[AMPS] Icom 756

[Garry & Yelena]

JOHN BASILOTTO wrote:
> 
> I have an Icom 756. I was using it with a Drake L4B. At first everything was
> fine; after about 30 days the reed relay in the Icom froze stuck. The Icom
> relay is rated at 16volts@2amps. The Drake amp has a 12 volt relay but has a
> potential of 26volts with no load. Consider using an aux. relay; otherwise
> you too may have to replace an Icom relay.
> 73, John w5GI
> 
I  have an L4B. The changeover relay has a 24 volt coil in my amp--if
yours is 12V, perhaps someone botched a repair. My L4B did not have a
quenching diode across the relay coil--perhaps it was the transient that
fried your relay! Install a rectifier, backwards, across the relay coil,
either directly, or on a small terminal strip.

One alternative to avoid problems like you describe is to install a
small circuit in the amplifier to allow control of the amplifier's relay
with low voltage and very low current. This is readily accomplished with
a CMOS inverter (4069), NAND gate (4011) or NOR gate (4001) driving an
N-channel MOSFET. Radio Shack's IFR510 will do nicely. Put a pullup
resistor in the gate input, and bypass the input to ground. For the L4B,
power the gate with a dropping resistor off the relay supply (about 150
ohms, 0.5W), 12 V zener and bypass caps. With a pullup of 22K, <.5 mA
controls the relay. If the radio also provides a lead from the collector
of the transistor controlling its internal relay, you can use that and
disable the internal relay. Ground the inputs of the 3 unused gates or 5
unused inverters.

As long as you are at it, you can add two diodes, an electrolytic, a
resistor and a small NPN to "kick" your amp's relay with a
double-voltage transient, which will speed up the changeover and
increase the life of your relay contacts. This has been covered before
and is on K6XX's web page--ask n0ss for the URL (n0ss@socketis.ne).

The small board can easily be mounted on standoffs near the relay
supply, running the lead from the relay control jack along the existing
harness and securing it with cable ties. As an added precaution, run the
input and output leads through a ferrite bead where they connect to the
board. This and the bypassing should preclude RF pickup problems.


-- 
Garry Shapiro, NI6T
Editor, "The DXer," newsletter of the NCDXC
Visit the Northern California DX Club 50th Anniversary page:
http://www.aa6g.org/ncdxc50.html

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