[Boatanchors] 75A-4 Mechanical Filter Shunt-Feed: Precaution

[Donald Chester]

The original circuit in the 75A-4 receiver has the full B+ flowing through the input coils of the mechanical filters
to the plate of the 6BA7 2nd mixer. If the plate of the tube ever shorts internally, or somehow the wiring to the
plate circuit shorts to ground, or as the (now 60 y.o.) insulation on the coil inside the filter deteriorates with age,
the full 200 volts B+ from the power supply will appear across the filter, likely resulting in its instant destruction.
In later production runs of the receiver, the circuit is changed to shunt-feed, using an RF choke as a plate choke
to carry the B+ to the tube, with B+ voltage isolated from the filters with a 1000 pf disc ceramic blocking capacitor,
C144. A 62pf mica capacitor is wired in parallel with the rf choke to broadly resonate at 455 kc/s. The cold ends of the
mechanical filter input coils in the revised circuit are grounded directly. This upgrade was designed to prevent
damage to the filters in the event of any one of the above-described failures.

Collins regularly published "service bulletins" to the receiver, incorporating the latest production changes,
and often a set of components to retrofit the service updates could be purchased as a kit directly from Collins.
This production change appeared on the schematics in later users manuals, but was never the subject of a
service bulletin nor were the components offered as a kit. However, descriptions of the upgrade quickly
circulated around the amateur community, and many 75A-4 owners purchased the components and
incorporated the change in their receivers.

The original Collins upgrade suffers a fatal flaw, since a shorted blocking capacitor (C 144) would bring 
exactly the same disaster that the revised circuit  was designed to prevent. The capacitor used in the later 
production is a nondescript 1000 pf 500v disc ceramic, and more than once I have seen similar capacitors 
in various equipment fail by developing a dead short. In my 75A-4 I replaced the original blocking cap with 
a .001 mfd 3 kV disc ceramic, after first testing it at 2 kV for leakage or short circuit, using my hi-pot tester. 
The new disc ceramic is the same diameter as the original, but several times the thickness. The likelihood 
of a 3 kv capacitor testing good at 2 kv and then developing a short at only 200v DC would be small, 
although not beyond the realm of possibility. A more failure-proof approach would be to wire two .002 mfd 3 kV
disc ceramics in series, since simultaneous failure of two separate capacitors operating at less than 10% their
nominal working voltage should be minimal.

In summary, if your 75A-4 does not already have the shunt-feed upgrade, this modification should be installed
sooner rather than later. If it does have the revision, replace the original .001 mfd disc ceramic capacitor
(C144) with a new one rated at 2 or 3 KV nominal working voltage. Better still, replace C144 with two .002 mfd,
2 or 3 kV disc ceramic capacitors connected in series.


Don k4kyv