ManhattanUp
Active Member
Today I went through and measured all of the resistors forward of the pre-driver transistors on both Power Output PCBs---just to eliminate them as a cause for biasing problems---and discovered something rather odd. Diodes D215, D216, D217 and D218 (070-047) have each had a 100K resistor (not shown in the schematic) added in parallel to it---the resistor has actually taken the place of the diode in the PCB and diode rides piggyback on it. In later revisions of the PCB (045-144 etc) this change would have been one of several made to this section of the circuit. However, on my PCB this is the ONLY change that's been made.
I'm thinking that either McIntosh issued a service bulletin that called only for this one change or someone went in and tried to upgrade the board (from 044-626) without making any of the other changes. I'd appreciate any insights or opinions that folks might have about the matter. Thanks!
P.S. For comparison, below is a link to photo of a later revision of the board where you can see the 100K resistor formally added to the circuit:
http://home.comcast.net/~mc_audio/MC2205_Love_RIGHT_installed.jpg
I'm thinking that either McIntosh issued a service bulletin that called only for this one change or someone went in and tried to upgrade the board (from 044-626) without making any of the other changes. I'd appreciate any insights or opinions that folks might have about the matter. Thanks!
P.S. For comparison, below is a link to photo of a later revision of the board where you can see the 100K resistor formally added to the circuit:
http://home.comcast.net/~mc_audio/MC2205_Love_RIGHT_installed.jpg
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