[Hammarlund] HQ-170 Repair Saga

[Elmore, Kim]

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”/