Eddystone "Cat 732" mains filter - what's in it?

[G6Tanuki]

The notes for my Eddystone 840A list a "Cat 732" mains filter as an option; given that my 840A suffers a lot from random SMPS- and thyristor-dimmer noise on the mains (proved by running it from a non-mains supply) I was wondering if such a filter might help.

But what's in it? There's a picture of one here:

http://www.mullardmagic.co.uk/eddyst...-unit-type-732

I've built a simple 'best guess' at it - just a commercial "RS" filter in a diecast box, but it doesn't seem to have much effect.

Were Eddystone being more cunning??

[turretslug]

I wonder if the 732 filter was aimed at the likes of the 670 cabin receiver (the 840 supposedly being a radio amateur-aimed derivative) originally? If it was intended to reduce commutation/switching noise from a ship's DC supply, it mighty have quite hefty chokes and biggish capacitors inside, rather than the tiddly L's and C's in a packaged filter intended for relatively high frequency suppression.

I suppose it's a potential shortcoming of any set with this type of PSU. There's a 670 here, for both safety and interference suppression I run it through an ex-BT "Mains Conditioning Unit 250VA"-fancy name, just a 1:1 230V transformer with earthed inter-winding screen, primary MOV, and secondary resonator C and RC snubber. I might have got away with a 125VA version with this c.60VA set, but was concerned that the half-wave rectified HT implementation would be a bit of an ask for the smaller transformer. A few years ago, these were common as muck and cheap as chips, I bought 2 for £10 (possibly less than their metal value!) from a guy who had a garage-full and was uncertain as to their purpose. Unfortunately, the snake-oil crowd subsequently cottoned onto them and they seem to appear for sometimes bonkers asking prices with outrageous (impossible) claims attached. You might get lucky and find one at a rally or the like, it's a classic "misc" sort of thing and lots appear to have been made.

[Barraman]

Here's a picture

Richard

[G8BBZ]

Hi,
It may be that your "RS filter in a box" isn't as effective as you had hoped because the filter is designed to stop noise generated within the equipment from leaking out into the mains wiring. Most packaged filters are of this type as a means of reducing spurious outputs to meet CE requirements.
What you need is a filter that does the opposite - stops noise in the mains wiring from getting into the radio.
If you are in an area where the mains distribution uses the PME system, it may well be that most of the noise is on the earth wire, as this is likely to be far from "earth" in RF terms.
The mains isolation transformer mentioned by turretslug is probably the best overall solution to the problem. Critically, it is the isolation of the earth conductor which is the most important feature of this arrangement. The transformer primary, screen and frame must be connected to the incoming mains earth to comply with the Regs., but the two secondary live conductors are then fully floating and one side can be connected to your RF Earth, together with the chassis of the receiver, without the risk that you are providing a low impedance earth for the whole street through the PME system.
If a mains isolation transformer is not to hand, you can try building a filter like the Eddystone one, this would have capacitance (X-rated) across phase and neutral at the mains input to provide a low impedance "shorting" path to noise, series inductance in both the phase and neutral leads to increase the series impedance to noise and a second phase to neutral capacitor (X-rated) across the output to the receiver. You can try capacitors from phase to earth and neutral to earth (Y-rated) but if the earth wire is very noisy this may make things worse rather than better.
cheers
Peter G8BBZ

[G6Tanuki]

Thanks for the comments. From the photo it looks like there are 2 chokes in series with the L and N lines, with a capacitor across each side, and another capacitor to ground.

The RS packaged filter has - according to the diagram on the case - a bifilar choke then three caps, one directly across L and N, the other two being from L to E and N to E respectively - a pretty much standard setup. it doesn't have a capacitor across L and N on the side I am using as "input" though - I may try adding 0.1uF here (I've got a box of X and Y caps spare...) or even reverse the input and output sides of the filter.

