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plessey: Plessey PR-2250 repair

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » plessey: Plessey PR-2250 repair
           
Hubert Eisner
 
 
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08.Nov.17 13:08

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The Plessey PR-2250 was obviously built for communication units of the British Armed Forces and it may safely be assumed that it was also planned for diplomatic listening purposes. Produced between 1978 and 1985, it was quickly replaced by satcom units and thus was also offered on the open surplus market. As cost did obviously not play any part in its development and production it was, and is, certainly one of the best shortwave receivers ever built.

The generous use of integrated circuits is typical for this period when higher-integrated IC were not yet readily available. Lower-order logical components had to be used, generating a kind of ‘IC cemetary’ within this receiver. Delving into the deeper parts of the (excellent) service manual enables entering the developers’ minds and offers an incredible view of the early stages of digitalisation.

At the time of production, more and more tantal capacitors were used, replacing the bulkier electrolytic capacitors. As no real long-time experience with either integrated circuits and tantals was available, this seemed an intelligent choice. Nobody at that time could probably have guessed that most of these components were not as long-time stable as desired; the dire consequences were obvious to me when my PR-2250 stopped working in 2010.

A friend and I were beginning to work on the receicer, but what at first seemed to be a routine repair – replacing the power supply caps and looking for other damaged components – quickly developed into a nightmare.

Whenever a tantal cap was replaced, another one exploded with a decent ‘plop’ in a not-so-decent ill-smelling blue cloud. We never counted the number of tantals we had to replace in this receiver but our estimate goes into the hundreds …

The reason for the mass extinction of these otherwise quite reliable components seems to be its coating. Over time cracks may have been developing, enabling humidity to enter the caps’ bodies, leading to their explosion when the full load was applied.

But the worst was yet to come. Apparently, many IC’s coatings also leaked, leading to internal corrosion. Unfortunately, some of the ICs used by the manufactorer are no longer freely available and had to be purchased at a rather high cost at internet auction platforms. An additonal purchase of four PR-2250 from the humid basements of the Britsh Navy did not much to supply us with spare parts, as in every single one of these receivers the same parts werde defective.

The receiver uses a highly modular approach to facilitate field service. As adapter cables are of course unavailable, many different multi-wire adapter cables had to be built In order to be able to get to the printed boards inside the shielded modules. Nor was it possible to buy RF connectors and connecting cables for the wiring between the modules, so improvisation was asked for, and we had to re-build the connectors.

After nearly one full year of highs and lows, the receiver is working fine now, but up to this day I’m still waiting for the ‘plop’ and the blue cloud …

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