PLS help with this Behringer failure - what is wrong?

[gnd]

Hi.
I have this handy rack mixer Behringer RX1602, and it worked fine some years. It is turned ON all the time, 24/7, I use it for monitoring in studio.

Recently it started making weird noises. As if it would overheat or something. If it turn it off, and wait, then some time all is fine. And after some 10-15 minutes there is few seconds of slowly increasing buzz (50Hz spikes), then big spike (positive), then 1second of silence, then one big spike (negative). After that all is fine again few seconds, then again buzz comes in, spike, slience, spike,..... It is getting worse every day.

I recorded this to another computer, from headphone out, but same thing is going on on main outs.

Can you please lok at it and hear it, and maybe you can suggest what is going on, and where to start troubleshooting. Maybe PS regulators overheating? Or PS capacitors?

Here are some screen grabs in Audition, at different zoom levels.
http://84.255.203.119/failure1/f1.JPG
http://84.255.203.119/failure1/f2.JPG
http://84.255.203.119/failure1/f3.JPG
http://84.255.203.119/failure1/f4.JPG

And here is how it sounds like ( :!: WARNING!!!! - LOWER YOUR VOLUME - DANGER!!! :wink: ) - first is silence, then big FS pulse up and down.
First spike cycle is without music, just silence so you can hear buzz. Second spike cycle is with quiet music, so you can hear how it cuts off inputs.
mp3: http://84.255.203.119/failure1/failure_01.mp3
same file in original wave: http://84.255.203.119/failure1/failure_01.wav

thnx
gnd

 

[okgb]

B'ringer seems to underbuild some of their things to the point of failure
, works long enough to get over warrenty .

Investigate the power supply , good luck finding schematics but check
for burnt or stressed parts , check voltages

regards Greg

 

[gnd]

Hi again.

Please, any advice on this is most welcome. I never really fixed any gear, just building, modifying... But why not try fixing something.... :?

I measured PC regulators, 7815 and 7915, and something is wrong there. 7815/7915 are not hot, but just mildly warm. Both are same temperature. So there is no overloading.
7815 is looking good on scope. But 7915 shows problem. It keeps switching on and off.

After several minutes warming up, on 7915 output appear spikes (sawtooth) in 100Hz freq. Their level increases over few seconds, and when they come to certain level, then 7915 shuts down for a second, then it turns on again.

I can actually hear buzz comming in (not on speakers, but out of circuit, no speakers connected) - probably thats condensers "singing", is it?

Here are scope photos, showing what is going on:
http://84.255.203.119/failure1/20071221_0002.JPG
http://84.255.203.119/failure1/20071221_0003.JPG
http://84.255.203.119/failure1/20071221_0007.JPG
http://84.255.203.119/failure1/20071221_0008.JPG

Negative voltage is -15V, and when it shuts down, it goes to approx 0.7V positive, then switches back to -15V.

Would that be broken 7915, or is something causing short circuit to it, and it shuts itself down? But short circuit would cause overheating, is it?

What are those increasing 100Hz spikes (sawtooth), that cause repeated shutdown of 7915?

thnx
gnd

 

[rodabod]

Have you had a look at the voltage which is feeding the 7915 regulator?

 

[gnd]

[quote author="rodabod"]Have you had a look at the voltage which is feeding the 7915 regulator?[/quote]

Yes, it looks fine. There is +/-26V coming to regulators. Transformer is 22v/0v/22v toroid. Voltages a bit high, typical for Behringer, it seems.

So it must be something is loading 7915 in some way, and causing 100Hz spikes, is it? It is increasing with time, so would that be electrolyts?

But it doesn't start immediately after power up, it takes some 10-15 minutes before it starts doing this, and it starts repeating slowly, then faster and faster, untill it repeats every few seconds. This would look like overheating somewhere, is it?

How to proceed troubleshooting it?

...

 

[clintrubber]

[quote author="okgb"]B'ringer seems to underbuild some of their things to the point of failure
, works long enough to get over warrenty .[/quote]
I see a similarity with the transformers IKEA uses for their halogen stuff

 

[hugoderwolf]

Did you try throwing in another 7915? Maybe the regulator is just crap. I would check that first before searching for some unusual load.