[The mains here is not PME - there is no utilico-supplied earth; this is generally known as a "TT" system and means I can safely have RF earths separate from the mains-earth]

I don't think the problem is 'noisy earths' somehow - as I don't have the same noise issue when feeding other receivers from the same antenna/earth/power-feed.

As an experiment I'll try running the 840A with it set to 110V and fed through my isolating transformer. My previous 'isolated' test was using a little 2-stroke generator.

[Synchrodyne]

Originally Posted by turretslug

I wonder if the 732 filter was aimed at the likes of the 670 cabin receiver (the 840 supposedly being a radio amateur-aimed derivative) originally? If it was intended to reduce commutation/switching noise from a ship's DC supply, it mighty have quite hefty chokes and biggish capacitors inside, rather than the tiddly L's and C's in a packaged filter intended for relatively high frequency suppression.

My understanding is that that was the case. See the attached snip from the Eddystone 670 brochure. The later 670C cabin receiver had the “brute force” power supply filter built-in. A snip from the schematic is attached, although it may not be of much help as the inductance values are not shown.


Cheers,

[G8BBZ]

Hi,
I am also supplied by a "TT" system and in my case power comes from a pole mounted transformer a couple of fields away via a 2 conductor feed line to 3 cottages. In each cottage Distribution board, the two conductors provide the Phase and Neutral connections and there is a 3rd Earth wire which runs outside and is connected to a single earth rod by the side of the building.
This earth connection is often very dubious - a single rod close to the building foundation is not going to have a very low earth resistance and the connection to it is often ancient and corroded. In the case of one of my neighbours, the earth wire was actually broken off the earth rod and clearly had been so for many years! In the abscence of an earth connection, any noise on the live conductors will clearly propagate.
If you have the option of operating the radio from 110v. then the type of isolation transformer designed for use on building sites is attractive, as the 110v secondary on these is actually 55-0-55v with an accessible centre tap. The centre tap can go to your RF earth and any noise which is present on the live conductors is then common mode which should aid in its suppression.
Interesting that the Eddystone is more affected by noise than other equipment - I look forward to hearing how your tests progress.
cheers
Peter G8BBZ

[turretslug]

There is an easy, link-set 110V position on the dropper. 110V would certainly help the UL41 from getting too sweaty, but it might also show up a tired triode in the UCH42. At least they're not too difficult or expensive to track down.

The 670A here (yes, a later ruler-scale -A version, not a plain 670 half-moon scale- apologies) has been granted temporary stand-down from France Inter 162kHz listening for a photo of the input choke, which measures at 2mH per section- so not that high a value, and comparable with what would be found in a typical package filter. I'd be surprised if the 840A didn't have this as standard- though the bean-counters might have deleted it for a set with a domestic, rather than shipboard, intended market.

For orientation's sake, the smoothing choke is on the left, coil-box at top and fuse/rear panel at right of picture. The original 2x 50nF capacitors, as per. Synchrodyne's circuit snippet, were rather nice brass-case cylindrical types but were also rather leaky so were ousted by modern "X" types.

[G6Tanuki]

I've got a good 'local earth' for my mains: having had noise issues a few years back I had the powerco visit and do various loop-impedance checks/tests all of which passed with flying colours [the noise was eventually tracked to a fizzy insulator on one of the 33Kv overhead lines at the bottom of the garden].

The 840A doesn't have any significant chokeage on its power supply as standard - hence my interest in an outboard filter. The only concession to RF-proofing the mains input is a 0.001uf capacitor in parallel with the rectifier valve [presumably to control 'modulation hum'].

I'm definitely going to try using my 110V isolating transformer to see what happens.

The biggest noise issue seems to be around 7 to 9MHz.

Another thing I plan to do is see if the noise is still there if I replace the antenna with a 'dummy' resistor - if the noise is getting in through the power supply then it should still be there when listening on the dummy antenna.

