I find myself a TV tech, getting dumped on with older HAM rigs for repair
by newbie licenced HAM's with no vacuum tube experience.
I am in need of a decent 50 or 100 watt dummy load to properly align the
transmitters at full power.
I have stuff for trade or am willing to pay up to $20/USD for one shipped
to Canada, postal V0X 1X0.
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Assuming that the HAM rigs you get for repair are for the HF bands (up to
29.7 MHz) and this being a radio homebrew NG ,may I suggest you make such a
dummy load yourself.
Ingredients :
- 20 pcs 1KOhm-2W - 5% carbon- or metal-film resistors
- 1 pce Cacoa (all metal) circular tin or similar for example a paint
tin.
- 2 pcs Jam jar lid having a diameter slightly less than the opening of
the tin .
- 1 pce SO-239 socket with square flange and 4 holes ( + 2 suitable
screws/nuts/seal washers) to suit PL-259 coax connector
- 1pce 150 mm (6inch) long thick copper wire ,dia 1 to 2 mm ( not
critical)
- 2 pcs Metal stand-off each having 2 tapped ends with screws ( length 25-
50 mm).
a) Drill 20 one mm holes (equally spaced) near the edge of both jam jar
lids and tin area around the holes on both sides.
b) Drill a concentric hole ( 16 mm diameter) in one of the jam jar lids and
a concentric 2mm dia hole in the other
c) Drill 2 pcs diametrically opposed 3mm dia hole in the lid with the 16mm
hole using the SO-239 socket flange as a template.
d) Drill 1 concentric 16 mm dia hole in tin lid and after that use the
SO-239 socket flange as a template to drill 4 holes (dia 3mm)in the tin lid
e) Fit the 2 stand-offs to 2 diametrically opposed holes in the jam jar lid
with the 16 mm concentric hole with the lid's inside surface towards the
stand-offs.
f) Cut resistors' wire ends leaving approx 5mm on both ends
h) Protrude remaining resistor wire ends through tinned holes in lid with
fitted stand-offs and solder.
i) Do the same with the other jam jar lid (the one with the 2mm dia
concentric hole) by pointing the lid's usual top towards the resistors and
solder these to usual inside of lid . The 20 resistors are now sandwiched
between the 2 jam jar lids .
j) Solder one end of the thick 6 inch long copper wire to the SO-239
connector and fit this connector with 2 diametrically opposed
bolts/nuts/washer to the tin lid
k) Protrude other end of 6inch long copper wire through concentric holes in
resistor package and fit the 2 stand-offs to the 2 remaining holes in SO-239
connector flange.
l) Finally solder copper wire to now bottom jam jar lid and cut-off excess
wire.
By putting the package inside the tin you now have a 40 Watts dummy load
It should be obvious that the resistor package must not make direct contact
with the tin.
You can increase the power rating of the dummy load by filling the tin with
oil . I have used motor oil for the past 18 years in a particular dummy load
with the resistance changing from 49.8 to 50.6 Ohms over that time.
However before using oil I would recommend sealing the tin's seams with
epoxy glue ( the 2 components stuff) on the inside to prevent 'oil sweating
' through the seams.
With all the ingredients and required tools available ,the above should be a
1 evening's job.
Measured SWR ( with MFJ 259B antenna analyser) is 1: 1.2 @ 29.7 MHz and
less at lower frequencies.
Although it is unlikely you will be testing the transmitters at full output
for any length of time ,the above dummy load will happily absorb 100 Watts
for 15 minutes if accommodated in a tin of not less than 1 litre (about 1
quarter US gallon)
Good Luck with this simple homebrew project.
Frank GM0CSZ / KN6WH
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