"Well, i have a Heath HA-14 and i have an isolation relay between the rig
and the amplifier. The relay in the amplifier is quite noisy, however
when i'm not using the amp, the relay in the rig seems to be quite loud.
Since i have the isolation relay at the power supply to the amplifier, i
was thinking of maybe using a 4N33 optoisolator to replace the relay in
the rig, but was thinking that maybe someone else has come up with
something better."
You should avoid using an opto-isolator by itself as the sole keying
mechanism to the amp. The opto-isolator should be used as the interface
between the rig's internal key line and a robust switching device like a
MOSFET.
If I was to go into an Omni Six again, here is the tack I would take: To
simplify the job, I would use the Jackson Harbor Press "Key All." This is
a
small PCB with an opto-isolator feeding a pair of high-voltage,
high-current
MOSFET transistors. You can construct your own, but this product provides
a
very clean installation whereby the PCB can be affixed to the side wall of
the rig with 3M double-sided tape. The Key-All is USD $15.00 and includes
shipping CONUS.
http://home.att.net/~jacksonharbor/keyall.htm
I no longer have an Omni Six manual so perhaps others can apply the
schematic numbers to what I am describing from memory:
1) Lift one of the base biasing resistor going to the small TO-92 style
transistor. The transistor is used as a current driver for the noisy amp
keying relay. This will prevent the relay from becoming active if you
forget to disable it from the rig's menu. If you're careful about leaving
the relay disabled in the menu, this step can be bypassed. Still, I would
lift the transistor biasing resistor. The lifted end can be soldered back
down again if you ever decide to sell the Omni six;
2) Find the "T" voltage buss on the Omni Six schematic. It appears at
several locations on the Control Board. Find the buss, and connect one
side
of the key-All input (R1) to the "T" buss. The other Key-All input lead
of
goes to the Omni's circuit ground. R1 on the Key-All is the
opto-isolator's
LED current limiting resistor. The 120-ohm value should be fine, but I
would probably ensure it is valued correctly so that roughly 17 mA flows
through the LED from the "T" voltage source (+5VDC I believe). That said,
I
would verify the "T" voltage with a DVM. If it is higher than +5VDC, R1
needs to be increased accordingly to maintain roughly 17 mA through the
opto-isolator's LED (Ohm's Law still works in 2007);
3) Ideally, the Key-All's MOSFET pair should remain floating above ground
for use with AC-switched amp key lines. What I am not sure of is whether
or
not the Ext. AMP RCA connector on the Omni Six's rear panel can isolated
above ground. If it can't, then you may be limited to amps with only a DC
key line.
As an alternative, the "T" voltage could be brought out to the RCA jack
and
the Key-All could be placed in a small plastic project box, external to
the
rig, where the MOSFETs can stay floating above ground. If that is done, I
would definitely move R1 from the Key-All PCB to the inside of the rig as
a
current-limiting buffer against accidental shorts.
That's it. If you can tackle this, you'll have the best output key line
known to mankind: Totally silent and capable of switching moderately
high-AC/DC-current key line amplifiers.
In other Ten Tec's, I have also replaced the TO-92 style relay driver
transistor with a 2N5551 (good for +140 VDC), bypassed the relay and then
brought out the 2N5551's collector to the rear panel RCA jack. That works
well with most amps, but the Key-All solution is the "real man's" way to
tackle the problem.
Paul, W9AC
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