Hi,

So, I'm all full of hope, glory and enthusiasm from my previous success at getting an old Radiola 573M working, that I'm going to have a bash at an old Eddystone 670 Marine Receiver.

I got the circuit diagrams from Kevin Chant's website and cross-checked the initial components I need to obtain, such as the caps (electro and paper types) as well as valves.

Looks like I can get to most of the components without fully dismantling this beast, which highlighted the internals of the radio are shielded from the external case, via bakelight. Does this mean the internals are floating at a voltage that could hurt me?

The original power lead is missing. It's a two wire socket, so I am thinking of putting in a proper 3 lead (with earth) power cord, and connect the earth to the external chassis, as I assume the internal part is at voltage and earthing it could be an issue?

Asked Evacto about new valves. I need 4 x UAF41, 2 x UL41 and 1 x UCH41. Evacto said they have the UL41 valves, but not the others and there doesn't seem to be a substitute for them. So, I'm doing some searches to see if there is indeed a sub for the other valves. I hope there is.

Also, the schematic shows this unit to use only half-wave rectification, through some big diode (with cooling fins) and a large resistor. I will take pics and send them through as I am not sure about this diode and resistor as they're pretty old and don't know if I can get them replaced, or maybe have to rig up some alternate solution.

As before, your guidance and advice is needed and greatly appreciated.

River

River.
The valves you mention are the types usually found in AC/DC receivers. AC/DC sets were common in Europe & US but not here in Australia. A WORD OF WARNING:- these radios can be lethal. They do not have a power transformer and therefore the chassis could be live at mains potential. To work on them safely you need an isolation transformer. The diode you are talking about is called a selenium rectifier and Is used for the half-wave rectification of the H.T. supply. Selenium rectifiers of this age would probably not work and are no longer available. A replacement would be a silicon diode and possibly a dropping resistor. The valves heaters are joined together in series with a ballast to make up the 120volts. The valves have no equivalents, except the UL41 has an American designation: 45A5. The UAF41 heater volts is 12.6v; UCH41 is 14v & UL41is 45v. I don't wish to be a wet blanket, but you really need to know what you are doing when working on AC/DC radios. Australian Electrical safety standards were a lot more stricter than other countries. That is why you hardly see these type of radios here. Also, if you were to get this set going, you must not earth the chassis to anything, including the earth on the mains lead.
Cheers.

Hi,

Hmmm, that doesn't sound very nice.

Maybe it's something I need someone else to look at, or just put it under the house to fade away.

I'll think about it.

River

PS: Some intense Web searching came up with the following valve substitutions...

UL41 - 45A5 (as mentioned by Monochrome), N142, 451PT and MUL41
UCH41 - CF141
UAF41 - D121

It's probably time I wrote an article regarding AC/DC sets.

Whilst they weren't common from a manufacturer's perspective, they were as a percentage of market share for a time.

In many places where DC was the only mains option, you had to buy an AC/DC set or stick with the more expensive option of using batteries.

There were no grids, county councils, electricity commissions, etc - power companies usually owned one power station with a handful of substations containing battery banks, mechanical DC-AC converters step-up transformers and the like.

Some companies supplied AC only, others DC only and the remainder the option of either or both. In Sydney's CBD for example, large buildings were often supplied with both. AC for general light and power and DC for pumps, lifts and escalators. All of the old lifts at Farmers, Grace Bros., Myers, David Jones, Mark Foys, Marcus Clark's, Anthony Hordern's, Coles, Woolworths, Waltons, et al, that required drivers had a DC machine to raise and lower it and compressed air for the trellis doors.

At one time I've owned up to six AC/DC sets. A lack of time more than fear was the factor in why I haven't restored any of them and I recently sold one - an Airzone Radiostar.

I don't wish to be a wet blanket, but you really need to know what you are doing when working on AC/DC radios.

No warning about AC/DC receivers can be given often enough I think. The fact that there is no electrical isolation will always be a problem with them and unless a restorer has some experience under his/her belt I usually recommend that the receiver be cleaned up and perhaps the cabinet and dial glass restored and the radio put on display. It can always be returned to in later years when the owner's experience with high voltages has grown.

Hi.
On the subject of AC/DC radios in Australia.
After doing a quick trawl of the AORSM's it appears that they were more common pre WW2. This was due, as Brad said, to the haphazard mains supply at the time. Some radio's in the 1930's, like particular models of Bandmaster, were DC mains only. By the 1950's the mains situation in Australia (with some exceptions) had become more standardised and local AC/DC mains radios were only being made by a couple of the smaller manufacturers. Whereas in Europe and particularly the US, AC/DC radios were much more common and were made right up to the end of the valve era. My understanding is this was more to do with cost cutting rather than catering for mains supply variation. A power transformer would be the most expensive single component in a radio. With this item deleted the product could be sold to the consumer at a more attractive price. In Australia, safety was more of a consideration than cost cutting. Nowadays, thanks to the Internet & Ebay, we are seeing more foreign AC/DC radios falling into the hands of buyers and collectors here. It makes me shudder thinking that these radios are potentially a lethal trap for the unwary and inexperienced. So whilst I don't want to alarm anyone, if you have a AC/DC radio - please be careful.
Cheers.

