MFA Lumi started "motor-boating" today

[marknoir]

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]

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]

Fixed

Fixed - bad zener

 

[kevinkr]

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]

Hello Marknoir,

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

Craig

 

[marknoir]

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]

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]

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

Craig

 

[kevinkr]

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]

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..

 

[llwhtt]

Any mods out there moderating would you remove/delete my email. Thanx, I'll do as Kevin said.

Craig

 

[SY]

Your wish is my command.

 

[llwhtt]

Thanx SY.

Craig