April 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
"Ground is ground the world around," is an oft repeated saying when
talking about making electrical connections to Earth ground. In
a general sense that is true, especially when referring to electromagnetic
radio signals and antenna systems that are in some manner dependent
on the common connection. However, when you are working within the
confines of a localized electronic circuit such as on a printed
circuit board or inside a chassis, there is no guarantee that without
proper precautions ground is not at the same potential everywhere.
Poor (high impedance) soldered, crimped, and bolted connections
are among the prime offenders that cause voltage differentials to
arise between points intended to be equipotential. RF frequency
signals are particularly sensitive to even a minor divergence from
the designed impedance because standing waves and reflected signals
can and will generate signal distortions. This brief article runs
through some of the more common causes of grounding problems. Anyone
working with older electronic equipment might find it especially
Uncommon Ground Difficulties
isn't always easy. Even an inch of wire can cause trouble
By A. R. Clawson
Technical workers, experimenters, hobbyists and even some engineers
consider an electronic chassis a common ground. This common-ground
idea often proves false. Case histories, including causes and solutions,
of a few uncommon ground troubles are presented here.
Fig. 1 - Hum modulation, due to an inch of wire.
Fig. 2 - Equivalent circuit showing filter capacitor
return to lug and then along common impedance of converter
Fig. 3 - Common and nearby grounding of horizontal oscillator
bypass and IF bypass capacitors results in jitter.
Fig. 4 - Mutual interaction of ground currents from nearby
Fig. 5 - Capacitive coupling between a hot chassis and
a hot conductor; a - actual circuit; b - equivalent circuit.
Fig. 7 - Poorly soldered lug introduces a high common
impedance between the common joint of two capacitors returning
Fig. 8 - The joint in Figure 7 after resoldering.
Even the smallest part of a metal chassis presents some opposition
to the flow of current. This is its impedance and for DC is equal
to the ohmic resistance. There is no need to get involved in the
calculation of impedance, but we must look into some of its effects.
To AC the impedance is a combination of AC resistance and net
reactance - primarily inductive reactance. AC resistance is not
the same as DC resistance, but always greater. The skin and other
effects add to the DC resistance. This may increase the resistance
factor by as much as 12 times at television frequencies.
The other component of impedance, the reactance, may reach rather
high values with increasing frequency for inductive circuits. A
1-inch piece of ordinary hookup wire can have 4 ohms of inductive
reactance at 30 mc! Yet its ohmic (ohmmeter) resistance is too small
to measure with an ordinary meter.
An example of this type of trouble turned up in an ordinary broadcast
receiver. The set had a bad case of hum modulation, indicated by
no hum when the set was tuned off station.
Troubleshooting was rapid since the hum increased as the higher-frequency
stations were tuned. The variation ruled out the if stages, leaving
only the converter. The trouble was found to be an incorrectly grounded
lead from an electrolytic capacitor.
Fig. 1 shows a wire running from a ground lance (punched-up lug)
to a socket lug of the converter stage. Note the cathode resistor
of the tube and the ground return of the electrolytic capacitor
returning to the socket lug. Both cathode current and capacitor
current flows through the wire from socket lug to lance.
Capacitor current has two components of interest: reactance flow
equals the ripple voltage divided by the capacitor's reactance;
AC leakage current, equal to ripple voltage divided by the electrolytics
leakage resistance. There is also a DC component of no concern here.
The wire offers some impedance at 120 cycles - we need not worry
about calculating it. But we do have to note that there is a voltage
E = I x Z (or E = IZ)
where I is the combined alternating current and Z the small but
definitely present impedance at 120 cycles.
Fig. 2 shows the circuit. The wire and, to a lesser extent, the
lance form the common impedance, with some small contribution from
the socket lug, making Zcommon. Its equivalent circuit
is shown as an insert. The leakage resistance and capacitive reactance
are shown as resistances. The voltage developed, Ecommon
is applied to the cathode of the tube in series with a cathode resistor.
The ripple voltage modulated the cathode of the converter in this
The value of Zcommon increased with increasing frequency,
permitting more of the developed hum voltage to be effective. This
follows from the formula:
XL = 2πfL
Inductance L remained constant while frequency f increased 3
times from 500 to 1,500 kc, with a similar increases in X1 - the
reactance-and the impedance of Zcommon at these frequencies.
