GEC 4940 Radio

[sobell1980]

Good morning all.
Thankyou for all your replies last night. In response to your questions my DMM is an expensive automotive snap on type bought last year. I will include a picture of it and it's tolerances below. It is also tested for calibration twice a year. I believe the loading of the meter is 10 mega ohms when using acv. The bulbs are rated 6.5 volts 0.3 amps and both blew after an hour or so.

Phil I have a very cheap and nasty Rolson, analogue moving coil meter if that is of any use? Perhaps now you know the loading of my DMM you may be able to suss out if my 7 volt reading is ok or not?
Many thanks
Dave.

[hansomcommon]

I would put the set on 240-260v anyway as it will reduce the stress on an oldish set, performance will be barely changed.

[ColinB]

The readings shouldn't change with the meter type; you won't pull down the line voltage with a low impedance meter....!

This only affects voltages that come from a high source impedance.

Set the transformer tapping so the set is being slightly-under-run and not slightly-over-run...

If you have 241v coming from the wall, set it to 240 - to - 260.

[Herald1360]

Humph! A 6.5V bulb that blew on 7V after an hour or so is pretty rubbish, I would say. A 1000hr life bulb on 10% overvoltage should still last about 250 hours. Whatever, if a bit dimmer doesn't matter, stick a 3.3R 0.5W resistor in series with each lamp, that should increase longevity considerably.

Your meter sounds absolutely fine for the voltages you've measured unless your mains supply is seriously non sinusoidal.

[sobell1980]

I shall have a dabble later on this eve. Dreaded chores first according to the good wife! Thanks for the replies. I shall set it to the highest voltage rating and measure again. Could be also cheap bulbs from Ebay. However, I feel and agree the set would be better slightly underun.
Phil, I think I will be needing a period type meter to prevent further confusion, especially when comparing against service data that were measured with this type of meter. I'm still a little confused about how different types of meter can ultimately give you different readings and how the impedance of the meter can cause thus? An Avo meter with a moving coil will give a different voltage reading to other types of moving coil snd DMM ?
If anyone could elaborate for me I would be most grateful.
Sorry to mods if this is moving away from topic but it will help me to understand measuring this particular part of the circuit correctly. Many thanks.
Dave.

[Herald1360]

The only significant effect on readings for different meter types is the loading on the circuit being measured. So, for example, an AVO 8 on its 100V range will look like a 2M resistor, whilst a cheapo DMM will be 1M and a decent one 10M. If you're looking at a circuit like the anode of a valve with a 47K feed, the AVO will load it down by about 2.5%, the cheapo DMM by 5% and the decent DMM by only about 0.5%. At lower voltages and higher impedances like an output valve grid circuit, the effects are much more noticeable.

Conversely, if figures are given for say an AVO 7 readings, the DMMs will read considerably higher (closer to the actual circuit operating values, in fact).

If you know the meter originally specced and its range, you can work out the loading resistance and either calculate what the DMM should read, or you can shunt the DMM to reduce its resistance to what the original meter's was. The latter is probably more useful since it will provide information related to the actual state of drifted resistors etc etc.

Lots of good clean fun!

[sobell1980]

Thanks Chris for that info. Would it be possible to give an example of this calculation with my DMM so I can figure out what the readings should be on my DMM . This calculation wouldn't be needed though in every part of the circuit I measure? For example if I were to measure the LT to the heaters on this set with my DMM and then an Avo I would expect to see the same readings? So where and how do these differences come into play and when should I decide to make a calculation to be sure my readings are correct compared to that quoted in the repair data . Sorry if I am repeating myself but I'm just trying to get my head around it. I'm going to get myself an Avo meter to make my life easier I think rather than confusing the situation. Many thanks.
Dave.

[SteveCG]

Unfortunately Dave, different AVO models have different circuit loading effects, as Herald1360 has already mentioned. I would suggest that your DMM is already as good as any - unless you really want to work out the finer points of circuit loading, which I don't think you do!

