The Tek 7x0
One of the best portable
devices on the electronics bench
over the last 20 years was the
Tektronix THS700 series (Figure
1.) Not only are the THS700s
portable, but the dual channel
inputs are independently
isolated to 600V. This is quite
valuable in debugging off-line
electronics in a safe manner.
THS7X0 scopes can be had
for $500-$1,000 on the used
equipment market. However, the
battery packs — four C-size NiCd
cells — are only good for about
three years. OEM replacements
from Tektronix resellers run
about $200, and after-market
packs cost $80-$100.
Battery Pack Description
Most battery packs are fairly complex arrangements of
tabbed buttonless sub-C size cells all welded together. The
THS7BAT turns out to be much simpler construction. The
4.8V pack contains four C-size cells in a straight stack.
Further, the pack is held in place
in the scope by spring pressure
similar to that of a flashlight; this
works to our advantage in
fabricating a rebuilt unit.
Figures 2a-2c show external
views of several OEM and
aftermarket battery packs. Note
the metal band near the
negative end. This
circumferential terminal is
connected by a metal strap to
the positive end of the top
battery and facilitates charging
the pack in an external charger.
At the bottom of the THS700
battery chamber is the negative
terminal spring; the positive
terminal is visible as a tab
contact about 1.5” above the
bottom. While the locking cap
has a metal inner surface, there
is no electrical connection to the
scope at this point.
The only item we want to salvage from the old pack is
the metal band with its connecting strap. Before removing,
We are accustomed to using portable battery powered devices in most aspects
of modern life. With some devices, the batteries are not accessible, rendering the
device useless when they fail. Others — such as portable drills — have
replaceable battery packs. When they fail, they can be replaced or rebuilt at
considerable expense. Seldom is the rebuild option tractable for the do-it-yourself type person. However, I have one application where a very expensive
battery pack can be easily rebuilt at a fraction of the replacement cost, returning
the device to useful portable service.
Portable Oscilloscope Battery Pack
By Kevin J O’Connor
FIGURE 1. Tektronix THS720 scope.
February 2018 19