DM4800 Phantom Power Kaput

[Herve QUEVAL]

Hi Dave, about your questions:
When the board is powered on and I have the mic/line input fader up, #16, I move the 48v phantom power switch off, on, off, on and I get signal bumps on the meter bridge for that input. This appears to be normal. Same result on every mic/line input with 48v power. If the Amp boards are not receiving 48v power, would I still get those signal bumps on the meter bridge?

Normal result because we switch the input load from ground to +48V. Even if +48V is missing, it creates a "pop", especially if +48V is open or show a different impedance than gnd.

If I plug in a run of the mill dynamic mic, (AKG D190ES) the channels all perform normally with good signal output. Does this indicate anything to you?

No, all seems OK on Input side ( except we have no phantom power...).

Need some measurement...

About DeoxIT, I use the products and are very efficient: Contact cleaner product or Fader Lub. It is a good idea to use it especilly when all is open and you can access to the board, fader or contact.

 

[DaveDC9]

Morning Herve,
"Normal result because we switch the input load from ground to +48V. Even if +48V is missing, it creates a "pop", especially if +48V is open or show a different impedance than gnd".

Okay, this makes total sense to me. And this "pop" was the beginning symptom. I'd be sitting here at the board noodling around on the guitar and the channel would glitch or pop. Sometimes it would do this and other times it wouldn't. 48v power was on so it appears that it was either cycling off and on or the impedance was intermittent when this pop would happen.

Okay my friend, I'm gonna get the setup torn down and the board open when the multi-meter and cleaner shows up and start the search.

You've given me a really good road map and starting point for all of this Herve and I want you to know how much I appreciate you taking your time to help me with this. Once I get some measurements I'll report back with the results.
Thank you sir!!!
David

 

[Herve QUEVAL]

Many thank's. We will wait the multimeter...

 

[DaveDC9]

Hello Herve,
I have the multi meter now and I've done a few test. I unplugged the mic cable from the board and turned the board on. I then turned the 48v power on and took measurements at several of the XLR inputs on that Amp board. It's reading 48v or a little above. Turned the 48v power off, reconnected the mic cable, unplugged the mic, turned the 48v power back on and took measurements at the female end of the cable and again, 48v power is registering.
I am starting to suspect that it is my microphone that has bit the dust. If 48v registers at the inputs and registers at the end of the mic cable then it's got to be the mic. What do you think?
David

 

[DaveDC9]

Hi Herve,
Another wrinkle in the mystery here and I think what you responded with below herein, lies the answer.

"Normal result because we switch the input load from ground to +48V. Even if +48V is missing, it creates a "pop", especially if +48V is open or show a different impedance than gnd".

Come to find out that this "popping" I mentioned above, I believe, damaged my AKG C414. I have a friend who lives just down the street from me and his son has a descent recording set up. I took the 414 down to their house and we plugged it into his system, applied 48v power and nothing. The 414 is dead. His son, gave me a mic he didn't want any longer, a Audio Technica AT 2035. Large diaphragm condenser mic. I came home and set it up and played through it a little while and for a $160.00 mic, this damn thing sounds good. BUT, the "popping" returned, intermittently. I think somewhere along the line and over time, this popping shorted out my 414.

I was partially happy because I was going to report back to you that it was the mic that was the problem and that I wasn't going to have to tear down the board but when I got the AT mic plugged in and played for awhile and the popping returned, it appears that I am going to have to open it up and see what's what. If the popping is being generated by a grounded input being switched to 48v power are we looking at the last part of your sentence above?
"especially if +48V is open or show a different impedance than gnd".

What would I check on the Amp board for that?
Thank you Herve!!
David

 

[Herve QUEVAL]

Hi, Good job Dave. Now we have to find the origin of the pops. Possible reason I have in mind:
1) 48V power supply "cut" some ms. (connection, switch, SW Power supply condensator failure.
2) Condensator failure at the input of the channel

IS this pop audible on all channel or one or some specific channel?
The answer give the reason 1 or 2

My approach is to do a test without mike:
Create a plug with a XLR male and 2,7K or 4,7K resistance between 2 and 3
Activate the Phantom power, connect the plug on the channel to test (CH 1)
Power on
hear the channel. If pop appends, repeat the test on other channel CH 5 . If appends will be a power supply problem.

Normally I will monitor the Phantom power supply with a Digital scope and will trigger if less than 47V and memorize the signal. Doing it this way, you can do other activities..

Waiting your answer to decide how to repair.