[Hammarlund] HQ-170 Repair Saga

[manualman at juno.com]

I haven't read the history of this Hammarlund HQ-170 repair yet but this
is one of the reasons why I also mention that Deoxit should never be
sprayed on to the paper phenolic switch wafers which were quite common on
many receivers, transmitters, etc. in the 50's and 60's. 

The Deoxit is absorbed into the wafer  material and the chemical agents
that bond the paper phenolic together react with the composition
chemicals of the Deoxit and can form minute resistive streaks in the
material and can often short one switch contact with another. If voltage
gets presented on a wafer terminal where there an internal resistive
streak has formed, maybe to another terminal, it's possible to  pass
smoke from the switch wafer.

Deoxit should be applied minutely to the actual terminal carefully and
not sprayed on the entire wafer.

Pete, wa2cwa

> On Fri, Mar 20, 2026, 13:43 Elmore, Kim via Hammarlund <
> hammarlund at mailman.qth.net> wrote:
> 
> > Well, following up on my foray into my HQ-170: I kept getting very
> > confusing reading when I checked for shorts to ground meaning I 
> simply
> > couldn't reliably isolate the issue. I'd check for shorts to 
> ground
> > through the cold side of R9 and find it open: no shorts. 
> Sometimes, I'd
> > see wildly fluctuating resistance readings, but they'd quickly 
> resolve
> > is I jiggled the test leads. This puzzled me, but I ultimately 
> became
> > convinced that something else was happening, such as fingers on 
> probes,
> > etc. It's a Fluke 79 so pretty responsive.
> >
> > Following an earlier suggestion, I decided to measure the current 
> though
> > R9 using a 1500 ohm 5W resistor simply because I had one. The idea 
> was
> > to simply get a glimpse of what I'm dealing with. I put the test 
> leads
> > in series with it and gingerly turned on power keeping my fingers 
> art
> > the ready to turn it off. I saw about 170 mA.
> >
> > 0.17 * 1500 = 255 V. That's essentially the B+ value. A short 
> *clearly*
> > exists somewhere.
> >
> > But when I did this I INSTANTLY saw arcing in the front wafer of 
> the
> > band switch. Power on to power off spanned 1-2 s. I removed tubes 
> in
> > case there's an intermittent interelectrode short. Same result. 
> With
> > power off, I moved the band switch and tried again. Same result.
> >
> > Two things are now irrefutable: the problem is in C134, a 2 pF 500 
> WVDC
> > mica coupling capacitor between T1 and T2 that I have yet to 
> locate. But
> > the other, more ominous issue is that the front wafer of the band 
> switch
> > was damaged when C134 shorted in the initial episode that 
> incinerated
> > the original R9. At that point it was destroyed for all practical
> > purposes. C134 is the only capacitor I have not been able to find 
> and
> > test that is in the path through the band switch. It has to be the
> > source of the problem. Now, if I could only find it C134...
> >
> > The damage to the band switch prevented me from making reliable
> > measurements.: the voltage used by the ohmmeter was insufficient 
> to
> > bridge the damage to the switch. But, at 250-300 V it's bridged 
> easily.
> > In one sense, I'm fortunate that it's the front wafer because it's 
> about
> > the only accessible one. The band switch is the heart of the radio 
> and
> > the entire thing is literally built around it. Yet, the only way I 
> can
> > repair that switch is to find a front wafer from a donor HQ-170. 
> Even
> > then, I'm not entire convinced I can get enough access to do the 
> job.
> >
> > This particular HQ-170 has considerable sentimental value to me 
> because
> > it was my dad's and so was my very first receiver as a Novice. It 
> is
> > cosmetically perfect and was aligned almost perfectly, a job that 
> took
> > me couple of days to perfect. I admit that I'm emotionally 
> attached to
> > it. That said, I am not left without a receiver: a good friend 
> gave me
> > his HRO-60 in perfect electrical condition with ALL the coil sets, 
> and I
> > also have an electrically perfect Drake 2A. Performance-wise, both 
> of
> > these are superior to the HQ-170 in many respects.
> >
> > So, I'm in no hurry. I'll look a bit more at my '170 to find C134. 
> If
> > anyone has a front wafer for an HQ-170 band switch, please contact 
> me.
> >
> > 73,
> >
> > Kim N5OP