[BoatAnchors] re: Problem with my Hallicrafters SX-43

[Steve Harrison]

At 07:47 PM 2000-02-26, Glenn Little wrote:
>...THe only other thing that I know that
>you could do is try substituting tubes one at a time.  Start with the audio
>output tube.

If the hum varies with the volume control setting, it's likely a non-audio
stage tube such as IF, VFO, BFO, RF or mixer. This is because the hum would
be generated in a stage before the volume control. If the hum does NOT vary
with the volume control, it's 95% certain to be an audio tube, or possibly
a power supply problem.

Also, if the hum really is being caused by a filament sagging against a
cathode, it's very likely that gentle tapping against the side of the
culprit tube will cause the hum to get very loud and then decrease as the
filament is "shocked" by the tapping. But do the tap test to ALL the tubes
as even a gentle tap could be transferred to the culprit even when not
tapping it directly. Use the eraser end of a pencil or your finger nail.

If you cannot obtain a replacement tube immediately and cannot stand the
hum, you can try to "shock" the filament of the culprit further away from
the cathode. Put on a pair of gloves and even a pair of goggles if you have
them. Then grasp the tube firmly in one hand, and very lightly "tap" it
against the open glove-covered palm of the other hand, rotating the tube 90
degrees each time. Be gentle- it shouldn't take much of a "shock". This
treatment could actually damage the tube; but if you have no recourse, it's
worth a try. Be certain to write down the tube type before you try this so
if the glass breaks, you know what it used to be ;o)

I don't suggest doing this with voltages applied to the tube, as you can
cause the internal elements to momentarily contact one another and weld
theirselves together due to power being applied.

It's possible, but unlikely, that the problem could be components mounted
on a tube socket under the chassis which are now, after decades, shorting
against one another, whereupon tapping on a tube in that socket might seem
to indicate a bad tube but replacement does not fix the problem. If
components against each other are the actual problem, then simply removing
and reinserting a tube could cause the components to be moved slightly,
making the problem appear to go away or appear permanently. In that case,
you will probably want to enlist the help and guidance of a local
tube-experienced Elmer to carefully look over everything with you under the
chassis.

73, Steve K0XP