Allen sys 100c making noise

[Everwalk]

Our small church has an Allen sys 100c that has started making a static, crackling noise when the keys are pressed. This happens on both manuals and the pedals with any stop. It also happens when all stops are off and one or several keys are pressed. When all stops are off, the noise seems to dissipate after pressing keys, but comes back after playing normal again. When one RCA output from the dac-4 is disconnected, the noise is not there. Switching output cables always points to the same dac-4 output as causing the problem.
History. In 2010 an Allen tech placed an M5 amplifier, am 100 preamp(?), and power supply for am-100 and dac-4. The 27v power supply was rebuilt in 2015 by a local shop and reads 27v + and -. The 15v power supply to the dac-4 reads 14.7v + and -. The MOS board was replaced in 2015 to correct another noise problem. The dac-4 is dated 1997 and I did change it with one thought to be good that I got while troubleshooting the MOS board problem. The noise is exactly the same with this dac-4.
Thanks for any help you may offer.

[jbird604]

Since you've done great troubleshooting and eliminated both the DAC and the audio system, that leaves nothing but the MOS board to be the source. Everything else in the system simply supports the MOS, so it must be the source of the noise.

However, it might not be the FAULT of the MOS board. The MOS is very sensitive to voltage, and if the -27 volt output of the power supply is not pretty close to perfect, the MOS board can send out noise. So I'd use a meter to check the -27 (and check the other voltages too while you're at it). There should be a knob or shaft to regulate each voltage unless the power supply has been replaced by a more modern one without the regulators. Use the knob to set the voltage at -27 or at least within a tenth of a volt either way, which shouldn't matter. Also switch to the AC mode on your meter and make sure there isn't more than a few hundredths volt of AC on the -27 or either of the 5 volt terminals.

Sometimes I encounter a MOS board that works best with the -27 slightly higher or lower than spec. Test yours by clipping meter leads (set for DC volts) to the ground and -27 terminals and blocking down a note with some stops drawn -- at least the Prinzipal 8 and the flute 8 in the great. Slowly turn the regulator shaft clockwise to make the voltage increase. You will find a spot (perhaps about -29 volts) where the MOS board completely breaks up or shuts down. Make a note of the breakup point. Now do the opposite -- turn the knob counterclockwise so the voltage decreases and take note of the point at which the MOS breaks up or shuts down, which might be around -25 volts. Calculate the halfway point between these two voltages and set your supply at that point, which is the optimum voltage for your MOS board.

If the MOS board is still misbehaving even after you have optimized the voltage, you probably have a bad MOS board. If so, you may be forced to find a replacement board. They are sometimes offered for not much money online, but you need to be careful to buy one with the correct CODE numbers. Otherwise you may find that some of the stops no longer make the correct sound. The correct CODE for the system 100C is B-1-4-6 unless your organ has the optional card reader, in which case you need the A-1-4-6. (The A-1-4-6 will actually work with your organ whether or not the card reader is installed. The organ will simply not use the extra chips on the "A" version of the board.)

[myorgan]

John,

Since the M5 and AM100 were replaced approximately 5-10 years ago, could it be volume switches on those power supplies? I'm also wondering about possibly deteriorating speakers. That organ would have had HC-12 speakers or earlier vintage speakers.

Just random thoughts on possibilities other than something holistic like replacing MOS boards.;-)

Michael

[Everwalk]

Further troubleshooting took me to the MOS board. By tapping on it the noise would change. I was able to isolate the tapping to one chip. I removed the board and noticed crud on the soldering of both sides. I cleaned it and then noticed a couple pins that didn't appear to be soldered. They were exactly where touching the board while playing made a difference. This is a used board I got in 2015 and appears to have been repaired sometime. This chip may have been replaced as it is whiter than the rest, and there are foil traces repaired. I was able to get a spot of solder on three pins, and now the organ sounds normal again. Time will tell.
Thank you both for your input!

[jbird604]

I'm betting that was it! Great job!