Roberts R66

[LyntonP]

Photo of valve

[ppppenguin]

The resistor values are on the Trader sheet and I've quoted them at least twice in this thread. If you're epxerimenting, start high and work down to get the right voltage on the filaments. The voltage will be misleadingly high if any of the valves are removed.

DK96 has listed equivalents of 1AB6 and CV9026 and should be fited in V1. V3 is a DAF96, equivalents are 1AH5 and CV9024. I think you said that the set worked at one point so the valves can't be too far wrong.

[Terry]

There's only one +ve (cathode) connection tag on the LT rectifier, and the two "spare"  spare ones are a through connection in the rectifier so far as I can make out.

The three that are clear in the pictures are green-red-green which makes perfect sense for the biphase rectifier but one of the other two is connected to earth, we're told, so what is the fifth tag for?

I can't see it being used as an anchor tag although I can't really be certain where it is but now wonder if the wiring is correct, after all.

Perhaps the first thing we need is voltage readings across C29, C30 and C31 to give us a clue as to what is really going on here?

[pwdrive]

There's some pics here from an unmolested one (Post#7)

https://www.vintage-radio.net/forum/show...hp?t=63559

Lawrence.

[Terry]

As for the voltage required by the valves themselves, 1.3V is a good choice to aim for.

I know that Michael Watterson went into this in some considerable detail some while ago but I'm not sure if it was here, on VRAT or UKVRRR.

Found it!

Take a look here: https://www.radios-tv.co.uk/community/ra...post-20506

[pwdrive]

Having a read around and another look at various pics etc...my latest take.... there's no resistance wire that I can see that's connected in series with R16 (see Trader sheet) but R15 has been increased from 3.9 Ohms to 5.6 Ohms and another diode (in the same assembly as the existing rectifier) is or should be connected in shunt  with the filament supply, the cathode of one of the full wave rectifiers is connected to C31/R16,  the cathode of the other one connects to C29/R15 and the shunt diode.

EDIT: Post crossed.....Link relating to the above added:

https://www.vintage-radio.net/forum/show...?p=1113928

Lawrence.

[Terry]

... the cathode of one of the full wave rectifiers ...

But what is very clear in Lynton's picture is that there is only one common cathode terminal - which makes obvious sense for a bi-phase rectifier - and it can also be seen in the edge on view (picture 4) in the UKVRRR thread. I can accept, though, that the other two tags are a completely isolated protection diode, in case one of the bi-phase pair develops a short although I would have thought that the terrible hum that would result the instant the set was turned on would warn the user/owner to switch it off again pronto!

I don't think the idea of an integral - but completely separate - protection diode would ever have occurred to me, Lawrence, but, with the evidence of an earth wire and the connecting point of the fifth wire it all makes sense at last!

[pwdrive]

The extra diode is in shunt with the heater supply as I see it but it could be mistaken as a cathode from a bi-phase arrangement.

Lawrence.

[ppppenguin]

If it is a protection diode, it's certainly not on the Trader sheet. I've also checked in R&TVS (1956/7) and the LT arrangments are identical to the Trader sheet, except that both LT smoothing resistors are 3R9 without any mention that one might be higher.

[pwdrive]

Photo of valve

It's a DK96, the markings are the manufacturing code.

Lawrence.