Bogen CHB-20A Rebuild

[physics]

Hi Guys

Note that some PTs have insulated bolts holding the laminations together - although, once the unit is vacuum varnished, the varnish is what holds the laminations together and the bolts are just for mounting. If there was tape over the threads through the lams and/or nylon bushings and/or fiber washers at either end, these should all be incorporated with the new longer bolts.

An alternative to the long bolts is to use long threaded standoffs screwed onto the stock bolts. Then use short bolts to secure the card. A quick look at the Keystone catalog shows threads up to 8-32 and lengths up to 2". PT bolts are often #10, so this may not be an option. Only short threaded spacers are threaded the entire length; from 1" up the thread at each end are about 3/8" deep.

The PSU card in The Standard carries the fuses and bias supply and is more or less full. How is your card empty?

Regarding the toothed lug for use as a chassis ground: In my amps and kits I use a flat washer over the lug, then the nylock nut. This way, the teeth on the lug does not interfere with tightening the nut.

In many of the amps I've built where a board sits over an OT, say, I add "captive bolts" to the card. These are comprised of a 3/8" long bolt with a nylock on the underside of the card facing the standoff. I tighten the nylock then release it very slightly so the bolt can turn in the hole in the board - not wildly, but with a screwdriver. This also works nicely when there is a stack of cards.

Yes, this transformer had fiber washers under the old bolt heads, which I made sure to put under the new bolt heads as well.

I considered using stand-offs to extend the stock bolts like you mentioned, however it ended up being cheaper to just buy new bolts.

The card is currently mostly empty since the only thing I've decided to move to it so far is the voltage doubler portion of the supply, maybe a cap for the screen, and probably the bias supply. There is enough space around the edges of the card that overhangs empty space instead of the transformer bell that I might be able to build everything on those edges if I didn't want to have eyelets over the bell, but that might end up requiring some contortions. The caps I have to work with are axial so they're amenable to that, but aside from that it might get messy. Hence inquiring about required clearances between eyelets and the transformer bell.

Good to know about the ground lug, I was wondering whether a washer would be necessary.

That's a nifty idea with the "captive bolts", I'll keep that in mind.

Thank for the info!