Mainline Problem [resolved]

M

Mordicai

New member
After a week of playing detective, I am unable to get the amp up and running.
Resistance reading are all ok except for 1 & 5 which both read 26.6, instead of39.7.
Voltage reading are all ok except for Kreg Reg ( both sides) which read 4V instead of 8-12.
The only other issue is the bias trim potentiometer's which I installed backwards. I realize it at the time posted on the Forum . Doc said he didn't think it made any difference so I never fixed it.I have 0 voltage in trying to adjust these.
Only the B side potentiometer is backwards, and the B side LEDs do NOT light up. The A side meter is  correct and the A side LEDs DO light up. But again O V when adjusting
 
Mordicai said:
Hi Paul, Breg on regulator board measure 220v both a & b
OA & OB MEASURE 2v
Click to expand...
OA and OB on the high current C4S boards.

I'm guessing the 2V is the OA/OB on the center board.

-PB
 
OK, that's helpful information. 

4V on the cathode of each half of the 12AU7 (Kreg) and 220V means that the 12AU7 is sucking down all the current. This means that the 6C45PI in each channel is drawing no current.  Here's a list of causes for this:

1.  The 6C45 tubes aren't being heated (shielded twisted pair to pins 4/5 on each socket).  If you can see them glowing, they are being heated.
2.  The high current C4S boards have a miswire.
3.  The MJE5731A transistors on the high current C4S boards need to be resoldered.
4.  The wiring on the center section of the center PC board is incorrect and not allowing the 6C45s to draw current.

Those are the ones that come to mind most immediately. 
 
The trim pot is wired as a variable resistor.  If you connect the middle leg to either of the outer legs, it will work in that fashion.

Also, check the jumpers on the big C4S boards very carefully to be sure they are all in the proper positions.
 
I've gone through the instruction page by page. All the joints seem good but I have resoldered many just to be sure. I can not find any errors except for the Trim Pots. I must have been tired when I did these for I not only put one in backwards, but totally mised..." Folds the leads directly under the adjustment screw over to the adjacent lead, so the lead under the screw is touching the center lead. " The question is how to fix this and could this be my problem? Should I Jank these  trim pots out and replace, or can I just solder a jumper in? My feeling is this isn't the fix I'm looking for!
 
Those leads have to touch each other to turn the pot into a variable resistor. If they are not connected the part won't function as needed.
 
Well...did it work???

I hope its up and running. If you lived close to Raleigh, NC I would be glad to help!!

Alex
:>)
 
Thanks for asking.....worked on the base today ..... I have high confidence the trim pots were the problem, so I want to finish everything , put Eva on in jriver , a glass of wine, warm up the amp and enjoy. This is my fourth amp and by far the best looking. I'm sure it will sound great. the power of positive thinking.
 
Great!!

Positive thoughts here as well!!

I do enjoy listening to Eva as well!! Awesome vocals, so sorry she is not around to make more beautiful music!!

All the best!
Alex
 
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