MFA Lumi started "motor-boating" today - diyAudio
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Old 13th June 2010, 12:30 AM   #1
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Default MFA Lumi started "motor-boating" today

MFA Lumi started "motor-boating" today :-) Any ideas what is causing it? I have the diagram. My thoughts are: 1) regulator tubes 2) filter caps 3) coupling caps 4) who the hell knows...
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Old 13th June 2010, 04:31 AM   #2
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I would check C35 & C36 for leaking....Going "high".
I would also drop a cap at pin three of your 6L6GC to ground, Appx 1.5uF400+VDC(The 'perfect' capacitor)..I'm suprized there wasn't one in the first place.
Just throwing ideas out.

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Old 26th June 2010, 03:06 AM   #3
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Default Fixed

Fixed - bad zener
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Old 26th June 2010, 07:15 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by marknoir View Post
fixed - bad zener

vr3?

I capitalized the above reference designator, but the forum software all on its own has decided I shall NOT be allowed to do this alone..

Oddly enough if I write an entire sentence I am allowed to do so, however a single word (reference designator) followed by a ? is not permitted.. Strange...
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Old 26th June 2010, 07:34 PM   #5
llwhtt is offline llwhtt  United States
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Hello Marknoir,

What are the chances of getting the rest of the schematic, the audio portion?

Craig
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Old 26th June 2010, 08:06 PM   #6
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Originally Posted by kevinkr View Post
vr3?

I capitalized the above reference designator, but the forum software all on its own has decided I shall NOT be allowed to do this alone..

Oddly enough if I write an entire sentence I am allowed to do so, however a single word (reference designator) followed by a ? is not permitted.. Strange...

No, VR2, one of the pair (chain) providing reference 340V dc voltage. I replaced them with a chain of five 68V zeners.
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Old 26th June 2010, 08:08 PM   #7
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Hello Marknoir,

What are the chances of getting the rest of the schematic, the audio portion?

Craig
Sure. Just send me your email address.
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Old 26th June 2010, 08:56 PM   #8
llwhtt is offline llwhtt  United States
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Here's my email- (removed by mods). Thank you very much!

Craig
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Old 26th June 2010, 10:25 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by marknoir View Post
No, VR2, one of the pair (chain) providing reference 340V dc voltage. I replaced them with a chain of five 68V zeners.
VR1 and VR2 actually clamp the voltage on the grid of the 6L6GC during warm up, when the 6EJ7 has fully warmed up and the feedback loop is closed you will see that the voltage at that point is below the zener threshold by some 40V or so depending on the condition of the particular 6L6GC and the line voltage.. They also will clamp the output voltage in the event the error amplifier tube (a 6EJ7) fails - in this case the output voltage will be much higher though than is the case for normal operation, however it is below the rated voltage of the caps that follow the regulator. (I've worked on a number of MFA Luminescence pre-amps over the last 15yrs or so..)
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Last edited by kevinkr; 26th June 2010 at 10:28 PM.
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Old 26th June 2010, 10:30 PM   #10
kevinkr is offline kevinkr  United States
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Here's my email- deletedatyahoo.com. Thank you very much!

Craig
Might want to ask a mod to remove this as it is an easy target for spambot harvesting.. Don't know about you but anything to avoid additional spam seems worth while..

Better practice would be llwhtt_at_yahoo_dot_com for example..
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