|JANUARY 1st 2013- article rewritten from the old Oct 2012 article. Adapting a BF981 dual Gate
Mosfet - to have 4 radial leads in order to replace a common dual gate mosfet (Q102). -- I was
sucessful the first try at it! The FT301 on receive now seems a bit more lively with S meter readings
a bit higher than before..and the receiver seems quiet as well. More testing and comparisions are to
be done. I am happy with this part swap. See picture story below. NOTE: This methodology..(a bit
like tying a fly!) could be used for other radio projects and rigs like the FT101 for example. JAN
1-2013..built another "radial lead" BF981 type dg mosfet to replace a 3N211 in an FT101E I am
fixing for someone. NOTE: I think this BF981 works as well and maybe better than the 3N211 I
replaced it with in the RF amplifier stage of the 101E! BF981s are only about $1.80 or so as
compared to the 3N211 which sells for around $8 to $11 or so.
I worked from a pictorial printed off from a data sheet for the BF981. I
often use www dot datasheetarchive dot com to download and print data
sheets and pictorials. The long lead is the drain. When I began to work at
doing the new radial wire leads..I drew a couple of sketches of the BF981
top view and the 3SK45 (original dg mosfet) and lined everything up so
that it would make for a direct plug in replacement (seamless!).
Above you see the RF board of the FT301. This board of my particular FT301 has two sockets for each
of the dual gate mosfets. I plan to replace both of these dual gate mosfets with my new modified BF981s.
First I replaced the RF amplifier stage Q102 then Im going to try replacing the mixer. I may move on and
try replacing one in the IF stage (perhaps later on).
I figured out..that I would work with scetches I made from the data
sheets showing the overhead view looking down at the parts in
question. When done this way..The metal can top view and the layout
of the top view of the BF981 lead configurations matched up. The
clamp used was a soldering aid clamp I bought from Radio Shack
some years ago.
The bare wire I used was from radio shack as well. I frequently grounded myself and the
soldering iron I was using. Most dual gate mosfets have protective diodes but I always
observe this precaution. In an industrial shop or lab..I would be working in a clean room
or at least..I would be wearing a ground strap and I would be working on an ESD safe
work mat as a working surface. -- At home here..for this hobby work..in my
basement..which is not all that dry and static prone..I work a little more casually.
The drain lead was soldered (above pic) and I cut off the tag ends of the wire. It is important to
note: the leads are necessary to connect the device to the circuit but as well..they draw heat
away from the tiny silicon chip that is encased in the plastic covering. So a little extra lead left
uncut is ok as long as there are no shorts to adjacent leads or adjacent circuitry in the radio.
A very simple loop is made in the wire (formed with the clamp tool in the
pictures) and I clamp the loop to the original lead as solidly as I can without
clamping things too hard. Solder completes the joint.
To the left is pictured the BF981 with the new
drain lead now soldered in place. The device
is resting on the pictoral I drew of the over
head view of a typical metal can style dg
mosfet (ie 3N211 or 3SK45 or RCA 40673).
Many (if not all) of the metal can type dual gate
mosfets that I have worked with so far..all
have the same pin layout. The small metal tab
that juts out a bit from the rim of the metal can
of a common dg mosfet sort of sits close to
the source and in between the source and the
In order for me to "plug"this new modified
BF981 dg mosfet into the same socket as the
old can mosfet ..I had to have a marker of
sorts. So..I put a red dot (as in the photo)
which will help me orient the new modified
part so that it plugs into the socket correctly.
--NOTE: being careful like this paid
off..because I plugged the new part in and it
worked right off the bat!
|Q102 dual gate mosfet, the RF amplifier stage. Shown above and above right (to the left of the black transistor with the Y on the top). The other
dual gate mosfet is a low level mixer. I'm going to try replacing that one as well with this modified BF981. I got this idea from reading some
comments on the internet. Some of the comments on various Yaesu forums (perhaps the Fox Tango Site as well. ) There seemed to be an
opinion amoung users that a BF981 would be a good subsitute for the older metal can 3SK40 types that the FT301 and FT101 and perhaps other
older rigs used. To my knowledge..no one tried attaching radial leads like I have. Perhaps many trying this would just solder the axial leads from
the BF981 to the printed circuit board foil side to get it to connect up.
