Saturday 29 September 2012

Rewiring the Epidiascope

So I have carefully looked over the 'scope and gave it a clean and brush down. More or less everything was in order, including the bulb which on testing it with my multimeter looked like the filament is still good. It's time to try to get it going.

It isn't just a matter of plugging it in. For one, the flex had been cut to the side of the 'scope and there was no plug. But more importantly the internal wires need checking over, are probably not safe, and probably need replacing. Especially on an old metal object such as this. As a minimum requirement, metal appliances need earth connections, which were not normally fitted in the "good old days", and should be fused properly too.


Apologies to people who know all this, but I will say it anyway. "Earth" is the third wire in the mains plug and is an important safety feature on appliances that are metal. The problem is that, without earth, all too often a wire gets frayed or damaged and accidentally gets out of place. Usually it falls onto or otherwise ends up touching metal somewhere, such as the metal of the case, or something connected electrically to the outside of the object. So anyone coming along who picks it up gets electrocuted. The solution is to wire the metal parts that people might touch to "earth", then if a wire gets frayed and loose it makes a connection to the metal, which is also connected to earth, so a short circuit occurs. This is still dangerous without fuses, but if fuses are fitted properly they blow on a short circuit, and the appliance fails, but at least it fails safely.

This old electrical appliance, like most old appliances, suffered two problems: no earth connection and very badly frayed electrical cable. In other words it was not safe. Here is the wire behind the main switch.

Switch before rewiring

The switch itself is metal and electrically connected to the case, so in the event of a wiring failure, touching the switch would be very dangerous.

So the wire needs replacing, earth connections need to be made, and the appliance must be run from an adequately fused line. (The British system is to have a fuse in the plug at the right rating for the appliance, as well as circuit breakers on the system at the distribution box.)

Wiring is not rocket science. You must assess what current will flow and whether the wire will heat up or not. Actually, all wire heats up slightly when used, but often not very much. Hot wire has higher resistance and resistance makes wire get hotter. So when wire gets hot, it heats up more quickly still. This can "snowball" (an unfortunate phrase in this context) into a fire. For my item, I expect that the bulb will get hot - but the wiring is not near the bulb, it is away from it and in the cooler area below the bulb. The internal case is large with a decent volume of air and there is also a fan. So heat is not a major problem. The bulb is rated as 1000W from a 240V supply so we expect 1000/240 or about 4 amps to flow. Play safe and make sure all the wiring and connectors are rated at at least 5 amps. (I used stuff I had in my shed that was good for 13 amps.) The fuse should be 5 amps, not the common 13 amp ones.

Apart from that, it's just a matter or replacing the old wire with the new, and making earth connections to the metal case in strategic places (i.e. electrically connected to any metal the user might touch, but hidden from sight as far as possible.) Here is the re-wired switch.

Switch after rewiring

I tested the wiring with my multimeter. I was also pleased to find that the switch switches both the live and neutral wires. (A useful safety feature in case that someone accidentally swapped over up live and neutral somewhere in the house's electrics.)

First light

The bulb in this projector is good sized 1000W tungsten filament lamp with a P40s connector. It is very delicate and was removed in advance of all the other work, so it's time to put it back and try everything out.

1000W tungsten bulb

I was lucky, and it worked first time!

Epidiascope: first light

But even 1000W is not very impressive in full sunlight... I must try it at night time next, and with some slides.

Sunday 23 September 2012

The Ross epidiascope

It's been a long time since I posted here - sorry folks. The day job got in the way, and then somehow I got out of the habit. I'm going to try and post about once a week for the next few weeks. (Maybe once a fortnight might be more realistic.) Let's see. Anyway the plan is to try something regular, if not frequent.

A Ross Epidiascope

I like old gear, and I like fiddling with old stuff to try and get it to work. And I like my photographs big. So when the possibility to buy a beast like this came up I went for it, even though I hardly need another project, and have only just enough space to keep it.

Ross Epidiascope

OK. What is it?

An Epidiascope is like an old-style slide projector or magic lantern and overhead projector built into one. It would have been used mainly for displaying images during lectures, talks and so on. So education was the main market, though no doubt they were used in cinemas, military etc. The gadget has a dual function, and I guess this is made economical because of the powerful light source.

Dia function

The more familiar function is that of a slide projector. That's the "dia" part of the name, from the Greek for "through", and in most languages but not English the word for "slide" is based on "dia". ("Das Dia", in German.) This slide projector is no ordinary 35mm projector like I remember being terrorised with by neighbours just back from their holiday. ("You must come round and see our slides.") Instead of projecting a 24x36mm image (or even a 56x56mm image if you are lucky enough to work with MF slides on a Hasselblad) the slides are three and a quarter inches square. So images of 80x80mm or so can be displayed, with correspondingly better resolution and punch. And instead of a perhaps 250W bulb, the epidiascope has a 1000W bulb for four times the brightness, or four times the area. That at least is the theory. I want to try it out.

The lens used is a 10.5 inch projector lens of about f/5. That was obviously intended for a fairly long throw, as well as coverage of the larger slide. I will have to see how it performs for me, but a wider lens might work better in my limited space. This is for experimentation as well. In any case the lens seems a good one and may also be useful as a taking lens for 4x5. Watch this space.

Epi function

The other function is the "epi" one, and this refers to projecting the surface of an opaque object or drawing. So in this respect the epidiascope behaves more like a modern day visualiser. The object to be projected is placed on a movable tray under the projector and can be brought flat against a glass underneath it. The light from the huge bulb is split into two paths and reflected off two mirrors in the side of the gadget. It illuminates the object, and the light reflected off it is reflected by a mirror and projected by a huge 17inch lens. The combination of the mirror an the lens restores the object to normal way round. (As opposed to upside down or mirror image.)

The area projected is about six inches (150mm) square. So quite large images should be projected. Once again, I need to try it to see how well it works in practice. Watch this space!


I have no idea how old this object is. Ross marketed models very like this as early as 1932 and up to the 1960s. Mine appears to be a later model as it has a fan added on the front. Mine is similar but not quite the same as this one and a contemporary advert for a very similar one is available on-line here: Mine is Type B.E model 2, which suggests it's not the oldest. More information would be gratefully received!

My plans are firstly to get it going and see how good it actually is. That means cleaning it carefully and re-wiring it inside. If the bulb inside is no good or blows, and it looks like I will continue to tinker with it I will probably replace the original bulb with a modern and more easily replaceable halogen one. I would also like to try making old-style three and a quarter inch slides. If all else fails these are very interesting lenses to try out. We will see...

Ross Epidiascope: open