[AMRadio] ham AM rig mod. iron


Rob Atkinson ranchorobbo at gmail.com
Sat May 14 15:03:24 EDT 2016


Someone asked about the mod. iron in a B&W 5100 blowing and the
failure mode from going above 100% modulation.

It wasn't stated whether the modulation was positive or negative.  I
do not have much experience with the vintage ham AM rigs at all so I
was waiting for someone else to reply but since no one has I will
comment.  My impression is that many of these transmitters had iron
that was just enough to handle the audio from a microphone into the
rig but not much more because of size or cost.  One problem in the old
days was that hams operated without oscilloscopes or other modulation
indicators so there may have been a lot of carrier clipping which
eventually led to winding insulation breakdown since with the B+ to
the final cut off on the secondary, the transformer was unloaded.   In
1955 a new mod. transformer wasn't that big a deal.  It's a bigger
deal now, especially if you want a replacement to fit in the rig
right.

I would start out testing with a VOM.  I'd put the Ohm meter across
the primary and secondary and see if it is infinite resistance or none
at all.  There should be some nominal dc resistance.  I'd see if there
is infinite resistance between each winding and the core.  If all that
checks out okay, and the transformer is disconnected from everything
you could try putting 120 v. AC on the secondary via an
autotransformer (variac) and  put an AC v. meter on the primary,
increase the secondary v. and see what the meter on the primary does,
measuring between CT and each side.  You should be able to run the
line v. up to 30 or 40 v. and see the transformation on the VOM.   I
think I'd just do the initial resistance checks and if I don't see
shorts or opens I'd try it in the rig.

I put a 2500 ohm 10 w. wire wound resistor from the secondary to
ground in my 100 watt ham rig.  Its value is the same as the anode
impedance of the final PA and it is connected at the mod. transformer
secondary at a junction between the secondary winding and the PA
anode.   one side of the resistor is grounded; the other side connects
to a string of 1 KV PIV diodes, 3 in series in my case, with the
cathodes on the B+ side.  The idea is that when the B+ is modulated
down to zero, the diodes conduct and the mod transformer sees a
resistance load briefly instead of no load at all.   The diodes don't
matter as  long as the total PIV is roughly double the peak positive
modulated B+.

In the June ER, Phil Legate AC0OB has an article on modifications to
the EFJ Viking 2 and one of them involves a resistor diode string
across the mod. transformer secondary.

73

Rob
K5UJ


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