SB-200 ALC mod for use with Icom 746

[John Farrington]

At 11:13 PM 03/06/2000 David A. Jaffe wrote:
>Has anyone used the SB-200 ALC with Icom equipment?  If so, what mod
>did you do to the SB-200 (if any)?

I connected my IC-720A's ALC input directly to an unmodified SB-200
and it works OK, but the 720A is a lot older than the 746, so I don't
know if they differ. One thing to watch out for is dual-use jacks on
Icom stuff - for example, my ALC jack is primarily factory set for
another function and has to be changed to the ALC function by moving
an internal jumper. This kind of information is usually hidden in
fine print in their manuals.

>Details: I am trying to set up my SB-200 ALC to work with an Icom 746,
>which wants an ALC voltage in the range of zero to minus 4 volts.

Does your Icom manual specify needing 0 to minus 4V at its ALC input
jack?  My manual doesn't mention the level anywhere, it only says that
if SWR (without a linear) gets to 3 or above, it's own internal
circuitry generates an ALC voltage to decrease the xmtr drive. I have
heard that Icom's require 0 to about -4V on the external ALC input,
with the -4V reducing the ouput RF power to a low level.

The SB-200 ALC output is negative-going:  as SWR goes up the ALC output
goes more negative to cause the driver (Icom, in your case) to reduce
power. The way to check would be to vary the swr by detuning an antenna
tuner connected between the SB-200 RF output and a dummy load, while
monitoring the ALC output voltage with a scope or meter. The SB-200
manual mentions that its ALC threshold is set to 10 V by a low tap on
the HV power supply, but I don't know exactly what they are referring
the 10V to.

I measured the SB-200's ALC output voltage, and it goes from 0V with
no SWR, to around minus 3.5 VDC with a high SWR. If you're getting
10V all the time then there is something wrong with its ALC wiring:
maybe the ALC modification for a pot wasn't done right.

73

John Farrington  KE5ZB

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