AC / DC Radio's are not the only radio related items that are potential killers.

There are several items that I have here that are of US design, but not neccesarily made there. These have two wire mains cords and metal cases.

This can happen on radio sets and on things like signal generators. You just need to make absolutely sure.

What is done is a filter / decoupling capacitor is run from what is supposed to be Neutral to the metal case or chassis. Rarely are thes mains rated (approved type caps). As the power can be inverted and thes old caps are not reliable and leak, there is every chance that the chassis / case can become live.

Clearly a dangerous state of affairs.

If this is a transformer apparatus, you can cut that cap out, however, in my book a metal case that has any chance of becoming alive, should be grounded to the mains earth. All such working apparatus here has been retro fitted with a three wire cable. or IEC socket.

The Europeans and US did also make transformerless Radio's with PC boards ... Beware.

Marc

Hi,

Thanks for the info and warning about the lethality of these radios. I did some reading and checking and now know what a selenium rectifier looks like, and judging by the age of the radio I dare say it's best days are long gone, and it would fry or burn or whatever on power up. I did see a youtube footage of a selenium rectifier destroy itself.

Now, permit me to try and describe how the power supply on the Eddystone operates, and please correct me if I get it wrong.... Only two power leads go into the radio - only active and neutral. The neutral connects to the internal chassis, which is insulated from the external chassis to avoid electricuting the hapless operator. The active goes to the power switch and into the selenium rectifier. This does a half-wave rectification and this voltage goes into a large multi-tapped resistor. I assume the various taps would be to provide the required voltages for the different valves heaters, as well as cathode voltage, plus ballast. The electro caps filter some of these voltages - but need to recheck which ones, as there are only 2 electros. Is this basically correct?

You mention AC/DC radio. I assume that means it can take batteries or connects to the mains. However there is no facility to put batteries in the unit, so I would assume there is a separate input for DC only voltage, which would be supplied by the building/ship/whatever? Does the DC go straight into the selenium rectifier, just like AC does? And due to forward bias of the DC the voltage goes through it and into the resistor as per AC?

If, by chance, I decide to have a crack at this, I assume the starting point would be to use a silicon diode and the necessary number of resistors to provide the ballast as well as necessary voltage taps to emulate what it has?

In the meantime, I am going to get new caps to replace the current electro and paper units. Try and get a set of valves, and give the whole unit a detailed clean and restoration. The power supply part, for the moment, is a tad scary and something I'm not thinking of doing until I get more confident and get more information. Besides, I dont have detailed information on what voltage comes from what taps on the resistor, as well as expected current - and I think powering it up to measure the voltages/currents could be a failure as I'm sure its all a little too old and fragile to take the voltage without some form of self destruction.

River

PS: Oh and another question.... why half wave rectification? Wouldn't full wave rectification provide a better, less ripple power?

It would be quite usefull to get a circuit of that to confirm if the rectifier is half or full wave. There is a difference in the ripple and rectified voltage comparing half wave to full wave.

For those unfamiliar again the circuit would confirm the heater wiring. It may have dropping resistors but it is not unlikely that one valve is actually a barettter (ballast tube) that keeps the current constant through the valve heaters. In that instance all of the sets tubes will be designed to draw the same heater current.

While Selenium rectifiers have a reputation for creating toxic waste clouds, do we have any evidence that it has failed?

Marc

River,

I don't know whether or not you have stumbled across these sites yet. Both of them deal with the issues you face in restoring your Eddystone.

http://www.geojohn.org/Radios/MyRadios/Safety.html.
http://w3hwj.com/index_files/RBSelenium2.pdf.

Cheers.

You'll want to have the radio's power switch in the hot side of the line. Most American hot chassis radios had this switch on the radio's ground side of the line. Partly as the power switch was part of the volume control, the designers wanted to avoid having wires bearing 120V60Hz (compared to the low level audio signal at the volume control pot terminals) near the high impedance low level audio signal. To avoid picking up hum. You may have to do some shielding of the 120V wires when you place the switch on the hot side of the line, wrapping a grounded to the radio chassis wire around the 120V wires will do that. And the neutral wire of the power cord now will go directly to the radio's ground. The best place to connect this neutral would be the negative lead of the filter cap. And I usually disconnect the ground end of the heater string from any local grounds at the audio driver tube, and connect a wire from that ground end of the heaters directly to the same point the neutral connects to the filter cap negative lead. This keeps AC currents from making hum voltages in the rest of the radio circuits. "Star grounding" they call this in tube hifi amps.