Cossor 487U

[Wellington]

I thought I'd share some photos of a Cossor 487U I recently acquired. I've never encountered this model before, and it exhibits a few curious features I thought would be of interest. The set was released in April 1948, according to the Cossor service manual.

I was surprised when I removed the back and saw that there was no transformer, as I assumed I'd bought a (possibly more common) 487AC. Looking inside, I could see that a replacement speaker transformer (not pictured) had been screwed (badly) to the speaker board and the output valve had been removed from its socket and was just lying on top on the chassis.

Valve line-up is OM10, OM6, OM4, 332 PEN (aka CL33), and OM1. Does anyone know the logic behind the Cossor "OM" valve numbering system? Leon wondered here if "OM" stood for "Octal Mains". The set exhibits that curious Cossor characteristic of the low-power valves being enclosed in brown paper bags!

I was surprised to see the on/off switch in (what should be) the negative 'side' of the mains, which means that the top of the dropper resistor would be live regardless of the position of the switch. This seems rather dangerous, but I suppose it's less dangerous than having the chassis live.

Photos below, hopefully! I'll include some of the cabinet in a subsequent post…

[Wellington]

One corner of the cabinet has received a visit from our old friend, the common furniture beetle . There looked to be no recent activity, but I decided to treat the cabinet before putting the set on the roundtuit pile. To do this, I removed the chassis from the cabinet before applying a woodworm killer. I'd assumed the cream-coloured bar across the front of the set would be wood, but it was actually metal.

Finally, does anyone know what the mysterious symbol is that appears on the underside of the cabinet and the tuning pulley? It's like a capital 'T' inside a diamond.

[TonyDuell]

Watch out with that mains switch arrangement.

If the lower mains wire in the diagram is live, then the chassis is (obbiously) live when the set is switched on.

But if the lower mains wire is neutral (so the upper one is live) then when the switch is off, the chassis is connected to the live side of the mains via the dropper and heater chain, a relatively low resistance, so the chassis is then very much live.

You lose either way...

[Wellington]

Thanks for that info, Tony. I hadn't considered that, with the switch open, the chassis could be made live via the dropper and heater chain! This set is in the roundtuit pile, so no risk at the moment, but I think I'll print out a copy of this thread to put with it.

[Boater Sam]

The T could be nothing more than the marking on the steel sheet before it was pressed.

A small mains double pole relay hidden under the chassis is the answer to the live no matter what chassis problem . Use the original switch just to energise the relay, a fuse could be included.

[ms660]

T = Tested?

Lawrence.

[Herald1360]

Originally Posted by Wellington

The set exhibits that curious Cossor characteristic of the low-power valves being enclosed in brown paper bags!

Perhaps someone thought they were really ugly?

[Wellington]

Originally Posted by Boater Sam

The T could be nothing more than the marking on the steel sheet before it was pressed.

Unlikely, since it also appears on the underside of the cabinet in the penultimate photo.

Originally Posted by ms660

T = Tested?

More likely. I wonder if it occurs on any other Cossor sets of the same vintage.

Originally Posted by Boater Sam

A small mains double pole relay hidden under the chassis is the answer to the live no matter what chassis problem . Use the original switch just to energise the relay, a fuse could be included.

That is a good idea.

Originally Posted by Herald1360

Perhaps someone thought they were really ugly?

Eye-rolling, indeed…