FW: HW-101 step 4

[Steve Harrison]

At 08:06 PM 5/10/99 -0600, Clark, John wrote:
>Second call.
>
>No replies?
>
>Receiver Alignment heterodyne oscillator.
>Page 120
>
>Other than "because the book said so", why am I setting
>these heterodyne values on "TP" to a "DC" level of
>" -.5 to -2" ?
[snip]
>Can anyone explained what I'm seeing here?
>
>Do I add small ( 100pf )cap to the crystals to bring them up?
>
>What is the relationship between the -.5 to -2 vdc setting and the
>peeking of the coils to the crystal frequencies?
>
>Any reason why the -1.5 vdc setting did not result in a good frequency?

John, since nobody else has dared stuff their foot in their own mouth, I'll
see if mine will still fit in my mouth :o)

I've never looked at an HW101 myself, but have played with SB300s and a
couple of SB101s and the like; I recall similar tuning instructions for
several of those models. I presume that small negative voltage you're
measuring is a test point on the grid of the heterodyne oscillator (if it
were the cathode, I'd think it would be slightly positive). The voltage is
developed when the tube oscillates and draws grid current; you are
measuring the voltage drop across the grid bias resistor (if you divide the
voltage by the value of the grid bias resistor, you'll come up with the
actual grid current in amperes). The readings given are those that the
Heath engineers found resulted when the crystal and tube was oscillating
properly. If the crystal is not oscillating or possibly is oscillating on a
spurious resonance, or if you happened to tune the plate circuit to the
second harmonic, you may see a very different voltage. So the voltage given
is a guideline to assuring proper oscillation, nothing more. Measuring the
voltage that is mentioned in the manual in your own radio is no guarantee
that everything is hokey-dokey, nor is reading some other voltage any
guarantee that anything is necessarily wrong.

Thus, that voltage is a measure of the effectiveness of EVERYTHING in the
oscillator circuit, from the crystal activity through the leakage of the
mica caps across the grid to the cathode and ground to the value of the
grid bias resistor itself. Not to mention, of course, the tube and the
plate circuit. Perhaps, ESPECIALLY the tube, since as a tube becomes leaky
(or gassy), the grid tends to SOURCE current which would make the voltage
on that test point slightly positive.

So, you can expect that with all new components from the same stock that
Heath engineers first breadboarded that circuit, those voltages are what
you most likely would see.

Today, some 10 or 20 years later, there will, obviously, be some
differences, and those differences may not even appear the same for each
band, just as you are seeing. For example, some of the crystals will age
differently and so oscillate on slightly different  frequencies from the
original marking. They may have slightly different activity, meaning that
they may oscillate more or less strongly than when new, and thus they will
affect the tube's grid current to a large extent. And the tube, itself,
obviously is likely a bit different from when it was new.

You may be making much ado about nothing. What you should be looking for is
that the heterodyne oscillator works well enough that the rest of the radio
meets specification, particularly if you cannot seem to get the precise
measurement readings that are called for in the manual, which, again, are
guidelines for ALL-NEW components, including the crystals.

If the crystal frequencies are sufficiently far off that you are bothered,
then go ahead and try to "net" them in or ask around for replacements (I've
noticed a few HW/SB101s parted out hereabouts lately). But you may not be
able to "net" them in since as crystals age, they get harder to "pull"
reliably, and may not start reliably as you switch bands and the radio is
either hot or cold. Remember that back when the radio was new, we would
occasionally encounter the exact same problems, and for much the same
reasons: components would be slightly out of spec, or leaky, or whatever.
Those voltage levels were a guideline to helping us figure out whether or
not the heteordyne oscillator itself was at fault when the radio
malfunctioned, but do not point to the exact component, which is what we,
as technicians capable of assemblying and tuning the radio, were expected
to be able to find.

Hope this helps put your mind at ease! 73, Steve K0XP

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