I am in the middle of repairing an old Waters 334 dummy load wattmeter
for a local ham. He had picked this up at a hamfest and the meter
didnt work. I have removed the oil tank and cut an opening in the back
to access the internal components. The oil has been drained into jars
and I will put the oil back after I make the repairs.
Ok, here is the question.
The failure is the diode. it looks to have the original diode. Black
glass marked SG5409. I do not see a modern cross or the exact type
still available. Is there a better or more durable RF diode that will
work in this circuit? Temporarily, I soldered in a 1N4148 and this
seems to work well at the 100 watt level. My concern is at a KW will
this hold up? What about the possibility of stringing several 1N4148's
in series to increase the voltage capability? I know that most of the
power is going into the 50 ohm resistor and the diode is just seeing a
small sample. The diode is in series with four 2.7 ohm resistors and
then has a 15K fixed and a 10K pot before it gets to the meter. This
is on the 1KW position. Any suggestions will be appreciated.
73, Randy, w5kcm

w5kcm wrote:
I am in the middle of repairing an old Waters 334 dummy load wattmeter
for a local ham. He had picked this up at a hamfest and the meter
didnt work. I have removed the oil tank and cut an opening in the back
to access the internal components. The oil has been drained into jars
and I will put the oil back after I make the repairs.
Ok, here is the question.
The failure is the diode. it looks to have the original diode. Black
glass marked SG5409. I do not see a modern cross or the exact type
still available. Is there a better or more durable RF diode that will
work in this circuit? Temporarily, I soldered in a 1N4148 and this
seems to work well at the 100 watt level. My concern is at a KW will
this hold up? What about the possibility of stringing several 1N4148's
in series to increase the voltage capability? I know that most of the
power is going into the 50 ohm resistor and the diode is just seeing a
small sample. The diode is in series with four 2.7 ohm resistors and
then has a 15K fixed and a 10K pot before it gets to the meter. This
is on the 1KW position. Any suggestions will be appreciated.
73, Randy, w5kcm

1KW into a 50 ohm load is.... P=V^2/R... 224 volts.

Pick the fastest diode you have in the junk box with a breakdown voltage
of 250V or higher. A 1N4007 is probably not fast enough. One of the
high tech HexFREDs pulled out of an electronics fluorescent light
ballasts might be great. You don't care about current.

This is assuming you're measuring directly off the load, of course. If
there is a step-down transformer, you'll need even less voltage.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."