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Old 6th August 2014, 05:39 PM   #1
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Default Oscillation found in Luxman LX33

Hi all,
I have an old Luxman LX33 which have considerably noise at the output (around 20mVAC, measured by my Agilent 34401A).

Traced around and I found oscillations at the Preamp outputs (actually the Baxandal-type tone stack): same frequency, amplitude and characteristics for both channels. The oscillations are at approx. 37kHz with amplitude rise and fall pretty fast (several times/second) from 0 to about 4mVpp (bass & treble controls at 50%; volume at min.).

So far I have tried and observed the following, but the result is still fruitless:
- swap/replace the 12AX7
- disconnect the amp's PSU, power the tonestack with separated HT & heater supplies
- oscillations seem to appear somewhere the output triodes
- amplitudes of the oscillations is at minimum (approx. zero) when Treble knob at min. and Bass knob at either min. or max; max amplitude is achieved at max. Treble and 50% Bass

Now I'm stuck here, please help me solving this.
Attached is the schematic of the entire amp and the tonestack (B+ for the tonestack is approx. 300VDC).
Thanks,
Duong
Attached Images
File Type: png LX33 tone stack.png (153.0 KB, 227 views)
Attached Files
File Type: pdf luxman_lx33_schem.pdf (163.2 KB, 33 views)
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Old 6th August 2014, 09:41 PM   #2
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Might be too much gain in B4/2.
Try removing C112 to reduce gain a bit.

I see B+ for tone stack goes to two valves.
I usually separate the B+ to stop feedback between stages.
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Old 7th August 2014, 12:55 AM   #3
kevinkr is offline kevinkr  United States
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Given the age of this unit (30+ yrs) I would replace the power supply cap feeding the tone stack before proceeding with any modifications. I would probably replace all supply caps at this point.

Reducing the gain in either stage is going to seriously impact the performance of tone stack, in particular source impedance due to reduced OLG in the first stage and in the second stage would impact the contour as well as distortion characteristics of the tone control.

I am sure the amplifier worked properly at one point, and something is now broken - diagnose and fix that. Check for audio (37kHz osc for example) on the supply to the tone stage. Modify only after original problem is understood and solved.
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Old 8th August 2014, 02:25 AM   #4
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Quote:
Originally Posted by nigelwright7557 View Post
Might be too much gain in B4/2.
Try removing C112 to reduce gain a bit.

I see B+ for tone stack goes to two valves.
I usually separate the B+ to stop feedback between stages.
What Kevin says.
I was in design mode !

You should be replacing all electrolytics if it is of any age.
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Old 8th August 2014, 02:55 PM   #5
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Quote:
Originally Posted by kevinkr View Post
Given the age of this unit (30+ yrs) I would replace the power supply cap feeding the tone stack before proceeding with any modifications. I would probably replace all supply caps at this point.

Reducing the gain in either stage is going to seriously impact the performance of tone stack, in particular source impedance due to reduced OLG in the first stage and in the second stage would impact the contour as well as distortion characteristics of the tone control.

I am sure the amplifier worked properly at one point, and something is now broken - diagnose and fix that. Check for audio (37kHz osc for example) on the supply to the tone stage. Modify only after original problem is understood and solved.
Thanks all for your ideas,

Indeed since oscillations appear on both channels with similar characteristics, the first thing I suspected is the supply, hence I replaced both heater and HT with my bench PSUs but it didn't eliminate the oscillations.
However, I accidentally found out that the oscillations came from the ballast of a florescence lamp on my bench!!! Right after I turned off the light, all the oscillations disappeared. Quite embarrassing haiz.

Nevertheless, listening test still showed audible noise from the tone stage (noise disappeared when I shorted the PA input to gnd), so the issue is still there. PSU caps replacement is on the list again, but the original ones are triple sections 3x47uF/450WV & 3x47uF/550WV so replacement parts will be difficult.

(to be continue)
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Old 10th August 2014, 02:35 AM   #6
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Ciao, hoang duong go
Pls check pcb Clear or not ?
Step by step replacing power tubes, driver , tone, input... For check osc signal
And , and,,, take my home if you want
B rgds
Thuylt from vnav
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Old 10th August 2014, 03:00 PM   #7
kevinkr is offline kevinkr  United States
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Hoang, I've had the same exact experience with an electronic ballast in a fluorescent lamp on my bench so I can commiserate!

Glad you found it was an external disturbance..

Would still be advisable to replace the supply electrolytics, at best they are tired, at worst they could be a real liability.

Both JJ and FT make multi-section caps, you might find something that works well enough..
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Old 10th August 2014, 03:03 PM   #8
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I had the same problem wit ha new SS design.
Oscillation everywhere.
My desk lamp had a CFL lamp in it and when I moved the lamp away the oscillation went.

Many years ago I designed an electronic dog flap.
It had a huge inductive loop in the door to read the dogs tag.
In the mornings it would work fine but in the afternoon it picked up oscillations.
Turned out next door turned their TV on in the afternoons and I was picking it up next door !
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Old 11th August 2014, 02:12 PM   #9
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Case closed!
Luckily enough the LX33 has one spare hole for another PSU caps so I replaced the 2 triple-section (3x47uF) cans with 3 Mundorf 2x50uF/500V caps. The pesky noises are now almost gone.
I also found how sensitive the Mundorf EVO caps are, I used them to replace the original coupling caps in PA stage. Just removing the old caps and putting the new ones in place still leaves some back ground noises, but when I moved them to under the PC board, all the remaining noise went away.
Thanks all for your inputs as well
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Old 11th September 2014, 02:44 PM   #10
Norelco is offline Norelco  Philippines
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Hi Hoang.. What is the correct bias setting, because I just buy a Luxman LX33 today and i'm going to put on new matched quad of Mullard EL34 reissue,

Thanks


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