Peavey XR-600B Thermal Issue

[rf7]

This PA head was in before for a general maintenance, dirty pots, etc. It came back because it is going into thermal shutdown after 30 minutes of use. I ran it for 50 minutes at 20% power and it held up fine, but when I bumped it up 50%, it immediately shut down.

The thermal breaker says 434-4 L230 84/J on it. Maybe that's a 230 deg. F breaker?

Is he just driving it too hard or is that breaker potentially bad?

Is there anything else that I should consider looking at? The amp puts out 261Wrms into 2 ohms before clipping.

[Enzo]

So what power amp is on the back of that thing? Is there a row of transistors under black plastic covers across the rear? Or does it have them inside? 400 series? 400B/G? 400BH?

Immediately shut down? usually it takes a little while for heat to build up.

What exactly happens? The entire amp goes dark? Have you verified that thermal breaker is indeed what is opening? The heat sink has to get darned hot to open one of those, so is the thing too hot to touch? The thermal breaker kills power to the entire amp.

If that is what is happening, then you need to determine if the amp is really overheating and the thermostat is just doing its job, or if somethig else is the matter and the amp is shutting down but not really overheating.


Here is one for you, you crank it up to 50% and it shuts down. How long does it take after you remove the signal for it to come back on? If it is thermalling, then it takes time for the heat sink to cool.. If it comes back immediately, it doesn't sound like thermal.

[rf7]

So what power amp is on the back of that thing? Is there a row of transistors under black plastic covers across the rear? Or does it have them inside? 400 series? 400B/G? 400BH?

The transistors are on the inside. It's a 400B/G PCB.

Immediately shut down? usually it takes a little while for heat to build up.

No, I ran it for 50 minutes at 20% power into a 2 ohm load. It got pretty hot, but seemed stable, until I bumped it up to see if it would shut down. It will push all the way to clipping no problem, when it is cold.

What exactly happens? The entire amp goes dark? Have you verified that thermal breaker is indeed what is opening? The heat sink has to get darned hot to open one of those, so is the thing too hot to touch? The thermal breaker kills power to the entire amp.

The output went away. The heat sink was too hot too touch for a quite a while before it shut down.

If that is what is happening, then you need to determine if the amp is really overheating and the thermostat is just doing its job, or if something else is the matter and the amp is shutting down but not really overheating.


Here is one for you, you crank it up to 50% and it shuts down. How long does it take after you remove the signal for it to come back on? If it is thermalling, then it takes time for the heat sink to cool.. If it comes back immediately, it doesn't sound like thermal.

It took a few minutes before it would output again. It was not immediate at all.

I can do it again and check the thermal breaker right after it happens.

[Enzo]

Thermostats are available in the parts marketplace, but I'd just call PV and get the exact part if I needed one. But from your description it sounds like the amp is overheating, which is not the thermostat's fault.

Does yours have the output auto-transformer? If so, unplug it. COnnect the speakers directly to the amp. ANy difference? Before that, disconnect the autoformer and see if any DC offset exists on the output unloaded.

Q2, Q12 are the predrivers, note the schematic says voltages on their bases of +1.17 and -0.6. Yours look like that? And if they are skewed, what REALLY matters is that the two bases stay about 1.8v apart. ANything more than that will make the amp run hot.

[rf7]

No auto transformer that I can see. There is the Inductor L1 in line with the output. I measure 3mV DC on the output, pretty negligible.

Q2 and Q12 bases are 1.786Vdc apart, so that's OK.

[Enzo]

The autoformer wouldn't be on the board, it is an iron core transformer that plugs onto the little posts where the speaker connects. It is only used on certain models with the 400BG.

OK, no DC offset, good. Bias looks OK, good.


Just sitting there idle, does it get hot? Or only while playing through it?

