Another R&S CMU200 repair thread.

[CJay]

So, my bargain R&S CMU200 is working for the most part but is doing some odd things.

On the face of it it seems to work OK, I can see signals where I expect them to be on the spectrum display (power level I'm not sure about yet, I'm still getting used to driving it but see below), the B41 audio analyser would appear to be working as well which is nice.

But!

The RF signal generator does not work as I expect it to, there is no carrier if I don't turn on modulation,  and none in AM, SSB or DSB modes regardless of modulation, even with FM modulation enabled and 1KHz tone/1KHz deviation (or any combination) I get a silent carrier on a separate receiver.

Setting FM modulation and coupling the output to any input on the CMU I can measure the carrier generated to within 1.5dB in the power meter mode, I am happy to accept that error is in my cables/adapters so I suspect the Front end and RXTX boards are OK which is quite a relief.

Running the self tests gives me numerous failures with the internal RF path and RXTX board, relating to no measured carrier.

I have a gut feeling this is due to the no carrier issue in anything other than FM modulation mode as it seems logical to test using an unmodulated carrier.

FM modulation test of course also fails.

From reading the service manual I think the plug in TXDSP1/AUC1 modules within the Digital board (DIG?) are possibly to blame, does this sound correct?

Also, does anyone have a good picture of the RF front end board as seen from the bottom of the instrument? There are four solder pads on the exposed PCB which look to have had a component or connector knocked off of them, next to a small aluminium electrolytic capacitor.

[CJay]

OK, so, fixed.

it now passes FM cal, Internal RF Loop Path 1 and al the other tests I can throw at it without having the calibration fixtures.

I've got carrier in all modulation modes now and with modulation off.

I had to clean the contacts and reseat the TXDSP and AUC boards in the Dig module, once I'd done that it requested FM calibration and away it went, fully working, I just need to lay my hands on some decent cables to run the remaining self tests as some are falling outside tolerance but count me one happy user.