In article ,
Roy Lewallen wrote:

I thought I'd blow my little converter. Anyone care to educate
me as to why it didn't blow? Would it have blown up if I had
hooked to the other side of the ladder line? Did the 2 five pole
low pass filters save it?

My money's on the filter.

Yeah, there would be quite a lot of attenuation... five poles, and the
40-meter signal is about four octaves above the filter's nominal cutoff.

I thought that the converter might possibly include a set of
anti-parallel diodes between the filter and the SA604 to provide some
additional strong-signal protection, but I don't see any. So, it must
be that the filter attenuation was sufficient to keep the SA602 input
levels below the danger threshold. I seem to recall that these little
Gilbert-cell active mixers are reasonably rugged.

In looking at the schematic, I do see one thing which concerns me.
The filter is a pi configuration, with a .01 uF cap to ground at
either end. The cap at the antenna end of the filter (C4) would tend
to shunt a lot of the 40-meter signal to ground, with L1 blocking much
of the rest.

Seems to me that there could be two bad effects from having this
circuit shunted across the antenna when the 40-meter transmitter is
keyed up. For one thing, it could present a low impedance at its
attachment point (depending on the length of the feedline from the
upconverter to the attachment) and might adversely affect the
transmitter. For another thing, if there's a significant amount of RF
power getting to the front of the filter, it'll be shunted to ground
through C4 (a little monolithic ceramic by the look of it) and losses
in C4 might be sufficiently high to cook it and let all of its magic
smoke out.

If C4 does pop, the converter would still work, but there'd be one
less pole of low-pass filtration.

It might not be a bad idea for the OP to check C4 to see if it shows
signs of overheating, and perhaps consider adding another inductor
between the antenna terminal and the C4/L1 junction. This would
convert the filter from a two-Pi to a three-L configuration, and
increase its antenna-side impedance at HF.

--
Dave Platt AE6EO
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