Beginners Road To Repair

[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]

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.

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

[Dubbie]

Start by looking up the datasheets for the voltage regulators. You can see what pin is supposed to be the output. Then you can check if there is a plausible voltage on there. If so, check the op-amps (google their datasheets), and make sure they have the right voltages on their power pins.

[don@cascadefellowship.org]

Start by looking up the datasheets for the voltage regulators. You can see what pin is supposed to be the output. Then you can check if there is a plausible voltage on there. If so, check the op-amps (google their datasheets), and make sure they have the right voltages on their power pins.

There's printing I can read on one of the regulators but not the other.  The one I can read says the following:

PM32AJ
LM34OT12
7812 P+

Which part of this text is used to correctly find the data sheet?  I see that the shape of the regulators is not the same.  Without the text, how can I find the information on the other regulator?

[Dubbie]

that regulator is a very common one. The number you will come to recognise is: 7812 P+

That means the 12V positive variant of the 78xx family of regulators.

The other regulator will be most likely the negative one.
It's probably a 7912 which is the negative version of the other one.

[don@cascadefellowship.org]

that regulator is a very common one. The number you will come to recognise is: 7812 P+

That means 12V positive.

The other regulator will be most likely the negative one.
It's probably a 7912 which is the negative version of the other one.

Gotcha!  I've found a datasheet for the L7812 series.  Here's a https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwj8sd2Zk_DbAhXCjlQKHeWQAFYQFjABegQIARAB&url=https%3A%2F%2Fwww.hobbytronics.co.uk%2Fdatasheets%2FL7812.pdf&usg=AOvVaw1wjXOcxDJQGNbPlMPjpT_e.  Is this what I'm looking for?

[Dubbie]

Yep.

You can see the pinout on page2.
Skimreading the datasheets for the various parts will give you lots of info and help you figure out what is doing what on the board.

[don@cascadefellowship.org]

Yep.

You can see the pinout on page2.
Skimreading the datasheets for the various parts will give you lots of info and help you figure out what is doing what on the board.

Ok.  So both regulators seem to be input, ground, output (with pins down from left to right). While testing the 7812 with my DMM (black lead on ground) I'm getting 3.4 volts on input and .08 volts on output.  On the 7912 I'm getting 16.9 volts on input and 5 volts on output.  However, this pinout doesn't jive with the 7912 data sheet which shows ground, input, output (again pins down from left to right).  I'm not sure which model the regulator without the printing is.

If I'm reading the data sheet on the 7812 correctly, I don't think it's operating up to spec. 

[Old Printer]

By putting the name of the unit into Google it first took me to their website. First thing they said was this is an all analog device, which will make troubleshooting easier. By adding "schematic" to the search phrase I wound up at a site called Freestompboxes where one of their pass-times seems to be hunting down schematics and reverse engineering pedals. You need to join to get complete access, but one poster seemed to have either the factory schematic or had reversed it himself. That would really give you a leg up. It is amazing the info that can be ferreted out with some perseverance. Good luck with your repair, it looks like a pretty straight forward circuit.

[don@cascadefellowship.org]

Update:  After looking at the other voltage regulator (the one that I thought didn't have any writing on it) with a flashlight, I was able to see some printing on it.  It's indeed a 7912 regulator.  I've got my DMM set to DC and I don't think either regulator is outputting 12 volts.  Are they both bad? Am I doing this right?

[Dubbie]

Ok, so now there are a couple of possibilities, there could be a fault with the power regulators or their supply. OR there could be a short somewhere in the rest of the circuit that is pulling the voltage right down. If would be good if you could identify a way to disconnect the power supply from the rest of the board. That will allow you to see where the problem is.

[StillTrying]

Ok.  So both regulators seem to be input, ground, output (with pins down from left to right). While testing the 7812 with my DMM (black lead on ground) I'm getting 3.4 volts on input and .08 volts on output.  On the 7912 I'm getting 16.9 volts on input and 5 volts on output.  However, this pinout doesn't jive with the 7912 data sheet which shows ground, input, output (again pins down from left to right).  I'm not sure which model the regulator without the printing is.

Some of your voltage readings seem like you might not have the DMM black on a 0V/gnd.
I'd check if the 7812 and 7912 have a pin connected directly together, that will be 0V/gnd and then find which of the 3 pins on the nearby socket is 0V/gnd, and measure ALL voltages from there.

[BrianHG]

It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps).

