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Beringer powered PA goes into Over Load

#41
Hi.

agedhorse said:
Just a caution when using test equipment with many of the new (especially class d) amps is that the speaker - terminal often is not ground if its an internal bridge configuration. If you connect this to the scope ground you can essentially short the amp to ground and often destroy a pc based scope instantly. You need a floating, isolated or differential input setup.

Another caution is that the measured waveform voltage may be (a lot) higher than a pc scope can handle without damage.
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Excellent points, hence the suggestion to buy a cheap ~20MHz scope that's pretty much useless in anything but class A/B AF amp duty.

Regards
Sam
 
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#42
Back to the problem...Here's what I'd do:

1) Double-check the speaker cables to make sure one of them isn't intermittently shorting out (say at the connection inside the plug, I've seen that multiple times) and check the terminals in the speaker, is one loose and perhaps shorting internally?

2) Confirm the speaker impedance is not below the amp's recommended load

If--and only if-- the above two items are OK... then plug mixer into an AC receptacle that is known to produce adequate voltage and current and:

3) Hook both output channels up to dummy loads at the lowest impedance supported by the amp

4) Run sine wave input into mixer. Observe output signal on scope.

5) While running amp at max rated output, measure DC rails. Any AC riding on DC rails?

6) See if amp cuts out, then diagnose further based on readings when amp cuts out.

I cannot stress enough that any further conjecture on solder joints, etc is useless if the speaker cables and speakers have not been thoroughly checked.
 
#43
T-Bird said:
Hi.



Excellent points, hence the suggestion to buy a cheap ~20MHz scope that's pretty much useless in anything but class A/B AF amp duty.

Regards
Sam
Click to expand...

A 20MHz scope is an ideal tool for industrial control analysis, low frequency PWM work, servo work, etc. it's a highly under rated, under utilized tool IMO.
 
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#44
Hi.

agedhorse said:
A 20MHz scope is an ideal tool for industrial control analysis, low frequency PWM work, servo work, etc. it's a highly under rated, under utilized tool IMO.
Click to expand...

Yes, 20MHz suitcase style scopes do have their use (if anyone still has one that is ;)), but decade or so back they were practically giving them away.
Up to a point that the current price level is where it is just because so many were scrapped.
At least over here.
I'm a hoarder, and every time the subject of scopes have came up, the aplogetic reply has been: "we just recycled 10 old ones that had been sitting on a shelf for who knows how long"
IIRC I paid 20€ for the 20MHz I have and 5€ for the 40MHz one. Without probes of course.
Heck, I paid more for my first one in the late 80's, a Telequipment 5Mhz single trace :).

Hand-held ones are still pretty pricey, and IMLE the lower priced ones are rather crappy for audio work.

Regards
Sam
 
#45
My first scope was a 1960's-vintage Hewlett Packard. Cost me over $100 and it was good for all of 500 kcps !! Worked good enough for audio use though. About 10 years ago I replaced it with a BK which cost me all of $40. It's good to 15 or 20 MHz. Works more than enough for anything I'm doing, I'm not aligning CD players or FM tuners.

So let's think about the PC-scope concept: plug an unknown voltage/current source into a PC which costs $600 or more, and which happens to be pretty unforgiving of static and voltage blips, and which happens to contain lots of other stuff like pictures and files and documents that would be a PITA to load onto a new computer should my existing PC fry.

Or, buy a $40 scope off of Craigslist, which has overvoltage and overcurrent protection, and which is easy to adjust for time and volts/div. Gee. What a difficult decision.
 
#46
Here's an example of the most standard of all industrial process & control scopes, 20MHz is plenty for the vast majority of the installed infrastructure, anything more and it moves to a totally different class of product. Invalid Link Removed

I use a 20 and 40MHz scope for everything I do other than the occassional DSP project, and I greatly prefer the analog scopes because when looking at carrier residuals the sampling of a typical digital scope may make something look better or worse than it really is. I'm sure some of it goes back to the look & feel of the hardware, the interface and experience reading between the lines.

I do have some laptop based tools but I am very, very careful not to make mistakes and often I will transformer isolate the audio between the analyzer and the unit under test if I am not 100% sure of what is going on. I never measure the output of anythig that can drive beyond 15V above and below ground maximum, often I will add a pad in front if I am going to be lookig at large signals over a couple of volts.
 
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#47
In a previous life as a bench tech I had a 20MHz (if that even) scope that I used for everything. Now 30 years later I'm wishing I still had one for sporadic use on audio circuits. Any recommendations on reliable and cheap models to look for on Craigslist? Thanks!
 
#48
froghat said:
In a previous life as a bench tech I had a 20MHz (if that even) scope that I used for everything. Now 30 years later I'm wishing I still had one for sporadic use on audio circuits. Any recommendations on reliable and cheap models to look for on Craigslist? Thanks!
Click to expand...

Even refurbished Tek scopes are pretty cheap these days -- hard to go wrong there. I've had a 30Meg Hitachi V-302F for around 30 years, still going strong after many thousands of hours of use. Can't imagine they would go for much at all as it was only around 5 bills new back then.
 
#50
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?
 
#51
No, because there shouldn't be -40V on the input of the regulator IC to begin with, not without preconditioning.
 
#52
Maybe I wasn't clear on my initial post. There isn't -40V on the input of the IC when the amp is first powered up. 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. The IC7815 voltages do not change at all.
 
#55
As was noted a bit earlier, this is a Behringer, and while I have no prejudice against Behringer (and have used a number of their devices with great results) spending a lot of effort and time on it is a losing proposition.

I appreciate helping a friend, and it seems that AgedHorse has contributed some great advice and commentary, but it appears to me that the law of diminishing returns says it's time to walk away and say "It's dead, Jim."

55881466.jpg
 
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#57
bluebird28 said:
I offered to help a friend that is using a Beringer powered PA that keeps going into Over Load after a short time period. I pulled the amp section to look at the circuit board. There are no burn marks or burned resistors or transistors, the board looks fine as do all the components on the board however the power transistors (4 ea.) attached to the heat sinks are soldered on both the top and bottom of the board the top solder joints are dull gray and the bottom are shiney. Do you think that resoldering the top solder joints would help this situation, or do you have any other suggestions? Your help is greatly appreciated.
Click to expand...
I experienced a similar issue with an HK audio amplifier some time ago. Nothing wrong wrong with the device acccept an little bit too conservative designed heat protection for the amplifier. Nothing wrong with the device but it helps to protect from abuse.
 
#59
Pilgrim said:
As was noted a bit earlier, this is a Behringer, and while I have no prejudice against Behringer (and have used a number of their devices with great results) spending a lot of effort and time on it is a losing proposition.

I appreciate helping a friend, and it seems that AgedHorse has contributed some great advice and commentary, but it appears to me that the law of diminishing returns says it's time to walk away and say "It's dead, Jim."

View attachment 3224672
Click to expand...

Lol! I get it. The deal on these speakers is that my friend gave them to me when he left town, so they don't owe me a thing. One works great, the other is as previously described. I'm just trying to figure out if I can fix it myself and if there are any common failure with these types of amps.
 
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#60
vernster08 said:
I can't tell if it's getting that warm to be honest. The #1 pin is the common ground on the 7915.
Click to expand...
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 :D 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 :DThat's can take some time, but you might nail it. Just be careful!
 
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