ZC1 Radios are now at least 65+ years
old. The seemingly endless stream of ZC1 radios and parts at HAM
sales tables and junk fests, traditionally the most likely places to
find sets or parts at reasonable prices, is showing signs of drying
up. Websites such as Trademe sometimes offer sets or parts suitable
for restoration use, but because of the wider audience prices tend to
However, provided most of the major
parts of your set are still there you should be able to restore it to
working condition. The biggest dilemma you will likely face is
how original to keep it. Modern components such as resistors and
capacitors are typically much smaller than the original components and
often quite different in appearance with bright shiny plastic
Such components can look out of
place amongst the original components but may be the only option
unless you have access to a stock of original or same era parts. New
Old Stock (NOS) parts may look OK, but often because of their
age, can be as faulty as the part to be replaced. This is also true
of recycled parts so always test or check a part, if you can, before
buying such parts and always before installation unless you like to
The most important
piece of advice we can offer is that when you get your new ZC1 home
resist the temptation to power it up the minute you get it in the
door. The only exception to this rule is if you know the set and
know it has been regularly used up until the day you got it.
The following precautions are
strongly recommended before applying power to a set that is known to
have been in storage or any set that has an unknown history even if
you are told it worked OK when last used.
Any set not used for a while is
likely to have dried out or shorted electrolytic capacitors. If power is
applied to the set they can overheat and explode or if internally
shorted draw enough current to destroy other associated parts.
This is particularly true for the capacitors in the Power Supply of
the set. These capacitors if faulty often cause the vibrator
power supply transformer to go open circuit. Replacement transformers
are now hard to find especially for the Mk-1 set.
Your first step should be to
remove the set from its case and carefully inspect it for signs of
damage. Electrolytic capacitors should be checked for signs of
bulging or leaking. Small components such as resistors and
paper capacitors should be checked to ensure that their leads are still
attached and the cases not split or chipped especially at the points
the leads are connected to. There is widespread belief amongst
members that any capacitor bearing the name "Hunts" should be
replaced on sight. As opinioned by one member, "Mr Hunt
invented the short circuit and called it a capacitor!"
Wafer switches should be checked
for dirt or corrosion on the contacts and if necessary cleaned.
Do Not use fluids such as CRC to clean contacts! They
dry out leaving a sticky residue behind which attracts dust like
iron filings to a magnet. A strip of clean copy paper
carefully drawn through the paired contact faces on the wafer will
remove most oxide coatings and dust buildups. If you must, use
a reputable contact cleaner designed for use with sensitive
electronic equipment sparingly.
Operate all toggle switches.
These are normally reliable long lasting components and if their
action is firm and smooth and they "snap" from one position to
another they are most likely OK.
Visually check as best you can
that there is no damage to wiring looms or wiring insulation but
avoid disturbing them as you may cause problems.
Check that there are no obviously
missing parts or components. If you are not sure if something
is missing, compare your set with the chassis views in the relevant
manual, excellent diagrams showing the appearance and position of
most parts are provided.
Check that all valves are firmly
pressed into their sockets. If you remove a valve don't just
tug it out or you could end up holding the glass envelope with the
base still firmly in the socket. Remember the valves are most
likely 60 something years old and the glue holding the glass
envelope and bakelite base together may have failed or weakened with
time. Carefully "wiggle" the valve from it's socket avoiding
excessive force and strain.
Any faulty components found should
now be replaced.
Applying Power to the Set
Remove the vibrator unit from it's
Then with no filament voltage applied, initially
put a low HT voltage from an external power supply on the HT line.
Gradually increase the
voltage, a bit at a time while monitoring the leakage current, just in
case one of the power supply electrolytics has a short or breaks down
when voltage is applied.
If you get to full HT
voltage without problems leave it applied for a time. Be
careful, if a capacitor fails it will happen suddenly without warning.
Wear safety glasses. An exploding power capacitor is both
awesome and dangerous.
If you have made it this
far without problem turn the power off. Check that any charge on
the power capacitors has bleed away before touching them or associated
circuitry or you may get a shock you won't forget. Don't
deliberately short the capacitors out to discharge them, you may cause
damage to them.
Replace the vibrator in its
Turn the TX switch off
Check Fuse, replace if necessary.
Apply 12 volt DC to the
sets battery terminals the vibrator should start making a humming
sound. If it doesn't the contacts may be stuck. Turn the
set off and remove the vibrator. Holding the vibrator at the
base end try slapping it into the open palm of your other hand in
short sharp movements. This will often free up stuck vibrators.
Refit and turn on the set, hopefully you will now hear a buzz from the
vibrator. If you don't, you could try the process again but if
after a couple of try's it still doesn't work a replacement vibrator
is called for.
Check for hiss
in the earphones. Shows at least
the audio stages have life.
Check volume control works. If
Attach aerial or signal generator to
set and tune across full tuning range. If the hiss in headphones
changes or stations are heard as you tune across the band it's a good
sign and your restoration job may not be too hard.
Follow the performance and
adjustment guide set out in part 2 of the manual
Fix any faults found. Shorted
coupling and bypass capacitors are common, as are resistors gone high
in value. The Beehive trimmers used often become shorted from
internal corrosion. If components check out OK possibly a valve
or valves has low gain, try changing the relevant valves. Just
swapping around valves of the same type can sometimes work.
If you have problems follow the
procedures in the relevant manual. Typical voltages at various
points are given. Just remember that the original voltmeters
used when the set was new were mainly low impedance devices, typically
1,000 Ohms/Volt. Modern high impedance voltmeters, 100,000
Ohms/Volt or higher will tend to give slightly higher readings than
those given in the manual.
Once you have the receiver and audio
stages performing it's time to move on to the transmitter section.
Remove Signal Generator or aerial
Fit dummy load between Antenna and
Earth terminals. A small torch bulb can be used and power out
indicated by lighting of the bulb.
Unless you have access to a
"Screened Room" testing should be confined to Amateur Bands only and
then only if you hold an appropriate operators license.
Turn set on and allow receiver to
Turn transmitter section on and
allow valves to warm up. Check current draw to the set is not
Follow the procedures set out in the
manual for checking performance.
Checking the voltages and comparing
them with those given for those locations in the manual is a useful
place to start if problems are found.
Get the CW mode going first then
move on to AM and MCW.
When you have finished your small
light bulb should be glowing brightly indicating good power out.
Check with an oscilloscope that you
are producing a good clean signal.
Remember the AM modulation
percentage is low by design, don't expect more than about 30% unless
the set has been modified.
To Modify or Not.
Ideas and circumstances have changed
a lot in the last 60 years in respect of radio design and technology.
What was considered adequate and good practice 60 years ago for a
Wartime set can look quaint and technically inadequate to modern eyes.
There are many modifications that can
be made to improve the performance of ZC1's and it's up to you
to decide if to keep your set in it's original state or to modify it.
Many worthwhile improvements to performance can be made simply by the change of values of
a few components others require more extensive modifications.
A number of modifications that have
appeared in Break In magazine over the years are attached in a pdf
file at the top of this page.
These restoration tips are offered to help those
restoring, or thinking about restoring, a ZC1.
They are based on the experiences of members and
contributors and are offered in good faith.
However the ZC1 Club accepts no responsibility for
the accuracy of information provided or it's suitability for use in
your particular set.
The ZC1 Club strongly recommends that only those
with suitable training and experience of working with the potentially
lethal high voltages employed in these sets attempt such restoration