My Allen MOS-603C (MOS-2 series, manufacture date not yet determined) is playing through its original S100 amps. One of the amps has developed a consistent hiss, while the other three are as quiet as can be. I have removed the line input and the hiss remains. All amps are properly grounded. Can anyone offer thoughts or suggestions about the source of said hiss?</p>
I'd check the transistors in the pre-amp stage of the S-100, ahead of the volume control. Does the hiss go away or decrease if you turn the volume knob down on this amp?</P>
Because Allen used a low-level signal on analog and MOS organs (to be compatible with the photo-cell expression system), they had to have a very high gain pre-amp stage in the amp. This opens the door to an aging transistor become a white noise source.</P>
John</P>
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John
---------- *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!
I checked, and the hiss does not change with adjustment of the gain on the amp.</p>
While I'm at it: What can you (or anybody) tell me about the RCA jacks labeled "EXP -- IN -- IN" on the amp?</p>
I'm planning to rearrange all the cable layout in my little 11x20 apartment living/dining room and getting the organ line cables isolated in some PVC traces along the baseboard. That should clear up some interference from various AC sources.</p>
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