Amateur Radio Homebrew (Shack Culture)

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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

I ordered my 250W resistor on Saturday and when I checked with the supplier although the description said the item was in London the delivery date given was 27th June. This led me to believe it was actually coming from China, it's country of origin!

I had a tracking number from Yodel this time so I punched it in on Monday and it said that the item was waiting to be delivered to Yodel. This reinforced my view that it was coming from the other side of the world. I received an email though this morning that said my item was on the van and due to be delivered tomorrow. Knock on the door at 2.14 this afternoon and I have it in my hand, it's come from Birmingham which I suppose is another outlet for the importers.

Baking this afternoon so I will put a picture up later of the little beast. :smile:
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

OK here is the resistor pack, just 1" wide including its integral metal heatsink mount. Amazing that a package as small as this can dissipate 250W. I have no doubts as these same devices are used in Bird Through Line watt-meters which are regarded as top line equipment.

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Before we go any further drilling mounting holes and such, good practice says it is best to test the component before installation. My Beckman digital bench meter shows it reading 50.3 ohms, near enough for me.

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Re: Amateur Radio Homebrew (Shack Culture)

Post by Stanley »

Amazing how components have shrunk in my lifetime....
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

As resistors go, small as this one is, it is actually massive compared to the lower power surface mount components. Lots of these are no more than a couple of millimetres in size. Some semiconductor components are now "printed" and actually integrated directly onto the surface of some printed circuit boards.

When I was TV servicing a 100W resistor or dropper stack used in the valve based TV's for multiple voltage rails would be between 6" to a foot long. If one section of it burnt out you chopped out a section of the winding to make sure it was open circuit then attached a replacement high wattage wire wound resistor between the section terminals. My 100W instant heat Weller soldering gun came in handy for jobs like that, you generally had to use high melting point solder for the connections as they ran at such high temperatures. You needed clean contacts and a lot of heat to get that stuff to run, it wasn't flux cored either you had to use separate flux from a tin or tube.
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

This is the external profile of the extruded box section. I need to drill two mounting holes through for the resistor pack. The diameter of the drill needed is greater than the gaps between the flutes so without the benefit of a pillar drill and drill vice the bit is going to jam with a hand held drill.

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I decide to start with a countersink bit in order to clear the flutes back a bit, I ran the countersink down to the base of the flutes then drilled the clearance holes that i needed for the mounting screws. The screws are pan heads so I followed the first hole with a bit slightly bigger to open up the base of the countersink. This allows the screws to sit down a bit further in the base.

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I mounted the resistor with a thin film of heatsink compound and secured the 3mm screws with nylock nuts. The heatsink of the resistor package is connected to one end of the resistor. The centre tab which is the other end of the resistor is connected to the centre pole of the SO239 using a short length of solid 2.5mm mains cable. I installed the resistor pack with it's tab away from the socket to give a bit more room to solder up the connections. Here it is looking inside. I checked on the meter again that I have not damaged anything using my 100W iron for the joints.

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Job done apart from additional internal heat shunting material. The inside void available is essentially 1.5" high x 2.5" wide and a maximum of 2" deep from the back of the case to leave a sufficient gap so as not to foul the "live" side of the resistor pack. Any additional metal does not need to be in one solid piece as long as components are securely fastened to create a thermal link to the outer case.
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Re: Amateur Radio Homebrew (Shack Culture)

Post by Stanley »

Good trick with the CS bit!
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

Needs must Stanley. This is one project that doesn't really need to be pretty just robust and functional. It will only get occasional use as a piece of test kit. I have done a tentative test of the unit from my handheld transceiver which puts out 4W on high power. I shoved a connector adaptor on its micro antenna port to convert it to SO239. Then used a line coupler to attach it to the dummy load. I stuck my finger on the top of the resistor when I put the handheld into transmit. As expected, no detectable warming of the component with full power indication on the transmitter. In this test x 62 overkill, I would expect nothing less. :smile:
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

I have passed on my project to Stanley today with a fag packet drawing. This is to make and fit some internal cooling vanes.
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Re: Amateur Radio Homebrew (Shack Culture)

Post by Stanley »

I must be slowing down a bit because even though it is a good drawing I had to have it explained to me. Hardest part will be identifying the threads on the tiny pan headed screws he has given me!
It will be a nice bit of distraction therapy from the valve gear. Ian agrees with me that the drawing is a nightmare!
But first I must cook veggies!
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

I bought a whole range of standard metric thread screws, nuts and washers for the fastenings on the motorbike. As far as I am aware they are bog standard metric thread. I think the pan head ones I supplied are 3mm. I have matching nuts if that would be any help?
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

Furthered the dummy load a bit more by modifying the side panels for the case. I marked one side up then bolted the two sides together using the corner fastening holes before drilling them out for more ventiation to the case. I ran the countersink bit over them to break the edges. When I get the main unit back from Stanley I may put some holes in the top as well, there is a flat panel section that will be easy to drill. It will depend where the internal cooling fins are fastened as to where I put any extra holes.

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Re: Amateur Radio Homebrew (Shack Culture)

Post by Stanley »

No need for the nuts, I'll manage something Ian.
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Re: Amateur Radio Homebrew (Shack Culture)

Post by PanBiker »

I collected my modified dummy load from Stanley this morning so it's back on my bench for the finishing touches.

Unit with extra heat-sink material installed.

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There is scope on the small flat top surface for a few more ventilation holes.

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Three more holes for extra air circulation.

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Side panels on and it's now complete.

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Thanks again to Stanley for his metalwork skills.
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Re: Amateur Radio Homebrew (Shack Culture)

Post by Tripps »

Looks like a really nice job - well done both. :smile:

Could do with one (and a fan ) for my mobile. I've been using it for Skype calls, and it gets worryingly hot after only about ten minutes. I take it out of its case for calls now and it feels better. Google says it's not uncommon.
Born to be mild. . .
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Re: Amateur Radio Homebrew (Shack Culture)

Post by Stanley »

Don't even think about it David! I have an engine to finish.
Ian, keep your eyes open for a nice piece of hardwood 7" X 15" X 3/4" Finished, I'll be wanting a base for the next beam engine in about 12 months and I have run out of wood.... :biggrin2:
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