AML 1073 Pre troubleshooting / no input signal

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drum_flo

New member
Joined
Jan 21, 2018
Messages
2
Dear Group DIY members,

this is my first post in this forum and I'm very much looking forward to participating in the discussions taking place here! :D

I just finished soldering my  first DIY project and did two AML 1073 Pres. It was great fun and basically I thought that I really did a decent job.
One of them is working just fine and sounds great.

Unfortunately the other unit isn't working as expected :-(. It doesn't really amplify the microphone signal.
The ground noise is increasing as I turn up the gain knob and I can hear a very thin and quiet microphone signal which is just barely audible and not changing in volume weather the gain knob is turned up or down.

I have absolutely no knowledge in electronics (although I'm about to learn how to read a multimeter :eek:) and no idea where to start troubleshooting....
As a first step I was told to measure the resistance between the hot and cold pin of the XLR connector in the two different modes.
The results where as follows (within the 2k range):

1200 ->  .058 Ohm
300 -> .018 Ohm

Do you guys have an advice where to start looking for errors and how to do it?

Thank you very much and again thanks for having me in this great forum!

Cheers
Flo
 
I don't mean to discourage you but it's a little difficult to build something like this without a vague idea of how to debug circuits and the tools required to do it. Even if you have a multi-meter, by itself it's not enough. You need something to do a noise measurement and probably a frequency response measurement which requires at the very least some kind of audio interface. Why go through the trouble of building something like this if you cannot verify that it's actually working properly?

If all you have is a DMM, at least sanity checking voltages. Check to see if 24V is making it to the various points. Check resistance between the gain switch and the gain selection point on the gain stages circuit (BA283 terminal T on the original boards).

Usually this sort of problem turns out to be a missed a solder joint or a missing connection. Scan the bottom of the board using a magnifier and light and look for missed solder joints.
 
You can easily create more problems than you have now by poking around without knowing how everything in the circuit interacts with each other.

Like squarewave said, look for solder joints or connections...

Testing voltages is tricky if you haven't an idea where to probe so, get familiar with a schematic and how to interpret it.

You have a working unit so, without any power connected....you could use the continuity function and poke around spots and compare with the working one as well as checking resistance at different points of both units and compare.....

I'd be hesitant to put any power to the unit while testing it until you feel comfortable with what you are doing.......

Post some findings and maybe someone will be able to interpret them for you....

Post a schematic.....
Good Luck!
 
scott2000 said:
You have a working unit so, without any power connected....you could use the continuity function and poke around spots and compare with the working one as well as checking resistance at different points of both units and compare.....
Click to expand...
Good idea.

scott2000 said:
I'd be hesitant to put any power to the unit while testing it until you feel comfortable with what you are doing.......
Click to expand...
Good advice. But touching leads or pads with a DMM should not damage anything. The problem is if the probe tip slips and closes two circuit points together (like a power lead to a grounded metal part) causing a short. So just be careful! I just got new probes from ProbeMaster and they come with little plastic shields that screw on so that only a small bit of the probe tip is exposed. Neat.
 
squarewave said:
The problem is if the probe tip slips and closes two circuit points together (like a power lead to a grounded metal part) causing a short. So just be careful! I just got new probes from ProbeMaster and they come with little plastic shields that screw on so that only a small bit of the probe tip is exposed. Neat.
Click to expand...

Yeah....I just spent time replacing parts on my working amp when I slipped with my probe trying to bias and shorted a transistor...... 

I saw those Probemasters mentioned by Gold I think......and now you............Guess I'll have to check them out....

Thanks!
 
drum_flo said:
Dear Group DIY members,

this is my first post in this forum and I'm very much looking forward to participating in the discussions taking place here! :D

I just finished soldering my  first DIY project and did two AML 1073 Pres. It was great fun and basically I thought that I really did a decent job.
One of them is working just fine and sounds great.

Unfortunately the other unit isn't working as expected :-(. It doesn't really amplify the microphone signal.
The ground noise is increasing as I turn up the gain knob and I can hear a very thin and quiet microphone signal which is just barely audible and not changing in volume weather the gain knob is turned up or down.

I have absolutely no knowledge in electronics (although I'm about to learn how to read a multimeter :eek:) and no idea where to start troubleshooting....
As a first step I was told to measure the resistance between the hot and cold pin of the XLR connector in the two different modes.
The results where as follows (within the 2k range):

1200 ->  .058 Ohm
300 -> .018 Ohm

Do you guys have an advice where to start looking for errors and how to do it?

