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Technical Topics => Equipment => Topic started by: Kage on August 18, 2021, 2243 UTC

Title: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: Kage on August 18, 2021, 2243 UTC
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  :o

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.
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: PXR-5 on October 06, 2021, 2128 UTC
Good catch, you are braver than I to solder on these SMD PCBs

Glad you got it working :D

-Jeff
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: redhat on October 18, 2021, 0553 UTC
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
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: M R I on October 25, 2021, 0546 UTC
I have a Sony ICF-2010 that had low receive. I replaced two FETís 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.
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: East Troy Don on October 25, 2021, 2030 UTC
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.
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: M R I on October 25, 2021, 2054 UTC
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.
Title: Re: Tecsun PL-880 AM BC band died sudderly and found out why
Post by: Charlie_Dont_Surf on October 25, 2021, 2104 UTC
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 (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 (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/ (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 (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 (https://www.nxp.com/products/cross-reference:CROSS-REFERENCE)