Lafayette HA-460,HA-410 Audio Problems:

[Richard Carroll]

Joseph Cro wrote:
>
> Hello,
>         Name here is Joe (N3IBX) and I have a peculiar problem with my
> Lafayette HA-460 and HA-410 (six and 10 meter AM respectively). It seems that
> when I first turn each radio on the audio output is strong. After about 15
> minutes or so of operation the audio output falls off. I can still copy
> stations, and sensitivity doesn't seem to be affected, but audio output is
> low.
> I first checked the obvious.... the audio output tube. They test VG on each
> rig. Another thing I've noticed is that the audio output tube plates glow
> orange when they are in this condition. I thought a resistor or cap may have
> failed, but I have the same condition with both the HA-460, and the HA-410.
> Could it be a component failure coincidence? or was this a characteristic of
> this series radio?
> Any insight will be most appreciated.
> 73,
>     Joe Cro N3IBX
>
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Two possibilities come to mind:

If there is a electrolytic capicitor from cathode to ground on the audio
amp stage (would be across a resistor in series from cathode to ground)
it may have lost it's capacity, introducing degeneration and reduced
amplification. This is a common cause of loss of audio gain in tube
stages
 But:
The tubes' plates  glowing orange suggest another problem: A shorted or
very leaky coupling capacitor from the previous stage (driver) to the
grid of the amplifier tube (in a capacitance coupled amp). This places
plate voltage (high positive) on the grid of the amp and drives it into
saturation, causing the tube's plate to turn red, along with much heat.
It is no longer linear amplifier stage at this point and much distortion
will occurr, as will little or no amplification. Older paper capacitors
are commonly leaky or shorted.
73, Dick W0EX

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