SX-115 Selectivity Switch
This is a common complaint regarding the Hallicrafters SX-115
“The Selectivity switch positions don't seem to be effective.”
Simple fix for the problem:
To fix the problem, simply replace the Z5U Ceramic disc capacitors on the selectivity switch along with C73 and C86 with Polypropylene film capacitors and realign the 50Kc I.F. 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.
- Measure the the capacitors. The .01uf caps are easy to reach. Measure them with the radio cold. They will be very close to .0082uf not .01uf --- (-5%/Decade). That is a result of aging. 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.
- 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 Z5U 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.
The values of those capacitors are Critical, they control the frequency offset for the five selectivity settings as well as coupling.
Hallicrafters made a component selection error. They apparently decided that the capacitors used in the BAND PASS FILTER circuits of the SX-115 could be class 3, Z5U capacitors.
Every engineering or design document explicitly warned the use of Class 3 ceramics in tuned circuits, timers, and analog applications were patently poor design practices. 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 paper or film capacitors that did not have the instability with voltage or temperature that the ceramic Z5U has.
Hal used .01uf and .0047uf 10% ceramic Z5U capacitors to fit out the BAND WIDTH switch. I believe they meant to use high quality Class 2 caps that would probably work but were also a poor choice.
The ceramic Z5U is one of the most nonlinear, volatile ceramic capacitors ever available. The schematic calls for a 10% ceramic disc capacitor. It does not specify the class 3 Z5U. What does the 10% mean? It says 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 resonant point in each of the band width center positions to shift, coupling to increase, and band width to increase.
In the .5Kc selectivity position, the anchor position is set by alignment to center at 50.750Kc 750 cycles above the BFO. This results in a band pass from 50.5Kc to 51.000Kc. The 1Kc, 2Kc, 3Kc, and 5Kc center points are further shifted higher by the capacitors used on the selectivity switch; If the capacity changes, so too does the frequency offset center position and band width.
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 that 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, an additional 20% due to aging, and who knows what due to temperature change.
HOW THE SWITCH LOGIC WORKS:
The .5Kc position:
The 390pf capacitors control the filters resonant center point at 50.750Kc and the 2.2pf cap connecting the two transformers provides coupling. The .5Kc position shorts around all of the other capacitors. This is the unaltered center of the band pass and the point that you aligned first. It is at 50.75Kc and is 500 cycles wide. The band pass is from 50.500Kc to 51.000Kc 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 on the switch in parallel inserted in series with the 390pf fixed caps and removes the short around. This reduces the effective capacity, moving the resonant center point upward from 50.750Kc to 51.000Kc The removal of the short and insertion of the capacitors not only anchors the new band center at 51.000 Kc but increases coupling, widening the band pass from 50.500Kc to 51.500Kc centered at 51.000Kc.
NOTE:
The 1Kc position does not use a resistor in series with the secondary of the transformer. The design centers on this position.
The load impedance and resultant Q combined with the k (coefficient of coupling) resulted in critical coupling rendering the resistor unnecessary.
The 2Kc Position:
Removes a .01uf capacitor moving the resonant center point to 51.500Kc. The increased coupling to the secondary expands the band pass to 2.0Kc. A 180 ohm resistor is added to reduce the Q to a level that results in critical coupling. This creates 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. The increased coupling to the secondary expands the band pass to 3Kc. A 220 ohm resistor is added to to reduce the Q to a level that results in critical coupling. This creates a band pass from 50.500Kc to 53.500Kc at 6db rendering an audio band pass of 500 to 3500 cycles.
The 5Kc position:
Removes the last .01uf capacitor and reinserts the .0047 capacitor moving the band center to 53.000Kc. The increased coupling to the secondary expands the band pass to 5Kc. A 390 ohm resistor is added to reduce the Q to a level that will render critical coupling. This creates a band pass from 50.500Kc to 55.500Kc at 6db rendering an audio band pass of 500 to 5000 cycles.
The above information 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 and band widths will deviate significantly from where the are expected to 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.
MORE INFORMATION FOLLOWS:
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 .00071 warm. They don't affect the .5Kc position because it is a short around, 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; at 5%/decade.
The baking process is how the MFRS used to alter the capacity to the target value. You would always find a 10% capacitor at =< 2%.
The aging process combined with nonlinear –10,000 ppm Temp comp personality of the Z5U Ceramic will result in the .01uf cap becoming a .0071uf Cap and will do nothing but get worse over time.
Conclusion:
Replace the two .01uf and one .0047uf ceramic disc caps on each selectivity switch as well as C73 and C86 .02uf ceramic disc caps.
C86 is a sleeper. It is used to bypass the screen BUT is also a critical functioning member of the tuned circuit coupling the components to both sides of the second 50Kc I.F. circuit. It must be replaced with Polypropylene. Both C73 and C75 are exposed to +215 volts which further reduces their capacity after the accumulated reduction caused by aging and temperature.
NOTE: The four other paper looking capacitors in the SX-115 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 nonlinear in every sense and should not be used to replace the film caps in this radio.
Kindest regards Jim K9AXN