AML EZ1073- 500 kit - Low bias and relay not clicking on.

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BeatStudio

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Nov 22, 2010
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Just put together the EZ1073-500 kit however the max bias I can get with the trim pot is 24mV (should be 90mV) and the relay is not clicking in.

With the vertical boards it's hard to trouble shoot but from the few spots a could check it appears the proper 16 volts is reading at points I can measure.
Before I start removing boards It would be nice to narrow the problem down and pretty sure the relay not clicking in is a clue.

Also it does power a condenser mic with 48 phantom and passes signal, albeit a little crunchy. It is also quiet so no noise or hum, so I don't think completely screwed it up. Pretty sure something in the output stage is botched.

Link to schematics. http://www.audiomaintenance.com/downloads/ez1073pre_colourbook.pdf


Any ideas or advice greatly appreciated.

Thanks,
Scott
 

BeatStudio

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Nov 22, 2010
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Got it working! Good news is I got to practice trouble shooting and was fairly certain it was the surge control board. It was, and it was something stupid.  Because the resistor leads were pre-bent I couldn't flip the board over and because the roll of tape was nowhere to be found, I soldered from the top.  Nothing wrong with that except I missed one and it's harder to catch looking from the top.  Why did I not inspect the board before putting everything together?  Because I inspected from the back of the board and when I saw a missed solder joint I figured it was  a resistor.  :eek: Did I double check the top? No. Lesson learned? Don't be a dumb ass, Inspect Every Thing!

 

rockinrob86

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I just built a pair of these, and one is having this issue.  I pulled the surge board and checked the solder joints, it looked fine.  Reinstalled and problem was still there.

Doing some more digging, I figured out pin 12 (+16v) is measuring 10mv.  If I unplug the module, it measures 16v, and it measures 16v on my working unit.

What could cause this?  Bad regulator?

 

rockinrob86

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Did some more poking around.  If I measure resistance between pin 12 on the card (+16v) and the pins on the MC7808 regulator(U1), on my good unit I get 1 ohm (for the 1 ohm resistor between the regulator on +16v), 56K ohms in the middle, and 50 something K that rises on the 3rd pin of the regulator.

On the bad unit, I get 1 ohm,  0.2 ohms and 280K ohms. 

Schematic is on page 39.  http://www.audiomaintenance.com/downloads/ez1073pre_colourbook.pdf

I am not very familiar with the innerworkings/fail modes of regulators, but this looks like a good place to start.  Should I order more of these and just replace it? 

 

Walrus

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The middle pin of the 7808 regulator is the ground/common pin i.e. Pin 13.
Therefore you have a short between Pin 12 + 16v and Pin 13  0v, nothing to do with the regulator.
 

rockinrob86

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Walrus said:
The middle pin of the 7808 regulator is the ground/common pin i.e. Pin 13.
Therefore you have a short between Pin 12 + 16v and Pin 13  0v, nothing to do with the regulator.

Thanks a lot!  I did some searching and I hadn't trimmed some of the card pins on one card, it was shorting against the input transformer.  Fixed this and I was good to go.  Well, after I found another solder joint I missed, which I found after realizing I had no sound, and digging out my oscilloscope and signal generator and trying to move through the circuit.  I may have also caused a resistor to smoke a bit from my poking...


Anyways, I got both units biased and the gain adjusted, and both are confirmed fully functional and sounding excellent.  Thanks for the help!
 

greyskull

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Jul 28, 2011
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Hi all, I hope you don't mind me piggy backing on this thread as opposed to starting a new one!
Just finished up my first ez73 500 and am starting initial testing now. I definitely should have order a 500 extender card! on that now.

However...

Is it normal for the Bias to drop a little after the first power on?
I got the unit up to voltage no problem, but after a few minutes the voltage started to work its way down, not by much but from 90 to 89.9 after a few minutes and then 89.8 and so on?

I adjusted the trimmer to get back up to 90 and it seems to be settling down. Just wondered If this expected behaviour?

Very best
G
 

noiro

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Oct 22, 2011
Messages
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Hi all, I hope you don't mind me piggy backing on this thread as opposed to starting a new one!
Just finished up my first ez73 500 and am starting initial testing now. I definitely should have order a 500 extender card! on that now.

However...

Is it normal for the Bias to drop a little after the first power on?
I got the unit up to voltage no problem, but after a few minutes the voltage started to work its way down, not by much but from 90 to 89.9 after a few minutes and then 89.8 and so on?

I adjusted the trimmer to get back up to 90 and it seems to be settling down. Just wondered If this expected behaviour?

Very best
G
yes this no problem, unless it was something like 20mv off, then i would suspect something but otherwise it's fine, how do it sound. if you've got distortion then you may have an issue.
 

Joeblutak

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Jul 24, 2021
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Hey all! I hope you don't mind me resurrecting this thread! I've just built my first one of these and I'm having a few issues with it - it sounds kind of tinny and when I run pink noise through it there is a pretty defined slope off in the low end, it also is 6db quieter when it is in positive phase compared to negative phase? Is this an issue with me toasting some parts or something? I've gone over all my solder joints and they seem to be all good but I'm a bit stuck for now!
Joe
 
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