Originally Posted by Don Solaris ➡️

Roland S-550 fixed!!

Ok i got this fixed. After some more inspection and running oscilloscope through complete S-550 signal path, it turned out that what was "distorting" were my outputs. Plus volume was low as ****. This instantly rang some bells, but i wanted to be sure. So after inspecting the A/D section and confirming the problem was not there i've moved do DAC area. It looked ok, so i've moved to output amps and filters. Clean too. Then i've reached the last stage, which are output transistors. I assume these automatically cut the signal once the voltage is out, to prevent that horrid audio click during power off stage. Up until that point audio was perfectly clean, but right after, i would get distortion once the volume was put above 50%. So i replaced all nine of them and the unit now works perfectly. I get 2Vpp as described in service manual and no distortion whatsoever. Neither the volume is low. I can't believe i've fixed this thing.



One thing i should point out, these output transistors are common thing in samplers (and other machines) and are first to be damaged if the equipment is inappropriately connected! The reason is simple - these are first active components in the path between the incoming voltage and the ground. Most common cause is connecting it to non grounded equipment or mixer, because S-550 is not grounded neither. Since some of these machines feature voltage protection through the exterior case, that means the case will be at some voltage, ie 110V AC, and these voltages can easily damage transistors that are specified for 50 or so volts.



I had very similar situation with Mackie CR1604 and RS-7000 of which none were grounded. I've killed one output transistor in the RS and one input op amp in Mackie just by connecting them together! Both were using step down converters, none were grounded, one was "floating" on 110/120V. So, i've learned my lesson there. And since then i've used an additional cable that provides grounding to both devices. Always ground your equipment, particularly if you use step up/down voltage converters! At this point i'm now totally removing step converters from my place! I will simply have transformers re-winded (thanks to Roginator for the tip!) which i just did for my MKS-80. Mackie next!!