October 2014 41
Preserving a piece of stereo history ...
a tube console stereo repair story.
I think that’s one of my favorite things about this stereo. It
has quite a few functions: there’s the function selector on
the left; the tuner, loudness, and balance on concentric
controls in the middle; the stereo function switch; the
power switch along the bottom; and along the side,
separate treble, bass, and presence controls. It’s fully
customizable (Figure 3).
Digging into the Repair
The amplifier chassis — hidden inside the cabinet — is
a lot more minimal. There’s the output tubes: a pair of
6BQ5s for each left and right. You might know them as
EL84s, and in this amp they produce a nice clean sound
with plenty of power for the efficient speakers they’re
driving (Figure 4).
It’s pretty chaotic underneath. By this time, they’d
invented printed circuit boards but they hadn’t taken over
yet, and many consumer electronics just used good old
fashioned point-to-point wiring. It works, but it can look
like a hot mess! With the components spaced so far apart,
though, it’s easy to give it a quick look-over for obviously
bad parts — and there’s a problem that shows up right
away (Figure 5).
This capacitor is attached to the power amplifier
circuit — and it’s blown a piece of its plastic casing clean
off! Modern capacitors are made of durable and reliable
materials — often plastic and metal films and ceramics.
Back then, though, these materials had just started to
become available, and most electrical components were
still ultimately made of paper and foil with special
packaging. This one used a plastic and epoxy package but
over 50 years, the paper and foil degrade and start to leak
electricity. This creates a heat build-up that can release
gasses which expands in the sealed body, and ... pop! Off
goes a piece of plastic.
Even with this failure, the amp continued to try and
play — vintage gear can really take a beating. Once the
parts get to this stage, though, there’s no sense in just
replacing a few. They’re all the same age and they’re all
Don't just plug in a piece of vintage gear you find to see if it
works! This can be dangerous and might cause a fire or shock,
depending on its condition. Solid-state gear after about the mid
'60s can be powered on safely to see if it hums or makes any
sound, but most tube gear from the early '60s and before
probably needs service before it will work safely and reliably.
Early electronics used parts which break down with age just
sitting on a shelf, and it only takes a few seconds for something
to go seriously wrong and burn up an important component.
Figure 3. The amplifier chassis model 7K31.
Figure 4. Wiring around one amplifier
channel's output tubes.
Figure 5. This capacitor blew a piece of casing clean off.