[Hammarlund] HQ-170 Repair Saga

[Elmore, Kim]

Thanks, Pete! The arcing isn't to the phenolic at all: it's from the 
small "finger" wiper contact onto the larger contact surface. I've been 
very careful with any DeOxIt applications. Whatever it is, it's a 
symptom of the problem, not the cause of it. There is no carbon track at 
all. When I measure it, I see pretty much a dead

I need the expertise of someone with some deep HQ-170 repair experience 
to tell me where, physically, C128 is located. It's the coupling 
capacitor between T1 and T2 and, if shorted, would short B+ to ground 
through R9. I'm beginning to wonder if it's *inside* either T1 or T2 
because under the chassis I see only a straight wire connecting the two. 
It's not shown that way on the any of the four versions of the schematic 
available and I can't imagine why Hammarlund might have done that 
*unless* T1 or T2 were specified that way.

73,

Kim N5OP


On 3/20/2026 3:20 PM, manualman--- via Hammarlund wrote:
> 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
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-- 
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”/