Thanks for this nice write up Dave. A copy has been saved to my Larcan files
for future reference.
de K3SK
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-----Original Message-----
From: 222Activity@groups.io <222Activity@groups.io> On Behalf Of David Olean
Sent: Friday, September 1, 2023 12:18 AM
To: vhfcontesting@contesting.com; 222 >> 222Activity@groups.io
<222Activity@groups.io>; NEWSVHF@mailman.qth.net
Subject: [222Activity] The 222 MHz Amplifier Project Milestone
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 measur
ing 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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