Well gang, I have a real interesting problem for you that's about to
drive me nuts.
I have a MSR-2000 repeater that I'm putting into the 2M ham band. This
is one of the radios removed from service by the Ontario Provincial
Police, so it is a Canadian Motorola VHF low split, originally
transmitting at 141.xxx.
Following the suggestions on the Repeater-Builder web site, I have
converted it to a ham style controller (CAT-700). The radio tunes up per
the book, with all meter readings very nominal, and it makes full power
(100 watts) easily.
Here's the issue... it will not deviate the transmitter more than about
2.7 KHz using a 1 KHz tone before it starts severely distorting. The
problem appears to be in the exciter, which is a TLD9241A. There is a
sticker on the exciter shelf that says 0.260 volts = 5 KHz deviation.
Anytime I put more than about 0.140 volts into the exciter, it starts to
distort. I am using an IFR-1200S to send and receive the 1KHz tone, and
I'm looking at the wave form of the transmitted signal on the 1200S. I
have verified the 1200S is clean by looking at its output on another
service monitor, and it is very clean to beyond 6 KHz deviation. I have
looked at the audio going into the exciter on a scope, and it is very
clean to up around 0.400 volts, so I would say that the receiver,
controller audio, and transmit audio up to the exciter input is not the
problem.
I tried changing the exciter to another identical board, same problem. I
changed the channel element to a known good element, same problem. I
have changed the audio input transistor on the exciter board (Q501),
same problem. I have tried to adjust the IDC on the channel element.
While it does change the deviation, it has no effect on the distortion.
Here is some additional info I just ran down to the shop to check.
Sending a 5 KHz deviation at 1 KHz signal into the IFR1200S from another
service monitor shows no distortion, so no problem there. Putting an
audio generator right on the audio input to the exciter shows the same
issue, but here is where it gets interesting. Changing the audio
frequency, I am seeing a definite pre-emphasis network somewhere in the
exciter, as a tone of 1 KHz gives about 2.7 KHz deviation, but a tone of
3.2 KHz gives right at 5 KHz deviation, with NO distortion.
So, here's my question. I always thought you set deviation on an FM
transmitter using a 1 KHz tone, setting a maximum deviation of about 4.5
KHz. I can see with this exciter that doing that will result in much
more than 5 KHz deviation at frequencies above 1 KHz. Yet the Motorola
book calls for setting the deviation to 5 KHz using a 1 KHz tone.
What am I missing here?
Army - AE5P
Nacogdoches, the oldest town in Texas
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