Nuts and Volts - February 2018

The Tek 7x0

Series Scopes

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.

Disassembly

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.