NC-270

[rayfri]

Noticable hum in the audio, especially when the volume control is turned down
or all the way down, is usually a sign of a leaky or bad filter capacitor in
the power supply.
It's not filtering out the AC component left after rectification....
I'd start checking there first...     bridge the filter caps with a known good
one..
if the hum decreases or disappears,  you've found your culprit...



Brian Wood wrote:

> Thanks to everyone who helped me figure out what the problem was with the
> DX-100 grid current flicker. Turned out to be the 47 pf loading cap on the
> final grids. Bob, W3CD gets the prize for that one.
>
> I'm now working on my National NC-270. The receiver has a lot of hum in the
> audio. I don't know if this is normal for a boatanchor receiver, since hum
> was something we just lived with when using ac filament power, if I recall.
> But I thought I'd ask if anyone on the list has an NC-270 and could tell me
> if they have noticeable hum when the audio gain is turned down. I also have
> a fair amount of microphonics from the two audio stages (6T8 and 6CW5). A
> three-prong cord helped a little, but not enough. I also added 3 big filter
> caps to the supply, and that had no effect. Would appreciate any ideas. AS
> this is off-topic, please reply off-list.
>
> 73,
>
> Brian, W0DZ
>
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