The new BF981/radial leads. Radial leads are up and down vertical whereas the axial leads are horizontal like the X axis on a graph.
The new part is a bit of an ugly duckling when viewed up close like this..but it works good..and it is relatively inexpensive.
I just went shopping on ebay and saw the BF981s for as low as 10 pcs for $12.90 plus free shipping from China. In USA try Dans Small Parts and
ebay as well.
The Fox Tango web site is offering a replacement DG mosfet HIGHER Gain and LOWER NOISE for $13.99 plus shipping of about $4 or so.
IT MIGHT be a 3N211 but the part number is not given.
3N211s are quite a bit more expensive on ebay. You cant really buy them for less than $7 or $8 each and other sellers are trying to flog them for
IF YOU WANT TO TRY ONE OR TWO OF MY "MODIFIED BF981s"I can make a few up to sell and I would sell them for $4.50 each plus postage
Canada $1.51 and USA $2.80 (overseas- $5.50 or so).
I WOULD BUILD THEM TO LOOK GOOD as the one at the top of this page...and I would test them in the RF stage of either the FT101E or the
FT301. I have both these radios on my bench currently.
email me at this link here...EMAIL EARL
THE ONES I SELL WILL BE TESTED IN THE FT301 CIRCUIT BOARD ..SAME AS I did with this first prototype.
I will be trying perhaps a better method of attaching the leads and I may just cut the leads off some cheap 2N3904s to attach to the dg mosfet
OR..better..I may try and buy some lead material or precut leads. Leads from 2n3904s or other cheap transistors should be the same guage wire
as the 3N211 or rca 40673 metal can packs.
In my second Modified BF981 I used leads cut from 1n914s and they are a bit thick but worked in my particular application.
(mirror sites)..all free articles..no
advertising on my site!
Here I am working on PB1181 of the FT101E. The owner says the receive sensitivity is way down..so I am doing things such as cleaning connections on circuit boards and in
this step of the repair/analysis..I am trying out various different DG mosfets in the RF amplifier stage. NOTE: the owner had put a 3N211 in the transistor socket and I
thought Id try another different 3N211 ..which I did some maybe a wee bit of improvement..but then I thought...why not try one of my "modified"dual gate mosfets..a BF981
with radial leads soldered on! IT worked well before when I tried it a few months ago with my RF amp stage FT301..so why not try it now only..this time...I will make a neater
job of it and so I did.
I keep this sketch as it orients both the metal
can pack and the X pattern leads of the BF981.
The red dot is situated between the Drain and
Source Leads..much like the little tab on the
metal can of the 3N211 or 3N201 (or other
metal can fets). They all seem to pretty well
follow the same pin layout.
This time..I used 1N914 diodes and I formed
the leads to solder onto the leads of the
BF981. This time I did a simple solder bond and
I did a pull test on the leads and it seems to be
plenty strong when done correctly.
NOTE: Im working on my wooden desk here
and I ground myself to the equipment rack with
my hand (by touching) every once and awhile.
My shop is in the basement and there is
enough residual dampness down there that
static does not seem to be a problem at all.
Below..1 lead done. I cut the diode lead off and the diode is still useable actually with this much lead left. The thickness of
the resulting lead is a bit thicker than the standard dual gate mosfet lead..but not all that much more. I found I could slip
these leads into the socket of the transistor with no problems.
|Above: my final product. Plugged it in to the socket of the FT101E rf amplifier front end board and it worked well. Seemed to work even better than the 3N211 that was in
there before. -- My soldering was strong and maybe I could "touch up" the one lead where there is a bit of a solder BLOB ..but she works. My second modified
DGMosfet..and Im getting better.