Thank you very much and again thanks for having me in this great forum!

Cheers
Flo
Click to expand...

What scott2000 and squarewave said about this not really being a first time build. When taking on this build, you really should know things like using a DMM, component orientation, and proper soldering techniques to name a few.

Your problem more than likely is a bad solder joint (cold or bridged) or a misplaced component like the thousands of other problems (including my own) that you see here on this forum. So the fun part is that you get to check over all your work and compare connections to the schematic in the Colourbook. Your issue is probably related to the input and preamp gain sections but that is no guarantee. And you have to double check all your solder joints. This is a tedious and frustrating process when you've been at this DIY thing for a while. It's REALLY tedious and frustrating when you are new. But it's kinda the only way without physically putting it in front of someone to examine. Start at the XLR input and check for continuity with your cables and keep working your way down stream. Let us know how you do.

Thanks!

Paul
 
squarewave said:
I don't mean to discourage you but it's a little difficult to build something like this without a vague idea of how to debug circuits and the tools required to do it. Even if you have a multi-meter, by itself it's not enough. You need something to do a noise measurement and probably a frequency response measurement which requires at the very least some kind of audio interface. Why go through the trouble of building something like this if you cannot verify that it's actually working properly?

If all you have is a DMM, at least sanity checking voltages. Check to see if 24V is making it to the various points. Check resistance between the gain switch and the gain selection point on the gain stages circuit (BA283 terminal T on the original boards).

Usually this sort of problem turns out to be a missed a solder joint or a missing connection. Scan the bottom of the board using a magnifier and light and look for missed solder joints.
Click to expand...

Hey everybody,
thanks for your answers and help so far.
I just couldn't wait for my Capi 312s to get through german customs so I figured I start with the AMLs. Probably that was a bad idea.
The build itself wasn't too complicated, because it's, as you might know, very well documented. So actually I was very confident that I did everything right... I know that polarity matters and also the colourbook is making that very clear.

I have an Interface that I checked both units with. Thats why I could verify that the other unit works properly (brought bias up to 90V and measured In/Output gain using a sine wave and used it on it's first recording session yesterday :)).

The second, not working unit fires up, too. I could bring the internal bias up to 90V. Output stage seems to be ok, too. It's just not able to amplify the mic signal. I will check for dead solder joints and let you know.

Thanks
Flo

P.S.: Schematics can be found in this colourbook (section 3): http://www.audiomaintenance.com/downloads/ez1073pre_colourbook.pdf
 
drum_flo said:
I have an Interface that I checked both units with. Thats why I could verify that the other unit works properly (brought bias up to 90V and measured In/Output gain using a sine wave and used it on it's first recording session yesterday :)).

The second, not working unit fires up, too. I could bring the internal bias up to 90V. Output stage seems to be ok, too. It's just not able to amplify the mic signal. I will check for dead solder joints and let you know.

Thanks
Flo

P.S.: Schematics can be found in this colourbook (section 3): http://www.audiomaintenance.com/downloads/ez1073pre_colourbook.pdf
Click to expand...

I'm guessing you mean 90mv??


Like mentioned, probably something with the input but, it could be anything. You could send a sine wave and measure voltage to see where it drops off. Having a second unit is convenient .Should help you find the problem quicker.

Good luck!
 
to repeat some of what the others have said, you have 2 units. one woks one does not.  It would be very easy to compare he working vs the non-working. To say you did everything correctly is obviously not the case as you would have two working units vs 1 working unit and one not working properly. as mentioned check everything thoroughly.  As you mentioned you have a DMM, great. You can use that to confirm continuity and check the power voltages in various spots. As always do take proper precautions and if you need help with that we can help you, but you have to do some lg work too.
 
One thing you can do even with little knowledge of circuits (which the others above alluded to) is to reflow all of your solder joints. You can start by looking for any that are cloudy looking and not nice and shiny. But if nothing obvious jumps out, just go back around the pcb and reflow what’s there. You may have to add a dab or two more solder in places to get joints looking right. That’s the shotgun approach that anyone can do, and it may just work for you.

Also, did you clean the flux off of the PCB? Flux is slightly conductive and can cause weird issues if you don’t clean it off the board.
 

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