[K3PZN-List] 2M Repeater

[Lawrence J Solarczyk]

Greetings,

Wednesday night, Steve N3SB, Bill N3XLY, and I (WX3F) went up to work on
the 2 meter repeater.  After verifying that the spurs existed, we
separated the power amplifier from the exciter.  Using Steve's HT, we
confirmed that the power amplifier was clean and not the cause for spurs.
 We then looked at the exciter output.  After adjusting the power level
of the exciter to about 400 mW, we looked for spurious emissions.  We did
not see any spurs.  Since we had the exciter output terminated into the
station monitor, which is a good 50 ohm resistive load, we concluded that
the problem may be a result of the interface between the coupling of the
exciter to the power amplifier.  It appeared that the conjugate match on
the input of the power amplifier may not have been a perfect 50 ohm
resistive load.  So we reconnected the exciter to the power amplifier,
and looking at the configuration of the exciter from the repeater manual,
we noticed that there was a variable capacitor on the output stage of the
exciter.  Adjusting this capacitor maybe an eight of a turn eliminated
the spurious signals.  After several transmit attempts to see if the
repeater would again spur, a few times hold the transmitter on for
several minutes to having the fans on the power amplifier start up, we
verified that the repeater was clean.

In our testing, we did notice that there was no local microphone or
repeated audio modulating the transmitter. Although, the PL tone was
modulating the signal.  After going around and around in the schematics
for the repeater trying to find out where the audio was being lost, we
concluded that audio loss was at the input of the exciter module itself. 
The reason for this conclusion came from noticing that the local
microphone audio and the repeater audio entered the exciter module on a
different connector pin than the PL tone audio.  So, we were thinking
that maybe a transistor failed in the exciter.  But, while we were poking
around the back plane circuit board on the repeater, we would get an
indication that there may be a loose connection.  I then pulled the
exciter module out of its holder and reinserted it taking care to insure
that it was well seated in the connector.  Success, all audio returned.

Before closing up the repeater, we verified that the repeated audio level
did not exceed 5 kHz deviation and power output at about 100 watts with
about 80 watts out of the duplexer to the antenna cable.

Before we left the building, Steve mentioned that the power output level
of the 70 cm repeater may be low.  We found that the output of the 70 cm
repeater IS low.  The output is less than 500 mW.  We popped the cover
off of the power amplifier and notice a burn mark.  Checking the circuit
board, we found a tantalum capacitor blew its top.  That repair will be
for another day (another round-tuit).  The 70 cm repeater may be used,
just know that its signal and range is limited.

For the next couple of weeks, I would like all of you that have the
ability to monitor the output of 2 meter repeater for spurious emissions
to do so.  Until I receive any more notices that the repeater is
spurring, I declare that the repeater is available for regular use.

Thanks to everyone who helped in anyway to eliminate the spurs from the 2
meter repeater.

73,
Larry WX3F