[don@cascadefellowship.org]

Ok, so now there are a couple of possibilities, there could be a fault with the power regulators or their supply. OR there could be a short somewhere in the rest of the circuit that is pulling the voltage right down. If would be good if you could identify a way to disconnect the power supply from the rest of the board. That will allow you to see where the problem is.

Can I do that by unplugging the power supply?  It's a barrel connector on the small pcb board.  I've also just removed the large pcb board from the small board by unplugging the ribbon cables.  Testing the regulators this way yields 3.4 volts on input and .08 volts on output of the 7812 and 17.9 volts on input and 5.3 volts on output.  These seem identical to the readings I got when both pcb boards were connected.

[don@cascadefellowship.org]

Ok.  So both regulators seem to be input, ground, output (with pins down from left to right). While testing the 7812 with my DMM (black lead on ground) I'm getting 3.4 volts on input and .08 volts on output.  On the 7912 I'm getting 16.9 volts on input and 5 volts on output.  However, this pinout doesn't jive with the 7912 data sheet which shows ground, input, output (again pins down from left to right).  I'm not sure which model the regulator without the printing is.

Some of your voltage readings seem like you might not have the DMM black on a 0V/gnd.
I'd check if the 7812 and 7912 have a pin connected directly together, that will be 0V/gnd and then find which of the 3 pins on the nearby socket is 0V/gnd, and measure ALL voltages from there.

Ah! I think I may have been incorrect about the ground pin on the 7912.  With pins facing down the 7912 is ground, input, output from left to right.  I was testing that one incorrectly.  Both grounds are tied directly together by a trace on the board.  If I check both inputs and outputs using either ground I get the same results. Here are the readings from both regulators now tested correctly.

7812
Input 3.4 volts
Output .08 volts

7912
Input -17.9 volts
Output -11.9 volts

[don@cascadefellowship.org]

It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps).

I'll start learning how to check all of those components in that order and post my results.  I can verify that all the components and the boards do not show and burn marks or blow outs.

[don@cascadefellowship.org]

It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps).

I'll start learning how to check all of those components in that order and post my results.  I can verify that all the components and the boards do not show and burn marks or blow outs.

Checking the 3 input diodes with my DMM (with power off using the diode mode) yields between .56 volts and .59 volts for all three.  Reversing the leads on each one showed no flow.

[don@cascadefellowship.org]

It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps).

I'll start learning how to check all of those components in that order and post my results.  I can verify that all the components and the boards do not show and burn marks or blow outs.

Checking continuity on the 7812 regulator shows that the ground and the output are shorted together.  The ground and the input does not have continuity.  On the 7912 both the input to ground and the output to ground do not have continuity.  Does that mean I've identified a likely problem in that the 7812 ground and output show continuity?

[StillTrying]

"Checking continuity on the 7812 regulator shows that the ground and the output are shorted together."

If you're sure it's a dead short, for quickness I'd just neatly snip the output leg of the 7812 at the 90 deg. bend myself, to see if the short is the 7812 itself or somewhere on the 12V line.

[don@cascadefellowship.org]

"Checking continuity on the 7812 regulator shows that the ground and the output are shorted together."

If you're sure it's a dead short, for quickness I'd just neatly snip the output leg of the 7812 at the 90 deg. bend myself, to see if the short is the 7812 itself or somewhere on the 12V line.

I'm totally not sure of anything!  LOL! I popped the 7812 regulator off the board and checked it.  The short is not on the regulator but somewhere else on the 12 volt line. How does one go about tracking a 12 volt line looking for a short?  Is it a matter of just following the trace of the 12 volt and ground lines to find where they short?

[don@cascadefellowship.org]

By putting the name of the unit into Google it first took me to their website. First thing they said was this is an all analog device, which will make troubleshooting easier. By adding "schematic" to the search phrase I wound up at a site called Freestompboxes where one of their pass-times seems to be hunting down schematics and reverse engineering pedals. You need to join to get complete access, but one poster seemed to have either the factory schematic or had reversed it himself. That would really give you a leg up. It is amazing the info that can be ferreted out with some perseverance. Good luck with your repair, it looks like a pretty straight forward circuit.

Here is the schematic I got from the website.

[CJay]

There are two connectors from the 'regulator' board to the main board, disconnect those and see if the short disappears.

I have a suspicion it won't.

There are two yellow blobs on the regualtor board near the jack inputs, they're tantalum capacitors, C29 and the other next to the QC sticker.

I would be desoldering those if the short doesn't disappear when you unplug the main board.