[Hammarlund] HQ-170 mods (long)

[Craig Roberts]

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1)      the stock OB2 voltage regulator tube had been replaced by an OA2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