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 Good morning,

  

“On the SX-115, the Selectivity switch positions don’t seem to be effective.” 

  

The symptoms that you describe are not well documented but appeared in the late 60’s when the chassis heaters began to fail.  The heater in the SX-115 was more for the under chassis components than the VFO because the VFO has a drift correction adjustment unlike most other VFO’s.

  

The failure of the under chassis heater caused the problem with the selectivity switch to become noticeable.  When the radio was cold it seemed to work OK but as the radio reached operating temperature the selectivity seemed to lose its edge.  Over time it’s become worse.  

  

Simple fix for the problem:

To fix the problem is simply replace the Z5U Ceramic disc capacitors on the selectivity switch with Polypropylene film capacitors and realign the radio.  If you would like to understand why, read on. 

  

If there is any question in your mind that the Z5U capacitors need replacing, do two things.

1.Measure the capacity of the capacitors.  The .01uf caps are easy to reach.  Measure them with the radio cold.  They will be very close to .0082uf not .01uf.  That is a result of aging at -5% per decade.  Don’t think they age?  Remove one and place it in an oven at 280 degrees for about two hours.  Then let it sit for 48 hours and measure the capacity --- it will be .01uf like new.  Now it begins to age again.  The heating does not hurt it.  The baking process was used by Mfrs to adjust them to tolerance. 

  

2.Measure the caps while the radio is cold .0082uf, then warm the radio to operating temperature.  Operating temperature on the SX-115 is approximately 20 degrees C above room temperature.  Now measure the capacity.  It will be approximately .0073uf --- a long way from .01uf.

  

The following link is a video that was shot while heating a .01uf capacitor on the selectivity switch from 22 degrees C to 55 degrees C.  We recorded the capacity cold 22C .0083uf then at the operating temperature 42 degree C point .0071uf.  Then heated it further to 55 degrees C.

  

Click this link for a video snip of a .01uf  capacitor in the SX-115 being heated from 22 degrees C to 42 degrees C.

  

The values of those capacitors is Critical, they control the frequency offset for the five selectivity settings.   

  

      

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The Hallicrafters engineers made a component selection error.  

  

I believe the SX-115 was designed before the SX-117 by a year or two and the SX-117 is part of the story.  The SX-115 was the flagship and the SX-117 was supposedly the poor man’s copy.  

  

One of their engineers apparently decided that the capacitors used in the SELECTIVITY FILTER circuits of the SX-115 could be ceramic class 3, Z5U capacitors.  

  

Every engineering or design document explicitly warned that their use in tuned circuits, timers, and analog applications were patently poor design practices.  Hal ignored or missed this warning and went ahead with them anyway.  How they missed is a mystery because the SX-115 is the only radio designed with the 50Kc I.F. system that used the ceramic capacitors.  The SX-100 Mark2 and SX-117 used expensive General instruments film capacitors.  The SX-88, SX-76, SX101, SX-96, and early SX-100 used film or paper capacitors that did not have the instability with voltage or temperature that the ceramic Z5U has.  

  

The Hal engineer chose .01uf and .0047uf 10% ceramic Z5U capacitors to fit out the BAND WIDTH switch.  

  

Note: The SX-115 is the only 50Kc I.F. radio ever built by Hal to use the ceramic Z5U caps to implement the Band width logic.  This was a mistake as you will see below.     

  

The ceramic Z5U is one of the most non-linear, volatile ceramic capacitors ever available.  The schematic calls for a 10% Z5U ceramic disc capacitor.  What does the 10% mean?  It says that at room temperature, the capacity will be within 10% of the stated value, that is, DISREGARDING APPLIED VOLTAGE which has a profound influence on capacity.  All bets are off when voltage is applied.  With AC the capacity begins to vary at less than 1 volt.  This would cause linearity problems although not obviously perceptible.  The vast change in capacity with temperature and aging however, will cause the center point in each of the band width positions to shift.  

The 500 cycle selectivity position is set by alignment and the 1Kc, 2Kc, 3Kc, and 5Kc center points are fixed by the capacitors used on the selectivity switch; If the capacity changes, so too does the frequency center position.     

