Help with Eddystone 888A

[roberthewitt]

Hi all
Recently came into possesion of Eddystone 888A receiver .Cosmetically in great shape but when switched on the chassis becomes live even when rectifier valve removed. Only Live and neutral wires in ,no earth connected.
Am I missing somthing or is there a serious problem.

Any advice much appreciated

[MichaelR]

Without mentioning the obvious have you checked the wiring . A lot of Eddystone receiver do not have their original mains sockets , has the socket been changed?.

I would then check the isolation of the mains transformer , what size fuse have you got fitted.

Be careful

Mike

[roberthewitt]

Hi

The receiver had a bulgin plug with live and neutral going direct to transformer primary(the live via a single pole toggle switch )
The transformer is working in so much that the valves light and im getting 250-0-250 on the secondary. But as I say ..the chassis is live.Can you elaborate re checking transfromer isolation ?

Robert

[Station X]

I'd start by measuring the resistance from the L&N pins of the mains input socket to the chassis. The result should be infinite ohms. If you get a low reading check wiring from the input socket to the mains transformer. It may be in contact with the chassis somewhere.

I assume that if you earth the chassis the RCD in the consumer unit trips?

[roberthewitt]

Infinite resistance to chassis from L and N .

Ive not tried earthing the chassis..I was assuming it shouldnt be live even without the earth and therefore as you say ...would simply blow the mains.

Robert

[MichaelR]

Hi ,

As Graham in the post above has suggested is what I meant about isolation that the windings of the transformer have not some relatively low resistance to chassis. I am intrigued why you are not blowing fuses

There should be a fuse in the centre tap connection of the main secondary transformer winding to earth. Check the value of what is fitted ( for safety) it should be about 250ma.They were originally fitted with delayed Magnickel type.

Mike

[Station X]

If you're doing the test with an normal multimeter or DMM the results may not be conclusive. To be really sure you need to perform the test at high voltage using a "Megger".

I assume that you've downloaded the manual/circuit from the Eddystone User Group site?

As Michael says you now need to check the resistance of the HT winding with the centre tap fused removed.

As a matter of interest what voltage do you measure from the chassis to an earthed object such as the retaining screw of a mains socket outlet? Is it AC or DC?

[cmjones01]

What about the presence of mains filter capacitors between L and N and chassis? If the chassis wasn't earthed, then it would wander up to about 120VAC. That's plenty to light a neon screwdriver or show up on a multimeter.

What does 'the chassis becomes live' really mean in this context?

Chris

[Station X]

What about the presence of mains filter capacitors between L and N and chassis?

According to the circuit there aren't any. I assume that Live in this context means AC, as the rectifier is unplugged.

[Skywave]

Originally Posted by roberthewitt

The receiver had a bulgin plug with live and neutral going direct to transformer primary(the live via a single pole toggle switch )
Robert

Originally Posted by cmjones01

What about the presence of mains filter capacitors between L and N and chassis? If the chassis wasn't earthed, then it would wander up to about 120VAC.
What does 'the chassis becomes live' really mean in this context?
Chris

I know this receiver very well, in fact, I've owned for many years.

First, Chris: you say that the receiver had a Bulgin plug fitted. Do you mean that the set had been modified with a Bulgin socket fitted to the metalwork of the set, so that the mains lead was consequently removable or do you mean that the non-removable mains lead was still intact with a plug at the remote end of the mains lead?
Moreover, I see that you say "had" (past tense) a Bulgin plug fitted. So - have you removed this plug?

The original build of this reciver had the mains lead wired direct to the inner chassis: no chassis-mount plug / socket was fitted.

Finally, and this is the crux of the matter - between what points in the receiver have you measured a voltage and thus determined that the chassis is indeed "live"?

----------

Second, cmjones01: as per standard build, there are no capacitors fitted across the mains input, but if a chassis-mount plug / socket has been fitted, this may contain integrated filter capacitors.

Let's see what Chris comes back with, re: my Qs. above.

Al. / Skywave.

[HamishBoxer]

Will check mine as i am sure these are an earthed chassis.

David GM8JET

[roberthewitt]

Thanks for responses

The receiver mains lead(2 core black&red) is hard wired into switch (L) and transfromer primary (N)as per the cicuit diagram .At the other end is the small bulgin plug.There is no socket on the chassis .The circuit is as per the original from what I can see..no filter caps.

The original magnickle fuse is fitted and intact.

I have attached the 2 core to a normal 3pin plug. black (N) red(L)..No Earth
I am assuming the chassis is live because when touched with a neon screwdriver it glows but having just measured the voltage from chassis to exterior earth point it is only 25v a.c !
Resistance across primary is 14 ohms across secondary is 75 ohms

Hope that answers some questions

Robert

[paulsherwin]

Originally Posted by roberthewitt

I am assuming the chassis is live because when touched with a neon screwdriver it glows but having just measured the voltage from chassis to exterior earth point it is only 25v a.c !

