Peavey XR-560 powered Mixer blowing fuse

[gcb34]

I cannot seem to figure this one out.

If I only have 1 power source from the transformer it won't blow. If they are both connected it will blow. Loud hum as well. Any ideas?

[Jazz P Bass]

Either your output section is blown or a bad bridge rectifier?

[gcb34]

going after thr bridge rectifier

thanks for the reply. the output transistors look ok

[Enzo]

LOOK OK?

Transistors don't LOOK good or bad, they MEASURE good or bad.

Disconnect the speaker, and measure for DC voltage on the speaker jack. Power off and measure those power transistors for shorted conditiion.

What power amp is on that model?

[gcb34]

power amp is

150M

I meant measured good.

[gcb34]

rectifier measured good.... I hooked everything back up to check the output dc voltage but the fuse blew again.

[Enzo]

OK, no speaker or other load until we are fixed.

Look up "light bulb limiter." Make one and use it. Saves fuses, saves circuits. simple to make.

If no outputs are shorted, check all those cement power resistors for opens, and checl the two ribbon cables for opens.

There is a special diode down at the end of the heat sink. It is a dual diode, forward voltage is twice a normal diode. Its leads feed directly back to the driver board through a ribbon. Check the diode for open, then check again at wherever it is wired to on th driver side, we need to have continuity.

Also on the output board there is a small 180 ohm resistor centered between the cement resistors. Check for open.

[gcb34]

can i use a variac for now?

[Enzo]

Yes, of course, if you have a variac and understand how to use it in this case, then certainly. You said fuses were blowing. I can;t assume you have any equipment or knowledge until you let us know. No offense meant. yes, I use a variac and mains current meter instead of a bulb myself.


If you can bring it up a little on a variac, you can usually see if the output is going to center or slam over to a rail.

My agenda is to look for opens, once you have checked for shorted transistors. The bias circuit holds the opposing bases together, if that opens, then both sides will turn on and thus draw excess current. SO that is the importance of the cement resistors and ribbon connections.

But do check the transistors on the drive board if you have not.

[gcb34]

The cement resistors, ribbon connections, special diode and 180 ohm resistor all measured good.
When I bring up the variac just before it rails I have 4 vdc on the speaker jack.
BTW thank you Enzo for sticking with me with this one.

[Enzo]

OK, so output section might be OK. Go to the main rectifiers, verify all are OK, and scope the two main rails. At the last safe point on the variac, are both rails about the same? Or is one seriously lagging?

Rule of thumb: if an amp just puts DC on its speaker leads, as long as there is no load present, the amp will sit there with the DC, fuses will be fine, because that 50vDC or whatever has no load drawing current. SO when fuses blow that generally means both sides of the output are being turned on at the same time, whether by the other circuits or through faulty transistors. Of course other conditions can occur, but that is where I start.

On the upper and lowr left corners of the basic power amp, there are 15v zeners for the op amp rails. Checked them? And speaking of, wouldn;t hurt to swap out U302.

CR304,309,311,317, plus that dual diode - they are your bias string. Make sure nobody is open along there.

[gcb34]

I don't have a scope. Where can I check with two dvm's?
CR304,309,311,317 replaced
U302 replaced

[gcb34]

I got to about half way on the variac the cement power resistors got very hot and somewhere it was smoking.
the double diode is good

[Enzo]

Well, if both sides are turning on at the same time, then those power resistors are having to dissipate all that failure current as heat.

Did you see if those zeners were at least not shorted or open? Could you get the variac up enough to get them to try to "zene?"

Are ALL FOUR cement resistors getting hot? Not just one or two? If so, then for sure both sides are turning on. And that means the bases of the + side are being pulled towards V+ and the bases of the - side are being pulled toward the V-. Q300 and Q308 are your drivers, so their bases should be held together by the bias string. Roughly 2v apart or +1v on Q300, and -1v on Q308. Just ball park figures. If the bias string is not intact, then those bases will be a lot closer to their respective V+/V- rails. SO look into that.

The drivers usually should be replaced if the output transistors were replaced. And always check the emitter resistors for them, R314, R319 100 ohms.

CR308, CR316 can be unsoldered at one end and lifted, that will disable limiters Q303,305 in case they are involved. Those parts are protection, but not needed for basic amp function, at least during tests.

SOMETHING is causing both polarities of output to turn on at the same time.

[gcb34]

The 4 resistors all got hot... the zeners were all good.
I will try the CR308 and CR316 lift next.
What can I substitute the output NPN and PNP transistors with?