far your marginal performance It had a production run beyond expectations that sold very well. Still a good performer for a portable.

[Charlie_Dont_Surf]

I can't help you figure out why it died but I will say that substitution tools are your friend.

The BF510 is a small-signal RF N-channel FET in a SOT89-3 package. It was made obsolete some time between 2010 and now. 

Last night I found that Digikey has a (new?) substitution tool. Do a search for BF510 at the "top-level" of Digikey. The search result indicates that it has an entry for the original and and there is a J-FET version.

https://imgur.com/a/ENVoFVD

and it indicates that the is obsolete but it also has a place to suggest substitutions at the bottom of the page. I assume that you are most interested in the one that is < $1.00. 

https://imgur.com/a/W4sR4na

You get to decide if that transistor or the the J-FET alternate is appropriate.

Some of the transistor manufacturers also have substitution tools to suggest drop-in replacements of similar parts that the manufacturer makes.

Infineon has one here but they do not make RF transistors (I think) so I'm only mentioning this for reference only: https://www.infineon.com/cms/en/product/search/cross-reference/

Toshiba does make RF transistors but no joy on on a BF510 substitute: https://toshiba.semicon-storage.com/us/semiconductor/product/mosfets.html

NXP, who made the BF510, has a cross-reference but they don't cross-reference to NXP products: https://www.nxp.com/products/cross-reference:CROSS-REFERENCE

[Kage]

Well it happened AGAIN!

Even after replacing the FET with another temporary part that was "good enough" I still managed to fry the input and you know what I narrowed it down to?...

Lack of back to back protection diodes on the built in antenna so that it could limit how much was going into the first stage transistor.
I tried to describe this in this video.

I re-edited my video and uploaded a newer version without so many mistakes...
https://www.youtube.com/watch?v=-2q8uIG3XK0

I'm to the point where I am testing ideas to replace the FET altogether with a BJT and so far it's working.
I do realize I can simply replace the FET with an equivalent "new part" and that will remain an option (if one can be found) but for now I just want the thing to hear stations on MW and this simple change that only increases the part count by 2 components (capacitor, resistor) not only completely replaces the FET but handles huge RF fields far better without distortion much like my DX398/ATS909 but still has the sensitivity to compare. I damn near feel like they should have used a BJT in the first place or at least spent a few more cents on protection diodes.

[Kage]

So I've been thinking about this a bit more...

Since M R I and East Troy Don mentioned the Sony ICF-2010 radio I decided to pull the schematic up on that and sure enough
it uses a circuit which is very close to the one used in the Tecsun for the front end MW/LW transistor which is a 2SK152
in that radio.

This has me thinking if I can find a 2SK152 I would bet it would perform similar to the BF510 and there is some
wiggle room in the PL-880 to solder it in on nearby joints if careful to preserve the original SMD PCB pads in case I find
an original replacement.
The figures of both FETs are close from what I can tell.

Interestingly the SK152 datasheet talks about it being useful in head amplifiers for video cameras or VTRs but no
mention of RF applications. I guess they are a bit like jelly bean parts as long as specifications are right.
On the other hand I've tried a handful of other FETs I thought would perform good as a drop in yielding various
results from bad to mediocre but nothing that compares to the original performance.

If all else fails I may just button up the radio for now with a high gain bipolar with a bias resistor and
base capacitor in place of the FET which seems to perform almost as good but it's an annoying hack/workaround.

On a side note it appears Linear Systems is now producing a replacement for the discontinued BF510 called the
LSBF510 so that may be useful information for those in the same boat with me down the road...
https://www.prnewswire.com/news-releases/linear-systems-releases-replacement-for-discontinued-nxpphilips-bf510-n-channel-jfet-switch-301498152.html


UPDATE:
I found a SMD 2SK932 in an old Pioneer car stereo "Super Tuner" AM front end that isn't pin for pin compatible but can be fitted in and now the radios MW/LW is alive again and working as good as it did new with the BF510! 
Or possibly even better?.. because of the higher forward transfer admittance Yfs rating which is a head scratcher.
Definitely soldering in protection diodes this time so hopefully this doesn't become an issue *again*.
Also I remade the video and replaced it in the link above because the old one had a lot of mistakes I made.

Whew!

[Kage]

Not pretty but it works...



Don't mind the triangle electrical tape, I explain that in the video. It's just an external connection I soldered in that has nothing to do with the failure.
SMD transistor looks hilarious being turned 45 degrees upside down, but got to do what you can when the pinout isn't the same as the OEM part

Protection diodes installed. All is buttoned up and ready to listen to pirates again 

[BoomboxDX]

I am amazed that even as late as the 2010s (when I think the PL-880 was introduced), back to back protection diodes were not included.

Glad to see you being able to fix your radio.