[Hallicrafters] SX-115 and HT-32B question

[Jim Liles]

Good morning Lee,

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 is more for the components used rather than the VFO because it has a drift correction adjustment that is simple to manipulate.

The problem that you describe happened when you unplugged the radio or the heater failed causing the internals to cool.  When you powered the radio up, the temperature change over the first hour heated the ceramic capacitors in the band pass filter.  The ceramic capacitors used were a design problem.   Read on.  
 

The Hallicrafters guys made one of two profound mistakes during the tubed era that jump out at me.  The first follows.

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 BAND PASS 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’s used film or paper capacitors that did not have the instability with voltage or temperature that the Z5U has.  

The Hal engineer chose .01 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 using the ceramic Z5U caps to implement the Band width logic.  This was a mistake as you will see below.     

If it were not for the under chassis heater failing, the error might have gone unnoticed.  Why is the Z5U a bad choice?

The ceramic Z5U is one of the most non-linear, volatile ceramic capacitors 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 ---- DISREGARDING APPLIED VOLTAGE which has a profound influence on capacity.  All bets are off when voltage is applied, and with AC the capacity begins to vary at less than 1 volt.  This would cause linearity problems, not obviously perceptible.  The vast change in capacity with temperature however will cause the center point in each of the band width positions to shift.   

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 less than 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.  Last, the Class 3 ceramic capacitors age loosing approximately 3% 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.  Audio 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 and voltage 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




--------------------Original Message-------------------------- 
From: kc9cdt at aol.com 
Sent: Saturday, February 14, 2015 10:59 AM 
To: hallicrafterssr2000 at k9axn.com 
Subject: SX-115 and HT-32B question 


Jim,
Today I am having a ball with  my SX-115/HT32-B.

A question please for each:

SX-115 seems to hear USB and LSB ...example...on 40 Mtrs, tune in a nice signal on LSB, without touching dial, change to USB...I can still hear the signal fine even though it is not as loud?? Problem or just alignment?

HT-32B tune up on 80M 3820, DSB or CW... max carrier out is 15 watts, BUT on SSB 120 watts peak out while talking loud or whistling?? Alignment?

Thanks,
73,
Lee

Lee Simmonds
Summit DCS LLC

260-799-4077 Office
260-403-6936 Cell