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Beginners Road To Repair
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don@cascadefellowship.org:
Hi everyone!  After watching some videos on the YouTube channel I feel this may be the right place to introduce myself and ask my question.  Here is my question:

What videos should I watch (and in what order) so that I may gain the knowledge needed to repair the piece of gear shown in this picture?

Now some background and additional information.  I have a beginners level of knowledge of electronics as it relates to audio engineering.  I have a DMM and an O-Scope as well as a good soldering iron.  The piece of equipment is a BBE Acoustimax which is an active DI box used for guitars and bass.  A DI box such as this does two things.  1. Outputs a mic level signal that an audio console can accept. 2. Allows you to change how your guitar sounds by using some effects.  I happen to really like this unit and was bummed when it stopped working.  I thought it would serve as a good project that I could use to further my knowledge of electronics repair, which can be really handy for an audio engineer.

The problems that I'm seeing is that the DPDT switches do not illuminate the green LED's next to each switch (the one on the right is partially obscured by the top of the switch) and the unit does not pass any signal from input to any output.

Here's what I've already done to help solve the problem.  I've tested the power supply to the unit (both on it's own and under load) and it seems to be working.  I've verified that the unit switches on by observing the lit red LED you see in the picture.  I've removed, tested and reinstalled the two capacitors next to the incoming power supply, although I only tested them with my DMM and not an ESR.  I've visually inspected the boards and found no obvious signs of damage (like burnt components or misshapen components).  I've hooked up the input and output and verified that the unit is not passing signal.

Here's what I would like to learn and some more supporting questions.  How do I learn to read the traces on a PCB board to get a general idea of how current flows from one component to the next and how do I test that along the way in order to determine where the problem is? How do I learn to test the components? What components fail more frequently and which do you usually check first?  How does one usually approach repairing a PCB board of this type when there are no visual signs of damage?

I know this is a lot and I hope I set the problem up in a simple and big picture way.  Thanks!

Don
Dubbie:
Is the input AC or DC?

It's hard to see anything on the low resolution photograph.
if it is AC, have you checked the negative rail as well as the positive?
don@cascadefellowship.org:
It's 12VAC.  I'm sorry about the small picture.  You should be able to click on it to view it larger in your browser. I had to downsize it to get it under the size limit.  If you'd like I can send you the original, it's quite larger.  I have not checked the negative rail.  How would you identify that and test it?  I have tested the power on the switch terminals but that's as far as I've gotten.
BrianHG:
With 3 diodes at the input, 1 facing up and 2 down, this preamp will only work properly with an adapter which has an AC output.  If he is using a DC out adapter, one of the power rails wont get + or - power.
don@cascadefellowship.org:

--- Quote from: BrianHG on June 26, 2018, 01:06:31 am ---With 3 diodes at the input, 1 facing up and 2 down, this preamp will only work properly with an adapter which has an AC output.  If he is using a DC out adapter, one of the power rails wont get + or - power.

--- End quote ---

I'm using the proper 12VAC power supply for the unit.
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