Tech Flashback: Tektronix 1106 Oscilloscope Battery

Tektronix 1106 Battery Front

Those who are my direct friends on Facebook and have been following the comments on the BWD 141 Function Generator posts would have seen my friend Charith talk about this. Also thrown out in the same pile of trash in Electrical Engineering at UNSW, I managed to salvage this home. Part of the lure was the Tektronix branding – anything Tektronix is expensive. Secondly was the fact that it was a battery pack. Pre-research, I thought it was a battery pack with inverter – so you’d charge from AC, and it would develop an AC output to power an oscilloscope. In fact, upon consulting documentation, I found I was wrong, and instead this develops a 24v DC output only, so it’s a large battery with charger in a pack.

The handle was very well moulded, the exterior in great condition. It weighs close to 8kg and made my arm rather tired. It has leg-clamps (the plastic things) which allows for you to place the oscilloscope unit on top and lock the legs into the pack so it could be transported as one unit. This is really the only way to power an oscilloscope out in the field to do some on-site testing – what a kludge. There was only one control on the front – a switch to change between trickle charge and full charge rates.

Interestingly, this unit was also used in the US Military and given a military designation with a service and maintenance guide written. It’s available online, and seems to be one of the only service manuals.

Tektronix 1106 Battery Rear

The rear of the unit is devoid of much – there is a voltage selector switch, a mains input lead and an output lead with a proprietary plug. There is also the input fuse.

The pretty stuff is inside … as usual. The screwheads have been rather stripped, so unscrewing it wasn’t that easy …

Tektronix 1106 Battery Internals

Now isn’t that pretty. An array of 20 D-size Ni-Cd rechargeable cells in series. The cells are covered in white shrinkwrap, so no vendor is visible. The hand-written date seems to allude that the unit had been tested 10/02/1976 (converted, assuming it’s American date). After all this time, none of the cells had leaked which is remarkable given most old Ni-Cds I’ve seen have corrosion from leakage, especially those which have been kept on float charge. It doesn’t look like the unit had got much use, the lack of corrosion implies it wasn’t kept on float and wasn’t being used or taken care of, and the label suggests the battery pack had never been replaced (likely being the same one installed when new). The circuitry itself is very crude for charging – it’s all “linear” style with transformer and series transistor control. As Ni-Cd’s had a fair amount of self-discharge, there is a trickle charge setting of 62mA to keep the pack topped up, with full charge being 620mA – a small current which can be had from any laptop charger nowadays!

The design of the pack is rather crude in the fact that the output 24v is permanently available to the rear socket and to the front panel meter, which slowly discharges the battery to 0v, making it permanently useless. As a result, I don’t feel like it’s even worth plugging in given the condition of the battery pack. The pack is rated to offer about 140Wh of storage – about two removable laptop batteries worth!

Oh how battery technology has advanced and energy density has increased. We can have the same storage in about 1.5kg rather than 8kg.

Of course, those guys could have gone with lead acid batteries, although the electrolyte and maintenance might be a concern, as is their desire not to be deep cycled. It would have been cheaper but also bigger. It’s remarkably good styling to have such a battery be so thin back then …

It was reported by Charith that the unit exhibited a bad smell – this could be because of several reasons – the fully discharged battery would put an immense strain on the series regulation circuitry making the transistors dissipate high amounts of current. Secondly, the capacitors on these seem to have oil stains on the sleeve, suggesting electrolyte leakage which may mean the capacitors are internally “arcing” and making a bad smell as well.

Tektronix 1106 Charger Circuitry

In true linear tradition, you can see the transistor mounted to an “unpainted” section of the metal case for better heatsinking, and a tubular style high-power resistor screwed into the side of the case, also likely for heat dissipation reasons. Examining the transformer also yields wiring diagrams for different voltages (as charging is predicated on the secondary voltage output being high enough to be above the battery voltage and the voltage drop over the regulating series elements). You can also see a label with the name of the tester, and the old Tektronix logo which looks rather comical …

Tektronix 1106 Battery Transformer

And on the heavy-duty aluminium cross-members which holds the cells together, a pertinent warning, as the internal resistance of Ni-Cds are very low, allowing for catastrophic amounts of current resulting in heating and welding of shorted conductors …

Tektronix 1106 Battery Cross-Member

So there we are, another very old piece of gear … the elegance is in the construction and the simplicity …

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One Response to Tech Flashback: Tektronix 1106 Oscilloscope Battery

  1. Eero A. Sarlin says:

    the cells are not D size but F size.
    cell capacity was around 7Ah or 7.5Ah way back then when it was made probably by GE .
    nowdays just a bit larger capacity cells are made in NiMH chemistry:
    http://www.batteryspace.com/nimhrechargeablecellf-sizelongd12v13000mah1pc.aspx

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