Tranny failure

[Mark Hennessy]

As the Marantz works ok with music, its probably ok

No!

Something is wrong, and you need to do diagnosis. What you've done so far has been very "general", and that's fine and understandable, but that approach doesn't deal with the difficult ones.

As hinted at earlier, the clue to this is the average level of sine waves compared to music. Ignoring modern hyper-compressed rubbish, music has a PMR of 20dB or more. 20dB. In power terms, that's a factor of 100.

To put that into context, a 100W amp that is on the cusp of clipping the peaks is only delivering an average power of 1 or 2 watts. Yes, incredible. But true.

Compare that to a sine wave, where the PMR is very much smaller. Just 3dB.

So as soon as you do sine wave testing, you're asking the amplifier to deliver much more power than music. That means that much more is demanded from the power supply, and the power transistors are going to get a lot hotter. Something is falling over, and you must determine what. Because if you don't, you'll find out the hard way that the owner like to listen to hyper-compressed modern rubbish through very inefficient loudspeakers while wrapping the amplifier in a duvet, having first placed it on top of a radiator! Well, not exactly, but you get the point.

Your fault reports are getting better, but we need to be a lot more specific.

When the output falls to almost nothing, what exactly happens? Does the sine wave become severely clipped (and equally so on both peaks)? Do the power supply rails stay where they should be, or does one (or both) fall accordingly? That's why - BTW - we need several multimeters

Does the input to the power amp itself stay the same?

Does it happen equally to both channels? That's a really powerful question, BTW, and one that is unclear to me so far...

What temperature are the output devices when this happens? (IR thermometers are good for this, and only ~£10 these days).

Is the current limiting causing the clipping? And if so, is that because of a dry joint on the output emitter resistors (reasonably common).

This amplifier has DC offset detection. Assuming this works properly, that'll fire in the event of many faults.

BTW, what's a "ripple current of 300mV"? That's a ripple voltage, and a perfectly reasonable one for an unregulated PSU. That ripple voltage will reach several volts at full output, but the amplifier will be designed to reject that. Not a problem. What's more interesting, however, is the frequency of the ripple. It should be 100Hz on both rails, but it it's 50Hz, that points to an o/c diode (or dry joint or similar). It happens.

Can we just check, BTW, that we're talking about the right amp. I got the 1070 manual from Hi-Fi Engine. It's only 38 watts per channel, and that's into 4 ohms. That's a long way from 70 watts. The rails are only +/-36V when idling, so it would struggle to reach 50 watts on a good day. If the "M" version differs significantly, have you got the manual? A link or similar would be good...

As for the Sony, you need to measure the DC voltage across the bias circuit as a starting point. There's little point dabbing the 'scope probe around the circuit as the negative feedback loop will confuse everything. If the adjustment is doing nothing, that points to a pretty specific part of the circuit. So let's focus on it, rather than everything else. (NB: I haven't yet looked up the schematic for the Sony).

Finally, the above was dashed out in a hurry, so apologies for the highly unstructured brain-dump. I'll have more time later this evening.

Don't give up!

Mark