Hi guys,
I have got faulty Shure BLX4 wireless system in for repair. The wireless microphone does not 'lock' to the receiver and hence no audio (the ready LED is not illuminated).
I have checked the wireless microphone whether it transmits. I was able to see and hear transmission using my SDR.So I'm assuming the issue is with the receiver.
I couldn't find schematic for this model and have checked out schematics of other models which are from Shure and they are not similar.
I'm pretty new to RF electronics though its something I am really interested in.
I was hoping to get some idea on how to go on troubleshooting with this.
Thanks in advance

 

Aside from some frequency-related things, RF troubleshooting is no different from audio troubleshooting at it’s foundation.

The usual methods of visual inspection, and signal injection & tracing, are the first line approach to any circuit.

Do you have an RF signal generator that can produce a signal of the right wavelength? Your SDR receiver should work to follow the injected signal.

Of course, check the power circuits first and make sure that the \( \mathsf{V_{SUPPLY}} \) is presents t the right voltage and goes where it needs to, and that \( \mathsf{V_{CC}} \) for the various chips seems right.

Given that you don’t see a status light indication, it is probably not a problem in the audio path.

Don’t neglect a very carefully visual inspection before anything else, and you can also try using freeze spray (or an inverted "canned air”) to see if any of the larger ICs responds to the thermal change. Just be sure that if you use the canned air expedient it isn’t one with bitterent in it—you are unlikely to make that mistake twice.

Just be methodical and follow the signal path as best you can. You may need to research some datasheets to get pinouts because that PCB looks pretty dense and footing through multilayers could be painful without some clue about where things likely go.

 

Depending on the test gear you have at hand, I would check to see if the transmitter frequency is what it is supposed to be, as it is possible the transmitter has gone a drift from its correct spot frequency. If that checks OK and it is on the right frequency then the receiver is the next thing.
Check power, most likely there will be a PLL chip, look through its datasheet, it will have some kind of lock detect output pin that confirms whether the local oscillator is locked on the correct frequency. Just got to go through the normal detective work when fault finding any electronic boards. Chips and their datasheets will help, knowing what each block does helps. You could look at other radio mic set diagrams, they still may help even if not the same unit.

 

Thanks guys for the insight. I have checked the Vsupply and Vcc at the chips and at every point it checks out, and I didn't measure any sort of ripple either. However I did measure a bit of ripple on two electrolytic caps inside the shielded box not sure if there on the VCC (yet to figure out what the are).
Even though I checked whether the mic was transmitting I didn't check if it was the right frequency, just found the frequency chart. Will double check that.
The PLL IC used in the unit is ADF4212L and there is another IC 1145M (Quadrature detector) I'm not really sure how this IC is suppose to work, have gone through the datasheet. I figure its something to do with IF signal.
Lastly one other thing I have noticed was the spot under the shielded box gets a bit warm (not overly hot), I checked if any specific component was getting warm and didn't find any. Is that normal?

Thanks again for the help.

 

See page 14 of the datasheet for PLL lock pin. The hot spot must have a component in its vicinity which should be even hotter than the spot, you need to find out what is getting hot and if it is normal or not. check both top and bottom side of the PCB.

 

If you spray the pcb with freezer spray, around the hot spot it will highlight what is causing it.