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Old 15th February 2011, 01:44 AM   #1
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Default MFA Lumi sings high-pitched tunes

Hello. My Lumi has began to produce very high-pitched, faint noise. It starts as soon as I flip the mute off, as soon as it warms up, it sings for about two hrs, then it disappears. It is in both channels, independent of volume control, and is in line section only, very faint but annoying non-the-less. I tried changing tubes, no change. Since it goes away in a couple of hours, am I correct to assume that some caps are gone? Unit is quite old, so it may be a good idea to recap it completely. My film caps are all new, but cans are all original (with exception of phono filament supply). Any thoughts? Thank you.

PS I have schematic diagrams for A-2 and C versions, if anybody is interested.
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Old 15th February 2011, 02:43 AM   #2
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Sounds like oscillation. Could be power supply caps (or many other things). Is there a regulator?
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Old 15th February 2011, 02:36 PM   #3
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Quote:
Originally Posted by SY View Post
Sounds like oscillation. Could be power supply caps (or many other things). Is there a regulator?
LOVE your France quote... :-)

It has tube regulated high voltage, zener regulated phono filaments, but no regulation on line filaments. I tend to think that it has to be recapped...
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Old 15th February 2011, 03:20 PM   #4
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Is the noise common to both channels?

I love that quote. Kept it longer than I have most of them.
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Old 15th February 2011, 06:10 PM   #5
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Is the noise common to both channels?

I love that quote. Kept it longer than I have most of them.
Yes. what throws me off is that it disappears after a while
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Old 15th February 2011, 06:15 PM   #6
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If it's in both channels, it's likely to be the regulator or some other power supply function common to both channels. Just out of principle, I'd replace the reg tubes and electrolytics. I'd also look at whatever circuit references the tubes' heaters (especially the pass tube) to a DC potential. Is the reference voltage derived from a Zener or from a VR tube?
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Old 15th February 2011, 07:28 PM   #7
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Quote:
Originally Posted by SY View Post
If it's in both channels, it's likely to be the regulator or some other power supply function common to both channels. Just out of principle, I'd replace the reg tubes and electrolytics. I'd also look at whatever circuit references the tubes' heaters (especially the pass tube) to a DC potential. Is the reference voltage derived from a Zener or from a VR tube?
I'm not home now, i will have to look at the diagram. VR tube, I beleive, only provides 330v... Zener provides 6.3v, I don't think it is floated at any potential. I have to check.

Thx!
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Old 15th February 2011, 07:44 PM   #8
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I'm not home now, i will have to look at the diagram. VR tube, I beleive, only provides 330v... Zener provides 6.3v, I don't think it is floated at any potential. I have to check.

Thx!
Sorry, I was unclear. The "reference" voltage I was asking about was for the regulator error amp. Different than the first thing I mentioned, the voltage to which the heaters are referenced (but that's also important).
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Old 15th February 2011, 08:57 PM   #9
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Sorry, I was unclear. The "reference" voltage I was asking about was for the regulator error amp. Different than the first thing I mentioned, the voltage to which the heaters are referenced (but that's also important).
I will post a diagram later tonight.

High voltage is referenced either by a zener chain, or a 6EJ7 tube circuit there,I don't remember which exactly now. I have replaced the chain once, because one of the diodes died and it started to motorboat.

There were suggestions to float heaters at about 150v. I think that in later versions they are.

Thanks

Mark
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Old 16th February 2011, 12:33 AM   #10
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Default Diagrams as promised

Diagrams as promised
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