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Old 13th Oct 2020, 2:53 am   #1
cernem1alt
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Default Eddystone 740/3 part confirmation ID?

/I know where this part comes from and hopefully goes back can someone confirm it is some form of balance wheel on the VFO? also as it was broken in 3 places was it a case of someone sometime apply brute force and it snapped ? the dial cord wire was also mangled so am going down the Pike fishing wire approach 24lb Drennan replacement, I am curious so as it wont happen again it seems a successful bond with araldite famous last words... Mike

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Old 13th Oct 2020, 8:29 am   #2
trh01uk
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Default Re: Eddystone 740/3 part confirmation ID?

That's a flexible coupler - and probably an insulating one too. This can deal with slight misalignment between (typically) a switch or a variable capacitor (etc) and a knob shaft going through the front panel of the unit. It also allows the part being controlled to be electrically insulated from the knob and shaft. That can be important for various reasons such as:
  • the part being controlled is at a high voltage relative to chassis (and there might a shock risk to the user)
  • the effects of hand capacity can affect the part being controlled

I've got a box of these I think. And by the way its not " balance wheel" - not enough mass in these to make any difference normally.

Richard
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Old 13th Oct 2020, 9:16 am   #3
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Default Re: Eddystone 740/3 part confirmation ID?

Flexible coupler.

There are many different types, but that's the most common. They're used to handle slight misalignment where two shafts are joined.

The insulated ones also have another reason. Where a variable capacitor shaft pokes through a panel and has a knob fitted to it, the grounding of the shaft at the variable capacitor isn't perfect and there can be a little RF voltage on the shaft. Putting your hand near the knob will shift the tuning a little. This can be enough to be very irritating. So insulated shaft couplers fix the problem.

In a set with a nice gearbox like the Eddystones, grounding of the output shaft is erratic due to the grease in the bearings etc. Having an insulated coupler stops the erratic contact making the tuning a little erratic.

Subtle, but problems worth avoiding.

David
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Old 13th Oct 2020, 9:49 am   #4
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Default Re: Eddystone 740/3 part confirmation ID?

The high-voltage isolation point made is a good one to be aware of with Eddystones- whilst the 740 has a mains isolating transformer, other models using the same central coil-box castings, tuning drive and general metalwork have the chassis connected to one side of the mains supply and are dependent on the plastic doughnut to keep the mains supply off the exposed casework (as well as a scattering of Paxolin bushes, strips and Nylon pot-mounting cups, all of which need to be present and correct. The 670A here had been potentially lethally mis-re-assembled by a previous owner).
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Old 13th Oct 2020, 10:37 am   #5
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Default Re: Eddystone 740/3 part confirmation ID?

Thanks, Turretslug, I'd overlooked the need for mentioning safety isolation in the AC/DC models, knowing the 740/3 has a mains transformer.

But it is definitely worth mentioning. Even some of Eddystone's grander models were AC/DC and ran a hot chassis inside a metal cabinet! More for shipborne use than for cost-cutting, I think. But they can still be deadly if seemingly trivial mistakes are made.
People need to know to check what sort of power arrangement an Eddystone set has before touching it.

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Old 13th Oct 2020, 11:16 am   #6
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Default Re: Eddystone 740/3 part confirmation ID?

Yes- as someone who started his career in a relatively blasé atmosphere re. electrical safety and saw it turn overnight into a cat's cradle of green/yellow bonding and PAT testers, live-chassis Eddystones give me a bit of a shiver. One of the most popular models seems to be the "universal"-PSU 840 series- popular implying that some could have had all sorts of bodges and misunderstanding inflicted on them.

Forgot to mention- there is a type of flexible coupler that has a sort of wrinkly brass "maggot", like a miniature concertina bellows, that doesn't electrically isolate between the coupler ends. Obviously not good in some circumstances!

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Old 13th Oct 2020, 11:28 am   #7
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Default Re: Eddystone 740/3 part confirmation ID?

Take the worry away and just use an isolation transformer when servicing.When I started servicing in 1961 we never had such luxury,just a piece of "pit belt" to stand on!
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