Graves Vulcan

[radiosoul]

Hi folks,

snowman_al Actually, I'm not really sure if there are any capacitors inside those components. I keep looking hard into them with a strong headlamp but they seem more like resistors to me.

Lawrence, thank you, catalogue entry noted. I see that the RC units they refer to are resistances with no parallel capacitors so that seems to agree with my findings so far.

Alan, the tests of the 2uF HT capacitor are as follows.

1) Using a Megger 1730 set to "Resistance" to generate 200mA current and a milliammeter in series with it, the capacitor drew about 0.25mA with a resistance measured by the Megger of 11k Ohm (including the milliammeter). A direct resistance test with the Megger revealed 10k Ohm. A quick calculation shows that the test voltage across the capacitor in this attempt is only 2.75 Volts - not much good for testing an HT capacitor.

2) Second test with a bench power supply and an analogue current meter in series with it and the capacitor. Voltage applied across the capacitor was 64 Volts (best I could do with the bench supply) and the residual current was 13mA. Doesn't look very good..

[radiosoul]

Hi folks

snowman_al, I'm pretty sure that those components are resistors only. I've looked at them with a strong headlamp but I can't make out exactly what's inside. A test with my ESR capacitance set revealed that they were only resistors so I'm going with that for the moment.

Lawrence, noted the catalogue entry, thank you. The RC components they refer to are called Resistance Coupling so that seems to confirm my findings.

Alan, I tested the HT capacitor with my bench supply dialled up to 64 Volts (best I could do with this supply) with an ammeter in series with it and the capacitor. With 64 Volts across the capacitor the stable residual current was 13mA. Doesn't look very good..

[radiosoul]

Argh! double post! I keep walking away from the laptop when posting to do more stuff on the radio only to come back and forget where I was. Ho hum.

[ms660]

The capacitance (capacitor) will be between the P & G connection I would imagine.

Lawrence.

[snowman_al]

Radiosoul, I think we have crossed wires here?

The two tubular components (pic.3) are just resistors.
The flat component in pic.4 is a capacitor, I think, though I have never had one to touch or test.

Without seeing the circuit, I'm assuming it is a grid leak detector like the diagram you posted, my guess is it is (or was) a 300pF cap ''connected across'' the 2 meg resistor within the circuit (not physically inside the resistor).
It is connected to the aerial input wire, so is logical.

Though now I am doubting myself, so can you measure the 'flat thingy' for resistance and capacitance again for us?

[radiosoul]

Hi snowman_al - yes, crossed resistors anyway.

I was distracted with working on the bench and didn't make things clear. As you say, the components are resistors and there are no external capacitors across them. The flat thing looks like a mica capacitor - I'll try another measurement and come back with it.

Should have a complete diagram soon.

[ms660]

McMichael for one did clip-in capacitors and holders in that style.

Lawrence.

[PJL]

If the flat capacitance measures only 28pF you could try dismantling and cleaning it.

[radiosoul]

I wouldn't have thought about that initially but when you mentioned it I took a second look at the capacitor and, indeed, it came apart. Two brass (gold?) plates separated by one piece of mica sandwiched in between two pieces of Paxolin. Great for servicing down to component level! Anyway, when I got it back together it still measured about 20pF. Any idea what a "normal" capacitance should be?

[ms660]

It would depend on its circuit function, I can see one end goes to the antenna but what does the other end connect to?

Lawrence.

[radiosoul]

..the grid of the front-end valve. So, one side to the tuned RF circuit and the other to the valve grid.

[snowman_al]

Like this then?

Your capacitor is CX and the 2 Meg resistor is either in parallel with it (across it) or from the grid to the earth line. Both work just the same.

CX needs to be 200 to 300pF as mentioned before.

Alan

[radiosoul]

That's pretty much it. Only difference is the long-wave coil in series with the medium wave one and the shorting switch across it. So far I haven't found any resistor across the grid capacitor or between it and ground. When I get back from work today I'll re-check my circuit diagram and post it.

[PJL]

Mica plate capacitors like that can't go very wrong. You need to make sure the faces of the plate touching the mica are cleaned of any oxidation and are flat. You could try using a vice to ensure the plates are making good contact with the mica.

