Hello VHFers
I am happy to report that over the next few months, there will be some
new stations getting on the 222 MHz air waves with high power in some
rare states. When I started the project, I had 13 X 1500 watt amplifier
pallets. As of August 31 all 13 amplifiers are completed and will soon
be on the way to their final destinations. Amplifier #13 is going to
Wyoming and should help to make Wyoming a household word among WAS
chasers. True to form, the last amplifier did not pass final inspection
as it would not deliver 1500 watts. It turns out that one of the MRF151G
FETs had one side of itself open up. This was discovered in final test.
For testing, the amplifiers are keyed and run in steady key down mode
for one minute on and one minute off sequences all while the heatsink
temperatures are monitored. I also measure the temperature of the DC FET
switch to make sure the mounting and heat sinking are proper. The last
amplifier worked great at 1000 watts, but could not deliver any more
power than about 1300 watts. Something was definitely wrong. I should
note that the heat sink temperature goes up to about 43 or 44 degrees C
while it is transmitting constantly at 1000 watts output. It seems to
top out around these temps for 1000 watts output.
The first tests on the ailing amp were to double check the BIAS
settings. In conversation with K5RUS, he mentioned that the Larcan amps
were typically set for 500 ma per side on each FET for a total of 1.0
amps per device. (The MRF-151G is a Gemini package in push pull
configuration.)The BIAS was upped to about 700 ma for digital TV use.
Russ said that the FETs can run nice and linear on SSB with BIAS set at
250 ma per side. The result, according to K5RUS is cooler operation,
better efficiency and slightly higher drive levels. Taking his advice,
I re set all the BIAS settings on the ailing amp at 300 ma, while I was
trouble shooting the amp. All the FETs had been biased at 700 ma.
Finding the bad FET was easy. It drew no current. I pulled out the bad
FET and put in a new one and re set the BIAS. A second power test noted
62.5 dBm output or 1780 watts at full limiting.
If you want to check the BIAS on your amplifer and reduce the resting
current, there is a simple method needed to check BIAS, Connect a 50
ohm load to both the input and output connectors. Remove all of the DC
fuses and connect an ammeter to the first fuse contacts on FET #1. At
the same time, you will need to short the BIAS level on the opposite
side of the FET that you are measuring. This is needed for accurate
measurements. You must short out the unused side during BIAS
measurement. There is a 10 ohm resistor that goes from the BIAS pot to
the actual FET gate lead. There are two BIAS pots per FET. One is used
for each side of the FET package. You should short the BIAS to ground at
that point where the pot and the 100 ohm resistor are connected
together. This will allow a proper reading of BIAS current on the
opposite side. A counter clockwise rotation of the pot will reduce the
BIAS current. When one side is set, move the short to the opposite side
and move the DC current measuring leads as well. It is a simple
process. You can go through all six FETs in about 15 minutes. It helps
if you make a short clip lead for shorting out the BIAS.
I have three 500 watt pallets left that will get converted next, but I
am taking a few weeks break. Two of the amplifiers are spoken for. The
third amplifier is something special and is not like any others. It has
a CATV hybrid amplifier driving a single MRF-148 FET that then drives
the two MRF-151G FETs for the 500 wattts output. Input drive is only 50
milliwatts or +17 dBm. This is a natural amp to use with a low power
transverter. I am in the process of building up a W1GHZ 222 MHz
transverter. Paul reminded me that his transverter is a cheaper
alternative to the other USA comercially available transverters. It
requires that you can work with surface mount techniques. WW1M has
already made up a W1GHZ transverter and is planning a better 222 setup.
He had no trouble with assembling the transverter. You can get all the
new components for Paul's transverter from Mouser Electronics. I just
got my order today. I think it cost me about $80 but it needs an LO. You
can buy 194 MHz crystals from Q5 Signal. You can also spring for a Digi
LO at a bit over $100. So add up everything and a low power transverter
will cost you about $200 plus your time. Contact W1GHZ for more info. Of
course, some people are not comfortable with surface mount assembly. In
the words of my old pal, K1GW, Glenn, his assembly tools revolved around
a blow torch and boxing gloves.
Anyway, when my W1GHZ transverter is finished it will be paired up with
the low power drive 500 watt Larcan amplifier for a very neat semi Big
Gun setup! Then we have to figure out where it should go.
73
Dave K1WHS
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