[Boatanchors] Hammarlund Super Pro 150 question

Tue Apr 13 15:54:28 EDT 2021

I appreciate all the comments on my SP-150 question. I've taken a good look
at the schematic of the SP110 (which is the SP150 model receiver in the
wooden console cabinet), and the SP200 and SP400.

The SP110 has a 5 resistor series string off the -50 volts from the power
supply. The SP200 and 400 have only 3 resistors in series.

In the SP110, the sensitivity pot is wired across two taps on the resistor
string, so actually pot resistance isn't that terribly important. If you go
too low, you'll mess up the voltage divider taps on the resistor string. If
you go too high, it could mess up the receiver AVC action as the
sensitivity pot is active while in AVC mode.

The resistor taps where the sensitivity pot connect represent about 5K of
resistance end to end in the divider network, so I'm going to order a 50K
pot and install it. That's high enough not to mess with the divider voltage
values, and low enough to not impact AVC behavior.

Thanks for all the sage advice and comments. It should be a good receiver
when I'm done, especially with that 15 inch speaker and push pull 6F6
output stage!

Wally GIbbons, AF7FH

On Tue, Apr 13, 2021 at 1:00 PM Richard Knoppow <1oldlens1 at ix.netcom.com>
wrote:

>     I had a look at the later versions. No values given in any of
> them for any resistor. In the military versions the value of the
> sensitivity pot is 50K, the voltage across it is approximately
> -22 volts. I was hoping I could find a tube pin where a
> comparative value could be found but connections are too
> indirect. I wonder if there is a Rider or other after market
> instruction sheet with values on it. Since the circuit is
> virtually identical with the later version my guess is that the
> value is the same. This brings to mind an interesting question;
> are RF gain controls usually linear or log? I never thought to
> check. The only one I have had to replace recently was on my RCA
> AR-88 and I was able to get an original part. I suspect they are
> log or audio taper pots. I have a BC-779 and am quite familiar
> with its insides but that is no help with this.
>     FWIW I consider the Super Pro to be a very fine receiver, its
> main fault is drift, they really must be allowed to run
> continuously to be stable. Also, I found on mine there is a
> considerable amount of pulling of the LO from the AVC. Installing
> a VR tube on the LO cures that. Hammarlund did this with the
> HQ-120-X, I wonder why they never did it for the Super-Pro.
>
> On 4/13/2021 10:49 AM, Al Klase wrote:
> > Gang,
> >
> > In light of the ensuing discussion, I took a look at my
> > SP-110X. Unfortunately, it's not easy to get to the sensitivity
> > pot.  So I made some measurements from the 3rd IF grid cap to
> > ground with the switch in the "manual" position.
> >
> > Sensitivity setting:
> >
> > 0    106K
> > 1    174K
> > 2    176K
> > 3    176.5k
> > 4    138K
> > 5    124k
> > 6    117K
> > 7    114K
> > 8    113k
> > 9    111k
> > 10   110k
> >
> > So my SWAG (Silly Wild-Assed Guess) is that the sensitivity pot
> > is about 150K with an /_antilog taper._/
> >
> > Circuit hot shots, please check this out.
> >
> >
> > 50-100K linear would not be disaster.
> >
> > 73,
> > Al
> >
> > On 4/12/2021 11:36 PM, Al Klase wrote:
> >> Wally,
> >>
> >> FWIW, the sensitivity control in the SP-200 is 50K per the
> >> manual.
> >>
> >> Al
> >>
> >> On 4/12/2021 7:06 PM, Wally Gibbons wrote:
> >>>   Greetings,
> >>> I'm overhauling a SP150 I got a few years ago. The
> >>> sensitivity control is
> >>> bad. The resistor element is missing pieces. Therefore I
> >>> can't measure the
> >>> end to end resistance of the original for a correct value.
> >>>
> >>> The schematic of the SP150 is apparently identical to the
> >>> SP110 minus the
> >>> crystal filter. But the sensitivity control resistance is not
> >>> called out in
> >>> the service data.
> >>>
> >>> Does anyone know the value for the sensitivity control?
> >>>
> >>> Thanks..
> >>>
> >>> Wally Gibbons, AF7FH
> >>> North Logan, Utah
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>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
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