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Use care when working with service manuals

December 19th, 2022, 1:21 pm

I have been helping one of the members in PAAG with a recalcitrant Crown Macro Reference amp. I have Macro and Studio reference versions in my system so I thought it would be a good chance to practice in case one of mine starts to work up. Like any good technician, I started with the service manual provided by Crown. When I got to the ODEP circuit which is a device that predicts the stress on the output devices and runs the fan as well as folding back the drive if the devices are in the area of stress, the manual gave instructions on setting the null for that section.

Following the instructions I could not get the voltages specified on the pins of the differential amplifiers. To make a long story short, that section had bad part designations, bad pin designations and pot numbers that did not relate to the devices they were supposed to control. I got the hint when I looked at two voltage values that should result when setting the adjustment pots and realized that it was impossible to get those readings. What they specified was essentially a 22 volt drop from emitter to base on a PNP transistor. That junction acts like a forward biased diode and there is no way to get that level of voltage drop if the junction still exists. I figured out what the problem was and then was able to get the correct voltages at the appropriate points. I spoke to one of the repair techs at Crown who agreed and basically said the engineers who wrote that section must have been smoking green stuff. I corrected the section and sent it off to the technician, but I'm sure they will not re-do a manual for a 30 year old product. So, just because a service manual comes directly from the manufacturer. don't assume everything is correct.

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