A neon tester will light with a tiny leakage current if the chassis isn't earthed. There may even be enough leakage to cause a slight tingle. This isn't in itself dangerous.

You are right to be very cautious if the tester is lighting though.

Paul

[Skywave]

I have the cct. diag. of the 888A in front of me. It shows the incoming mains as two-wire with no connection to the supply earth. The chassis metalwork is shown connected to the "E" spring-loaded terminal at the rear of the set next to the A1 and A2 spring-loaded terminals.

I expect that the 25 v.a.c. you are measuring is simply due to capacitance from the mains transformer primary to the unearthed chassis and is, therefore, not a cause for concern, since the available current by this route will be very small. However, it would be wise to replace the 2-lead mains supply cable with a 3-wire type and connect the chassis, therefore, to supply earth. Alternatively, you could supply the receiver from an isolation transformer using a balanced two-wire supply and connect the "E" of the receiver terminal (and thus the receiver's metalwork) to a seperate R.F. earth. Personally, I would prefer the latter version, since it helps keep mains-borne noise out of the receiver's sensitive 'front-end'.

Al. / Skywave.

[David G4EBT]

Originally Posted by roberthewitt

Thanks for responses

The receiver mains lead(2 core black&red) is hard wired into switch (L) and transfromer primary (N)as per the cicuit diagram .At the other end is the small bulgin plug.There is no socket on the chassis .The circuit is as per the original from what I can see..no filter caps.

The original magnickle fuse is fitted and intact.

I have attached the 2 core to a normal 3pin plug. black (N) red(L)..No Earth
I am assuming the chassis is live because when touched with a neon screwdriver it glows but having just measured the voltage from chassis to exterior earth point it is only 25v a.c !
Resistance across primary is 14 ohms across secondary is 75 ohms

Hope that answers some questions

Robert

I don't know if you have the circuit, but if not, you can download it from this link:

http://www.eddystoneusergroup.org.uk...e%20OE3SBN.pdf

The socket on the rear panel is for connection of an external Eddystone S-Meter (which occasionally appear but cost a fortune and confer no real benefit. There is also facility for connecting an external Eddystone PSU powered from a 6V accumulator. No relevance to the present day use of the set.

In its day, it was often used with a transmitter as the receiving side of an amateur radio station, and several of the facilities were designed with this in mind. It was assumed that a good earth would be used as part of the station, and personally, I wouldn't be happy to use this set without providing that earth via a conventional three wire mains lead/plug.

Eddystone valve radios are notorious for their little red globular Hunts capacitors.

Generally, if the set is working well it's best left alone. They're invariably quite complex, and once you start meddling with them for no good reason, they can lead you a merry dance.

The Eddystone User Group website had members with a wealth of experience and can give advice on the sort of faults and shortcomings of sets which - when new, gave a good account of themselves, but are well past their MTBF (mean time before failure).

Hope that helps.

David,
G4EBT

[roberthewitt]

Thanks to all for advice.
I remeasured the voltage drop from unearthed chassis to earth on wall socket and it was in fact 167 v ac.
Decided to earth the receiver anyway and stood back. Lo and behold ..no explosion and
everything seems to work !!.

By the way there is an S -meter on ebay at the moment .£55

I wont touch the wiring as it looks original to me ,but the tuning dial would benefit from a clean...any ideas on what is safe/best to use ? what about isoprop.(99% alc)

Robert

[MichaelR]

Hi Robert,

Looks like it was leakage as Al suggested.

The "s" meter is a great looking accessory, I have one however not much use in its original form as the dial is calibrated at 3db per s point as opposed to the normal 6db per point. This means the needle spends most of its time on the end point on 80 metres

You can shunt it easily enough though.

Mike

[Sideband]

Probably nothing to worry about but top marks for being carefull! All you are probably getting is capacitive coupling between the mains transformer primary winding and the metal core. Think of it like a capacitor between mains and chassis which will 'leak' a minute amount of current to the chassis. It's probably only a few microamps and completely harmless to touch but it will make a neon screwdriver glow.

Earthing the chassis will do no harm but may increase the risk of primary to core breakdown in the mains transformer which will cause the RCD to trip. Better that than a live chassis though!


Rich.

[Skywave]

Originally Posted by MichaelR

The "S" meter is a great looking accessory, I have one however not much use in its original form as the dial is calibrated at 3db per "S" point, as opposed to the normal 6db per point.
Mike

4dB per "S" point, no?

Al.

[MichaelR]

Well spotted Al.... 4db per S point it is.

Mike