MFA Lumi started "motor-boating" today

marknoir · 12th June 2010, 11:30 PM

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...

Richard Ellis · 13th June 2010, 03:31 AM

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.

__________________________________________________ ____Rick.....

marknoir · 26th June 2010, 02:06 AM

Fixed - bad zener

kevinkr · 06:20 PM

Quote:

Originally Posted by marknoir

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...

llwhtt · 26th June 2010, 06:34 PM

Hello Marknoir,

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

Craig

marknoir · 26th June 2010, 07:06 PM

Quote:

Originally Posted by kevinkr

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.

marknoir · 26th June 2010, 07:08 PM

Quote:

Originally Posted by llwhtt

Hello Marknoir,

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

Craig

Sure. Just send me your email address.

llwhtt · 26th June 2010, 07:56 PM

Here's my email- (removed by mods). Thank you very much!

Craig

kevinkr · 09:28 PM

Quote:

Originally Posted by marknoir

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..)

kevinkr · 26th June 2010, 09:30 PM

Quote:

Originally Posted by llwhtt

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..

26th June 2010, 02:06 AM	#3
marknoir diyAudio Member Join Date: Feb 2005 Location: Brooklyn, NY	Fixed Fixed - bad zener

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13th June 2010, 03:31 AM	#2
Richard Ellis diyAudio Member Join Date: Sep 2006 Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.	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. __________________________________________________ ____Rick.....

26th June 2010, 06:34 PM	#5
llwhtt diyAudio Member Join Date: Jun 2008	Hello Marknoir, What are the chances of getting the rest of the schematic, the audio portion? Craig

26th June 2010, 07:56 PM	#8
llwhtt diyAudio Member Join Date: Jun 2008	Here's my email- (removed by mods). Thank you very much! Craig

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