ma 1200 resurection?

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#1 fixitguy1ca

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Posted 15 July 2006 - 10:58 AM

I have an older  macro tech 1200 amp that smoked one channel. The other one is fine. I've replaced the outputs, 4 diodes that were shorted, the pre drivers and a cap and 0 ohm resistor that was heat damaged. the channel now powers up, the odep light comes on and it shows signal as normal. When you connect a load to the amp the supply rails power goes from +42, -42vdc to -5 +79vdc and the green light (signal) indicates distortion. There is output at the speaker jack, but it sounds like only half the amp is working(clipped, snappy, distorted) I put 6 hours in to this thing last night and am now ready to use the thing as an anchor. Please someone.....anyone..... save this amp from as brutal a death as I can muster.

#2 KGring


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Posted 17 July 2006 - 09:03 AM

You are almost there. The pre drive portion of that part of the output circuitry is located on the main board. Assuming the repair is channel 1, ( replace the 100 numbers with 200 for channel 2) Q115 And Q116 are the predrivers for the output. Resistors R147 and R149  will be wither 100 or 200 ohm depending on the vintage. If either of these are open, replace both transistors Q115 and Q116.
Kevin B. Gring
Crown Audio