Audio characteristics of National NC-57

[Ian Webb]

Stewart,

If you haven't replaced all or a majority of the capacitors, particularly the
coupling capacitors and cathode bypass capacitors do so and then worry about the
audio.  Any paper capacitors and those black tubular ones that look so nice and
that the audiophools are still wanting to buy have got to go as they're little
more than a resistive path all too often and will upset the grid bias something
fierce with HV from the previous plate upsetting the bias for the next audio
stage.

You'll likely find a fair number of resistors are 50% or more high as well so
check every one that you can reasonably get to and replace as needed.  Also make
sure you measure control grid,  screen grid, cathode and plate voltage on each
stage against the voltage chart in the manual and if anything is too far off find
out why.  Don't be too fussy about it being right on as + or - 15% is probably
quite sufficient and it's apt to be on the high side with today's higher line
voltage.

Of course you've also replaced all the power supply electrolytics I would hope
and, if like my NC-125 of that era, it doesn't have a line fuse, you've added one
even if you bury it under the chassis as I did in my NC-125.  Not doing so and
putting in an appropriate sized fuse runs the risk of losing a hard to replace
power transformer and not replacing the electrolytics makes that doubly likely.

I suspect any audio problems will disappear when you do that if my own experience
with a couple of NC-125s that I brought back to life are any indication of the
problems you likely to encounter.

Good luck and most of all have fun.  I really love those vintage NC receivers as
my first commercial receiver was an NC-125 bought new in 1952 direct fromt he
factory.

Ian, K6SDE


C Stewart wrote:

> G'day folks,
>
> Got my hands on a "dead" NC-57.  Not really dead, but on life support.  It's
> audio is very low, even for local stations on the broadcast band.  All the
> tubes appear to be working, and tested as "okay"... Was this beast known for
> weak audio?  I've got a transistor battery rig that puts this to shame.
> Also, the BFO doesn't seem to oscillate, or at least not the way it should.
> I'm printing the manual as we speak.  Just wondering if there was any
> collective wisdom floating around.  Thanks!
> --
> Science has proven that the human body can tolerate nearly everything,
> except death.
>
> C. Stewart (VE9CES)
> nemisis at brunnet.net
>
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