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Restoration of a ZC1-Mk1 type 3

 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Some 60 years after they were manufactured the chance of obtaining a ZC1 Mk-1 in reasonable condition  is now fairly rare.  Members of the ZC1 Club had been on the look out for one suitable for restoration for some time.  Hence when this set was spotted on the auction site "Trademe" the word circulated rapidly around the members.

 

 

ZC1 Mk-1 as first seen on Trademe

 
     
 

Of particular interest was the fact that it was a version 3 production model, a version the club, despite it's best efforts,  had been unable to get photographs of.  Although originally the most numerous version of the Mk-1, with 4,000 produced, judging by the difficulty of finding one to photograph, they now seem rarer than the earlier versions produced in much smaller numbers.

A careful study of the posted photographs showed that a number of modifications had been made, and a number of significant bits such as the battery switch and battery terminals were missing.  However, overall the set appeared to be in reasonably good physical condition and a good potential candidate for restoration. 

In due course the set was acquired at auction and whilst it's delivery was awaited, club members dug through their ZC1 spare parts collections for parts identified from the auction photographs as being missing.  In a surprisingly short time most of the missing  major parts were successfully unearthed. These parts included the battery terminal posts, power switch, watch holder, fine tuning knobs, keying relay, vibrator and the 6X5 GT rectifiers.  Bits not immediately found were the bakelite battery terminal escutcheon and the internal top cover shield..  In the end a replica escutcheon was made and a replacement top cover is still being sought.  (A replacement top cover was found May 2011)

Once the set was received and on the bench, it was removed from it's case and thoroughly inspected.   Fortunately the photographs on the website proved to have given a fair idea of the condition of the set and at this stage no great surprises were received.

 
     
 

 
 

Two unwanted holes

 
     
 

It was decided to get the front panel looking right first and everything not standard was removed.  Fortunately only five things had to be removed, the red terminal post, the potentiometer just below it fitted with a ZC1 Mk-2 style knob, the two  silver fine tune knobs and the terminal post in the bottom left hand corner of the set.  The hardest part to remove was the terminal post in the bottom left hand corner which defied all attempts to unscrew it and in the end it had to be cut off. 

The removal of these bits left only two unwanted holes in the front panel, those left by the red terminal post and  the potentiometer shaft as shown in the photo above.   Fortunately the larger hole is covered by the watch holder but the smaller hole left by the red terminal post will require filling.  Two methods suggest themselves for filling the this hole.  The simplest is to carefully countersink the hole then fit a countersunk screw of the same appearance as other screws used on the front panel, such as the one showing above and slightly to the left of the hole to be filled.  The end result looks unremarkable and blends in, even to those in the know, and is unlikely to be noticed by casual viewers looking at the set.   The second method is to fill the hole with "bog" and paint over with matching paint.  To do this properly takes time and a lot of skill, there is a real possibility that the end result will be less than perfect and draw the eye to it's imperfection..  It was decided to use the first method on this set.

 
     
   
 

Starting to look more original

 
     
 

Next the modifications to the circuitry were examined.  The receiver was examined first.  As can be seen from the photo below a miniature tube had been added.   As well as the extra valve extensive modifications to the receivers circuit had been made.  Fortunately most original parts, even if now disconnected, had been left in place..

The extra tube appeared to be for a type of "Q" multiplier.  On testing, the capacitors associated with the tube were all found to be faulty with low capacitance.  Other capacitors of the same type added elsewhere in the set were also found to be faulty.

The audio stage output had been converted to a fixed output arrangement.  The volume control had been crudely modified into an RF gain control connected to the RF and mixer stages using "washing-line" construction! 

The BFO had been modified to operate the same as the Mk-2 with a switched pot used to vary the BFO frequency.  The Mk1 has a BFO switch only with no provision to alter the BFO frequency.

All these modifications were removed and the circuitry converted back to original.

Next the valves were tested, all were found to be OK.

