Your driving the transmitter directly from
your repeater controller..? ... using the
back plane method?
Sounds like your repeater controller tx output
doesn't have enough drive power (capacity) to
directly drive the tx channel element IDC
(channel element modulator circuit).
Cheers,
skipp
www.radiowrench.com
> "Army Curtis" <[EMAIL PROTECTED]> wrote:
> 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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