RadioBanter - View Single Post - Help with HQ-160 slot filter and product detector

February 17th 04, 05:08 PM

In article ,
r (Mike Knudsen) writes:

I finally got around to working on an HQ-160 I picked up a few years ago. It
works well, except the CW/SSB is very weak, and the slot (notch) filter
doesn't work at all.

I expect to check voltages around the 6U8 "linear detector" which I assume is
a
product detector, though how it works is a mystery to me, being just a
pentode
used as a triode. I know the BFO half of the tube is working, since I can
hear
it change pitch with the pitch control on an AM signal.

I've worked on the PD circuit this AM, and it appears the primary of T8 (an IF
transofrmer used only to feed the PD) has a non-tunable primary. Since it
isn't open, I assume the internal shunt cap is shot.

For now I will just bypass the primary and feed the coupling cap (5 pF) from T7
to the secondary. That produces a CW/SSB audio level comparable to the AM
detector's. There will be nothing out the IF Output jack on the rear apron,
but I could move its pickoff to the secondary also.

What a shame the AVC doesn't work on SSB/CW. The circuitry is all there to
keep it on, but this being a transition-era rx, Hammarlund figured users would
want it off. The 170 and 180 got it right!

Haven't got to the slot filter yet. Dreading an open coil (like what else
could be wrong?) I've shot the Slot Depth pot full of DeOX-It, no help.

Again, thanks for any suggestions. 73, Mike K AA1UK

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