Ceriatone Forum

Hello,
I'm working on a OTS FM 50 Mod Eagle.  The parts list says the two backer boards supplied are for the Relay PS board and the PAB board.  I assume that the 8 long brass standoffs are for the PS and Main boards mounted on the floor of the chassis.  Then I assume the 6 short standoffs are for the three boards mounted on the sides.  So I'm wondering, why does the Relay PS board need a backer if it's mounted on standoffs?  Also if the PAB and Channel boards don't have standoffs, why don't they both have backers?  I figure I will use longer screws and double nuts to get these two small boards off the floor and keep them from grounding out.  Maybe even get a little crazy and cut a backer for the Channel board.

I very much welcome some good advice.

Thanks

Check out the pictures on the web site - if you look closely at the right ones you can figure out how and where to use the backing boards. I don't know exactly how the mounting situation is with your kit - mine didn't come with anything that worked, so I just used some other stuff that I had on hand.

The pics on this page - http://www.ceriatone.com/productSubPages/OTS%20Series/OTSFM50WPics.htm (http://www.ceriatone.com/productSubPages/OTS%20Series/OTSFM50WPics.htm) - it looks like the backing boards are used on the 2 boards that are on their sides over on the PT side. That's how it is in my BM 50w. The other small boards are mounted up on standoffs, so don't need backing boards.

You might look around on the board for an assembly picture thread - here's one - http://ceriatoneforum.com/index.php?topic=1160.0 (http://ceriatoneforum.com/index.php?topic=1160.0) - I also had one, but the site where my pictures were hosted kacked, and I haven't bothered replacing the pics yet.

Thanks!  mcinku's photos were helpful.  His HRM layout is a bit different than my FM, its got 3 small boards on the ground and two on the wall by the PT.  It looks like four of his boards, the Relay PS, Bias and the two Relay boards are all mounted without standoffs so I assume they all have backer boards.
So, if I assume the two boards I have with backers ( Relay PS and PAB) get bolted down without standoffs then my 6 short standoffs will take care of the Bias, Channel and FET boards.  The 8 long standoffs are obviously for the PS and Main boards.  Problem solved!

I wonder what was Nik's logic in using the backer boards instead of standoffs?  The Relay PS board is awfully close to the pilot light and voltage regulator, lowering them give you a bit more clearance.   Similarly the PAB board's relay sits just under the three small switches on the front face.  So maybe clearance issues on top of the boards were the driving factor.

thanks again!

I don't know about you but I'd want a backer board for the Bias/Rectifier board. You have close to 700 VAC entering that board from the PT and 450 VDC or more heading over to the filter board. Using standoffs it is possible (although remote) that some conducting material could get between the underside of the board and the chassis and short something to ground. With a backer you pretty much have eliminated that. I've built five of these type amps from Nik and received backers for all boards except the internal tone stack board that comes with the HRM. I just went to the Depot and bought some appropriate size rubber washers and used them as standoffs.

The same thought crossed my mind.  Even though an air gap is a fine insulator it's disconcerning to see only a 1/4" clearence.  If you think about it, if you have a short you would want it to ground.  Shorts to ground clear the over current protection quickly, in our case the fuses.  I much rather blow a fuse then become the human fault path! 

It's interesting that your kits had backer boards for almost all the boards.  Maybe Nik doesn't see the shorting issue as a problem.  My guess, based on the two boards that have backers is he is concerned with clearence above, not below the boards.