[Hammarlund] HQ-170A mods?
Kim Herron
[email protected]
Fri, 02 Jan 2004 13:47:30 -0500
Hi Craig,
>While re-tubing my "new" HQ-170 VHF I discovered that it has been equipped
>with an OA2 VR tube rather than the OB2 called for in my schematic. The
>OA2 tube seems to work okay. So, for that matter, does an OB2 (while
>glowing a very bright orange). Underneath, I've noticed what appears to
>be a modification to the power supply circuitry (perhaps to accomodate the
>OA2 ?). Two additional diodes -- besides the stock pair -- have been
>added. And then there's this gray "thing" down there. It's not the 3K 10W
>power resistor but it is near it and resembles a "cement" power resistor,
>too. It is small in diameter, somewhat long and cylindrical in shape. This
>thing is as mysterious to me as a 1920's grid leak (which looks like a
>fuse) was years ago.
>
>What is this device? A choke? (I don't have the radio with me here at
>work so I can't tell you exactly where this thing is in the circuit). The
>device is a bit scorched and cracked, which is why I'm worried about
>it. If it need replacement, I have no idea what to replace it with (!).
The best way to deal with this type of thing is to put it back
stock. There are people here who will disagree with me on this,
but I've found that the design that Hammarlund used works well, when all
the components are in spec. So usually the mods
are covering up something deeper in the circuit that is actually broken/out
of spec. Put the supply back to stock,
check your voltages and then fix the offending components. I've found
several partially shorted P/S chokes over the years
that will cause you fits but aren't readily apparent. Make SURE that you
are using the right voltage chart for an A model (and
schematic). The P/S values are different because the supply is using the
diodes instead of a 5U4GB. That does make a difference,
and the wrong service info will lead you down the "garden" path.
Thanks!!
Kim Herron
1-616-677-3706