Over by the output, see R47? 5.6 ohm 5w. Make sure it is not open. And just because, measure the resistance of R32, right next to and in parallel with the inductor. If it reads 22 ohms, the iinductor is open or broken free. The resistor should read as a short. R47 is the stability network. If the amp is oscillating at RF, you wouldn;t hear anything but it would get hot.

[soundguruman]

This PA head was in before for a general maintenance, dirty pots, etc. It came back because it is going into thermal shutdown after 30 minutes of use. I ran it for 50 minutes at 20% power and it held up fine, but when I bumped it up 50%, it immediately shut down.

The thermal breaker says 434-4 L230 84/J on it. Maybe that's a 230 deg. F breaker?

Is he just driving it too hard or is that breaker potentially bad?

Is there anything else that I should consider looking at? The amp puts out 261Wrms into 2 ohms before clipping.

If you run it into 2 ohms, no wonder it overheats. 4 ohms would be the lowest you could run it.

[Enzo]

The XR600B is rated at 300 watts into 2 ohms. Printed right on the panel and in the factory specs. It will be happy as can be at 2 ohms.

[rf7]

Just sitting there idle, does it get hot? Or only while playing through it?

No, it does not heat up while idling. Only when it is being played through.

Over by the output, see R47? 5.6 ohm 5w. Make sure it is not open. And just because, measure the resistance of R32, right next to and in parallel with the inductor. If it reads 22 ohms, the inductor is open or broken free. The resistor should read as a short. R47 is the stability network. If the amp is oscillating at RF, you wouldn;t hear anything but it would get hot.

Those resistors are all fine. I did not see any oscillation on the scope. There is some noise on the output if you crank the treble on each channel, since the EQ is after the volume pot for each channel, but the amp does not heat up because of that.

[Enzo]

OK, so it works all day on 4 ohms? But as soon as you drag more than 130 watts from it on 2 ohms it shuts down? (130 = 1/2 of yo0ur 261)

[J M Fahey]

1)

434-4 L230 84/J

if anything, sounds like 84ºC to me, which by the way is a reasonable temp value.
Popular ones usually run from 75ºC to 85ºC.
*Maybe* 230ºF is the equivalent, too lazy to check now, but values are *usually* in ºC, so ....
2) as of your working all day on 4 ohms, but not on 2 ohms, remember the *cabinet* impedance is nominal, and some are lower than others.
So maybe you are using, say, 2 x "4 ohms" cabinets but each is ,say, 2.5 ohms or less at least at certain frequencies.
Have seen it many times to ignore the possibility.
Happened to me short ago: I shipped an amp to a guy who blew it in the first week. 100W into 4 ohms.
I repaired it but added a "fool/dumb proof" protection I only add in certain cases, when people is too dumb to follow instructions.
The amp drives happily 4 ohms all day long (2 x 8 ohms speakers in parallel) , and up to (or "down" to) 2.6 ohms, which means *3* speakers in parallel, but triggers at around 2.2 ohms or less: a relay cuts the speaker out, a Red "Overload" Led lights and the amp stays mute until you turn it off, wait a minute and turn it on again.
With the proper load, of course!!!
The guy complained that it drove one of his cheap Chinese "8 ohms" cabs but turned off with two.
My answer? : "simple, both your cabinets are less than 2.2 ohms .... period".
So each one is less than 4.4 ohms, at least at some frequencies !!!
I suspect the 5 parallel Piezos present in each "PA/DJ " cabinet.
Maybe something similar happens to your customer.
3) repair should not mean re-engineering, but maybe you can add some small PC type fan and cool things off.
Definitely won't hurt !!!

[rf7]

OK, so it works all day on 4 ohms? But as soon as you drag more than 130 watts from it on 2 ohms it shuts down? (130 = 1/2 of yo0ur 261)

It seems to run all day at 50% power into 4 ohms. It ran for 20 minutes at 130 watts into 2 ohms before it shut down. The customer says he's only running one cabinet, but he does not know the impedance of it.

Yeah, a fan inside would be a good idea!