Restoration Examples Page
RCA 77-DX & RCA BK-5B Ribbon Mics - (See below for the "Before"
& "During" pictures)
The "After"
Pictures:
AND: An RCA BK-1A Dynamic & KS-11 desk
stand:
The BEFORE & DURING Pictures of the above 77-DX &
BK-5B:
The pictures don't really do justice to how
bad these were. On the 77-DX, the surface rust on the center label ring
meant removing the nickel plating (note it's brassy appearance in the
disassembled picture below), as did removing the rust, oxidation & pitting
on the shock mount parts, yoke thumbscrews & cable clamp. This mic
prompted me to learn how to do auto-catalytic nickel plating. On the
center body, which is a bakelite cylinder that is riddled with holes inside to
form a transmission line labyrinth, the top section had been covered with a
piece of copper sheet metal, to hide a 2.5 inch long section of the top edge
that was chipped out. I removed the plate & re-constructed the chipped
out section, then primed & repainted the entire body (to the proper
color). The bottom housing was pitted from oxidation so badly that I had
to get it sandblasted. Then, I polished back to it's original
shine.
The BK-5B shows it's damage pretty well. It was mainly
just a matter of stripping every bit of paint off of every part, polishing out
the oxidation pitting, then priming & painting. The trickiest part of
doing a BK-5 is the ribbon installation & tuning. It's ribbon has a
width of 0.052", vs the 77's 0.056". Not a big difference, but the BK-5
has much less clearance between the ribbon & the walls of the magnet gap,
making it much much harder to keep perfectly centered during clamping &
tension tuning. In the pictures below, note the ingenious use of the
outside wall of the BK-5 center body to create the labyrinth, necessitated by
the fact that the transformer & low cut inductor occupy the middle
space.