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Old 13th Apr 2021, 9:25 am   #1
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Default Plessey PRS2282A BPF-Change Issue


On my bench at the the moment is a Plessey PRS2282A HF Rx which has very noticeable RF ‘clicks’ as the band-pass filters change. On some bands these clicks reduce to inaudible if the filters are changed a number of times quickly, but return after a short rest period. On other bands there is no noticeable change in level. Without an antenna connected the RF ‘meter’ shows a corresponding sudden signal as the relays operate. I’m presuming it’s the releasing relays causing this but due to its construction, without the right extender card, I can’t get a scope in to look closer.

Attached is a copy of part of the circuit. Reverse diodes have been added across the ULM2002 relay drivers and on the 5V relay rail, with no change to the clicking.

Any thoughts on a cause / cure please?

Many thanks,

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Old 14th Apr 2021, 6:03 pm   #2
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Default Re: Plessey PRS2282A BPF-Change Issue

The mention of the clicks changing if there's a number of rapid changes - and looking at the circuit - suggests that the problem may be contact-resistance.

I came across this in a bit of quasi-military gear and realised that the problem was that the relays were purely switching low-level [microvolts, millivolts] of RF or audio; there was no DC component passing through the contacts [which would have helped break-down any surface tarnish-film].

Sometimes after a period of prolonged disuse [like sitting on a shelf in stores for a few years] the equipment would be spectacularly-deaf, only to return to normal sensitivity after some vigorous frequency-changes-involving-filter-switching.

I re-engineered things to bleed a low current from a +300V current-limited source [a high-value resistor and series-choke] through the relays, which cleared the tarnish and reduced the number of 'new' boards that were returned-to-the-supplier for being defective!
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Old 14th Apr 2021, 6:09 pm   #3
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Default Re: Plessey PRS2282A BPF-Change Issue

There are bits of those filter circuits floating at DC, with only capacitors to ground. You could have DC leaks charging things due to dirty boards or absorbed humidity, charge builds up, and when you switch a relay, you get a small zap producing a click. Move relays around quickly and there is less time for charge to accumulate, so a smaller zap.

Best quality guessing, but something to check for.

Relays have wonderful isolation at DC like all mechanical switches with plastic insulation. So good design is to make sure there are DC paths or leak resistors to keep all switch selections not in use at the same DC as the one in play. Avoids clicks and thumps on switching.

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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Old 15th Apr 2021, 7:21 pm   #4
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Default Re: Plessey PRS2282A BPF-Change Issue

Just noticed in my first post I said the added diodes were across the relay driver devices. They aren't, they are across the caps.

I had to replace a couple of the relays in this as the contacts had gone high resistance, no difference. So will try some DC wetting and a number of coil operations for a while and see what the result of that is. Thanks for the thought. I may also remove one of the capacitors and leave the reverse diode in, see if that makes a difference. The steady reduction in the click level when a relay is operated seveal times in quick succession does have a capacitor feel about it...!

This is a really impressive HF Rx and has a much better operator feel to it than the Racal RA1792 which I have used in the past, and which is probably the same class and era. Would be good to get it running without the annoying clicking!


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