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

[Barry L. Ornitz]

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.