SB-200 ALC mod for use with Icom 746

[Gary Schafer]

John

Swr has nothing to do with alc on an sb200 amp. The only way to generate alc voltage is to
overdrive it. That will happen if you do not have it loaded heavy enough. If I remember
right the alc is generated from the grid current in the amp. The lighter the loading the
more grid current it will draw, generating more alc voltage.  Again if I remember right,
the 10 volt threshold that they are talking about is how high the internal alc voltage has
to go inside of the amp before you see any output alc voltage.
You do need some sort of way to limit it so that it does not go to high and damage the
Icom radio. Either with a pot or a zener diode. There are several circuits floating around
to do this.

73
Gary  K4FMX


John Farrington wrote:

> 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
>
> Sponsored by the City of Tempe
>
> Listserver Submissions:  heath at listserv.tempe.gov
> Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
> Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"