[R-390] Alignment

[Bill Smith]

Hi, Jim

A couple of observations.  150uV is .000150 volts, which could be difficult
to observe on a VOM, at least on an analog meter type.   Typically a one
volt signal is fed through a switched attenuator to obtain a calibrated
signal.

Regarding the Heathkit, is it a SG-8?  I used one for years and used it to
tune up all sorts of equipment, from AC-DC radios to 450MHz two-way radio.
For a general purpose signal source, or a local AM transmitter (play music
through your radio) they are great.  However, they really aren't calibrated.
Here are a couple of things to consider.  First, the oscillator is a
modified multivibrator, with a pulse-shaped output.  Much of the output
energy is at harmonics of the fundamental, so even if you were able to
measure the voltage, the reading will probably be misleading because much of
the energy is at frequencies other than the fundamental.  To be meaningful,
the generator should produce a low-distortion sine-wave.  Remember that a
diode detector is a peak reading device, that is the detector rectifies the
waveshape, and a following capacitor stores the resultant voltage.  With
little or no load, the voltage will probably represent the peak voltage of
the waveshape (less 0.7 or 0.2 volts lost in the diode).  Actually there are
a number of variables in a RF diode detector, but that is another story.

Another issue is shielding.  Although you need a relatively strong signal
(150uV), the SG-8 and similar generators leak signal through the cabinet,
out through the line cord and through the mounting feet.  :-)   Just moving
around can affect the signal pickup in the receiver.

Also, although probably not an issue here, the output impedance of the SG-8
is a total unknown, which manifests itself in SWR (radiation) on the output
cable as well as generating an unknown match to whatever you connect the
cable to.  The attenuator (switch and pot) are for all purposes useless, as
they are not shielded, and are not an effective design to attenuate RF
signals.

In short, the generator is fine for applications where you can place the
unit next to the device you are working on, and tune with whatever signal
the equipment picks up, but it is a difficult device with which to make any
sort of measurements.  Any attempt to modify it or overcome the SG-8's
serious weaknesses make me reflect, today on April 1, of all the times I
have embarked on a modification project to make a pearl out of a rock.  I
ended up with a broken, bruised rock every time.  ;-)

73 de Bill, AB6MT
billsmith@ispwest.com


> >
> > I did the alignment, before my latest escapade, and it went well.
> > However, I estimated the 150uV needed to obtain the 7volts diode load.
> >
> > The signal generator is a Heathkit that has no meter, so the output is
not
> > calibrated.  I measure the frequency with an HP freq counter, so I have
> > accurate frequencies.  I know that a calibrated output is not needed for
> > any of the alignment, except to set the 7volts at 150uV.
> >
> > Can the 150uV output be accurately measured with a VOM??  I think the
> > output to be measured is RF voltage, so would a small RF detector, such
as what
>    is in the ARRL handbook be suitable and accurate enough for the
purpose.
> >
> > Jim.
> >