Home HISTORYRESTORATIONDESIGN REVIEWSERVICE NOTESSCHEMATICSPHOTOSENGINEERING CHANGESFOR SALEContact Me

Multi-Tech makes a great switch and the Chinese switch is also well made.  Problem with the Chinese switch is it’s made with the same design flaw that was in the original switch. 

  

Note the original OAK and Centralab switches made for the Hallicrafters SR-2000 and National NCL-2000 used coin silver for the rotors and clips unlike the silver plated copper Chinese and Multi-Tech switches. 

  

The vintage Oak, Centralab, and Chinese switches were designed with 9 amp clips and max 1500 volts stress.  The wafers used 2 clips in parallel for each band on the rear wafer as well as two in parallel on the front for the 80 and 40 metre high end helper caps.

  

The parallel clips will support 18 amps about the same as the Multi-Tech.  Problem is you will not likely find a 30 degree 12 position rotary switch with a gap between clips to support over 1500 volts unless you add corona shielding or configure the wafer and rotor plate to provide double or quad spacing.   

  

The Multi-Teck front wafer for the TL922 has a corona shield and different design for the clip/rotor configuration that adds more arc protection than the rear.  The rear wafer does not provide the same voltage protection as there is no need.  At 2Kw the antenna side will not likely see over 600 volts even with with an ugly SWR.   The rear wafer didn't need the corona shield, just the front.

  

The TL922 made the same error as the NCL-2000 and pre-production SR-2000.  There was only a single gap on the high voltage wafer.  The production model SR-2000 changed the configuration of the high side wafer to a 2 gap switch after determining the single gap switch failed when the line voltage was increased to 122 volts and the radio was very lightly loaded.  The NCL-2000 was never re-designed and history tells the story.  You will find the National designers did recognize the inadequacy of the single gap switch in the NCL-2000 and changed the design of the NCX-1000 to a 2 gap highside wafer before it went to market.  

  

It is easy to change the original switches from 1 to 2 gap switches.  Simply reverse the rotor plate and trim the tail so it disconnects at the same time the main does.  If your radio adds capacity for 80 and 40 Metres jumper 80 and 40 together and simply tune the main cap higher.

  

The SR-2000 took the configuration a step further using the same switch wafer.  They designed a 4 gap switch in 1971.  None have failed from arcing since that change.

Short answer to a long story; the original switches did not fail due to current.  The high voltage side simply arced because of an inadequate gap.  

  

If you do a post Mortem on all of the failed switches from the various manufacturers, it will become apparent that the vast majority of failures occurs with the wafer that implements the extra capacity on the high side of the final tank, not the switch for the antenna side.  

  

I’I've seen only two radios that used a two gap switch; The National NCX1000 and Hallicrafters SR-2000.  The SR-2000 evolved to a 4 gap switch in 1971 and has not failed since.

  

Note the added gaps are provided by configuration not corona shields or resizing the switch.

  

The SR-2000 design also added logic to measure the continuity of the antenna switch contact that fed the antenna.  That measurement was used to gate the drivers when in SSB or high power mode.  With this design the antenna relay timing is not a problem --- the drivers are disabled when the contact is not made.  

  

Conclusion:  The original switches will work fine.  They simply need a fairly simple modification.

  

Regards Jim  K9AXN 

  

ON POSITION

  

  

Created with the QTH.com SiteBuilder.