  

Data sheet information for the Z5U.  The Z and 5 are the low and high charted limits of temperature which are +10 to +85 degrees C.  The U states that the capacity will vary from +22% to –56% over the temperature range.  That’s the charted range.  Anything beyond will vary radically more.

  

The Polypropylene film capacitors vary <=2% over a wider temp range and the capacity does not vary with voltage whereas the capacity of the Z5U will vary up to 60% over the voltage range, +22% to -56% over the temperature range, and an additional 20% due to aging.  Class 3 ceramic capacitors age loosing approximately 5% of their capacity each decade; the film caps do not age. 

  

HOW THE SWITCH LOGIC WORKS:

  

The .5Kc position:  The capacitors on the band width switch control the position of the filters center point and the resistors flatten the response.  Example:  The .5Kc position shorts around all of the capacitors.  This is the unaltered center of the band pass and the point that you aligned first.  It is 50.75Kc and is 500 cycles wide.  This covers 50.500Kc to 50.1000Kc at the 6db point with the carrier at 50.000 Kc.  The 6db audio band pass is from 500 cycles to 1000 cycles.  

  

The 1Kc position:  Introduces all of the capacitors paralleled on the switch in series with the fixed cap reducing the effective capacity moving the band pass center upward and removes the short around, increasing the center frequency to 51.000Kc but does not add resistors yet.  However the change from short to capacitors does widen the band pass to 1Kc.  This expands the audio band pass to 500 and 1500 cycles.    

  

The 2Kc Position:  Removes a .01uf capacitor moving the center point to 51.500Kc and adds a 180 ohm resistor widening the band pass to 2Kc, creating a band pass from 50.500Kc to 52.500Kc at 6db rendering an audio band pass of 500 to 2500 cycles.

  

The 3Kc position:  Removes the .0047uf capacitor moving the band center to 52.000Kc and adds a 220 ohm resistor rendering a band pass from 50.500Kc to 53.500Kc for an audio band pass of 500 to 3500 cycles.  

  

The 5Kc position:  removes the last .01uf cap re-inserts the .0047uf capacitor and adds a 390 ohm resistor.  This moves the band center to 53.000Kc and widens the band pass to 5Kc creating a band pass of 50.500 Kc to 55.500Kc for an audio band pass of 500 to 5500 cycles. 

  

The above paragraph says it all as to why the Z5U is a design error.  The variation in capacity with temperature, voltage, and age disqualifies it.  The band width center positions will not remain where they should be using the ceramic capacitors.

  

I hope this helps and hope you change those caps out for Polypropylene.  That is a fine instrument --- keep it that way.

  

Kindest regards Jim K9AXN

 

ADDITIONAL INFORMATION:

If you should choose to replace those capacitors, Please measure them before you remove them.  The front .01uf is disconnected when the switch is in the 2Kc position and the second when in the 5Kc position.  Measure when cold and when it reaches operating temperature --- after an hour.  The .01uf should measure .0082 cold and .00074 warm.  They don’t affect the 500 cycle position because it is a short around the capacitors, however the other positions will overlap each other to varying degrees.

 

Here’s an interesting experiment.  After you remove the ceramic caps, put them in your oven at 280 degrees for 2 hours then take them out and let them cool and settle for 48 hours.  Now for the magic.  Measure the capacity.  They will be .01uf like new and the aging process begins again ---  5%/decade.  

 

The baking process does not harm them, the MFRS used to alter the capacity to the target value by baking them.  You would always find a 10% capacitor at =< 2%.        

 

The aging process combined with non-linear –4000 ppm Temp comp personality of the Z5U Ceramic will result in the .01uf cap becoming a .0074uf Cap and will do nothing but get worse over time.

 

NOTE:  The four paper looking capacitors are not paper but high quality Mylar film.  If you have already replaced them with Ceramic, remove them and replace with them with Polypropylene.  C100 and C111 are timers for the fast AGC which controls the first RF amplifier.  Using ceramic caps relegates the fast AGC to a second rate performer.  All of those Mylar caps were deliberate choices used in critical circuits requiring linear time constants.  The Z5U is patently non-linear in every sense and should not be used. 

 

Kindest regards Jim K9AXN

COPYRIGHT  James Liles

  

  

  

  

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