[Hallicrafters] Black Beauty capacitors Beware

[Jim Liles]

The ER article regarding Black Beauty capacitors has an assumption that needs attention.

All of the 50 Kc I.F. systems in the Hallicrafters receivers beginning from day 1 used Paper or Plastic film capacitors except one, the flagship SX-115 which used Ceramic disc Z5U capacitors.  This was a disaster that only becomes worse year after year.  

The selectivity circuits in the SX-117 using film handily outperform the SX-115.  We talked about the reason for this in an earlier post so won’t repeat all here.  

I have never seen a schematic drawn by the Hallicrafters Co. that referred to a film or plastic capacitor, just Paper, even though they switched to plastic film well before the SX-100 was built.  The reference to paper is to assure that Ceramic is NOT used in the noted circuit.  You will find paper and ceramic of the same value in these radios so why distinguish?

There is a notion that the paper capacitors had some peculiar property when used in the 50 Kc I.F. system or any other circuit that would enhance the performance of the radio.

The purpose of the capacitors in the 50 Kc selectivity circuits is but one.  Move the band pass center away from the 50Kc carrier to a point where widening the band pass with resistors would not require shifting the BFO.  Nothing more.

In the SX-115, using Ceramic Z5U capacitors resulted in shifting the band pass centers to anywhere but where they belong and it gets worse over time.

Why the problem with the SX-115?  Find out for yourself.  Measure the capacitors.  The .01uf caps will be approximately .0082uf.  That’s because Ceramic disc class 3 capacitors loose 5% of their capacity every decade.

Take it a step further.  The Z5U has a +20 –56 temperature coefficient.  If you heat the caps to 20 degrees C over room temperature which is the expected temp deviation in the radio, the capacitor will end up at about .0072uf to .0074uf.  The paper or film will be .01uf.  

Result, the band passes are significantly misplaced. Alas, lost in space looking for GPS coordinates!

In audio circuits the notion that paper has some magic and mirrors in comparison to film is campfire talk.  In comparison to ceramic class 2 and 3, it is no longer campfire talk but irrefutable fact.

Kindest regards Jim K9AXN