[Bazz4CQJ]

Originally Posted by G8BBZ

I am also supplied by a "TT" system and in my case power comes from a pole mounted transformer a couple of fields away via a 2 conductor feed line to 3 cottages. In each cottage Distribution board, the two conductors provide the Phase and Neutral connections and there is a 3rd Earth wire which runs outside and is connected to a single earth rod by the side of the building.
This earth connection is often very dubious - a single rod close to the building foundation is not going to have a very low earth resistance and the connection to it is often ancient and corroded.

Wow; sounds exactly like you are describing my house! All the houses on my lane are fed by overhead cables and there is a substation about 75m away from me. As for the earth, this house was built in the mid-60's and I think it's the original.

I'll have to explore the garage to see if I've still got a low voltage isolating transformer.

Adding to the noise issue, with the overhead power cable, a overhead phone line, and a fair number of trees, this house is pretty much doomed in terms of erecting good antennas.

B

[turretslug]

I have to confess to being slightly surprised at the absence of a mains filter choke with a power supply of this nature- maybe the argument was that it wasn't as necessary for the 840A's intended market as the 670A's, but a counter-argument would be that radio amateurs would be more likely than most to search out weak and tenuous signals that might be impaired by mains grot.

In my youth, I recall (a bit shamefully!) when dishevelled old radios were plentiful and got plundered with little conscience, it didn't seem that unusual to find mains filter bobbins (sometimes the RS retrofit types) fitted to AC/DC type sets, so they must have been fairly inexpensive at manufacture and interference a significant issue.

[Skywave]

Originally Posted by Bazz4CQJ

Adding to the noise issue, with the overhead power cable, a overhead phone line and a fair number of trees, this house is pretty much doomed in terms of erecting good antennas.

That description is almost identical to the location here, including the proximity of a sub-station - plus the very high level of PLT interference from less than 1 MHz to at least 30 MHz. This house is almost surrounded by overhead 3-phase power lines. The workshop here is on the third floor, so an effective R.F. earth is not possible.

Al.

[Bazz4CQJ]

I recently dismantled a piece of late 1970's lab equipment (a spectrometer) that had probably cost £30k, so the makers probably wanted to use a reasonably effective mains filter (for that era!), and they had used a unit made by TDK, as per attached.

My "To Do" list includes a line item to try this out and see if it has much effect on my HF recievers. I cannot claim to have great expertise on filtering; does anyone have an idea as to whether or not this might be useful in this role, and whether any additional components might compliment it?

B

[turretslug]

That certainly looks like a comprehensive item, with the two-section pi filtering. I suppose mains filters come under the heading "unlikely to be harmful, and may do good"- with provisos.... Some filter types had/have the pyrotechnic Rifas lurking within, they may have been largely European-made types but it's also possible that filters from other regions sometimes also used Rifas simply because they sported a wide range of national compliance badges. Some filters were also a little generous with the capacity of their "Y" capacitors, leading to leakage currents that could cause RCD tripping when several devices with filters resulted in a cumulative effect, or simply produce a "last straw" effect when lots of slightly leaky appliances (refrigerators, boiling rings etc. are present). The data for those TDK filters does state that they leak less than 1mA to earth at 60Hz, though.

I like to use these sorts of devices where possible- whether they universally reduce/eliminate RFI is doubtful, but they do have the appeal of adding HF source impedance to help a downstream MOV do its job. Whilst spikes are an obvious hazard to the likes of semiconductor rectifiers, I've heard it said that the brief current "blip" from mains spikes can also result in long-term cumulative weakening of thermionic rectifier cathode coatings- which at least sounds plausible, unfortunately mains filtering is yet another of those subjects that arouses both flowery and over-earnest unlikely BS and entrenched hostile camps in some circles....

[Keith]

Certainly a quality filter with common mode and differential mode sections. It will certainly clean up the mains going into your receiver. Unfortunately, if the mains borne interference is radiated by the house wiring, etc, then the rubbish will be coming in via the aerial socket. If you know of one particular interfering device then it's worth trying using the filter to reduce the RF getting out!