On 22/05/2017 17:43, Mark Zacharias wrote:
On Sunday, May 21, 2017 at 7:17:01 AM UTC-5, Lee wrote:
Have been asked to look at this Marantz PM6004 amplifier, report is it
"cuts out" if the volume is raised more than a third.

Had a Google and I get hits saying its "a common fault" but nothing
about resolutions.

Before tearing it apart, are there any common issues with this model?
(I found the auto shutdown "crackle" fix)
What it appears to do, on a dummy load, is the protection circuit is
kicking in, seemingly at random...

Obviously I'll get in there and measure voltages and waveforms when I
have it set up on the bench later

I would check to see if the output waveform for either channel clips prematurely; this is seen by the protection circuit as DC voltage.

Also check for bad solder connections involving the output transistors.

I would also check to see if the integration capacitor for the protection circuit might be open. In this case maybe C9003, a 47uF at 50V.

A good multimeter and oscilloscope are generally required to trace down these faults.


Right, I am now confused. Not that it's difficult

I think the original music source it was tested with was severely over
driving it...

If I feed it a 0.8v pp sinewave from my sig gen it doesn't clip at full
volume and doesn't go into protect.
If I feed it 1v pp it just starts clipping (symmetrically) at 75% volume
then "soft" protects if pushed to "full" - that is it goes into "mute"
(blue led) and lowers the volume then un-mutes itself. It has a
motorised volume pot so you can see it do this

However, if I feed in 2v pp* (max it says in the specs), then it *just*
starts clipping at half volume and if pushed further to about 75% volume
it goes straight into standby with "protect" (the standby led)
flashing, needing a power cycle to reset.

Other than it being a touch quick to go into protect, does this sound
like it's actually working properly? Or at least as it was designed?

Where it's normally installed, it's paired with some very expensive
speakers and I am loathe to poke about in the protection circuit. If the
advice here is that it's behaviour is still flakey then someone else can
take that risk

*assuming my sig gen is accurate, of course.