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Directorate of Telecommunications
Eureka Moment

I always enjoyed fault finding. The more obscure the fault, the better I liked it. Hercule Poirot when he solved his most difficult cases never derived more satisfaction than me but I must admit I often took longer than the one hour all TV detectives take to solve their cases.
However mundane the fault there was always that sense of satisfaction in repairing failed equipment.

I remember when at the Nottingham outstation spending more hours than I care to admit fault finding on a Crime Squad, STC AF101 that refused to work on the selected frequencies. Rapid fault finding was thwarted by a handbook, that if I remember correctly was one of the most confusing I had ever come across with no attempt on the part of the technical authors to simplify the schematics of the synthesiser nor provide any voltage table or measurement point references that would allow you to easily determine whether the operation of the frequency selector switches were applying the required conditions for correct operation.

I eventually traced the problem to the failure of not one but several steering diodes in the control unit and when these were replaced with much difficulty because of the totally inadequate access provided, not only had I restored a failed unit to full working condition but had also learnt much about the virtues of patience and perseverance. When that crime squad radio's synthesiser changed frequency correctly, that really was a personal eureka moment.

During my time as a CWT in charge of the Harrow E&D, UHF lab we received a number of reports from the Shapwick Depot that battery cassettes for the PL201 personal radios were exploding. The problem occurred some time after they had been placed on charge. It was not a case of the batteries falling off the charger and self destructing on the floor below, but more akin to being hurled several metres in a straight line across the room like gas propelled missiles before gravity took over.

In a number of cases these flying cassettes had narrowly missed police officers who were working in the station office at the time. No one had been hurt but that happy situation could not be guaranteed to continue indefinitely. As you can imagine we took these reports very seriously.

The battery cassette for the PL201 was quite large, rectangular in shape and was a very weighty item. The idea of being hit by one of these at any level of force did not bear thinking about.

Health and Safety was becoming increasingly important and I think these events occurred shortly after DTels had appointed its first Health and Safety SWE. I shudder to think what levels of panic would be generated today in our risk adverse culture.

The UHF lab at that time was manned by three individuals, myself, and two SWT's Harry Bridges and Baldev (Dave) Dahiliwal. It was Harry whom fate chose to undertake the investigation.

The CWT's job in E&D was very much hands on because not only did you become involved in the SWT's projects but were expected to undertake investigations and evaluation projects in our own right.

Harry's researches and enquiries took him into all possible failure modes of the battery, the radio and the charger, No reports had ever been received regarding the battery blowing up on the radio so the investigation early on centered on the battery and the battery charger. Enquiries were undertaken with the battery manufacturer in case the failure was caused by a manufacturing problem. As it turned out, the problem had occurred with cassettes from numerous batches and of different ages and therefore a specific batch problem was eliminated as the cause of the problem. I believe it was the battery manufacturer who suggested that a possible cause could be the reverse charging of a battery.

Reverse charging of a cell within a battery could occur if the battery as a whole was given a deep discharge. In that circumstance, one cell might be fully discharged before the others cells in the stack had reached their end point. The fully discharged cell would then be reverse charged by the remaining cells in the stack. A second way to reverse charge the whole battery would be for the battery cassette to be connected to the charger the wrong way round. I cannot now remember whether the PL201 charger was a proper battery processor or simply a charger.

3Ch UHF Portable

The failure problem with the batteries was reported to the equipment manufacturer. Their position was that they were willing to help but were convinced the problem did not rest with any of their equipments.

As far as I know, the PL201 was only ever in service with police forces in the Shapwick area. I do not know the reason for this decision on the part of Dtels but mention it here because some of the readers of this story may never have seen, let alone handled the equipment.

The PL201 radio was a very professional piece of kit in both its design and manufacture. One novel feature was that its antenna system was fully enclosed inside the case in order to give it maximum physical protection. The antenna was connected to the transmitter and receiver sections via a cleverly designed, thin film diplexer. I always felt that the performance of a radio with an enclosed antenna was inferior to one where the antenna was external to the case. I can offer no hard evidence to support that view because although I did many field strength measurements on personal radios whilst employed in E&D, I never did any comparisons between equipment with external antennas and the PL201.

The rectangular battery cassette case of the PL201 was made of a very rigid and quite thick plastic. My memory after thirty five or so years on the securing arrangements might be wrong, but I think the battery cassette slid over cheeks on the base of the radio and charger ports and was secured in position by two offset tangs on the base of the radio and charger ports that mated with corresponding offset slots cut into the sides of the cassette casing.

When the battery cassette was presented correctly to the charger or radio, connection and release could be undertaken with ease. When the battery cassette was rotated through one hundred and eighty degrees and incorrectly presented to the charger or radio then it was impossible to connect the two together even when applying considerable brute force.

The problem seemed intractable. To try to get some user perspective on the problem, I visited three police stations in Wiltshire that had experienced the exploding battery phenomena to look at their battery management arrangements. My aim was to establish whether the problem lay with the user. In each case there seemed to be good battery management and control arrangements in place. The battery chargers were wall mounted and in the larger stations stacked in columns and rows one above the other. At each station that I visited, considering that we had still not found a cure to the problem and that some of the officers I spoke to had themselves narrowly missed injury, I was treated very well indeed and was given full answers to all of the questions that I raised.

The eureka moment came a little while later. I was in the lab at Harrow with Harry and Dave. I was doing something unrelated to that particular problem when Harry gave a whoop of joy and exclaimed excitedly (no not eureka) but something along the lines of "I've done it" Enquiring what he had done he told me he had managed to reverse connect the battery cassette on to a charger and what's more proceeded to show me how it could be done several times more with several different batteries on alternative charger ports.

The technique Harry demonstrated was to place the bottom lip of the cassette underneath the bottom cheek of the rectangular charger port and rotate the cassette onto the port until it surrounded it. Applying pressure on the base of the cassette was then sufficient to fully engage the battery with the charger. The safety tangs on the side of the charger port were compressed and totally ineffective.

We were convinced that we had discovered the method for reverse connection of the battery to the charger. When I thought about it more it was perfectly plausible that the police were in fact encouraged to present the battery to the charger in the way that Harry demonstrated given that some of the chargers were mounted above shoulder height.

I arranged a meeting with the PL201’s manufacturer who were initially very sceptical that we had discovered the cause of the problem. That scepticism vanished when Harry demonstrated his now perfected technique. To say they were astounded would be an understatement.

We acted swiftly to notify the Depot of our findings so that they could warn the users and followed this up with the issue of a CCE modification notice that I think involved modifications to the charger ports. I believe the plastic on the cassette and charger ports was also strengthened in later production items. All in all a very satisfactory outcome and certainly one of the most satisfying eureka moments of my career even though it was Harry who found the cause of the problem and rightly deserved all of the praise for a job well done.

1.  John Leary for the article
2.  PL201 picture source: G8EPR Pye Museum

page updated: 22/07/19

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