[ICOM] Icom 765: Ant Tuner Detector adjustment requires a SWR3 150Ohm Dummy Load ??

[Roger (K8RI)]

On 10/25/2010 3:39 PM, rrocket88 at netzero.net wrote:
> Btw,
> One reason I'm Shipping my 765 off for adjustment is the Tuner Growls like a mad cat.. well 'el service manual page 6-16 (detector verify ) calls for a SWR3 150 Ohm dummy load at 100w power.
>
> I own No Such dummy load, so I can't complete the adjustment.
There are some large wattage, small size, surface mount resistors 
available from mouser and I believe RF parts. They look very much like 
the resistors used in the expensive Alpha dummy load. OTOH these are not 
exactly inexpensive, but are far, far less than purchasing the complete 
dummy load.
I'd have to look up the prices but I think they were in the $30 to $40 
range each.

IIRC these resistors which are in a heat sink about 2.5 or 3" on a side 
and bolt down to a heat spreader come in 50, 100 and 150 ohm ratings at 
something like 150 or 200 watts each. So a 50 ohm dry dummy load with 
800 watts continuous capability has a relatively small foot print and 
will go quite high in frequency depending on the layout and cooling 
fans.     I recall seeing a dummy load with 4 of these resistors in a 
square. All 4 tied together in the center where the center conductor of 
the coax also tied.  The other ends were all grounded to a ring so the 
distance from hot to ground was short.  2 or 4 could be positioned on a 
block in a coaxial arrangement to keep the swr down to even higher 
frequencies.

I've been thinking of trying the latter arrangement but I'm not sure 
about the cooling. They could be placed "base out" so the heat sinks 
would be around the outside, or they could be arranged around a water 
cooled copper block.  Just some thoughts.

73

Roger (K8RI)
> Well I've never been bashful about being dumb, but I find no search reference to construction , or purchase of any 150 ohm dummy load.
>
> Has anyone built, or seen one,  or Is this similar to an Auto Dealer having the "special Tool" to make the adjustment to your vehicle ??
>
> Any help appreciated just to ease my mind.
>
> Randy K4IMZ
>
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