re: Karl's 164 Radio Hum, and General Radio Topics

[David R Johnson <QA2282@domain.elided>]

In AD-V7 #1479Nizam Zambri wrote:

> Humming radio/car stereos in cars can usually be cured by a large inline
capacitor to smooth out the
> current.  When I had the problem with my Milano/75, I went to Radio Shack and
bought the largest
> noise-cancelling capacitor they had; aptly found in the car stereo section. It
cost me twenty bucks.
> Once in line, the noise was history.

Actually what you are referring to is a choke, a word used to describe a coil
when located so as to block AC signals.  A more sophisticated filter can be
built by using alternating series chokes and capacitors to ground, but since the
blocking devices I have seen do not have ground connections I assume they are
nothing more than a series coil, in which case, as Nizam suggests get the
largest one for the best effect.  Sorry to be a pedant, but as an EE I felt the
need to correct the terminology and it may help in obtaining the desired device
when heading to Radio Shack or whatever.

Regarding the use of a booster, it seems unlikely that such a device would be of
much use, as one poster experienced.  The reason an FM radio has trouble with
weak stations is much more to do with it's own signal to noise ratio and filter
quality than it's ability to amplify the signal.  Stick a booster in front and
mostly you will just bring up the noise floor with the signal, making the
receiver's job harder.  You may also overload the front end on stronger
stations.  The only time such a booster is useful is when you have a long
antenna cable with a lot of loss.  Then the booster is placed as close to the
antenna as possible, and _before_ the main cable.  TV boosters often have a
remote power supply setup so they can be mounted up in the attic with the
antenna.  Even then, the booster needs to have a low internal noise level, a
quality usually missing from cheap Radio Shack equipment, IMHO.

I would agree with Graham Hilder's comprehensive list of suggestions.  Also, if
the addition of the choke does not help, you could check that the spark plug
wires are of the proper suppresser type used by the factory and are in good
condition.  This is not possible to evaluate just by looking, nor with a
multimeter, so the best solution is to replace them if they have not been in
recent memory.  Another area to look into is the power connections to the
ignition unit.  I don't have a 164 but I would expect a 1991 model is equipped
with some kind of high voltage capacitive discharge unit, and these are more
difficult to suppress.  Unless it has an aftermarket unit the original
installation should be quite adequate, but is is worth checking the power
connections are still good.

I see from Karl's most recent post that the noise occurs on tape as well as
radio, which does seem to point to conducted rather than RF interference.  This
does not rule out ignition sources however since the ignition unit could easily
be putting interference back on to it's DC supply wires.  That is why it is
important to check that there is a good solid connection to the battery and
ground.  You could add a choke in the ignition supply also I suppose; it should
help but I have never seen it done for some reason.  As to whether the pitch of
the whine can be used to determine the source, I doubt it since a modern
electronic ignition may have a multiple spark feature, and contains an
oscillator to perform a DC-DC voltage boost.  Do not forget also that the
alternator rotates quite a bit faster than the engine speed depending on the
pulley sizes, and has a multi-element commutator, so calculating the frequency
multiple would be very involved.  It would be much easier to remove the
alternator belt and then run the engine to see if the noise is gone.  I would
consult the list before doing so in case this configuration could somehow damage
the electrical system, although I don't see how.

Other related but not necessarily helpful comments to the radio topic: FM
stations in the US are tuned in 200 KHz intervals, but appear to be offset from
integer MHz values by 0.1, so stations can be placed at 97.9 and 98.1 MHz for
example, but not 98.0.  In the UK I seem to recall the FM receivers would tune
in 100 KHz steps so all these examples would be allowable.  Whether this is just
a matter of convention or whether there is a real difference in the effective
bandwidth of the signal I could not say.

Hope some of this at least is useful.

Dave J. (BSEE and MSEE)
1982 GTV6

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