Beringer powered PA goes into Over Load

[vernster08]

Understood. But there can be some discrete components between pin 1 and the circuit's ground. That's usually only for precision applications and we are talking Behringer junk When it goes into failure mode, it looks like the unreg is sagging a bit. Makes me wonder if the unreg isn't being shunted to circuit ground through the 7915 and some discrete components, dropping the unreg a bit and the -15 VDC reg just drops out to 0. I kind of expect the 7915 or discrete components would heat up a bit, maybe even smoke or get discolored. Maybe not if the 7915 is on a hefty bit of heat sink. I think those are good for a couple of amps, so at -15V I'd expect something to get a bit warm. Maybe something on the output is shorting -15 to ground once it heats up. If you don't really care about the speaker, maybe get a can of freeze spray and cool off the 7915 after failure, then whatever runs on the 15 supply, one at a time until you find it. I've done more shameful troubleshooting than that in the past That's can take some time, but you might nail it. Just be careful!

Thanks for the suggestions! I'll definitely try the freeze spray and see what it gets me!

 

[BadExample]

Just don't get zapped!

 

[agedhorse]

I wouldn't recommend freezing ANYTHING until you actually know what is happening. There are shut-down modes on the regulator, there are also ways to isolate major circuit blocks to eliminate entire portions of the circuit from consideration (differential diagnostics).

Also, a general note to folks who may be following this, NEVER use freeze spray on a high voltage circuit board, the condensation that results can become a major problem at high voltage and cause considerable damage.

 

[BadExample]

I've rarely used it, never on high voltage. I used a temperature probe more. Temp probe works on live circuits, but you have to be at least as careful as with a DVM or scope probe. I knew a guy that just pressed his thumb on IC's, until e got a 3rd degree burn as big as a dime!

 

[fdeck]

I can't tell if it's getting that warm to be honest. The #1 pin is the common ground on the 7915.

This datasheet has the pinout as ground, input, output, with the tab connected to input. It's different than the 7815.

http://www.ti.com/lit/ds/symlink/lm79.pdf

 

[BogeyBass]

I would assume when it goes to shutdown it's designed to turn off the 15 volt supply to the preamp...idk

Explains atleast of why 0 volts at shutdown. input goes from 38 to 40 cause no current is flowing. Output 0 as designed

 

[BadExample]

Explains atleast of why 0 volts at shutdown. input goes from 38 to 40 cause no current is flowing.

Except the unreg drops from -34.7V to -33.5V. Looks like it getting pulled down.

The initial -ve voltage input on the IC7915 is -34.7V and the output is -14.9V. When the unit trips on overload, the input voltage goes to -33.5V and the output goes to 0V.

Probably best guess is the Uli-tronics overyly high voltage unreg for a +/-15V supply has murdered the 7915. Normally I'd think something shorted on the output. 3rd guess something funny between pin 1 and ground. I'm pretty sure the 30 + volts unreg is too high for the regulator design.

I'm experiencing the same issue with my B300. When the overload lights up, the -ve rail voltage goes from -14.08V to -40V. Is it a good assumption that the IC7915 regulator might be faulty or should I be looking somewhere downstream?

Maybe this post has confused the issue. vernster can you measure all 3 pins on the 7815 and 7915 before and after failure? Also check the AC reading (ripple) in and out of both regulators.

 

[agedhorse]

I would assume when it goes to shutdown it's designed to turn off the 15 volt supply to the preamp...idk

Explains atleast of why 0 volts at shutdown. input goes from 38 to 40 cause no current is flowing. Output 0 as designed

Interesting assumption that makes no sense.

 

[BogeyBass]

Assumptions are reduced with a actual schematic. It's probably straight forward n simple. Most people acting like both posts should give up on the units. When it's a easy fix

 

[agedhorse]

Assumptions are reduced with a actual schematic. It's probably straight forward n simple. Most people acting like both posts should give up on the units. When it's a easy fix

If you are so sure it's an easy fix, then offer to fix it for him or tell him what the fix is.

Without troubleshooting skills and an understanding on how the circuit SHOULD work, it's like coaching somebody on performing an appendectomy. Looks simple until you know more.

 

[vernster08]

Except the unreg drops from -34.7V to -33.5V. Looks like it getting pulled down.



Probably best guess is the Uli-tronics overyly high voltage unreg for a +/-15V supply has murdered the 7915. Normally I'd think something shorted on the output. 3rd guess something funny between pin 1 and ground. I'm pretty sure the 30 + volts unreg is too high for the regulator design.



Maybe this post has confused the issue. vernster can you measure all 3 pins on the 7815 and 7915 before and after failure? Also check the AC reading (ripple) in and out of both regulators.

You bet. I'll do so Thursday and post back the results.