[HBR] HQ-160 Osc Issue

[Walt Hutchens]

Byron said:

> So, I am starting to think that the HQ-160 has something inheritent
> in its design to cause the raspy tuning osc note?

You're not the only one to deal with this:

http://www.antiqueradios.com/forums/viewtopic.php?f=5&t=243032

However, I think that discussion heads in the wrong direction since
most of the focus is on the second converter -- 3.3 Mcs down to 455kcs
or something like that.  By listening to the 1st oscillator in the
R390A and finding it raspy, you've proven that's the place to start
work.

Also, substituting newer 'hotter' tubes in old sets isn't a good idea.
The challenge at HF is to handle signals of tens or hundreds of
MILLIvolts without trashing the whole band: Any new tube type requires
design adjustment that will rarely be simple. The 6BA6-6BE6 combo is
perfectly adequate for a receiver of this type and vintage.

Several things come to mind:

First I would check voltages in the oscillator and mixer stages with a
DVM looking not just for a correct number but also for a twitchy value
as that suggests that you're near the problem. Then, in order:

1. Bad rotor contacts on the tuning cap.  This problem, however,
usually varies substantially as you tune and may even go away
momentarily at certain dial settings (that you can never find again!)
Fool around with a screwdriver, shorting the rotor shaft to the frame
to take evidence on this one.

Crashing noises as you tune that cannot be eliminated by cleaning the
contacts suggest skipping to #5, VHF parasitics.

2. Bad oscillator or mixer tube. MANY of the NOS tubes have itty-bitty
chunks of cathode material adrift in there. Tapping lightly with a
screwdriver on the 6BE6 and 6C4 might be interesting -- any
substantial change when listening to the note suggests trying a
replacement in that socket.

3. Bad capacitors.  Every capacitor in an oscillator or mixer circuit
can cause this symptom and it will be dramatically worse at higher
frequencies because the amount of phase shift caused by whatever is
going on inside the cap requires a much larger frequency jump on
higher bands.

This is the most likely problem; I mention it third because the
best way to troubleshoot it is to replace the caps, one-by-one.

Sometimes a DVM will show that a bad cap acts like a battery due to
electrolytic corrosion: Any cap that shows even mV after the set has
been off overnight is a good place to start with replacement.

Those black plastic-encased paper caps in the BC-348 and other wartime
sets are famous for that.  .2V on those things, no load.

Use a fiber rod to press lightly on each cap while listening to the osc.
signal.  Pressure on any cap will move the oscillator around, but
anything sudden or crunchy sounding is proof beyond reasonable doubt.

If none of the above nails a specific cap then replace all the caps in
the oscillator and mixer circuits -- bypasses too. They're all
carrying RF so they can all cause a frequency shift if unstable.

Don't worry about temperature coefficients at first: Find the problem
with any cap that will handle the voltage and is close to the right
value, then go looking for the exact right part.

4. Bad resistor.  Why sure ... the old carbon composition resistors
all die badly, and sometimes with a poor contact between the lead and
the core.   If changing caps doesn't fix it then go through all the
resistors using modern carbon film units.

Here again pressing gently on the various parts may find a bad actor.

5. VHF oscillation in the 6C4 circuit.  That tube will perk to
somewhere close to 200 Mcs in a good circuit so long leads (as in an
HF set) could cause trouble.   Touching various points in the circuit
with the lead of a wooden pencil (use HV care ...) will often identify
these sorts of bad actor circuits.

In a new design a VHF parasitic would be a pretty good bet but in a
factory set with those 22 ohm resistors in the grid and plate
circuits, not so much. But I would poke it with the pencil lead before
spending a lot of time on replacing parts.

Drift and mechanical instability in sets of this vintage on the high
bands (compared to an R-390A or modern set) is probably built in. But
a note such that SSB cannot be understood and CW is raspy -- no.
That's a findable problem.

Walt
KJ4KV

Walt Hutchens