 

[dale116dot7]

Is the 7915 hot to the touch? If so, that's why. Don't burn yourself!!! If the reg isn't hot but is shutting down, replace the regulator for sure.

Feel the rest of the board for hot chips. If you find one that's hotter than the rest, that's your problem. Replace it.

If the regulators both run very hot, add heatsinks!!! Also, use a National Semi or On Semi 7815/7915 if you need to replace them.

-Dale

 

[Larrchild]

There is certainly a long history of unstable 3-terminal regulators.
Sounds like a fine starting point.

 

[Andy Peters]

[quote author="gnd"]After several minutes warming up, on 7915 output appear spikes (sawtooth) in 100Hz freq. Their level increases over few seconds, and when they come to certain level, then 7915 shuts down for a second, then it turns on again.

Negative voltage is -15V, and when it shuts down, it goes to approx 0.7V positive, then switches back to -15V.

Would that be broken 7915, or is something causing short circuit to it, and it shuts itself down? But short circuit would cause overheating, is it?

What are those increasing 100Hz spikes (sawtooth), that cause repeated shutdown of 7915?[/quote]

Actually, the spikes don't cause the shutdown, they're just a sign that the device is dropping out of regulation. I suspect that the regulator is bad.

Methinks the input voltage is too high but if properly heatsinked, it shouldn't fail.

Check around the regulator for the usual reverse-voltage protection diode. If the output capacitor discharges back into the device, it tends to die, and this diode prevents that. Maybe Uli's muntzing resulted in that diode getting the ixnay?

Good luck,
-a

 

[Larrchild]

For the benefit of those not acquainted with Earl "Madman" Muntz, ..."Muntzing" is explained by none other thaan Bob Pease here:
http://www.national.com/rap/Story/0,1562,17,00.html


Visualize World Pease!

 

[gnd]

OK, my first service adventure completed with sucess!!!! Yea, I may as well open repair shop now... :grin:

It was 7915, as you indicated. Thnx for all help.

It was quite easy work. It is all SMD, so boards are small, and attached to jacks/pots. I/O boars is attached to I/O jacks on rear panel, so I just needed to unscrew those jacks, and there it was. 10 minutes, done.

Heatsink is not really needed, regulators are really just mildly warm, slightly above bodily temperature. Probably they get hotter when all 16 channels are engaged 100%. They are attached to chasis bottom.

Otherwise, I must say I like this unit. Looking at it from my DIY perspective, it is like a little wonder. I could never build something like this. 30 pots on 1HE frontplate, plus transparent-plastic mute buttons with LEDs behind, indicating mute and overload. And every input has -10/+4 switch in the back. All balanced, headphone output, separate level for L and R output channel, some FX routing, level meter... Great solid build, looks great, .... I'd say it is the best 1HE rack mixer, if you need to join some synth or FX channels, or for simultaneos monitoring from multiple sources, like several computers.

And it has no sound, which is good for some uses. It is a tool for mixing channels transparently, not to add color. Obviously you will not use it for mixing your album, and I would not use it as active summing box feeding recorder, but for monitoring purpose it is great unit, and it costs 100eur. If I was to DIY something like this, I would never achieve this level of functionality, and even if I would, it would certainly not be for 100eur.

If you get it from eBay, like I did (for 50eur), I can only recommend it.

I think people are often unfair towards Behr. Considering price, they offer a lot, often much more than competition.

....

 

[gnd]

[quote author="Andy Peters"]
Check around the regulator for the usual reverse-voltage protection diode. If the output capacitor discharges back into the device, it tends to die, and this diode prevents that. Maybe Uli's muntzing resulted in that diode getting the ixnay?
[/quote]

Yes, you may be right. The box is closed now, and back in rack, but if I remember properly, there were just 4 diodes in PS section of rear board. May be those two protection diodes were muntzed off. Or may be they are there as tiny smd's, of which there is plenty enough allaround the unit. Who knows what is there inside.... :? I'm glad it works now, lets see how much longer it will survive....

...