The Tektronix THS710A handheld scope is a very useful piece of kit, but the Tek battery pack seems to have a fairly limited life span and is costly to replace.  The one supplied with the scope only performed adequately for a few months of intermittent use and since then I had relied on the mains adaptor - which rather defeats the point of a handheld scope...

Although the THS7BAT pack simply comprises four NiCd 'C' cells in series, it isn't straightforward to copy. The construction is unusual, with the top positive connection brought down the side of the cells in a thin metal strip to a circumferential contact band near the negative end. This is probably designed to prevent the pack making contact if inserted the wrong way round, but it makes the pack unique and therefore expensive.

I thought it would be a good idea to modify the scope to use a set of normal rechargeable cells. These are relatively cheap, easy to obtain, and charging cells individually is likely to provide them with a longer life than a series connected pack. Also, as a bonus it gives the option to use primary cells in an emergency.

Battery Compartment Modifications:

The original pack was pushed into the battery compartment against a spring at the bottom and retained by a bayonet fitting cap. The spring provided the negative connection, and can do the same for individual cells. The inner part of battery cap is metallic and provides an adequate contact surface for the top positive terminal, and just required an electrical connection. This turned out to be very simple to achieve...

Adjacent to the bayonet fitting in the case there is a moulded feature which looks suspiciously as though it once might have been designed to house such a contact. All that was needed was a small piece of aluminium and a phosphor bronze strip (cut from an old battery housing).

The aluminium simply slots into the feature and is held held in place by the top half of the case, while the phosphor bronze is bolted to it and suitably shaped to provide a wiping connection. The original positive side contact is removed.

So what sort of cells to use ? The 2.8AH capacity of the original 'C' cells can now be obtained in 'AA' cells (in both NiMH and NiCd). 'C' cells are available in higher capacities but are relatively expensive, so it seemed reasonable to allow for the use of the smaller 'AA' cells.

Rather than modify the battery compartment I decided to use cheap clip-on 'AA' to 'C' converter tubes, thus leaving the option to use 'C' cells if required.

Although the Tek pack may eventually become difficult to obtain and even more expensive as demand reduces, 'AA' and 'C' cells of one sort or another are going to be around for very long time...

Charging:

The internal charging circuitry of the THS710A was barely adequate for the original NiCd pack - the charge rate of approx. C/8 gives a fairly slow charge but was much too high for a NiCd trickle and probably contributed to the short pack life. It isn't at all suitable for NiMH as it simply charges indefinitely, and would have needed to be modified for charge end-point detection. This would not have been easy to fit in the available space and integrate with the existing circuitry. On the other hand the long-term availability of NiCd is doubtful...

A straightforward solution was to disable the internal charging, and rely solely on external chargers. For many jobs several battery changes might be required and the internal charger is then of no use anyway. The potential hazard of connecting the mains adaptor with primary cells fitted is also avoided.

Implementation:

The simplest approach of using a diode in series with the battery to block charging creates too large a voltage drop (even with a Schottky diode), resulting in early shutdown and a loss of usable battery capacity. I wasn't particularly happy with the thought of hacking into the multi layer main circuit board, so a quick and dirty solution was to add a 12V relay on a small stand-off board which simply disconnects the battery when voltage is applied to the external PSU input.

Initially this relay was wired directly to the external PSU connector. However, if the PSU lost mains supply with the scope powered up, the relay held on down to a rather low voltage and failed to reconnect the battery in time to prevent a re-boot. This wouldn't have been so bad but for the fact that many user settings are lost if the scope isn't shut down tidily via the on/off button (or automatically when the battery voltage falls slowly down to 4.4V).

The final circuit uses a zener and a transistor to ensure the relay drops out as soon as the PSU voltage drops below approx. 9V. The slow response of the relay still results in a re-boot if the PSU jack is simply pulled, but none occurs on loss of mains voltage, where the capacitance in the PSU slows the voltage drop sufficiently for the battery to take over in time. (It does however mean that the battery will receive charging current if the PSU volts should lie between approx. 5V and 9V.)

As a final safeguard a 'crowbar' diode and 2A fuse protect the scope in the (hopefully unlikely!) event that the batteries are inserted the wrong way round - this fuse is soldered in-line and inconvenient to replace, but better safe than sorry...

Other Issues:

During handling it became painfully obvious that the surface of the display module is extremely soft - even light use of a soft lens brush caused small scratch marks. Fortunately these were removed successfully using 'Autoglym Super Resin' car polish - a single drop applied and polished to dryness using lens tissues. (No other type of tissue or cloth appears soft enough not to create further scratches.) A lens blower was subsequently used to remove dust from the screen and the inside of the window during re-assembly.

There has always been a design problem in removing the bayonet cap if there was no battery pack present to push it out - the cap has no feature to grip and pull on. This was solved by making an insulated rod of the appropriate length to insert in place of batteries while the scope is stored or just running from the external PSU.

Conclusion:

The modification has been very successful - a set of '2700mAH' (a possibly optimistic figure) NiMH 'AA' cells operates the scope for approximately 1.5hrs. This implies the full charge isn't being used, but on the other hand a 1 amp re-charge from that state only takes about 2 hrs.  So - continuous battery operation would only require cycling three sets of cells through two 1 amp chargers. At approx £50 this is still less than the cost of one Tek battery pack...


Barrington 14/03/12