Looking for fresh eyes (or ears) on my AML 1073-EZ build

I have this AML 1073-EZ  (one of two) that i built a year or two ago. Both I seem to recall were initially working - though one of them for whatever reason had a buggered LED which i was able to fix (at the time I suspected grounding issues).  May or may not be relevant. Anyway - so HERE are the issues.

1. after not being able to get a mic signal through it (after fixing the LED) - I did a bit of snooping around and discovered that the cold line of the mic input was shorted to ground. I did more snooping and deciding it was quite possibly C28 or the input transformer decided to replace the input transformer at no small expense. After i put the new one in - SAME PROBLEM.

2. I did more snooping around and found the following:

        •  on the impedance switch both sides of the DPST are shorted together though i don't think they should be

        •  now HERE's the weird part - 48V rail measures correctly and is healthy with very little ripple - BUT I can only get it across my 'hot' input pin (which makes some sense because of the grounding problem) - HOWEVER - I can't switch it on via the phantom power switch BUT RATHER when BOTH the phantom and impedance switches are down (then I get my 48V across the hot input only)


I'm having an awfully hard time trying to figure out what is wrong - i've checked pretty much every component on the board (most certainly on the relevant circuit paths ....

i would be most grateful if someone could offer a suggestion to try ... far too much time has passed and nerves shot over this! thanks

Some of us don't have every detail of every project on instant-recall. (Some of us are on drugs.)

Show/link this "EZ".

PRR said:

Some of us don't have every detail of every project on instant-recall. (Some of us are on drugs.)

Show/link this "EZ".

Click to expand...

oh very sorry - what do you think might help? i was targeting the question at others who might have built one of these - though it's more of a general troubleshooting question - what would help? linking the schematic? should i try to describe the issue more succinctly? a picture of the board? (not sure you'd tell much from that but whatever works)

If things are shorting together and they are not supposed to you a have either a solder bridge or something is miswired. It is also possible that one of the transistors has been blown up and now two of the legs are shorted together as I have had that happen with things like power regulators. You're going to have to pull it all apart and scour the underside of the PCB. It sucks, and it always does, but there isn't another way unless you can see on top of the PCB that something is misplaced, which still will require desoldering from underneath. Start with the switches in question and work your way from there with the schematic and test connection points against the schematic. Another clue is that you are only getting +48V on Pin 2 of the XLR referenced to ground, but not on Pin 3. Both of these know things should expedite the troubleshooting process.

You may find the culprit in minutes you may not find it for a week.

Happy hunting.


Thanks!

Paul

leadpoisoning said:

i was targeting the question at others who might have built one of these 

Click to expand...

Why limiting only to those? there's a lot of people that might help and never build that. I
Also even someone that built it 2 years ago, I'm sure doesn't know at the moment what is C28

leadpoisoning said:

linking the schematic? should i try to describe the issue more succinctly? a picture of the board?

Click to expand...

Yes, yes and pictures of your build

Potato Cakes said:

If things are shorting together and they are not supposed to you a have either a solder bridge or something is miswired. It is also possible that one of the transistors has been blown up and now two of the legs are shorted together as I have had that happen with things like power regulators. You're going to have to pull it all apart and scour the underside of the PCB. It sucks, and it always does, but there isn't another way unless you can see on top of the PCB that something is misplaced, which still will require desoldering from underneath. Start with the switches in question and work your way from there with the schematic and test connection points against the schematic. Another clue is that you are only getting +48V on Pin 2 of the XLR referenced to ground, but not on Pin 3. Both of these know things should expedite the troubleshooting process.

Click to expand...

Thanks - already been over the underside (and overside) of the board with a fine toothed comb just in my post build checks but also in short-to-ground hunting for the LED issue i had. Already pulled a number of components for that (have the sneaking suspicion it was somehow in my last 'repair' that I did something else that may have caused this).  Thanks I clearly need at least to start doing this more systematically instead of hen-pecking at random suspects

If you have one box that works & one that doesn't personally I would try to compare the good against the bad with a meter.

thanks Rob - well yes - i did a fair bit of that already which is why i changed the transformer -

now discovered something TOTALLY out of the blue I didn't anticipate - it LOOKS as though I may have exchanged the transformer daughtercards and used the 'line in' one for the VTB9046 for the VTB9045 ... I suppose that might do it ... if the circuit layouts on in the drawings are to be trusted.

it's funny how one can get caught up in one's own assumptions! (in this case having made the assumption at some point that the daughtercards were identical) because the transformers look mostly the same - it's my first time working with signal transformers though (d'oh!!)

but if this is the case it may be an expensive proposition as I think desoldering the daughtercard from my original exchanged transformer fried the coils (overheating you know) - not so easy to desolder

YES most DEFINITELY seems to be what i did ... i was going to upload an image but it seems this site doesn't currently allow uploads - only links ... will see what i can do to show you my absent mindedness


well it's somehow a huge relief to no longer by treading the waters of confounded uncertainty - i'll say that much

TO upload an image click the "attachments & other options" tab below.  Then choose the pic you want to attch.  There is a size restriction on pics.  So rescaling is sometimes required.

> TO upload an image click the "attachments & other options" tab below.

If you are in the "Quick Reply" box, click "Preview" to get the file upload frill.

hmmm - never noticed that part before with the upload feature - it seemed to me it was all URL only - just having issues with my FTP software at the moment. thanks

semifinal report:

Thanks for the help everyone - well got all the 'daughter' boards put correctly on their respective transformers (boy what a doozy to desolder those with just a pump and wick! - harrowing!) so i got everything back together and to my dismay STILL had a short ... so went over every square mm of the board under the stereo microscope with a fine toothed comb and WOW - what did i find ...?? A SOLDER BRIDGE - but nowhere i would expect to find it - it was the tiniest bridge RIGHT at the pad where the cold lead leaves the board to the XLR input! My prior investigations in high sensitivity mode on my DMM was even TELLING me it should be pretty close to there ... I just didnt' think it was possible though - somehow there was a scratch in the conformal coating on the ground plane and it was only because the solder joint was SLIGHTLY misshapen that I thought to investigate - sucked up the joint - and  BINGO! no more short! wow! what elation. i've been trying to find this bugger for MONTHS. What's my take-away? I found two major gaffes created by making dumb assumptions - even though i thought i was being very fastidious. Also - i guess it's good to know to be extra careful with boards that have ground plane everywhere (esp when it's so close to the 'live' traces). Anyway - the phantom power seems 100% now so ... happy with that.  Hopefully i haven't ruined the second input x-former in the process ... we shall see ...