Exposure 7 pre amp help - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 24th March 2006, 07:22 PM   #1
diyAudio Member
 
Join Date: Jun 2004
Location: Hants
Default 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.

Click the image to open in full size.

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

Click the image to open in full size.
  Reply With Quote
Old 24th March 2006, 07:45 PM   #2
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
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
  Reply With Quote
Old 24th March 2006, 08:02 PM   #3
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
  Reply With Quote
Old 24th March 2006, 08:19 PM   #4
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
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
  Reply With Quote
Old 24th March 2006, 08:33 PM   #5
Pasul is offline Pasul  Czech Republic
diyAudio Member
 
Pasul's Avatar
 
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.
  Reply With Quote
Old 24th March 2006, 08:34 PM   #6
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
  Reply With Quote
Old 24th March 2006, 08:37 PM   #7
diyAudio Member
 
Join Date: Jun 2004
Location: Hants
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
  Reply With Quote
Old 28th March 2006, 08:25 AM   #8
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help needed with PCB UV Exposure box zeus_threat Everything Else 12 7th October 2008 04:50 PM
exposure XXI gio_b Solid State 23 15th December 2007 10:14 AM
DIY UV Exposure box with UV LEDs Nordic Parts 54 1st October 2007 09:58 PM
EXPOSURE X Integrated Amp Help arupg Solid State 3 21st February 2007 09:41 PM


New To Site? Need Help?

All times are GMT. The time now is 11:15 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2