Sony 2010 JFET problems

 The Sony 2010 shortwave receiver is one of my favorite radios.  It is synthesized, has memories, no menus, covers shortwave, air band and FM, has symchronous detection as well as a BFO for SSB and it has timers.  I blogged a little about it here:

https://wb9kzy.blogspot.com/2022/05/accessorizing-sony-2010-receiver.html

One problem mentioned in that post was the dreaded deafness on AM and shortwave due to the front end JFET, Q303, being damaged by static.  Here is a zoom into the Sony 2010 schematic:

Q303 is at the lower center right

Sony protected the built-in whip with diodes but the external antenna does not have diodes.  It's easy to add diodes to the included antenna assembly:

a zoom into the diodes, back to back 1n4148 soldered from the hot lead to ground:

Q303 is identified on the schematic as a 2SK-152-1, a Sony part.  When my Sony 2010 was damaged a couple of decades ago I was still able to buy these from a reputable place online.  I *think* this parts vendor was MCM Electronics in Ohio (now closed).  I can't find any paperwork or parts (believe I must have ordered more than one transistor).
 

The only claimed and still available substitute I've found is the Interfet IFN152 available from Mouser for $13.58 !:

Since the 2SK-152-1 may now be unobtainium, what is a good substitute ?  From the datasheet we can see a few things, the pinout is DSG (gate on the end) so pin compatible parts would include the J310, MPF102 and others.  The IDSS current is 9.5 to 14.8 mA at 5 volts, so the part is expected to work at low voltages (the Sony 2010 runs off 3 D cells).  Another interesting thing on page 2 of the datasheet is that the parts are spec'd in 4 groups depending on the IDSS - so that tells me that IDSS is going to be important for JFET substitutes, the MPF102 might not be the best substitute with a wide spec'd range of 2 to 20 mA for IDSS.  Here is the datasheet for the 2SK-152:

Fortunately IDSS is easy to measure, just a breadboard, power supply or battery and a DVM are needed.  The gate is jumpered to the grounded source.  The power supply is connected to the drain and ground through the DVM set to measure current.  Since this is application specific, set the voltage to +5 volts per the 2SK-152-1 spec.

onsemi test setup for IDSS

Here are some test results sorted by IDSS for some JFETs that I have:

Voltage  5.0 volts from a 78L05 regulator, DVM is the Metex set to the 400 mA range
part     IDSS mA
J305     3.0
2N5457   4.3
MPF102   6.5
BF256B   7.7     (reversed pinout: GSD)
MMBFJ310 35.2    (SMT equivalent of J310)
J310     36.8

If biggest is best then the J310 is the winner although it does exceed the 2SK-152-1 upper limit for IDSS.  However there are many other specs for JFETs, one of them may well be more important than IDSS.  I did try several of the JFETs in a 10 meter preamp that I use and there wasn't any noticeable difference between them.  I put the old MPF102 back into the preamp.

Hopefully the added diodes will do the job of protecting the Sony 2010 and I'll never have to get in there to replace Q303.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm