[Collins] 75A4 sensitivity

[Dr. Gerald N. Johnson, electrical engineer]

I don't think the molded oiled paper capacitors used in 50s vintage
receivers used PCB. I think they used plain kraft paper and mineral oil.
I've NOT detected the characteristic odor of the particular PCB used in
capacitors (and transformers) around those capacitors even when the case
is split.

While certain abuses of PCBs have caused increases in cancers,
particularly by ingesting the PCBs, its a wonder that the electrical
industry making capacitors and transformers using Askeral or Pyranol and
other brands of electrical insulating PCBs have never been charged with
causing excess cancers in their employees that handled very large
quantities of PCBs. Those companies have been charged with dumping stray
PCB into the rivers outside their plants so that the river beds and
water creatures do contain considerable PCB. Pyranol is more common in
metal cased capacitors, sometimes used in radios. Pyranol and the other
brands of electrical insulating PCB (there are four chemical compounds
called PCB, only one used for insulating oil) have a distinctive odor.
Pure mineral oil has no odor. Its against the law to dispose of such
capacitors in the common trash, they must be incinerated at designated
facilities. I don't know how to find those places.

I like Orange Drops because I've abused them beyond reason without
change in leakage. I've not tested other brands that way. And in the
past times there have been very compact mylar capacitors from the
Pacific rim with marginal insulation thickness and less than marginal
lead wire sizes.

The original capacitors generally did not have non-inductive
characteristics. In some cases the inductance and capacitance were
chosen to be series resonant, often at 455 or 500 kHz. Orange drops
weren't made with extended foil 30 or 40 years ago, but they are now
which cuts their inductance a great deal.

There's no need to remove the 5 volt leads from the rectifier socket, it
makes for excess wear on the socket and wires should some one wish to
return to using the vacuum tube rectifier in the future.

Most likely the 33 ohm resistor in the RF stage grid circuit was the
most effective change there. One could go to 100 ohms without damaging
the RF gain of the stage or it NF and have a greater oscillation margin.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.