About a week ago I posted the following:
"My PM-3 has a very pronounced hum on receive. The hum appears to get
stronger when
peaking the receiver tuning control. It is independent of power
supply. I've tried it with my 961, an Astron VS-35, and a Radio Shack
supply; all the same. I would guess that it's time to start replacing
electrolytic caps, but before I shot-gun it, I was wondering if any one
else had been down this same road."
After receiving a host of interesting ideas from the nice folks on this
reflector, I decided to try the simple things first, and was 99%
successful. I have included snips from most responses below for anyone
interested.
Most of the respondents mentioned radiated RF from the LO as the
source. It gets into AC power lines thus into the AC power supply, and
causes the hum. It was generally mentioned that good shielding, good
grounds should help, as well as the usual array of RFI prevention i.e -
RF chokes, by-passing the rectifiers, etc. The cure in my case was
switching to a dedicated Ten Tec 210 power supply for the PM-3, as
opposed to running it from a shared supply. The 210 is reasonably well
shielded, and includes .001 by-pass caps on the rectifiers. I did find
it necessary to shut off all the other gear on the desk, as it induced
hum, especially the 961 power supply. Once the hum on the PM-3 receiver
had been substantially reduced, I an S-30 Signalizer into the set-up,
and was pleasantly surprised to find the audio filter further attenuated
the hum. The S-30 had been sitting around for some time, but has
finally paid off. I'm now happily running this vintage QRP rig on both
20 & 40 M. Hope to work some of you there. Next step - a 509 !!!
73 all, and many thanks for the suggestions
George Maier - K1GXT
Responses. --------------
-----------------------------------------------------------------------------------------------------------
If you use pure DC for power and still get the hum perhaps you are
picking it up
from
something else. What happens if you move it to a different location /
antenna /
???
-------------------------------------------------------------------------------------------------------------
Do you have any stray magnetic fields around the rig (another power
supply
or the base of a fluorescent desk lamp, for instance)?
-------------------------------------------------------------------------------------------------------------
George,
Of course, you know a direct conversion receiver like this is all the
more
prone to 60 Hz hum due to high audio chain gain, where most of the
receiver
gain is located. Try the radio on a battery, like a fresh lantern
battery, or
storage battery, gel cell, etc. If it still hums, then check the
electrolytics.
One of the fixes for some AC hum is to bypass each leg of the AC-DC
bridge
rectifier in the AC power supply with disc caps, say about 0.01 mf. Use
of
twisted pair wires to power the PM-3 can also help.
-----------------------------------------------------------------------------------------------
George,
The hum is probably the result of the local oscilator being radiated
in
to the house wiring, modulated by the AC ( rectifiers, light dimmers
etc.. ) then being picked up on the antenna. This is common with direct
conversion receivers. Shield the oscilator, put chokes in the power
leads etc...
-------------------------------------------------------------------------------------------------
George, since it is a DC receiver, try a battery instead. You are
getting
power supply hum in the rx.
-------------------------------------------------------------------------------------------------
The PM-3 and other direct conversion receivers all suffer from something
called common mode interference when used with an AC supply. The easiest
way
to eliminate this problem is to use lantern batteries with the PM-3. If
you
want to use an AC supply, I think there's a way you can use ferrite
torroids
on the power leads to block the hum. There's a technical explanation of
why
this hum occurs. Check back issues of the Handbook and old QSTs. I
recall
reading about it somewhere.
-------------------------------------------------------------------------------------------------
Many direct conversion receivers have a hum on receive when
using an AC supply. Its caused by some common mode noise that
gets into the detector and is amplified by the high gain audio stage.
Different antennas may help or hurt, but this is basically a
characteristic of the direct conversion design. Similar receivers
like the Heath HW-7 and HW-8 will do the same thing. To fix it,
try winding two wires through a toroid core and connecting them
in series with both power leads. The best fix is to run the rig
on a battery.
-------------------------------------------------------------------------------------------------
I faintly remember this as being a problem with fairly simple (which the
PM-3
is)has) direct conversion receivers. I think it is some sort of common
mode ground loop problem. The easiest way to fix it is to get a 12V
battery
of some sort. A cheap 4 or 6 AH gel-cell would last for a long time
between
charges. I will do a bit of research and see if I can give you a
reference
for other possible fixes.
-------------------------------------------------------------------------------------------------
George:
This sounds like 'common mode' hum caused by the direct conversion
receiver.
The give away is that it peaks when you peak the receiver tuning
control.
What is happening is that some of the local oscillator signal is getting
into the power supply where it is modulated by the a.c. in the rectifier
diodes, then it is being re-radiated by the wiring and picked up in the
receiver antenna circuit. Of course, the local oscillator signal in a
direct-conversion receiver is on the receive frequency, so the receiver
finds a signal modulated by hum following it no matter what frequency
you
tune it to.
One solution is to run the receiver off of batteries. For a.c.
operation,
I've had success with many direct conversion (DC) receivers by trying
various combinations of grounds between the cases and the power supply,
etc.
One even worked much better with the ground removed from the system!
It's a
matter of tinkering.
One specific thing that you can do is to try to reduce the amount of rf
from
the local oscillator getting into our out of the power supply. You can
get
clip-on ferrite 'filters' at places like radio shack. Try using several
in a
row along both the mains and dc leads, close to the power supply. I have
one
setup that yielded most of the hum to wrapping the dc lead through a
large
toroid a number of times.
DC receivers are great performers, but this is one of their most
significant
(and frustrating) weaknesses.
--------------------------------------------------------------------------------------------------
Many direct conversion receivers have a hum on receive when
using an AC supply. Its caused by some common mode noise that
gets into the detector and is amplified by the high gain audio stage.
Different antennas may help or hurt, but this is basically a
characteristic of the direct conversion design. Similar receivers
like the Heath HW-7 and HW-8 will do the same thing. To fix it,
try winding two wires through a toroid core and connecting them
in series with both power leads. The best fix is to run the rig
on a battery.
-------------------------------------------------------------------------------------------------
George,
Your PM3 has what the OTs called tuneable hum. The cause and solution
for
this problem was well known in the 30's and even the 50's (see Don Mix's
"Novice Special" in QST about 1955 or 56). It was rediscovered when the
direct conversion RXs became popular in the 70s.
RF from the oscillator (or the oscillating detector in the case of a
regenerative receiver) gets into the power supply and is modulated by
the
rectifiers. This modulated RF is then detected by the receiver.
Naturally
this effect is frequency dependent and tuning the detector will make it
better or worse, hence the term tuneable hum.
Solutions include electrostatic shielding between the primary and
secondary
of the power supply, RF bypassing at the rectifiers, and RF chokes on
the AC
lines and the DC lines of the power supply. Usually the RF bypassing is
all
that you need.
Bypass the input and output of each diode rectifier to ground with a
0.01
mfd cap. I have also see articles where a bypass capacitor was put
across
each rectifier. The object is to suppress the modulation of the RF.
About 10 years ago I acquired a Heath GR81 2-tube regen. The tuneable
hum
was awful. I could not understand how anyone ever used a regen as a
regular
receiver in the 30s and 40s. So I bought two 0.01 mfd/1000V disk caps
from
RS and put them from the cathode and anode of the 35W4 to chassis
ground.
The improvement was INCREDIBLE. So if any of you find an old blooper at
a
flea market, that should be your first mod after you get it home.
I have read of tuneable hum cases occurring even when the RX was powered
from a battery! The solution in that case is usually to bypass the
rectifiers in the other power supplies in the shack.
--
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