[Hammarlund] Re: SP-600 Question - Actually GR-650-A

[jsheetz]

I have an addditional question about alignment of the 650-A
bridge. Does anyone know at which point each of the eight
tracking screws on the inside of the large main dial pot
are supposed to be set? These are obviously what enables
GR to use a preprinted main dial. I can't find any 
instruction on this. I wonder if it is in any version
of the instruction manual? Any help appreciated. I'm losing
track of who has a good copy of the instruction manual
but maybe I should obtain one.
  Thanks
   John, K2AGI

"Barry L. Ornitz" wrote:
> 
> Hi Roy and friends,
> 
> Roy Morgan, K1LKY, wrote:
> 
> > HAH!  They don't go out of calibration. Period.  Checking
> > them is an exercise in nostalgia. It's good for you. (Can
> > you tell I have great faith in these things?)
> 
> There is not really much to calibrate in these instruments.
> General Radio used precision parts including hand-wound
> resistors which were trimmed in manufacturing.  Unless
> corrosion has significantly eaten away the cross section of
> the resistance wire, there is little to put one out of
> calibration.  While these precision resistors do have a
> maximum current rating, even moderate overloads are not likely
> to change the calibration (unless wire insulation is melted
> and the resistance wires short to each other).
> 
> One source of error in these bridges, however, is the
> frequency of the oscillator used in AC measurements.  My GR-
> 650 does not have the 650-P1 oscillator amplifier; it uses the
> original "microphone hummer" instead.  In my unit, this
> oscillates at 1020 Hz giving a small error.  The oscillator
> components in the 650-P1 can drift, however, leading it to go
> off frequency so this is worth checking.
> 
> I learned that high impedance headphones must be used with
> this bridge.  I found low impedance ones would work if you
> used a plate-to-voice coil transformer to drive the
> headphones.  I had designed a crystal controlled solid-state
> oscillator along with a differential amplifier and phase
> sensitive detector to add to my unit.  But I found I could
> null the bridge audibly far faster than by looking at a zero-
> center meter display!  So much for technology!  :-)
> 
> Air core inductors are not affected by current, but iron core
> inductors certainly are.  As you magnetize the core, the
> incremental permeability decreases.  This is the whole
> principle behind magnetic amplifiers - a small DC current in
> one winding can control the mutual inductance of other AC
> windings.
> 
> Fixed power supply chokes generally specify a minimum
> inductance at a maximum DC current rating.  They generally
> have fairly large air gaps in the core to minimize
> saturation.  "Swinging" chokes can be much smaller physically
> as they have a smaller air gap and do not need as much iron or
> as many turns in the windings.  Here the inductance is allowed
> to drop as the choke DC current increases.  This is often not
> a problem in many power supply applications as the inductance
> needed for smoothing decreases as the current load on the
> supply increases.
> 
> The GR-650 does allow you to use some DC current in the
> inductor windings while measuring the AC inductance.  It
> requires an external supply and you must follow the
> manufacturer's maximum current limits.  This is discussed in
> the manual.
> 
> And speaking of manuals...
> 
> > Yes, I have the manual (which includes the Oscillator-
> > Amplifier), and also a calibration instruction I think..
> > 'Glad to make you a copy.  Send address.
> > Any one else need a copy?
> 
> My manual is a poor photocopy and it is missing a few pages.
> I would appreciate a copy of yours.  I'll be glad to pay
> postage and copying costs.
> 
>         73,  Barry L. Ornitz     ornitz@tricon.net
> 
> By the way, instruments like the early General Radio bridges
> show the true craftsmanship of both the designer and the
> actual builder.  They were constructed individually, hand
> calibrated, and were really precision instruments for their
> day.  The "consumer grade" Boatanchor radios we usually
> restore pale in comparison to the workmanship that went into
> these precision instruments.
> 
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