Radiophone Value Please.

[Techman]

Definitely don't even think about making a power supply for this radio until you've done the checks mentioned by others above.

1) Check the continuity of the speaker coil as I did (shown in the last of the five photos above) using a meter and report its resistance reading.

2)Remove the valves and check the continuity of their heaters in the same way. Look up data for the valves on the 'net for correct pins. DO NOT pull the valves out by the glass.

3) Check the continuity of both windings of that transformer - unfortunately they're often o/c as has already been mentioned and a re-wind will cost.

If any of the above items are faulty then it'll be a non-starter and the set will be just a collectors ornament, unless you're a real enthusiast and want to plough some cash into it and fix it up. However, seeing as there's some replacement wiring and also possibly a re-made speaker cone, then it could mean that your father has already had this set working, but make those simple checks first before you get too bogged down in it. I think it's a great set and a wonderful thing to keep in memory of your father, remember, if you sell it on, it's gone forever and you may eventually wish you'd kept it, regardless of it working or not.

[ultraviolet]

Thanks Techman - I'll go through the checks as advised.

[Techman]

Great! Just remember to make a careful note of any connections you remove and also to not mix the two valves up and note which one was in which socket.

If in any doubt about any of the tests, then just ask and members who have posted will be able to advise.

[ms660]

Were's the grid bias coming from?

Lawrence.

[Cobaltblue]

Its possible its been modified ir its a slightly different circuit.

LT will be 2V HT anybody's guess but start at 90V maybe its been modified for Auto bias.

Cheers

Mike T

[Ed_Dinning]

Hi, check carefully before powering up that grid bias is not required, as without it, even for a short time, parts can be easily damaged.

Ed

[Boater Sam]

A "complete noob" working on a period 20s radio, this could end in tears.
Let someone who knows what they are doing look at it.
End of rant.

[ultraviolet]

Originally Posted by Boater Sam

A "complete noob" working on a period 20s radio, this could end in tears.
Let someone who knows what they are doing look at it.
End of rant.

I do build DAC's and headphone amps as as a hobby so I'm not totally ignorant. It's just that I have no grounding in electronics, so switching from chips, smd and current bias to LT and HT and Grid Bias is a bit unfamiliar to me at the moment.

[ms660]

Originally Posted by ultraviolet

I do build DAC's and headphone amps as as a hobby so I'm not totally ignorant. It's just that I have no grounding in electronics, so switching from chips, smd and current bias to LT and HT and Grid Bias is a bit unfamiliar to me at the moment.

No worries.

HT = high tension (high voltage)

LT = low tension (low voltage)

Grid bias is also low voltage compared to the HT.

The valves in your receiver will be triode valves, that's three electrodes, anode, grid and cathode, the cathodes in your valves are directly heated, that's to say that the heating filament is coated with a substance that's designed to emit electrons and is therefore the cathode, valves that have a separate cathode structure are referred to as indirectly heated, these have a cathode consisting of a hollow tube, the heating filament runs inside the cathode tube, in these types it's the cathode tube (heated by the filament) that emits the electrons.

The grid bias is always -ve with respect to the cathode, if there is no -ve
bias voltage on the grid then unless limited by some other means the valves anode current can rise to a level that could damage the valve and anything that's connected in series with it.

As has already been mentioned the valves grid bias arrangement appears to have been changed from a grid bias battery supply to automatic bias, this is normally done by inserting a resistor in series with the HT -ve supply, the voltage (-ve with respect to cathode) developed across the resistor being used for the grid bias supply voltage.

Here's a typical 2 valve TRF schematic of the period from Mike Edwards excellent site, it show the typical arrangement for the grid bias battery connection:

http://www.kbmuseum.org.uk/kb_servic...KB_207_cir.pdf

Here's one showing automatic bias as described above, it uses pentode valves and there's no step up transformer for the audio stage but the principle is the same, you will see that a resistor (1.6k) has been connected in series with the HT -ve supply, the grid bias voltage developed across that resistor is fed to the output valve via a couple of resistors:

http://bungiehd.co.uk/twovalve.html

Lawrence.

[ultraviolet]

Thanks.

I have checked the continuity of the valves and speaker cone which are ok. I'd need to dismantle everything to get to the transformer.

A closer look reveals a note attached to the bright green cable stating GB Rover - so I suppose that refers to the grid bias.

[Cobaltblue]

You shouldn't need to dismantle anything if you follow the circuit in the Link that Lawrence posted which is very likely to be very similar to your set you can measure it between the Grid pin of the second valve and your green cable marked GB Rover.

The resistance may well be several KOhm but it should not be an open circuit.

Cheers

Mike T

[ultraviolet]

I don't get anything between pin2 and the green wire using the multimeter.

I think the guy who modded the set knew what he was doing so I'll make up a 50v and 1.5v power supply and see if the set works.

Many thanks for all your help.

[ms660]

The grid bias is normally fed to the grid via the secondary of the step up transformer, I would check both primary and secondary windings for their resistance readings.

Lawrence.

[ultraviolet]

Originally Posted by ms660

The grid bias is normally fed to the grid via the secondary of the step up transformer, I would check both primary and secondary windings for their resistance readings.

Lawrence.

That means taking the front knobs off to get at the transformer - they are firmly attached and I don't want to damage them.

As I say I'll risk it with a test voltage and see what happens.

Thanks.

[ms660]

Understood.

Lawrence.

[ultraviolet]

Just discovered two grub screws on the knobs so managed to get the board out - nothing shows up on the multimeter across the transformer and there is no continuity through the LT + and - so I think a bit more checking is needed.

[Ed_Dinning]

Hi UV, it was quite common for these intervalve transformers to go open circuit as the were wound with several thousand turns of very fine wire. They can easily be re-wound and if you look at the threads you will see some suggestions as to how they can be rigged temporarily to check if the set is working.

Ed

[ultraviolet]

Thanks Ed - the transformer had gone o/c so I'll do as you suggest.