My newly acquired Exposure 7 pre amp just blew up after I connected it to a power supply
On each pre amp board, I can only see a burnt out resistor marked "R77". One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground. It seems to me that the power supply ground is "floated" at a certain voltage about the signal ground.
I would be very grateful if someone who has a Exposure 7 pre amp can tell me the value of the R77 resistor. If you have a circuit diagram of the pre amp, I would also like to have a copy. Many thanks.
Hon
On each pre amp board, I can only see a burnt out resistor marked "R77". One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground. It seems to me that the power supply ground is "floated" at a certain voltage about the signal ground.
An externally hosted image should be here but it was not working when we last tested it.
I would be very grateful if someone who has a Exposure 7 pre amp can tell me the value of the R77 resistor. If you have a circuit diagram of the pre amp, I would also like to have a copy. Many thanks.
Hon
An externally hosted image should be here but it was not working when we last tested it.
Hugo
The pre amp is powered by a +/-24V power supply in a separate box. I found a fault on the +24V rail of the psu. A tantulum decoupling cap was faulty and it shorted the +ve output of the LM317 regulator to the 0V rail which is floating. I have fixed the power supply.
The R77 resistor connects the psu 0V to the true ground of the pre amp. The true ground is connected to the common earth for all the signals and chassis. My guess is the +24V rail was shorted to the resistor and a large current flows through the resistor to ground. I am no expert, just my pure guess.
Hon
The pre amp is powered by a +/-24V power supply in a separate box. I found a fault on the +24V rail of the psu. A tantulum decoupling cap was faulty and it shorted the +ve output of the LM317 regulator to the 0V rail which is floating. I have fixed the power supply.
The R77 resistor connects the psu 0V to the true ground of the pre amp. The true ground is connected to the common earth for all the signals and chassis. My guess is the +24V rail was shorted to the resistor and a large current flows through the resistor to ground. I am no expert, just my pure guess.
Hon
I got the preamp working at the end after working on it intermittenly for nearly 6 days! Tony Brady of Exposure replied to my query this morning about the resistor and the repair necessary to bring the 7 pre amp back to life. He is a great guy.
The R77 resistor should be 27 ohm which makes sense. Too high a value will create a large potential difference between the power supply and signal grounds. Now the resistor measures 0.68V which means that the preamp only draws about 25mA idle current.
Tony told me that the 10uF tantalum caps tend to short circuit which is exactly what I found, 5 out of 8 tantalum caps for the left and right boards were shorted. He suggested to replace them with electrolytic ones.
Thanks for the help from everyone.
Hon
Tony Brady of Exposure replied to my query this morning about the resistor and the repair necessary to bring the 7 pre amp back to life. He is a great guy.The R77 resistor should be 27 ohm which makes sense. Too high a value will create a large potential difference between the power supply and signal grounds. Now the resistor measures 0.68V which means that the preamp only draws about 25mA idle current.
Tony told me that the 10uF tantalum caps tend to short circuit which is exactly what I found, 5 out of 8 tantalum caps for the left and right boards were shorted. He suggested to replace them with electrolytic ones.
Thanks for the help from everyone.
Hon
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