I haven't seen a circuit for this model, however Lekmek use a 15k voltage divider in a few models around yours.
One end is to HT, so your 309V looks right. The other end should be ground, so the 1V is close. The middle taps should power the screens and the oscillator anode and I would expect to see something in the 70 to 100V range on these taps. I suspect it is open circuit between the HT end and the first tap.
You should have resistance of 15k overall and the sum of intermediate resistances should be 15k.
Harold

Edit:
I still haven't found a circuit, however AORSM lists the voltages as:
HT 225V
Oscillatot plate 150V
Mixer & IF screens 100V
These are taps on the voltage divider.

Hi Jamie, regarding scoping points in a radio, yes beware of voltage levels in valve radios and the voltage rating on the input of CRO's.
Refer to your CRO handbook for the ratings of the input and of the probe concerned.
A maximum voltage will be stated and its wise not to exceed the rating.
If there are any doubts DONT do it.
I regularly use a Jaycar CRO with 600 volt rated probes and input but always park the probe on the /10 position to reduce the level applied to the CRO socket. I am carefull not to go near the rectifier valve where you can have peak to peak voltages near 1000!
Again if in doubt DONT as I spiked the input of my favourite CRT CRO by accidently brushing the probe somewhere I should not have and spent a happy day replacing the FET inputs and downstream chips on the CRO pcb! Alright for me as I have half an idea about what I am doing but a tradgedy if you are not used to it.

Regarding what waveshapes you will see, you have to know what should normally be at the various grid and plate points on a typical or a specific circuit. Radio manufactuers never put this info on circuits, how strange, although I see a lot of TV circuits at least put a thumbnail sketch here and there.
Having built a few sets I have learnt what is typical and you could have a look at some photos in my posts where I show superhet waveforms and also discuss instability in IF stages. Have a look at the Little General set I built for starters.
You may also have a similar set that works ok and you can scope that at the various plate and grid points and build up a gallery of typical waveforms to expect.
Cheers, Fred.

Yes the voltage divider certainly appears to be the culprit, my multimeter is fairly reliable with resistance and the fact I have no reading sure indicates a problem. Hopefully I'll be able to get it re-wound. I have a couple of photo's pending which show it looking a bit howdy doody between the 309v end and first tap where I have about 4volts, not 150v as should be. Also I have new valves now installed and no difference. My oscilloscope is an old valve one I bought for $50 from a guy in QLD. I don't have a manual but will search for one, I suppose the best bet is to check for voltage with my multimeter and only probe where very little is present! But I'm thinking that the voltage divider is showing some obvious clues as to what's wrong, so I'll get that sorted first, once I can eliminate that as a problem hopefully it'll be apples!

However I'm wondering, if a section of the voltage divider is oc, could I patch a conventional resistor in to bridge the oc portion, and if so I need to work out the value and wattage.

Yes you can patch a resistor across the open section. If only the top section is open, measure from the tap nearest the top to ground and the resistor you need is 15k minus the value of the good sections. If a couple of sections are open, measure the length from the top connection to the bottom and you know this should be 15k. Then the patch resistors will be proportional to the length of each section and all should add up to 15k.
For example, if there are 2 intermediate taps, (3 sections) and are equally spaced, each resistor will be 5k.

To calculate the wattage, you have to consider the current through the divider plus the screen and anode currents drawn by the valves. This gets complicated and I would start with maybe 5W resistors and give it a try. If they remain relatively cool, leave them at that. When the radio is working correctly, you can then measure the voltage across the patch and calculate the wattage to put your mind at rest.

Harold

Umteenth error 500.

I did copy this to Clipboard

One must be cautious with all test gear. A lot of the new stuff is not made for Valve radios and there are traps. The more recent stuff or better quality stuff has a "class" and that relates to its end use. Some digital meters do not like RF and it can cause a flashover.

The oscilloscope if modern will have an input Voltage and the x10 is normally used at HV. The handbook should tell you. It is important to understand how an instrument works and its limitations. RMS & Peak (Vpk) are not the same.

