[VHFcontesting] The 222 MHz Amplifier Project Milestone

[David Olean]

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