Mackie HR624 protection issue

[rf7]

I've got a Mackie HR624 on the bench that, when given signal, the output pulses on and off. Eventually, the protection kicks in more and compresses the output to sit at 1/10 power (10W). The rails measure fine. Without a load, it pulses once and sits at 10W. How do I approach this?

Thanks in advance!

[Jazz P Bass]

Is it just the LF (Parallel TDA7293) section?
Does the HF (single TDA7293) section do it also?
Your going to have to narrow it down some.
Are the + & - 41 Vdc rails holding up good?
How about the preamp rails?
Can you verify that it is a preamp or a power amp issue?

[soundguruman]

Framing Hammer?

[rf7]

The pulsing only happens to the LF section of the amp. The 41V rails pull down about 5V when the amp is peaking, the 15V rails stay at 15V. The HF section just pulses once and sits at 5W output.

Without a load, the LF section goes up until it clips and then it compresses down to 10Vrms and sits there. The HF section behaves the same way with or without a load.

The preamp output (LF-Drive) looks choked (it's smaller and has a little dip near it's peak) a bit when the amp is peaking, but when the amp output is pulsed low, the preamp output comes up higher and looks correct. The HF preamp output looks OK.

It seems like it's the HF power amp, but the protection is in the preamp section.

[rf7]

Framing Hammer?

Ha-ha! I wish.

[Jazz P Bass]

Is there a power amp IN jack?

[Enzo]

I see two parallel TDA7293 ICs, I bet one of them is defective, in which case I;d change both.

Just below and to the right is a 0.1 ohm R73 in the speaker neg return lead for current feedback. I see a feedback defeat switch. I'd activate that and see if killing that feedback stops this. Make sure that R73 is healthy.

I see TPs J1 and J2, which would ground off any input to the power amp ICs when shorted together. SHort them together, does it still do it?

Without a load, it pulses once and sits at 10W.

I don't know what this means. I am also confusede by a similar statemant in a later post. How does an unloaded output produce 10 watts?

[rf7]

I see two parallel TDA7293 ICs, I bet one of them is defective, in which case I;d change both.

Just below and to the right is a 0.1 ohm R73 in the speaker neg return lead for current feedback. I see a feedback defeat switch. I'd activate that and see if killing that feedback stops this. Make sure that R73 is healthy.

I see TPs J1 and J2, which would ground off any input to the power amp ICs when shorted together. SHort them together, does it still do it?

I don't know what this means. I am also confusede by a similar statemant in a later post. How does an unloaded output produce 10 watts?

I will change the TDA IC's tomorrow.

Killing the feedback does not change the pulsing of the output waveform.

I grounded TP's J1 & J2 and the preamp output is good.

Sorry, I forgot it was unloaded. I should have just said 9Vrms.

[Enzo]

Oh, if you have DC on the speaker lines, then almost certain a blown 7293.

[Jazz P Bass]

Reposted schematic file.

[tmccartney]

Hello, I realize that this is a late response, but replace of the following capacitors. C65, C41, C37, C63, C64. C65 is the most important one as it is the AC power detect circuit. Like Bogen amps, they won't release the power mute unless they see adequate ac measurement.

[rf7]

It turned out to be capacitors C64 & C65 that were bad, actually.