CMU200 low TX modulation.

[g4wyz6]

Hello

I've just brought one a CCU200 and found that the TX analogue modulation is very low, if I set the CMU200 for 28.3kHz FM deviation monitoring using a Marconi 2505 Modulation Meter the deviation is only 5kHz  noisy but the modulation frequency is correct. If I switch the modulation of and look at the carrier its clean.

This CCU does not have the CCU-B41 audio generator option, but I believe the analog modulation is derived in the Digital board?

Is the deviation low?
As I maybe chasing my tail.

Any help would be appreciated

Mike

[g4wyz6]

Well I've discovered the CMU200 had more self-tests then I thought - used the scroll key!

Narrowed the fault down to the CMU-B21, MC68K module.

Mike

[g4wyz6]

The MC68K module (DSP) the data was reloaded and even tried the previous firmware.

Eventually thought this maybe the wrong branch, it turns out the fault was on the A7 'ADC MODULE1, DDC MODULE1, TXDSP MODULE1, AUC MODULE1 in the DIGITAL BOARD'.
The fine dust had turned into a dark grey glu as the previous owner had kept it in a cupboard for a number of years, but there was surface corrosion on the cover shells, more like a damp garage.


FM deviation is now OK, but low level deviation i.e. 5kHz is 6kHz (20% one of specification), tried re-calibration but no effect. Does anyone have any info on this module please.

73's

Mike G4WYZ

[RF_fanatic]

The root cause is most likely a defective: 1100.2500.02 board (add on board in the Dig unit, the board with the large heat conductive tape)

If the deviation or signal waveforms are distorted than the cause for the problem is in 90% this board

You can easy test this by :

Setting the generator at for instance 1200 Mhz
Set the analyzer at 1200Mhz
Set span to 1Mhz
And than modulate in various modes. DSB, SSB, AM, FM (and select deviation) FM Stereo (if K14 installed)

It must show the typical waveform responses, often the SSB and DSB are distorted, for instance a DSB response in a SSB mode. Or too low deviation

Sometimes it can also be detected during calibration if it fails to use the full 500khz