Moving the ground-return wire of the electrolytic to the lance,
where it had been prior to electrolytic replacement in another shop,
cured the hum. Just an inch of common ground wire caused all the
False Sync Pulses
Feedback of sweep or sync into the video or pix if or front end
can result in an unwanted and false sync pulse. Sweep oscillators,
even though afc-protected, may try to lock on the added pulse with
resultant symptoms of jump, jitter, and sometimes even vertical
Typical of this class (see Fig. 3) is the case of horizontal
oscillator feedback, or injection. A tubular ceramic (C1), a horizontal
oscillator bypass carrying sweep currents to ground and disc capacitor
(C2) were grounded to the same ground lance (lug). The lug acted
as the common impedance. Feedback of horizontal oscillator pulses
into the picture if resulted in jitter whenever the horizontal frequency
drifted ever so slightly. The design error was corrected in later
Fig. 6 - a) In-line layout minimizes chassis current interaction;
b) bent layout, with possible regenerative or degenerative feedback.
Not all feedback was due to the common impedance of C1 and C2's
common ground lance. Some feedback was caused by the current of
C1 intermingling with the chassis current of C3, a disc if bypass
capacitor. The common impedance was the sheet-metal chassis proper!
The solution was to move C3's ground to the same point as C1.
Fig. 4 shows what happens. Tube V1 has a bypass C1 and tube V2
has as its bypass C2. The tubes may have different functions like
the sweep oscillator and if just mentioned. The bypass capacitors
normally return their current through the chassis to the cathode
of the tube. Heaviest current flow is between the ground point of
the capacitor and the cathode or its bypass capacitor. Not all the
current goes in a straight line however, but forms a sweeping motion
in accordance with the low but existing chassis impedance. The lines
in Fig. 4 enclose approximately equal areas of current flow and
equal chassis impedance.
Note that the lines of flow (flux) of capacitor C1 intermingle
with those of C2. The result is a voltage, similar to that developed
in the hum modulation case. This time it is in the chassis instead
of a wire. The voltage is small and can be disregarded in many instances.
Sometimes, this voltage may inject another voltage, a false sync
pulse, for example. Degenerative or regenerative feedback may also
Worthy of note is the control of chassis currents and common
impedances by chassis openings. If a row of slots or holes is punched
between the sockets of V1 and V2 (Fig. 4), the common impedance
is interrupted to a large extent. Where rerouting the capacitor
is not feasible, this might be a satisfactory solution.
Cases occur of capacitive reactance between a wire or other conductor
and a hot chassis. The chassis may be carrying a large current at
a high frequency and, if a conductor is too close, a very low feedback
path may exist due to the capacitance formed. The reactance will
be low at high frequencies according to the capacitive reactance
Fig. 5 is a sketch of such a wire close to a chassis. Feedback
of deflection yoke currents into tuner (front-end) shields has occurred
by this method. The remedy is redressing the yoke leads. Commercial
equipment does not lend itself to layout changes but such information
is helpful to those that make their own. Careful layout can avoid
As an example, Fig. 6-a shows a straight-line amplifier - the
tube sockets in line. Given the same components, the in-line amplifier
can yield greater gain than the bent amplifier of Fig. 6-b. The
reason is that the bending crowds the current flux (flow) into the
corner, and input and output currents of the tube at the bend mix
in the common chassis impedance. The result may be instability.
Tube sockets are mounted in holes in the chassis. A ground point
may be selected on the opposite side of the tube. For example, in
Fig. 4, the common impedance between C1 and C2 can be greatly lessened
by using new ground points at X. The socket holes interfere with
current flow toward the other tube for each capacitor. This is more
effective than the slot method.
Another way to keep ground currents in line is to increase the
conductivity of the chassis or ground in the desired direction,
thereby diverting current from undesired paths. A strip of braid
may do the job very nicely. In Fig. 1. paralleling the inch of wire
with braid would probably reduce the common impedance to where the
hum modulation would not be objectionable. However, moving the capacitor
lead was the easy way.
A sheet of metal, riveted, soldered, or bolted to the chassis,
can serve to divert currents by lowering the impedance.
Fig. 7 shows the circuit formed by a poorly soldered connection
between a ground lug and two capacitors making; a return at that
point. Fig. 8 shows the actual parts after resoldering.
The impedance Zcommon should have been nearly zero
ohms in this case, but actually was about 0.5 megohm. The large
electrolytic and an agc filter capacitor in Fig. 8 were grounded
at the lance. Due to the common impedance, the power supply ripple
fed into the age line practically without hindrance-the only opposition
was the reactances of the capacitors themselves.
Common practice uses the chassis as one return leg for the heaters
of all tubes. These can intermingle with signal currents to produce
unwanted effects. Heaters should be connected directly to their
own ground rather than to another socket lug because the common
impedance may become a trouble spot that will be difficult to localize.
Assume the wire in Fig. 1 is the common return of a heater and a
cathode - the effect might resemble cathode-to-heater leakage and
no amount of tube substitution will remedy such a situation.
Posted May 25, 2014