At this point a lose glass in the meter became annoying so the meter was removed from the set, disassembled, the glass cleaned then glued back into place.  The meter was then refitted.

 
     

Using an external power supply, power was then applied to the receiver.  The receiver worked but performance was poor. 

Once the set had properly warmed up and stabilised, an alignment of the IF was attempted.  The tuning slugs appeared to have been untouched since manufacture of the set.   It was then discovered that a slug in the 1st I.F. transformer was broken, and from appearances likely had been since new. 

When the transformer was removed and opened so that the slug could be changed it was found that the coil former was broken as well.  Fortunately a spare IF transformer of the same type was available for replacement.

 

Once the broken transformer, which was cracked off its' base internally, was replaced it was found that when peaked up the stage was on the point of oscillation.  The problem was traced to a missing 0.1uF cathode bypass on the 1st IF 6U7.  The RF bypass capacitor on the 6Q7 detector/audio preamp was also found to be  missing. Both capacitors were replaced.

The next problem was found to be a faulty coupling capacitor to the grid of the RF stage.  The RF stage plate coupling capacitor to the 6K8 mixer was also faulty.  Neither capacitor had any measurable capacitance.  They were there physically, but that's all!   Both were replaced.  The receiver was then successfully re-aligned.

 

With the receiver now working it was time to try the transmitter.  First a number of modifications required removal including an interesting method of changing from VFO to crystal control.  To make the change over the oscillator tube was removed and an adapter plug used to replace it.  A tube was then plugged into the adaptor socket.  A second grid clip attached to the crystal oscillator circuitry is then connected to it's grid-cap leaving the original grid clip hanging free. A wire exiting the adaptor connected to the crystal socket and also performed a secondary function of retaining the adaptor.

How successful this arrangement was is not known but being definitely not standard was removed. 

Another modification removed was a resistor and electrolytic capacitor setup to reduce the plate voltage to the final, presumably to achieve better modulation.  This wastefully restricted RF power output and is of dubious value as there is normally ample modulation to do the job. 

With all modifications removed  the transmitter section could now be  turned on and the tubes allowed to warm up.  It was found when the transmitter was keyed on that initially the plate current was 22 milliamperes but rapidly sagged to around 10 milliamperes.

The problem was traced to the link to ground on the final grid drive resistor, (normally removed when checking grid drive with a meter inserted in its' place) which had been left disconnected, leaving the earthy end of the grid resistor "floating" above ground.  With this link replaced the transmitter was then aligned.

The set was left running in a "soak test" to see if anything failed.  As could be expected in a set this old probably running for the first time in years something did.  Initially the transmitter was "netting" OK but after a while this function failed.  This was traced to the 5K resistor switched into the plate circuit of the VFO for netting purposes having gone high to around 1 M..

     
 

Next the power supply received attention.   A vibrator and a pair of 6X5GT rectifier tubes were fitted to the original sockets which fortunately were still in place.   With replacement battery terminals together with a new power switch and 15 amp fuse fitted, a quick check of the large power capacitors indicated that they were likely OK.  12 volts DC was then applied to the battery terminals.  Output voltage from the power supply was fine but significant "hash" coming from the supply indicated more work to be done in this area.  However the hash gradually disappeared before it could be investigated, presumably as long unused capacitors reformed.

 
     
 

Finally the flick tune indicator which had been installed back-to-front on the transmitter dial was corrected.  Everything was checked one last time and then the set went to air making its first QSO probably in decades.  The report from the distant operator confirmed the transmitter sounded good which has been further confirmed in subsequent contacts..

This completed an interesting restoration in which many "oddball" modifications were removed and a number of faulty components replaced.  This set is now expected to see regular use on air.  Further cosmetic repairs to the set's case and paintwork will be undertaken at some point in the future.

 
     
 

This report was compiled from the restoration notes of club member George MacLean (ZL2GM) who carried out  the restoration..

 
     
 

 
 

Set Restored and Operational

 
     

top                                                                                         January 2010