Luxman R117 receiver with oscillation

Audio or ultrasonic osc? What happens with reduced rails via variac?

Both channels or just one channel?

All speakers, or just one particular model?

Does it occur with a purely resistive load? Does it occur when the amp is unloaded?

Are you using oddball speaker cables? (I've seen pathological interaction.)

Here's a thought... Could it be the amplifier rapidly switching between the high and low rails? Have you worked through the circuitry that "decides" when to make the switch? If the oscillation were at 2x or 4x the input frequency, that would strong evidence.

Is there a possibility that someone has changed the outputs or drivers with faster transistors than the OEMs? Chuck

A question... Have you stuck a 'scope on the output to see what the "oscillation" looks like?

Hello,

I've only run the receiver on test equipment. Sine wave in, 8 ohm non-inductive load (Dale NH250 resistors) and I'm looking at it on a scope. Both channels, both outputs. The test set up is the one I always use, and other amps do not have this issue. I have tried one channel at a time with the same result. Speaker cables are 16 gauge 'zip cord'.

I've never left it up for more than a moment, because the current draw is 8+ amps on the 120V line. It's a very high frequency oscillation, but I can't really see the waveform to describe it, as I don't want to blow up the amp. All semiconductors are original.

Thanks for your input, Tim Schwartz Bristol Electronics

Hello Nigel,

It's an ultrasonic oscillation, and a lower line voltage does not help.

Thanks for your time, Tim Bristol Electronics

Ouch! Full-power ultrasonic oscillation! I guess that pretty much rules out all of my suggestions.

Of course, the fact that it oscillates on both channels strongly suggests something common to both channels. I wouldn't completely rule out the possibility that something is wrong with the B+ switching circuit.

Maybe you should convert the amp to power an ultrasonic cleaner!

interaction.)

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Sorry to come back... You didn't say whether this occurred when the outputs weren't loaded.

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I would try some extra/changed C over preamp low voltage supplies or try some chokes in the prea rails

If these fix the problem, then the problem must be something fundamentally wrong with the receiver's design. I suspect that isn't it.

At one time it was very common to use 1uF tantalum capacitors to bypass various devices because they would go into oscilation or produce high frequency hash without them. In the 1970's they were "required" for both input and output sides of three terminal voltage regulators.

Over the years most of them have failed, some with a loud bang, some with no noticable effect. It's quite possible there are a few bypass caps that worked perfectly fine when the receiver was built that no longer do anything at all.

Geoff.

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No argument! But that wasn't what the poster said. (See above.)