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Modifications    PDF (3.2MB)
(Mods. Published with permission of Breakin Magazine)
Restoration ZC1 Mk-1 ver. 3
Refurbishing Tuning Dial
Capacitor reforming
Republished with permission 
of author who retains copyright and VMARS
Field Repair of Vibrator Units

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 be higher.

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 finishes.

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 live dangerously.

Very Important

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 socket.

  • 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 socket.

  • 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 all OK

  • 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 from set.

  • 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 warm up.

  • Turn transmitter section on and allow valves to warm up.  Check current draw to the set is not excessive.

  • 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 work. 




top                                                                                                  April 2009