Buc Builds a Relay Interface Assembly
Our Radio World colleague Charles S. “Buc” Fitch, P.E.,
recently completed a relay interface assembly for controlling an older
Late-model transmitters have remote control terminals
operating on 5, 12 or 24 VDC, perfect for interfacing to an external remote
control system. Many older transmitters don’t include this feature and instead
use 120 or 240 VAC for their control ladders.
Connecting these terminals directly to a remote control is
asking for trouble for several reasons.
First, the current rating used in the transmitter may exceed
the rating of the remote control relays. There is also the possibility that you
can short the control ladder AC to the remote control, causing serious damage
to the remote control, as well as yourself. Finally, good engineering practice
frowns on having raw 120 or 240 VAC on terminal strips in the back of an
equipment rack. It’s just waiting to “bite” you.
The solution is a relay interface, such as the one Buc has
built and seen in Fig. 1. The terminal connections are the correct orientation
for attachment to the inside front wall where the control wiring is located.
Fig. 1: This remote
interface mounts inside the transmitter.
The top strip of 12 connections are (in
pairs) from left to right for the 120 Volt AC transmitter control ladder. Buc
chose relays with contacts rated for this voltage. The contacts handle filament
on/off, plate (high voltage) on/off and power raise/lower.
If you set out to build such an
interface, add Buc’s confidence LEDs for each relay. The green LEDs are located
along the lower left of the board. This simplifies troubleshooting between the
transmitter and the remote control — no more worrying whether the relay is
actually pulling in.
Fig. 2 shows a side view. The metal
standoffs help with mounting to the transmitter wall.
Fig. 2: Side view. By
sandwiching the control relays under the terminal board, we get a compact
* * *
One of the best design criteria for broadcast equipment is
longevity in the field.
Although developed years ago, the Gentner VRC2000 Remote
Control System is still going strong at a lot of stations. Channel parameters
are programmed in a volatile memory. The memory is “kept alive” with a coin
battery inside the unit.
The pitfall is if the battery fails and you lose AC power to the unit, you must re-enter all the
programming functions, which is not a fun project.If you have one of these workhorses handling your remote
control functions, read on.
First, if you haven’t replaced the battery, it may be dead
and you’re skating on thin ice. But before you change it, see what Saga’s Ira
Recently on the PubTech listserv, engineers discussed adding
wires to the coin battery contacts and running them outside the unit to an
external battery holder. Not a bad idea; it eliminates removing the VRC from
the rack, popping the top and changing the battery on a routine basis.
Ira’s suggestion is to plug the VRC into a UPS, which makes
changing the battery a moot point. However, make sure the internal coin cell
battery is fresh before unplugging the VRC. You don’t want to lose the
programming in the time it takes to move the AC cable to the UPS!
He chose a UPS because of the risk of shorting battery leads while
soldering the wires, and subsequent loss of programming.
another solution is to piggyback new batteries inside the unit. But be careful.
One wrong move and you’re faced with a long re-programming task. (This may
happen anyway, if you ignore the battery and at some point lose power.)
me know if you have other equipment with similar issues and how you handled
them. Email suggestions and comments to email@example.com.
Fig. 3: These work
great for removing tarnish on contacts, especially tarnished copper
transmission line connections.
* * *
Brian Urban, chief
operator at the University of Texas at Austin’s KUT(FM), weighed in on our earlier
discussion of corroded batteries: “I’ve had much better luck recovering from
leaking batteries since I remembered an acid will neutralize a base.”
But in the case of seriously corroded
terminals, Brian has resorted to 3M Scotch-Brite pads to clean the terminals,
after using the vinegar or baking soda. Next time you’re at the store, pick up
a pack of pads. Be sure you get the ones that are not impregnated with soap!
Contribute to Workbench. You’ll help your fellow engineers, and qualify
for SBE recertification credit. Send Workbench tips to firstname.lastname@example.org. Fax
to (603) 472-4944.
Author John Bisset has
spent 43 years in the broadcasting industry and is still learning! He is SBE
certified and is a past recipient of the SBE’s Educator of the Year Award.