[Hammarlund] HQ-170 Repair Saga

[fernan]

Hi

  I'm at work, so I haven't read the messages completely yet. I just wanted
to mention something similar happened to me. A resistor blew and there was
arcing on the bandswitch wafer. The issue was that the wafer was soaked
with contact cleaner. After washing the bandswitch wafer and replacing the
resistor, everything returned to normal. This occurred on my main receiver,
the HQ145A.

Regards
Fernan

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
>
> --
> Kim Elmore, Ph
>
> Kim Elmore, Ph.D. (Cooperative Institute for Severe and High-Impact
> Weather Research and Operations, Adj. Assoc. Prof., OU School of
> Meteorology, CCM, PP SEL/MEL/Glider, UAS, N5OP, 2nd Class
> Radiotelegraph, GROL)
>
> /“I am no weather master, nor is aught that goes on two legs.” – Tom
> Bombadill, “Fellowship of the Ring”/
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