Tech Talks and Tips by K4KYV

(Cont'd from previous page)

Oldham Coupler
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...is the name of the little flexible shaft coupler found in the Collins 75A- series receivers, R-390 series and other equipment. The coupler, a mechanical device for transmitting torque between two shafts that are not perfectly collinear, was invented in 1821 in Ireland by an engineer named John Oldham, to solve a problem in a paddle steamer design.

Animation of the coupler in action

 

75A-4 Mechanical Filter Shunt-Feed: Precaution

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. See the attached schematic.

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 within the amateur community, and many 75A-4 owners purchased the components and
incorporated the change in their receivers.

Click on the attachment to compare schematics of the original Collins circuit (left) and the circuit after
the production change (right). L32 is the mixer plate choke, and C145 is the resonating capacitor.
C144 is the plate blocking capacitor.

The original Collins production change suffers a fatal flaw: a shorted blocking capacitor (C 144)
would produce exactly the same disastrous result that the upgrade was designed to prevent. The capacitor
used in the later production is a nondescript .001 mfd 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 a hi-pot tester. The new disc ceramic is the same diameter as the old one, 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 should be small,
although not beyond the realm of possibility. A more secure 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 would seem unlikely.

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 revised circuit, replace the original .001 mfd disc ceramic capacitor
(C144) with another one rated at 2 or 3 KV nominal working voltage. Better still, replace C144 with two .002 mfd,
2-3 kV disc ceramic capacitors connected in series.

Note:
The 62 pf mica capacitor and 2 mHy RF choke used by Collins in the upgrade may be difficult to obtain.
Readily available standard component values, a 2.2 mHy (2200 μHy) RF choke and 56 pf dipped mica capacitor,
should work OK. A small terminal strip is needed for mounting the parts.

 

Four-sided vs Triangular Towers

You may have noticed that many older towers dating back before WWII are four sided, while most newer towers constructed postwar are triangular. Classic examples of four-sided construction are the famous diamond-shaped Blaw-Knox towers at WSM, WLW, WFEA in Manchester NH and others. Photos of towers dating back to the 20s and earlier in old magazines and textbooks nearly always show four-sided construction. Here is the Rohn tower company's take on three vs four sided tower construction.


Square Angle vs. Triangular

Following are some points to consider when specifying towers. As you will see, the square angle design has no advantage whatsoever over the triangular design, and the triangular design has many advantages over the square design. This is evidenced by the fact that the majority of the world's major tower/mast manufacturers operate in an environment where the finest quality raw materials are available, and are not limited to a certain steel supply. These modern manufacturers provide triangular towers as their primary product.

1. Square tower design is old fashioned and antiquated. Some designs date back to the 1800s and were based on availability of material, existing design capabilities and ease of manufacture. Almost any steel fabricator can design and manufacture square angle towers or masts. Square angle towers are specified in many cases because of the long standing designs that have not been updated over the years. These structures have been the accepted standard, since many departments and users do not have the in-house structural engineering capacity to evaluate modern designs, and take the time to update their requirements.

2. Square tower design is very popular in developing countries where updated technology and sophisticated machinery are not available yet. It exists only because there is no other choice, and what was good enough 100 years ago, is all that is available today.

3. Towers and masts constructed from angle material have a much higher wind load than the more sophisticated triangular round member tower. A triangular round member tower is much more aerodynamic and therefore has lower wind resistance.

4. Because of the higher wind load on the structural members, more reinforcing pieces are necessary, and therefore the structure when completed has many more components and connections than a triangular tower.

5. A square tower with all of this extra material, is no stronger than a triangular tower designed for a similar load. There are international standards developed for tower design such as ANSI/TIA/EIA-222-F-1996 that govern proper tower/mast design for the communications industry.

6. As a result of the square angle design, there is more labor involved to assemble the material, more possibilities of pieces not fitting, more connections to become loose and require maintenance.

7. Triangular towers however are lighter in weight thus saving freight costs, and are constructed of fewer pieces. This is possible because of the higher strength steels that are currently available for the more high-tech tower/mast designs.

8. Triangular towers only require 3 foundations, square towers require 4. There are considerable cost savings in civil works and concrete using a triangular design.

9. From the standpoint of deflection and twist, the triangular pipe tower is stronger and more rigid pound for pound.

10. With round main members (legs) equipment such as dish mounts, platforms etc. are mounted with 'U' bolts, and therefore can be moved from location to location without drilling additional holes in the structural members of the tower. Antenna mounts for example can be added to the structure without any field punching, drilling or welding.

11. There is an old fashioned argument that pipe members corrode from the inside, and since the corrosion is hidden, it cannot be maintained or corrected. Back when pipe members were first used in construction, the material was not hot dip galvanized inside after fabrication. With today's modern fabrication procedures and galvanizing technologies, this condition does not exist. Back to back angle members can also corrode from the inside, and cannot be maintained. The secret is in the fabrication/galvanizing procedure. See more in FAQ.

12. Due to the availability of larger sizes of higher strength round structural steel shapes, round member pipe/solid towers can be designed with single piece main structural members. Angle towers require 'back to back' bolted or welded members ("built up" sections) to provide the strength required for some of today's tremendous antenna loads and tower heights.

http://m.rohnproducts.com/technical-info/square-angle-vs-triangular.html


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