[Boatanchors] Heath audio question

[Robert Downs]

The volume control pot apparently is not going to zero ohms to ground at
full CCW rotation.  One cause could be an open ground lead, possibly a
corroded ground tie point.

Robert Downs


-----Original Message-----
From: boatanchors-bounces at mailman.qth.net
[mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of RAY FRIESS
Sent: Friday, August 9, 2019 10:24
To: heathkit at yahoogroups.com; Boatanchors QTH
Subject: [Boatanchors] Heath audio question

Working on an SB300 and I suspect I have a bad tube, but I'm still puzzled.
I'm still digging through the junk box to find a replacement to verify it
one way or another.   V10 is the first and second audio amp with the volume
control feeding audio to the grid of the first audio section of the 6HF8.
The tube also contains the second audio or audio output section.  From there
it goes to the speaker and headphone jacks.
First, I notice the receiver seems more sensitive to speaker and headphone
impedances than most other receivers I've had.  Specs call for an 8 ohm
speaker, which I use with the SB600 speaker.  However, when I plug in a 16
ohm speaker in the headphone jack , I notice a decrease of 50% or more of
audio availability and have to turn the volume control clear up and even
then it's not real loud.   I've never noticed such a thing with other
receivers.  I know we should match the speaker impedance with the audio
output impedance, but I've never noticed such a dramatic cut when there has
been a mismatch.

SECONDLY and more important, the volume control at minimum volume is not
cutting off all volume.  The volume level does go down, but remains at about
25% (even with the correct speaker plugged in.)
So, I'm wondering if maybe the 6HF8 somehow is taking even the lowest level
audio going into it and somehow still jacking it up?  At the lowest setting,
the tube should not have enough, if any, signal to amplify, but somehow some
signal is getting past the volume control and the tube is amplifying it.  At
minimum, incoming audio should be going to ground through the pot.
YES, I have checked the pot ... it doesnt appear to be the problem.  It's
not even scratchy, indicating a dirty pot.
Ray