Examples: The Vertical System of Digitec QC1901 400V (DC & AC peak)
BWD 539CInput Voltage protection 400V DC 280V AC RMS; Trigger socket 100V peak to peak 30V RMS
Heatkit OS-1 had 400VDC (wax paper) caps on the input, they were replaced with 2KV types.

The big resistor is normally wire wound & notorious for corrosion and failing. If the wire gauge is the same I also use ohms per millimetre of the good bits (the longer the better) some circuits do quote the figures. I will often strip the wire off a failed on & use the clamps as tag strips.

What Lekmek model is it? I have serviced the odd one & m..a..y have data. Believe it or not one of the very early RCA sets has oscillographs on the data sheet. I actually use a CRO for aligning & signal tracer.

It's a model 540 from 1938-9 and my scope is an old valve jobby!

Photos uploaded to Post 15.

Have a look at 931T That has a 15K Divider & voltages on the AORSM scan. Many others had these, had one recently, some are 25K

Be careful with the voltages on DVM as it may have been measured with analogue of around 1000 opv (sometimes they tell you). DVM will read higher (not on B+).

Marc.

STC used about 25,000 ohm voltage dividers in some of their late 30's radios. Unfortunately that for my 830 had been replaced by discrete resistors when I got it. I have a ceramic former replacement for it when it gets a chassis look-over when I get a chance, since it was restored in the late 80's. As this replacement has only one take off point, will need to make two new take off points, for which I have been putting aside any thin sheet that I come across.

The failed voltage divider was caused when my grandfather accidentally replaced the 240v plug into the back of the chassis in the 120v orientation. So that was taken out in the repair. I haven't seen one of these in place, but I think that the socket in the chassis could be wired up safely to prevent a repetition, just by leaving off the 120v transformer tap. I am not sure that an electrician would want to put a tag on the lead however.

Well I got it working today, believe it or not the voltage dropper/ divider was open between all taps. As I only have 1 watt resistors I just bunged combinations of them in series together and bridged each section, I came up with 7.1k from ground to the first tap, with 85 volts, there, 3.4k to the next tap with 155 volts and 4.8k to the HT end with 255 volts thereabouts. The cathode resistor in the photo, 425 ohm 3 watt was totally open too, I made up a 390 ohm one out of four 1.5k's, well it's reading 390 ohm anyway, so anyways, I've proved where thee problem is and had it working, only the resistors I patched in got a little hot for my liking. Somebody is sending me a 15k wire wound one like the original, I just need to fit it and get the correct cathode resistor and figure out haw to tap the new 15k divider, hopefully it's not too hard!

I would suggest marking & noting the connections on that "candohm" (long resistor) & removing them.

These are notorious for going open, or corroding. B voltage is too high so something is not drawing current. One end of that should be grounded & zero volts. relative to chassis.

It is supplying screens at 100V see circuit. DVM will possibly read a little higher but I doubt that they will be remotely close if its failed.

I did find its circuit

Marc

Hi Marc, I have a circuit from Radio Museum, but it doesn't show the voltages. Yes the B+ is high, but It's way down from the 309volts it was? But if it's working and getting stations and music is playing nicely, it must be close to ok, so I'm guessing that the resistances I have used are close enough for it to work, but still a bit howdy doody. In any case I will need to find the right formula to get the right sections of resistance, totalling 15k for the voltages I am supposed to have to occur at the correct taps on the candohm, so I can fit and tap the new one properly, or order and fit resistors of the correct type and ohmage.

I wouldn't be surprised to find another high or open cathode resistor?

I think we need to sort out the candohm first. The high voltage is consistent with loading issues. the biggest load is the #42 it cannot burn out its cathode resistor. Its grid bias is that across the "back bias" resistor. I would expect 6.5 to 7 Volts CT to chassis if its all working.

N.B: CT is negative & chassis positive when measuring across the back bias resistor.

I have sent a copy of my circuit with voltages to Brad and Jamie Lee. That should qualify my comment about the voltages.