Troubleshooting Powered Mixer

[captntasty]

Hi all,
I am working on a friend's powered mixer. It is frying a resistor upon power-up. Here's the schematic with the offending resistor. How do I approach this for troubleshooting - should I jumper this 43R resistor then get some measurements?



Here also is a link to the entire schematic PDF - it's on page 5:

http://www.biamp.com/download_redire...ype=prod&tid=6

[Brad1]

It's only connected directly to four components. Q8 and Q9 at one end, and Q13 and Q14 at the other. I'd start by checking those to see if any of those are bad, and work out from there.

I wouldn't "jumper" it. It's there for a reason. Look at the flow.

Brad1

[chipprogr]

LOL i love that" offending resistor " statment.. try a railroad spike, it might hold...


Ok on a serious note, id pull Q 8,9,10,13,14,15 and check them with a meter. Chances are one of your predivers and/or outputs are burnt and causing this resistor to burn. Its never the resistors fault for smoking..

[Enzo]

It is pretty much a certainty one or more of those three transistors under it are shorted.

And the other set of three transistors up above it a ways? I'd be checking them too.

[captntasty]

I knew it had to be something on one or the other side of that resistor. I tested the transistors and it appears to be q9 or q10 (don't know which one) but one is a dead short - MJ15022. All the others seem to be okay.

One other question - on testing one of the voltage regulators, a LM7815, I get no continuity on any pins whatsoever but on the complementary LM7915 I do... what would be the proper test for these and what results should I expect? I know the 7815 is a + voltage regulator and the 7915 is a - voltage regulator... does this account for no reading or is the 7815 cooked?

[Jazz P Bass]

LM7815

That regulator should show something on the meter.
There is not any good test that you can do to prove it works other than hooking it up to 24Vdc.
On diode check the IC should read somewhat like a diode.
Use the ground leg as anode.
On resistance check it should read pretty high.
Resistance or diode check verify a short or open.
They can't tell if it will function.
(Look at the block diagram of the datasheet. One busy puppy)
Take the IC out of circuit. Check the input pad to verify 24Vdc (give or take ).
With the power supply turned off check the resistance of the output pad to ground. Low ohms is bad. High ohms is good.

[captntasty]

The diode test shows nothing on the 7815... on the other it acts sort of like a diode. Given that I figured it was gone... thanks.

[Enzo]

The 7x15s are ICs, not transistors. Once you have the power amp not blowing up, just plug the amp in, the 7x15s either make voltage or they do not.

[captntasty]

So while I am waiting for parts I decided to check the power tranny. On the high voltage leads (all disconnected from board) I only see about 1R(ohm) - barely any resistance. Doesn't matter which ones. This is bad right? Means the PT is cooked...?

[chipprogr]

. This is bad right? Means the PT is cooked...?

Its a decent size wad of just copper wire, value sounds absolutley right to me. If it were cooked, you would see the tranny at high resistance and if the wrappings were shorted, they would blow the main fuse in a 4th of july pyro show.

[captntasty]

Even on the 120v primary? I guess I just expected to see at least a little more than 1 ohm.

[captntasty]

OK - so R17, R18, R21 and R22 (all .33R 5W) between the two sets of transistors are gone... completely open. Would one of the transistors being blown cause that or vice-versa - or something else.

What about the bridge rectifier back at the PT? Could that be a problem? What should I expect when I probe that? It generally acts like diodes when probed, which is what it is...

Last question, would a blown speaker cause problems like this? The speaker the guy gave me to use is a 60w speaker and as I understand it this is a 300w output... if he's been using these speakers would that cause some of these components to fail?

[Jazz P Bass]

60 W speaker

Is the 60 watt speaker o/k?
Read the dc resistance of the coil.
4, 8 16 ohms?
The wattage rating on the speaker is how much power it can dissipate.
Running a 300 watt output amplifier into a 60 watt speaker load is alright if you do try to push too much wattage through it.
Yeah, fully cranked the amplifier will cook the 60 watt coil.
It most likely would fail open circuit, not shorted.
Most likely one of the output transistors went bonkers.
Then the house of cards came down.
Open this, burnt that, shorted this......

[captntasty]

The speaker - both the 10" and high tweeter are now open - completely cooked, the coil is completely frozen. They were working - I could hear hum, etc. but if they were in the process of failing could that have caused problems such as a blown output transistor and blown resistors on the output. I replaced the transistor that was definitely gone, and the 4 .33R 5W resistors, powered up, heard a crackle/pop and voila - fried speaker.

I tested the main bridge rectifier. Bridge Rectifier is ok... various components, diodes, resistors, etc, all seem to be ok. I'm going to test the power supply caps next...
What else should I be looking at?

[captntasty]

So I've been able to power it up without anything blowing up... some voltage readings:

PT/BR1 to output - should be +/-59v under load and +/-75v no load
I read +/-39v under load and +/- 62v no load
BR2 to driver - should be +/- 60v under load and +/-75v no load
I read +/-40v under load and +/- 62v no load
Low voltage compared to spec but relatively OK

Here's the problem:
BR3/Voltage Regulators to various IC's - should be +/-15v
I read -0.7v, -11.4v under load and -0.7v, -14.7v no load
The negative rail seems okay but the positive rail is totally off at -0.7v - I had replaced the 7815 positive voltage regulator.

Also, when I have a good speaker attached, no signal, the speaker is pegged with a bit of hum as if it had a very powerful and constant signal. I noticed on the LED Limiter display that it is showing nearly full power output with no signal in. Normally, only the first LED should come on to show power and the rest should remain unlit.

Any ideas where to go next?