Mesa Boogie MKIII bias supply issues. Please check my logic

[jukelemon]

Hi all!


I have a MKIII with not only a complete lack of bias supply but adding insult to injury...a bias supply of 186vdc :)


I have attached the schem as well as points 1 and 2 on another attachment.


Amp had a burnt screen resistor on one of the 6CA7s.  Repaired that and went ahead replaced all the screen resistors 5 watters for safe measure.


Bringing up the amp via a Variac I noticed a good NEG bias supply on the left pair but a climbing POS bias supply on the right pair.


So shut it down and remove the tubes.

In the middle of tracing the bias supply to Point 1 (on attachment), the left side pair goes positive as well :(

And by POS, I mean 180+VDC.

So... I can measure an expected -59vdc at Point 1 BUT when I cross the 220k resistors (Point 2), I get the 180+vdc bias supply and that follows on down to the pins.

So...I measure the .1 caps that are coming off the Phase Inverter.  Both (odd I know) are shorted.  I measure continuity between the leads on both.

So, I am assuming the shorted coupling caps are allowing DC into the bias supply and that is why I am seeing what I am seeing.  Make sense?

On another note...i don't think the schem is written correctly because those .1 caps are not on the side of Point 1.  They are actually on the side of Point 2 which makes sense as to what i am reading as far as component location/leads.

[stratomaster]

Boogies have a reputation for using under spec caps in this location.  They're typically 400V where a 600V+ cap would be better.  Replace with the equivalent value in a higher voltage and then check again with the tubes out. There's also a chance the filter caps in the bias supply were reverse biased if you had voltage leakage. You may need to inspect and possibly replace those.

[jukelemon]

Boogies have a reputation for using under spec caps in this location.  They're typically 400V where a 600V+ cap would be better.  Replace with the equivalent value in a higher voltage and then check again with the tubes out. There's also a chance the filter caps in the bias supply were reverse biased if you had voltage leakage. You may need to inspect and possibly replace those.

Thanks.  I have read that too and was planning on 600v caps for sure.

So....my logic is accurate then?

I traced the bias supply all the way from the PT so I am thinking I am good up to the junction of those .1 caps.

[shooter]

i don't think the schem is written correctly because those .1 caps are not on the side of Point 1.  They are actually on the side of Point 2 which makes sense as to what i am reading as far as component location/leads.

your logic is sound, redraw of find a better schematic though because the schematic makes your logical conclusions illogical 
this is one of their 50W  make it look like that

[jukelemon]

i don't think the schem is written correctly because those .1 caps are not on the side of Point 1.  They are actually on the side of Point 2 which makes sense as to what i am reading as far as component location/leads.

your logic is sound, redraw of find a better schematic though because the schematic makes your logical conclusions illogical 
this is one of their 50W  make it look like that

Thanks Shooter and yes, that schem looks more like what I’m seeing in real life.


Of interest .. the schem I posted is identical to what came with the amp :) The original yellow documentation papers are still with the amp.


The schem also doesn’t show the 2.7k screen resistors on the 6ca7s.


Weird

[jukelemon]

In the event others are in a similar boat…c511 and c512 are the offenders.


Pulled them out today and both are dead shorts between leads.

[Willabe]

In the event others are in a similar boat…c511 and c512 are the offenders.

I don't see them labeled on the schematics.

Which 2 caps do you mean? The power tube coupling caps?

[jukelemon]

In the event others are in a similar boat…c511 and c512 are the offenders.

I don't see them labeled on the schematics.

Which 2 caps do you mean? The power tube coupling caps?

Sorry. Yeah. The actual board is labeled in the amp. Why the schem is not and the board is .. not sure.


I’m referring to the the 2 x .1 caps off the PI and connecting to the bias circuit

[jukelemon]

Closing the loop...


2 x new .1 caps and the amp is back to life and healthy!

Can't say I like the sound of them and damn...how heavy can it get lol.