[ARC5] On the air

[AKLDGUY .]

Here is a description with photos of my AN/ARC-5 setup which I completed a
few days ago and operated for the first time on the New Zealand AM net on
Friday night, 19 May 2017. Operation was not very successful, but I was
heard. My antenna is very poor and I struggled to get into the local SDR
receiver, my primary aim.

I've owned the T-19 ARC-5 for a decade and a half, but never had the
enthusiasm for procuring HV transformers for a power supply. A few months
ago, I obtained a Thordarson mod transformer, and then a dynamotor. The
latter made the power supply so simple.

This view shows the setup exactly as used on the Friday night net. Power
supply at right is 0-16 VDC, 25 A, set to 13 V and connected to the 12 V
gel cell which provides ooomph for starting the 12 V, 14 A dynamotor. The
transmitter and MFJ tuner are sitting on a rack that normally mounts my
BC-AO-230 transmitter, and the lower shelf consists of the BC-230's power
supply and is not relevant here.

https://photos.google.com/photo/AF1QipOouvSiAsF0etl5eZE1tLY9bywxdc1AZgmwjUhf

https://photos.google.com/photo/AF1QipO9tKJVQgK28nV-7VkwbLE-3Y8MaO1oAxdZmQd2

The following view shows, at right, the heater/selector-relay supply (25.6
VDC regulated).

In the middle chassis at rear is the relay that switches the 13 VDC to the
dynamotor, and the heater transformer for the 2E26's (heaters wired for
12.6 V with junction grounded). At front is the transformer that supplies
the relays and the speech amp. On this front chassis is room for a 200 V
supply for my ARA receiver, which may be the next project.

 At left is the speech amp chassis (front) and the 2E26 modulator tube pair
and Thordarson transformer (rear). The 2E26 screens are regulated at 150
VDC by the 0A2 on the rear of the front chassis. The 3 dry cell batteries
provide 25 V bias to the 2E26's, adjustable by a pot in the left black box.
The right black box consists of meter sockets for PA and 2E26 plate current.

https://photos.google.com/photo/AF1QipPJ2aMdDeyN77Ew9xSVxWGv6CdkMxzHi_XHXgLN

The following photo shows the rear of the T-19. The black boxes contain
dropping resistors for the VFO (right, orange wire) and 1625 screens (left,
yellow wire). The resistors are made up of 4K7, 5W units in series (the
largest value of 5W readily available).

https://photos.google.com/photo/AF1QipPvtVCIDNtPPzz9EJ4yukETGynJlJzhKZrSbUd1

The last view is of the underside of the speech amp & modulator, by far the
most complex of the 3 twin-chassis units. Speech amp is 3 X MPF-102,
driving an LM-380N on the small perf board. This drives the 8 ohm tap of a
standard multitapped line transformer, whose "secondary" is 5,000 ohms.
Because there is no centre tap on the 5,000 ohm winding, an artificial one
was created by splitting the load resistance into two 2K7 resistors with
their junction decoupled with a 47uF, 35 V ESR electrolytic capacitor. The
2E26 grids are not to be driven into grid current in Class AB1, so they
must not be driven greater than 25 V (50 V grid-to-grid). This setup is
easily capable of developing 80 or 90 volts peak, measured from grid to
grid.

No attempt was made to make the speech amp compact; I had plenty of space
and my eyesight is not so good for fiddly stuff any more, hence the half
watt resistors.

https://photos.google.com/photo/AF1QipOlKXE3sldmC0Idq5egpoFXpqvVtU3icxGGJkDR

Hope someone picks up an idea or two.
73 de Neil ZL1ANM
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