[Hammarlund] HQ-170 mods (long)

[Roger D Johnson]

Craig Roberts wrote:

> As those of you who have been following my riveting commentaries know, I
> recently acquired an HQ-170A with some odd modifications in the power
> supply. I�ve now restored the circuitry to its original configuration and
> thought I�d share the experience just in case others find a receiver in a
> similar state.  Although most of the modifications performed by a previous
> owner on my radio were apparently not authorized or documented by the
> factory, the circuit changes may have appeared in a magazine article or
> other communication lost to history and, thus, may have been done to other
> HQ-170s.
> 
> When I took delivery of my eBay queen, its power supply circuitry showed
> evidence of considerable and sloppy tampering.
> 
> 1)      the stock OB2 voltage regulator tube had been replaced by an OA2
> 
> 2)      the dropping resistor to the VR tube had been changed from 3K 10W
> (or 4K in very early sets) to 2K (thus yielding the 150 volts needed to
> operate the OA2).
> 
> 3)      one diode had been added in series with each of the original two
> rectifier diodes (CR1 and CR2)
> 
> 4)      an �extra� 82-ohm 10 W power resistor had been added between the
> junction of the diodes and the choke L6
> 
> Since the HQ-180 series receivers use an OA2 regulator, I thought perhaps my
> receiver had been �upgraded�.  Maybe so, but there�s no evidence the factory
> ever thought of it.
> 
> Although my modified receiver worked fine, I decided to return it to its
> original state for three  reasons:
> 
> 1)      The former owner�s workmanship was shameful, featuring big, blobby,
> cold (or at least chilly) solder joints, uncrimped component leads and
> soldering iron-scarred wire insulation.
> 
> 2)      Several of you advised me that leaving the unauthorized mods in
> place would earn me eternal damnation (but � sincerely � thanks for your
> advice and useful information).
> 
> 3)      An OB2 glows prettier than an OA2.
> 
> It turns out that the �extra� 82-ohm power resistor was, indeed, suggested
> by Hammarlund.  So, I left the change intact, though I did replace the
> resistor itself with a new one (the old one looked undersized and was
> scorched).
> 
> I removed all four diodes and put two new 1N4006s in their place.
> 
> I replaced the 2K 10 W dropping resistor with a new 3K unit, thus dropping
> the VR tube�s operating voltage back down to about 105V. This allowed me to
> retire the OA2 in favor of a new OB2.
> 
> Since �unmodifying� the receiver, I�ve noted one major change in its
> performance.  The noise floor has lowered remarkably. Perhaps one or more of
> the added diodes was noisy, or perhaps just plain bad soldering was
> generating electromagnetic junk.  At any rate, if there was any benefit to
> be had by the modifications, I can�t imagine what they would have been. I
> feel better now.
> 
> Thanks again to those who offered their friendly advice. It is much
> appreciated.
> 
> 73,
> 
> Craig
> W3CRR
> 
Hi Craig....

I'm wondering if the power transformer has been replaced. The old non-A
receivers has p/n P26305-1 while the A series has P26305-4. I suspect
the -4 has a lower voltage output to work with the solid state
rectifiers. My manuals show approx 250 VAC from each side of the
secondary at the rectifier tube plates to ground with 117 VAC input.
I wonder what yours measures??

73, Roger




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