[Boatanchors] Trouble Shooting Tip: HQ-170 Bad IF

[David Stinson]

My estate-find Hammarlund HQ-170 rolled-over on me
last week.  No signals, S-meter pegged low.
Finally got a chance to look at it.  Since the S-meter pegged
is an indication of problems around the AVC line, I measure
the voltage at the AVC rectifier and YEOW!  250V B+
on the AVC line.   I started tracing and isolating branches
of the AVC line until I isolated it to the 1st 455 KC IF stage,
V-4.   The B+ on the primary winding was leaking to 
the secondary, putting the B+ on the AVC. 
Here's a diagram.  B+ line in Red, AVC in Green.

http://home.netcom.com/~arc5/HQ170IF.jpg

I do have a replacement transformer, but I'll have to 
go mining in "The Cave" to find it.  In the mean time,
I wanted to know the rest of the rig was OK.
I learned a trick from one of my Elmers and from
My Hero, Scotty of Star Trek:  Bypassing.
I disconnected the transformer primary and provided
B+ to the Converter via a 2200 Ohm plate load 
resistor, then coupled the signal via 150 pFd capactor
to the top of the secondary.  Bypass in blue:

http://home.netcom.com/~arc5/HQ170IFBP.jpg

The radio is now playing and sounds as good as ever.
There is of course some minor lose of selectivity and 
alignment will need touch-up, but from a practical 
standpoint I can't really tell any difference.
I will replace the IF can.... eventually. ;-)

GL OM ES 73 DE Dave AB5S