[Milsurplus] Tuning the Bendix Skipper 135's Output Tank

[David Stinson]

Thank you everyone who replied for your kind and
useful insights.  Our community is the best.

Tweeked on the Bendix some last night.  I have one of those
"Koolertron" Chinese signal sources you find cheap on Amazon,
and have found it reliable and invaluable.
Here's one:

https://tinyurl.com/rmw86adh

These have dual, independent outputs at up to 20V.
When Square Wave output  (or CMOS-level; square
waves from 0 to positive voltage rail) is selected,
and when coupled with DC blocking caps, I've used it
to substitute for TX and RX crystals in several
applications with success.

With the power supply electrolytics installed *correctly,*
the HV supply is working properly.  Disconnected the
switching transistors to disable the HV supply for this
tuning test.

Calculated the rated plate load as suggested  by our members
  and connected a 5300 ohm resistor
from the PA plate to ground, then connected a 50-ohm load on
the Antenna connector.  Whether I input the signal at 3885 KC
in the antenna jack (PTT relay bypassed) and scope at the plate,
or signal at the plate and connected the scope to the Antenna,
or put the signal in at the PA grid and scoped at the antenna,
it all worked similarly with plenty of signal.  The best results
were with the signal injected at the PA grid.  Filaments lit but
no B+ of course.  Set the output loading cap half-meshed,
then moved the tap up-and-down the PA Tank coil until
I found the max signal level.

I think the signal-to-PA grid configuration worked because
the little generator puts-out a whopping signal- A full Watt.
I bet I could make some QRP contacts with it ;-).

Removed all the test set-up and powered the rig.
The actual correct tap was about 4-turns more
inductance, which I found in a few seconds.

As with a TCS or ARC-5, the rig isn't designed for 50-ohms
at 75 meters, though it comes closer than either of those.
Like the ARC-5, when using no external matching, the max
power out is with zero loading inductance and max coupling,
which will deliver only a fraction of the rated power out
and reduce harmonic suppression in those rigs.
For this preliminary tuning, I intentionally set the loading
coil to zero turns; will add 100 pFd series to the antenna
output and retune later.  Once tuned, the rig, which is rated
at 33 W output, was delivering 24W out according to the scope
(100V p-t-p at 3885 KC).  Modulation with the original
Bendix-branded Shure CB6C hand mike looked good, though
the level is a little "hot."  The audio is what some would
call "communications quality," meaning it's serviceable
but it ain't no Dolby Stereo.  Might sound a bit better
when producing full carrier output.
When I get another few minutes to work on it,
I'll let you know.
I actually have the "Big Brother" to this rig-
a Bendix Skipper 430 which is a near-identical
design and cosmetics, with more channels
and 130W output.
https://www.ebay.com/itm/133789807781
Some time this winter.

If you ever decide to work on one of these
Bendix rigs,- they're fun and easy to fix-
*all* of the electrolytics, including the little
6-Volt and 12-Volt jobs in the transistorized
receiver, are all bad.  Change every one.

73 Dave AB5S


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