[Johnson] Inter-stage xformer (T3) for the Viking II

[Michael Sell]

Reporting on my recent experiences with changing out T3 while doing some
upgrades to a Viking II audio section.  I used the circuit modifications
presented in ER June 1998 by K6AD.  I had used this once previously with
great results.  However this time the transformer from the original article
was no longer available.  So, I tried three different ones from AES.

P-T156; used a voltage divider on the secondary as there is no center tap
and wired step down.  Frequency response was not what I was looking for.
-6db at 200hz, -3db @400hz, +/- 0db at 1000hz, +2db at 1800hz, and -4db at 5000hz

P-T124B by Hammond; again used a voltage divider on the secondary, step
down. While this transformer specs are much better, the actual frequency
response was very similar to the P-T156 with a bit of an improvement: -3db @
300hz and high end -3 db at 5000hz however with the same +2db bump in around
1800hz.  While a small improvement, not what I had previously obtained.

P-T124E by Hammond; here we go, this is what I was looking for.  Again wired
step down and using the exact diagram from the original
rticle.  -1db at 200hz, 0db at 1000, 0db at 2000 on up to -1db at 5000hz.  So this one
is the winner hands down!  No midrange bump and very respectable on both
high and low end.

Now the 124E is twice the money of the other two, around $28.  Worth it from
my point of view.  While on air audio reports using the 124B were very good,
the 124E has a much more open and natural sounding.

I don't see why the 124E would not work just fine to replace a failed T3 in
a non modified Viking II as well.  Your mileage may vary.  So the radio is
going back to it's original owner, who will be a new AM convert here on the
front range of the Rocky Mtns., Denver CO.

For reference, I use D104's on all my AM radios.

Michael Sell
k0com at comcast.net

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