Luxman R117 receiver with oscillation

[Tim Schwartz]

Hello and Happy New Year all,

I've got a Luxman R117 receiver here that breaks into a wild
oscillation when the amp is pushed. I believe that it occurs when the
amp switches from the low B+ rails to the high B+ rails. Turning up the
treble control seems to make it worse, but that might be a red herring.

I've changed all of the electrolytic capacitors in the tone circuit,
I've tested and substituted the 4 main filter caps. I've check the
suppression network on the output.I this this is some sort of decoupling
problem, but I've sent way too much time on this receiver. A friend who
worked on these when they were under warranty has looked it over as
well, and we're both stumped.

Anyone in the group seen anything like this on a Luxman R117?

Many thanks,
Tim Schwartz
Bristol Electronics

 

[N_Cook]

Hello and Happy New Year all,

I've got a Luxman R117 receiver here that breaks into a wild
oscillation when the amp is pushed. I believe that it occurs when the
amp switches from the low B+ rails to the high B+ rails. Turning up the
treble control seems to make it worse, but that might be a red herring.

I've changed all of the electrolytic capacitors in the tone circuit,
I've tested and substituted the 4 main filter caps. I've check the
suppression network on the output.I this this is some sort of decoupling
problem, but I've sent way too much time on this receiver. A friend who
worked on these when they were under warranty has looked it over as
well, and we're both stumped.

Anyone in the group seen anything like this on a Luxman R117?

Many thanks,
Tim Schwartz
Bristol Electronics

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

 

[William Sommerwerck]

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.

 

[Chuck]

Hello and Happy New Year all,

I've got a Luxman R117 receiver here that breaks into a wild
oscillation when the amp is pushed. I believe that it occurs when the
amp switches from the low B+ rails to the high B+ rails. Turning up the
treble control seems to make it worse, but that might be a red herring.

I've changed all of the electrolytic capacitors in the tone circuit,
I've tested and substituted the 4 main filter caps. I've check the
suppression network on the output.I this this is some sort of decoupling
problem, but I've sent way too much time on this receiver. A friend who
worked on these when they were under warranty has looked it over as
well, and we're both stumped.

Anyone in the group seen anything like this on a Luxman R117?

Many thanks,
Tim Schwartz
Bristol Electronics

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

 

[William Sommerwerck]

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

 

[Tim Schwartz]

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.

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

 

[Tim Schwartz]

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

Hello Nigel,

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

Thanks for your time,
Tim
Bristol Electronics

 

[William Sommerwerck]

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!

On 1/4/2011 9:35 AM, William Sommerwerck wrote: interaction.)



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'.

 

[William Sommerwerck]

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

 

[N_Cook]

Hello Nigel,

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

Thanks for your time,
Tim
Bristol Electronics

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

 

[William Sommerwerck]

I would try some extra/changed C over preamp low-

voltage supplies or try some chokes in the pre 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.

 

[Geoffrey S. Mendelson]

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.

 

[William Sommerwerck]

I would try some extra/changed C over preamp low-

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.

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