[ms660]

Originally Posted by radiosoul

So far I haven't found any resistor across the grid capacitor or between it and ground. When I get back from work today I'll re-check my circuit diagram and post it.

That's what I was thinking all along, the Ediswan RC coupling unit would normally have a coupling capacitor built in between connection P (Plate) and connection G (Grid) they were designed to couple the anode of one stage to the grid of the next stage, they weren't designed for grid coupling from a tuned circuit into a regen detector stage, it could be argued that the grid resistor in it can be used as the grid leak for the regen det but that would mean connecting the G terminal to ground and the -ve terminal to the grid and the P terminal to HT+ and the +ve terminal to the anode and then use a separate coupling capacitor to the grid of the next stage, but that would mean another separate grid leak resistor for that next stage... I can't see one as yet.

If you look at the pages in that Ediswan RC coupling unit link I posted you will see that in the schematic there are coupling capacitors, you will see that the Ediswan RC coupling units are used but you will not see any of the coupling capacitors in the board layout, that's because they are built into the RC coupling units, the clue being letters RC.

I also find it strange that there is no trace of this Vulcan "111" in the Wireless World archives or anywhere else for that matter except on radiomuseum where the photo's (to me any rate) show it to be the very same receiver in the flesh that the OP has.

I stand to be proved wrong but I'm wondering if the receiver started life out as the Graves two valve receiver and was got at by someone with a new cabinet and baseboard configuration (ie: rebuilt as a 3 valve job) but retaining the front panel and the panels components.....who knows.

The HT terminals don't make sense either, why just for HT and not LT?

Of course I could be miles out in all of this.

EDIT: Could the red markings on that flat plug-in capacitor be 0003 ?

Lawrence.

[ms660]

It also looks like the front panel of the receiver in question has been cut down at the bottom at some sometime to fit that fancy doo dah cabinet:

http://www.poppyrecords.co.uk/other/...webpage24.html

Lawrence.

[snowman_al]

Originally Posted by ms660

... I also find it strange that there is no trace of this Vulcan "111" in the Wireless World archives or anywhere else for that matter except on radiomuseum where the photo's (to me any rate) show it to be the very same receiver in the flesh that the OP has.

I stand to be proved wrong but I'm wondering if the receiver started life out as the Graves two valve receiver and was got at by someone with a new cabinet and baseboard configuration (ie: rebuilt as a 3 valve job) but retaining the front panel and the panels components.....who knows.
Lawrence.

Re-configured - now that would make a lot of sense. There were pictures on the net earlier that have now disappeared of a '3 valve' Vulcan. But that may have been this one too? Whoever did it certainly did a very neat job though.

Could be the detector has been changed to an 'Anode Bend' too?
And the OP is correct saying that one of the grid bias wires goes to the first valve.

I had best wait for the diagram, it is all very confusing! Alan

[radiosoul]

Right. Here's what I've drawn by inspecting the wiring and components. I've been over it at least twice but I'm more confused then before.

The front-end looks logical and is similar to the Graves II pointed out by ms660. It wouldn't surprise me to find that this set has been re-built to add the third valve, quite a neat job. So we need to entertain that theory.

Firstly, I cannot see how this can work. From what I see, there is no proper coupling between V1 and V2.

Also, there is nothing across the grid capacitors of V1 and V2.

Looking at the circuit diagram, if you took V2 and V3 and added the tuned circuit in front of V2, you’d have the Graves 2.

So suggestions and critique welcome.

Oh, and yes, ms660, that marking on the flat capacitor does look a bit like 0003 – hard to tell that last digit but it looks that way. Also, looking over the Ediswan RC unit I see no inbuilt capacitors. The base of it is solid so unless there are capacitors inside it there are none associated with it. I have read “RC” on the base as “Resistance Coupling”.

[ms660]

The coupling capacitor in the RC unit would be inside I would imagine.

Re: Your reverse engineered schematic, what's missing is a grid leak resistor for V1, the value of the grid resistor you've given for V2.....are you sure it's not megohms?

Lawrence.

[radiosoul]

First mistake. You're quite right Lawrence. It's 2.2 Meg. Even got 2Meg stamped on it! Just wrote it down incorrectly on the diagram. Will correct. Can't understand why there is no grid leak resistor for V1. Nowhere to be seen..