Exposure 7 pre amp help

hifienthusiast · 24th March 2006, 07:22 PM

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

Netlist · 24th March 2006, 07:45 PM

There's no indication from the picture that R77 is burnt.
You should be able to read the colors, it's hard to see from the picture. If you don't know the colour code, google will help you; if not I'll gladly post them later on.

/Hugo

hifienthusiast · 24th March 2006, 08:02 PM

Hugo

The photo was taken before the pre amp blew up. I cannot read the colour of the bands even if I zoom into the resistor. Now the resistor is burnt out and I can measure 3.3K but I doubt it is the original value.

Thanks anyway.

Hon

Netlist · 24th March 2006, 08:19 PM

I'm not sure what to advise. I have seen values of 10ohm to 1Meg in there.
Try 3.3k is what I would do and see what happens. Sure there must be a reason why the resistor blew up.

/Hugo

Pasul · 24th March 2006, 08:33 PM

hifienthusiast-hi,
If you say right: "One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground" this resistor protected from ground loop.And standart value is 10 Ohm.

hifienthusiast · 24th March 2006, 08:34 PM

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

hifienthusiast · 24th March 2006, 08:37 PM

Quote:

Originally posted by Pasul
hifienthusiast-hi,
If you say right: "One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground" this resistor protected from ground loop.And standart value is 10 Ohm.

Pasul

You are absolutely correct about the 0V connected to the signal ground via the R77 resistor. I will try a 10 ohm or 3.3K resistor and see how it goes.

Many thanks.

Hon

hifienthusiast · 28th March 2006, 08:25 AM

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

24th March 2006, 07:22 PM	#1
hifienthusiast diyAudio Member Join Date: Jun 2004 Location: Hants	Exposure 7 pre amp help 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

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24th March 2006, 07:45 PM	#2
Netlist diyAudio Moderator Join Date: Jan 2003	There's no indication from the picture that R77 is burnt. You should be able to read the colors, it's hard to see from the picture. If you don't know the colour code, google will help you; if not I'll gladly post them later on. /Hugo

24th March 2006, 08:02 PM	#3
hifienthusiast diyAudio Member Join Date: Jun 2004 Location: Hants	Hugo The photo was taken before the pre amp blew up. I cannot read the colour of the bands even if I zoom into the resistor. Now the resistor is burnt out and I can measure 3.3K but I doubt it is the original value. Thanks anyway. Hon

24th March 2006, 08:19 PM	#4
Netlist diyAudio Moderator Join Date: Jan 2003	I'm not sure what to advise. I have seen values of 10ohm to 1Meg in there. Try 3.3k is what I would do and see what happens. Sure there must be a reason why the resistor blew up. /Hugo

24th March 2006, 08:33 PM	#5
Pasul diyAudio Member Join Date: Dec 2004 Location: Czech	hifienthusiast-hi, If you say right: "One end of R77 is connected to the 0V from the power supply and the other end is connected to the signal ground" this resistor protected from ground loop.And standart value is 10 Ohm.

24th March 2006, 08:34 PM	#6
hifienthusiast diyAudio Member Join Date: Jun 2004 Location: Hants	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

28th March 2006, 08:25 AM	#8
hifienthusiast diyAudio Member Join Date: Jun 2004 Location: Hants	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

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