On 6/14/2012 7:54 PM, Jerry Kaidor wrote:
> Hello,
>
> To mess with amps, a good dummy load is a must. I went to my storage,
> and pulled out a Cantenna of unknown provenance that I had purchased about
> 30 years ago at a flea market. Not knowing what was in it, I assume it's
> poisonous PCB transformer oil.
>
Most came with non hazardous oil although many hams did put PCB oil in
them. Did it smell like oil or mineral oil? If so it was not PCBs.
> I had kept the dummy load in a plastic bucket
> with a hole for the coax, sealed with coax seal. Now I figured I'd
> dispose of the old oil, clean it out, and put in fresh mineral oil. What
> a mess!
>
> OK, the old oil is gone; it'll be headed to the local hazardous waste
> dropoff. The guts are all washed out in hot water and detergent. The
> heathkit can is toast - rusty and leaking. The silver
> straps around each end of the resistor are broken. So I can reconstitute
> this thing with new silver straps ( luckily I have some silver sheet
> laying around ). And the can - can be replaced with an ordinary one from
> the paint factory. Only trouble with that is - 1 gallon paint cans are
> now *plastic*, not metal. I guess it'll still work, just the heat
> transfer won't be quite as good.
Go to an industrial paint supply or even your Ace Hardware store. Both
carry metal paint cans.
>
> I also scored a 250W load cheap on Ebay. Seller said it came out of a
> cell phone installation. I popped it open - the resistor is the size
> of a postage stamp! I was messing with one of my SB2-LA's, used that
> load - figured a momentary toot from the amp wouldn't hurt it. I was
> wrong. Apparently these little resistors are not only heat limited,
> but *voltage* limited. I hit transmit, there was a small *pop* and a
> puff of smoke. Hole burned right through it.
Surface mount resistors suitable for dummy loads are about 200 - 250
Watts and about 2" on a side. You can create a 1 KW load with 4 of them
mounted vertically with the leads facing in to form a square tube.
Water cooled heatsinks on the outside, or just submerge them in DI
water....or oil.
Several on here or HamAmps have done something similar for some pretty
heavy duty dummy loads.
You may have to add some compensation, but that's not unusual.
Flat copper plate, solder 1/4" copper tube on and you're set to go.
73
Roger (K8RI)
>
> There are similar load resistors on sale on Ebay - 800W for IIRC $40.
> But that's just the resistor, you have to bond it to a heatsink. I
> have
> the heatsink from the defunct 250W load - I could machine off a big enough
> flat spot for the 800W resistor.... failing that, I have a really big heat
> sink in my junk.
>
> - Jerry Kaidor, KF6VB
>
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