replacement of batteries 7S14

Chris van Lint

Some time ago I posted a question asking for suggestions on how to replace the now banned 1.35V Mercury bias batteries used in each of the vertical samplers in this unit.
There were various suggestions and for anyone else who may be faced with this problem, I would like to share my final solution.When considering which battery to use, I initially focussed on Zinc/Air batteries, which also have a terminal voltage of 1.32V and would obviate the need for using some sort of voltage dropping arrangement. In the end I discarded this idea, as Zn/Air batteries have to be activated before use by removing a seal, which allows air to flow through the cell. Once activated the cell may have a life span of about 9 months, independent of the power consumed. Alkaline cells together with a germanium diode in series were another option, but I felt that as I was going to need a voltage droping circuit anyway, I might as well use something more lasting. The original Hg cells had been there for > 20 years and I was looking for at least that long for the replacements.
I settled for Silver Oxide batteries, however there was a problem in that they are not available with solder tabs (at least not here in Oz). A trip to a nearby battery rebuilder solved that problem and solder tabs were spot welded on for $5.00
I selected two Ge diodes which would drop the voltage to about 1.42V, which is the closest I could get. Once the whole network was soldered into place an I tested the unit and found it did not work. Perhaps another diode was needed, because the two cells are not merely in series, since the point where the cells are joined is also connected to the circuit (i.e. centre tapped). It is impossible to test the unit without re-attaching the top sampler cover, because it is in fact piece of PCB, which contains connector pins and bypass capacitors, which complete the circuit. With the top cover removed it is impossible to check performance.
It was time to be bold and check whether the 1.35V was really that critical. I soldered the batteries in place, without series diodes, put the cover back in place and found that after some adjustment the unit is working great on both channels.
It is fun watching a >1 GHz signal on a 7603

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