Left: the razor blade I used to clean the leads
of the diodes and the bf981. I cleaned to bare
shiny metal so that the solder would grab well.
Right pictured: the FT101E I am working on.
I have the HARD COPY SERVICE MANUAL for
the FT101, FT101B FT101E and I find it is not a
bad radio to work on at all! Plug in circuit
boards ..just like the FT301. Mostly solid state
but there are 3 tubes in the transmit stage.
Plug in circuit boards. I dont have any
extender boards for the 101 (yet).
It looks like I will be doing an alignment on
the receiver so I will be studying the manual
quite thoroughly in the next few days in
In my first attempt at making these radial lead mods to the BF981..I made a loop
around the BF981 leads and then soldered and trimmed. -- This is really
un-necessary and a bit ugly..actually.
Better to just use a type of lap joint and let the solder hold it together. IF done
properly it should be strong enough. You can do a 'pull test' on the joint. That is
what is done in the bonding rooms when bonds are made to IC wafers in delicate
circuitry. ..ONE NOTE: THE LEADS ACTUALLY are part of the heatsink. They conduct
heat away from the part and if you use a bit more solder or wire and make the
surface area of the leads larger..it will add power dissipation capability to the part.
|NOTE: I have spare parts for the FT301 and
the FT101s.. Parts like the japanese
transistors and Jfets like the 2sk19GR.
Recently..the prices went down on the
2SK19GRs. They used to cost an arm and a
leg..something like $15 each. I sell them for $3
each and I can ship them in an envelope for
only a few dollars.
I ALSO SELL ..TESTED ..BF981/modded with
radial leads for $4.75 each from my shop.
NOTE: ALL THESE ITEMS I sell go to help
finance this web site and as well..help me to
buy shop supplies ect! -- There are NO ADs on
my site..only me..tooting my horn every once
A simple contact joint of the cleaned leads seems to stick and hold well enough..if
done correctly. I did a little "pull test" just like industry does to test their BONDS
when ICs and transistors are bonded up.
If this were my FT101E..I would be now considering replacing the
electrolytic capacitors. I just visited an ebay store from Ontario where the
fellow is offering a complete set of replacement capacitors for about
$28.00 plus shipping.
The FT101E is a radio from the late 1970s and is about 30 years old if not a
bit older. So ..it is about due for capacitor replacement.
I recently pulled a couple of small electrolytics from an FT221 that I was
fixing and I measured the ESR resistance and it measured about 3 ohms.
The FT221 is the same vintage as the 101E here pictured.
Link back to my start..home page ..
(mirror sites).. 73 Earl VE3AB
September 16th 2013 -- Actually..while browsing ebay for dg mosfets and in particular 3N211s.. I came
across a listing for them from HONG KONG and I ordered 10 of them to try them out. They are just over
$5.00 each..and I didnt want to order too many in case they did not perform properly. Alot is said about
fake components and I wanted to test these ones out first to see if they worked.
They did work fine in my spot test. I placed them in the RF front end circuitry of my FT301. In fact..I
have taken some pictures of my tests and a couple of little movie clips of the 10 meter beacon I was
listening to when testing the parts. CLICK HERE TO SEE THIS NEW LITTLE ARTICLE.
note: I SELL these 3N211s for $7 each with economy postage ($1.55 in Canada and $3 to USA)..
small orders of up to $30 worth of parts (typically). My email link is above---
BF981 - Dual gate FET (from an internet forum) - general information.
It was an answer about my question for replacement of 3SK FET's in the Dutch ham-forum.
p.s. It has more dB gain and sometimes it needs a little ferrite on the drain due to oscillation purposes. (I had one situation where this occurred..earl)
This is a general note -- doesnt hurt to put a bead or 2 on the drain. -- I will sell you a few -43 mat'l beads for 50 cents plus cost of a letter in the mail.