alesis BRC repair

[cncjerry]

Hello people,
I have an Alesis BRC, short for " Big Remote Control".  Issue is it doesn't want to lock to the 48Khz word clock coming from digital devices.  I checked the word clock output of the BRC by itself and the 48.000Khz is sitting at 48.026Khz.  This is because the FPGA crystal that should be 20.000Mhz is at 20.010830Mhz.  The FPGA generates a 250hz signal to control the VCO and the 250hz is actually 250.125hz.  The 250hz PCM signal biases an FET that controls the VCO.  I guess I could search for the cap that it uses but no schematics.  I know it uses an MC108 in the VCO.

My options are to replace the crystal, I guess, or try to pull it down to 20.000Mhz.  I tried adding 10pf to the crystal loading, it uses two 10pf today, and it brought it down to 48.020Khz, so I guess I would have to add quite a bit of capacitance to bring it down to 48.000khz.  Then the risk is it won't start.  I have more accurate ways to generate 20Mhz and was thinking of using a DDS but that seems like overkill.  There are other options.

It's a FOX crystal installed, marked 20.000Mhz but it drifted over the years, I guess.  The reason I'm not jumping at just replacing it is because I would have to purchase a few and make a test setup.

Thoughts?

Thanks,

Jerry

[fzabkar]

I guess one reason for the increase in frequency could be that a "dag" may have broken off the quartz over time, thereby decreasing its mass.

[cncjerry]

I ordered a batch of crystals and will find the closest.  I also found a way to run the word clock to the sending recorder and that then syncs track 1, the one with the noise.  I don't get it, as usually the sending unit would send the word clock.  But I guess the clock being so far off it won't sync to any clock so I have to use the BRC clock until the crystals get here.

It's fun setting up this old gear.  Can't believe all the great music that was recorded this way and with the analog equipment before it.

Jerry