Vox AC15 PI Voltage Problem

[plexi50]

I have a problem with the PI voltage on only one of the plate resistors & coupling cap / V2A
I have a B+ of 232VDC on the supply of R18
R18 with the 12AX7 tube should be drawing this 232 volts down to about 159VDC
There is no voltage reading to be found on the plate side of R18
If i lift the plate side of R18 voltage passes the 100K plate resistor / I have changed the 100K resistor as well

V2B R17 is fine

I have lifted the coupling cap as well from the circuit still to find no voltage across the 100K to the plate side of R18 to feed the coupling cap and tube plate

I have traced this from under the board as well and everything looks to be wired and traced correctly
This is weird!

[tubeswell]

Maybe that 100k is 100R? Did you check it with your meter?

[plexi50]

Yes tubeswell i tested it before putting the 100K on the board. Ive seen some strange things before but this makes absolutley no sense. R17 & R18 both have the same B+ feed. Has to be a short some where on the R18 coupling cap circuit but so far i see nothing that like that. V2A should have plate voltage on pin 1

[catnine]

I'm no expert but it looks like r-17 and r-18 are the same color codes and if you have voltage through r-18 when you lift it off the tube what is that voltage reading ? If the resister and wiring are ok is it possible the tube is the problem or r-18 is on the wrong pin ? Why not pull the tube and then check the voltage on both plates ?

[VMS]

Do you measure the voltage from the resistor or from tube pins?

Amp turned off, is there a continuity to ground from the plate side of R18?

[VMS]

Also, check if you broke something when you changed the C57.

[plexi50]

Been looking for just that for the last hour. Pin 1 is grounded to the chassis. I removed all the screws from the chassis to board and pin 1 is ungrounded. Connected chassis ground wire to board and still ungrounded. Getting some where but still how does this happen?

[plexi50]

Oh your going to love this one. I havent confirmed this yet but this amp came in working but very distorted. The customer installed a choke. Look at the bolts facing the board when mounted. I believe there touching and grounding out the board when mounted and screwed down. Look at the board mounting holes! No ground tabs or metal to make a board ground. Thats why when i screwed down the board to chassis earth ground i got no continuity to pin 1 R18. So this tells me i think the extended bolts are touching the board and grounding out components. Dam that could have been bad if it was touching the B+ rail

Im going to Dremel off the bolts and see what i get

THAT WAS IT. Those pesky choke bolts were grounding out the B+ on the R18 rail. Im surprised the amp did not hum or R18 burn through. Maybe it was touching the coupling cap. Not sure. Sounds wonderful. Time to eat and do nothing else. I wish: I have a 5150 on the way here now. Work is going to kill me

[Willabe]

Dam that could have been bad if it was touching the B+ rail

He could have bought the farm on that, at least for the amp.    


Is that tape on the choke leads where they come through the chassis instead of a grommet?  


           Brad      




 

[plexi50]

Dam that could have been bad if it was touching the B+ rail

He could have bought the farm on that, at least for the amp.    


Is that tape on the choke leads where they come through the chassis instead of a grommet?  


           Brad      

Heatshrink tubing on the choke leads.




 

[catnine]

 I'm happy you found it.

[archaos]

THAT WAS IT. Those pesky choke bolts were grounding out the B+ on the R18 rail.

Frightening !  ...but all's well that ends well 

[plexi50]

Lucky know one got a jolt