SOUND DEVICES SD 301 MIXER.

[Robert Hart]

Some years back, I bought a used SD301. It turned out to have had a splash of seawater onto the display face it some tie in its life and some had crept into to its gizzards. There developed some really bad crackling self-noise after several months. The value and the possible cost of shipping and repair did not stack up so I tried fixing it myself. I spent much time, cleaning out every trace of whatever contaminants had got in and created fine crystal patches here and there. However that did not fix the problem.

Last night, I hooked it out of its storage and had another shot at it, the usual trickery of chill spraying and poking systematically across the PCBs with insulated probes to provoke any dry joints. The result was the same, no cure.

I noticed that after several years of storage, the sliding switches were a bit tight. That happens over time. If you keeping working tight sliding switches they may eventually burst the back out themselves. During my previous examinations, I tried the switches, thinking they would be the culprits. Whilst you get an initial pop when one is switched which is normal, the crackling sound seemed unrelated to any movement of the switches. It was stronger when "mic" was selected on any channel and phantom power was on.

This time round, I used a plastic straw to feed some WD40 into the switch cases and then some canned air through a straw to blast the WD40 into the cases and the excess off into a tissue wad. I exercised the switches then put another dose of WD40 and canned air through. That seems to have fixed the problem.

It seems that with tie and use and some contamination, the insulation between contacts in the switches may haver developed resistive conductive tracks. Small current leakages would certainly aggravate high gain preamps like those in the SD301. Time will tell if this is a peranent fix. However it is a good lead to eventually replacing those switches if the issue comes back.

The build of the SD301 is impressive. the PCBs are coated with a clear flexible sealant and this is no doubt why the mixer survived.

[Robert Hart]

For those who have a curiosity about the inner workings of these things, I managed to source replacement LEDs, rebuilt the display PCB and was well pleased with my work.

Now here's the go. I had not studied the 20 channel ribbon cable which ties the display PCB to the lower main board. It looked fine but on a really close inspection of the ends with a strong magnifier, I discovered that the conductive strips were loose on both ends of the cable. When inserted into the socket, the strips become misaligned or even fold. Once disbonded from the plastic ribbon they cannot be rebonded. An even closer look revealed that the piece was a wider cable that had been cut narrower with scissors.

The mixer has obviously had visitations within before I came along and violated its innards again. So much for going faithbased on ribbon cables. They don't have to look broken to be broken. I may not have needed to take out all the "failed" LEDs though there were some which were definitely done for. I am now waiting for a new ribbon cable to thread its way from the vendor through the Christmas shipping rush. Still it was a good opportunity to do a little more cleaning of debris that I missed first time around.