[Johnson] T/R relay

[Ed Tanton]

The only 'normal' reason for desensing with an electronic T/R Switch would
be operation on a band the TX is not tuned to. If that isn't what you're
doing, I would make certain your Fast/Slow AGC setting on the receiver isn't
the cause, then I would swap the 6BL7. After that, you start getting into
some component problem within the 250-39. One thing you can do about the
heat: paint the cabinet black. Most of whatever that finish is on my 250-39s
is crummy, one way or another (usually the front panel on mine.) So painting
the cabinet isn't going to hurt the value. This provides a nice opportunity
to 1st drill out the rivets and swap the connectors. If you care about
matching such things, DX-Engineering sells the Amphenol SO-239s with that
glass-reinforced PBT insulation material (as opposed to 'modern' TFE.) See:
http://www.dxengineering.com/parts/dxe-83-1r-kit 

About replacing the rectifier in any boatanchor: you can do it, but the B+
is going to go way up, so you need to make some measurements 1st. Measure
the HV, then the current. Replace whatever rectifier, then again measure the
B+. That will give you a value for an appropriate dropping resistor-both
ohms and power. Add the resistor, measure until you have the resistor value
right, and you are back to operation with the original voltage level.
Obviously, this is much more reasonable when there isn't a lot of current,
and when that current is steady-such as in a T/R Switch.

Other points about T/R Switches: 
	1) Fusing. I always add a subminiature 250VAC-rated TO-5
transistor-sized fuse (typically a 1 or 2A SB) internally. Fuses are
primarily disaster prevention devices, and for something like a T/R Switch 1
or 2A SB is fine. The EFJ 250-39 uses the EFJ fused-plug with a 2-wire cord.
By adding a small internal fuse, you can use a 3-wire cord which I am much
more comfortable with, and without violating my Rule 1-for-Boatanchors: I
don't make no new holes. 
	2) Rectifiers. I (pardon the heresy here on EFJ) really like the
broadband B&W 380B and the B&W 381-which adds bandswitching, and hence some
selectivity. The latter T/R SW is somewhat hard to find. The B&Ws use a
selenium rectifier and a 2 wire cord with no fusing (there MAY be an
internal fusible link (one individual wire from the stranded 2 wire cord
wire.) Replacing any selenium rectifier is always a good idea since the
fumes from a burned-out selenium rectifier are a definitely poisonous gas.
Same measurement techniques.

I have some of these same comments on my website which REALLY needs
attention now that I have retired:

http://www.n4xy.com/accessories-2.html 

Ed Tanton  N4XY  n4xy at comcast.net 
189 Pioneer Trail
Marietta, GA  30068-3466
U.S.A.

EFJ Reflector Manager

website: http://www.n4xy.com   
 
Member: ARRL(LM)  AMSAT(LM)
QCWA(LM) SEDXC NCDXF INDEXA(LM) 
OKQRP Elecraft K2 Beta Tester

All emails <IN> & <OUT> checked by 
Norton AntiVirus with AutoProtect 

-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net] On Behalf Of Charles Ring

I have used the 250-39 Johnson T/R switch since 1967. I've had 3 of them and
the power transformer on two of those have failed. Since returning to the
air a year ago I've been using one with my Globe King 500A and Drake R-4C.
It works, but unlike long ago, I can't get full QSK. The receiver is being
badly desensed. I don't remember if it did better then with the R-4C or if
that was only with the 2-C and the R-4B, both of which are waiting for
repairs. I am interested in changing to either a fast relay or a PIN diode
switch. I seldom do faster than 25WPM.

73 de W3NU