MTR2000 Troubleshooting Question

[LakeMan2]

I have a UHF 100W MTR2000 that is used as a GMRS repeater at 40W in a remote rural area. Had it now for about 5 years and it was working well until recently. It developed an issue where the audio was cut off and choppy. You could hear the beginning of a word (first couple of letters maybe) then nothing, then part of a third or fourth word etc. Basically it was so choppy that it was unintelligible.

Hooked it up to a service monitor. Output power is good right around 40W, carrier is on freq and looks clean. TX FM modulation looks to be for crap. Voice modulation looked odd on the scope so put a 1kHz audio tone @ -113dB into the RX. I have the station speaker. A good tone could be heard from the speaker indicating that the RX and SCM incoming audio processing was good. On the TX output though, the monitor showed that only the top half of the sine wave was being transmitted, nothing below the midpoint (or what should have been the midpoint). For the heck of it I hooked up a hand held to the service monitor RF In and put a 1kHz tone through the HH and got a perfect sine wave on the monitor. The MTR2000 TX only shows the top half of the 1KHz audio sine wave.

My guess is it is the exciter, but looking at the functional block diagrams, it could be the VCO and Ref Mod Audio out of the SCM I guess (even though the audio in and speaker out processing SCM look good). I have not been able to figure out how to narrow it down yet. I don't know if one or the other would be more likely to be cutting off the lower half of the FM modulation. Oh, and the RSS self tests did not show anything. I hate to buy an exciter and a SCM, as it is likely only one of those.

Has anyone seen anything like this or have any thoughts?


Thanks

 

[merlin]

Sounds like an op amp failure in the phase modulator. IE: one side of the op amp output is dead or biased beyond its range.
Had an old Kenwood radio went the same way.

 

[Tech21]

Put an external reference on the bnc connector right behind the front face plate. That is the proper way to retune the FM on an MTR repeater.

 

[LakeMan2]

Put an external reference on the bnc connector right behind the front face plate. That is the proper way to retune the FM on an MTR repeater.

I thought that was just for the Auto-Calibrate Alignment Procedure of the reference oscillator. After seeing the problem, I actually did a manual calibration of the reference oscillator per the manual (Station Alignment -> Reference Oscillator -> Manual Calibration). While it did bring the carrier freq on target, it did not improve anything relative to the audio modulation (still missing the bottom half of all FM audio modulation).

 

[LakeMan2]

Went ahead and tried an external reference alignment. No difference. Audio modulation still missing the bottom half. Pretty sure it is hardware, probably as Merlin suggested op amp failure in the phase modulator. I believe that would be the exciter in that case.