I, myself, can work out the loadings, but as a rule of thumb I just remember that the DMM will probably show a slightly higher voltage value for 95% of all the readings I take - and that level of accuracy is usually enough.

[Herald1360]

Sticking just to heater voltages, they come from a low impedance source (transformer, mains or batteries) and whatever the type of meter used will have an insignificant loading effect on the reading. Any differences in readings will be down to the basic meter accuracy or perhaps the shape of the ac if the local mains is distorted. In that case, you would need a true RMS reading meter to get a sensible result anyway.

[julie_m]

There is always the Bunsen Photometer method for measuring an RMS voltage! It's complicated, but it works like this:

Make a grease stain on a piece of paper. Shine a light bulb powered from your unknown AC voltage through it. Position an identical bulb powered from an adjustable DC supply the same distance away from the paper. Dim the room lights. Adjust the DC supply until the outline of the grease stain vanishes. Now the DC voltage = the AC RMS voltage.

This works by the fact that there is exactly as much light shining through the grease stain from behind (and adding to what is getting reflected from in front) as there is shining through it from in front (and so not getting reflected). Which means the total amount of light coming (by reflection and transmission) from the grease stain is the same as the amount of light coming (by reflection alone) from the non-greasy parts of the paper, so the transition from non-greasy to greasy cannot be seen. The bulbs are the same, and the distances from the paper are the same (this isn't strictly important, but it keeps the maths much simpler: energy depends inversely on distance squared), so the amount of light energy coming from each one -- and by extension hopefully the amount of electrical energy going into each one -- must be the same. And by definition, the RMS voltage is equivalent to the DC voltage that produces the same power in a resistive load.

[sobell1980]

Thankyou for your help on this Chris, SteveCG and Julie M. I appreciate your guidance on the meter loadings. I will continue to use a DMM and now I know to expect higher readings I will be able to accept this when measuring that this level of accuracy is ok as SteveCG suggests which works fine for me.

I have switched to the highest voltage tapping on the mains transformer and all appears to be well. I'm measuring about 6.3 to 6.4 volts a/c on the bulb holders and valve heaters. I've replaced the bulbs again which are 6.5 v 0.3 amps. I've had the set working out the case on the bench for half an hour on the highest voltage setting with no signs of any trouble.

Maybe as Chris said, someone may have moved the voltage selector as the set deteriorated to give it more gain on the valves and as the caps went leaky as this was full of wax caps. Then the set finally gave up and the voltage tapping was left as was till I restored it. I will put it on test out of its case for a few more hours to make sure the valves are not going low emission after a few hours play. Have enjoyed this project. Will let you all know over the weekend how the testing went.
Many thanks,
Dave.

[sobell1980]

The set was left on test out of its case all day with no signs of any trouble or bulbs blowing. The set is now complete and playing pride of place in my lounge.
Once again, thanks for all the help and input from the forum members.
Dave.

[sobell1980]

Apologies, I don't know why I can't get the picture rotation the right way round. Is this something one of the moderators could help me with?
Kind regards.
DAVE.

[Techman]

I think this thing with pictures posting on their side or upside down is something to do with using these new type phone cameras. I always use a (now quite old) cheap but proper dedicated digital camera and don’t have this problem, but I see it happen a lot on the forum. I have a very low setting on my camera of 0.3 Mpx and I take everything with this setting which means virtually no re-sizing and they up-load in seconds and are good enough for on here – I think

[Nickthedentist]

Well done, Dave. It looks very at-home on that dressing table/sideboard.

N.

[AC/HL]

See here: https://www.vintage-radio.net/forum/...d.php?t=118004

[Phil G4SPZ]

Originally Posted by sobell1980

I'm going to get myself an Avo meter to make my life easier I think rather than confusing the situation...

Hi Dave,

Now I'm back at the PC I have done a few figures on the loading effect of the Avo Model 7 on the 400 volt DC range, as specified in Trader service sheet 921 for your GEC BC4940.

If you look for example at V1 triode anode, its measured voltage is stated at 68 volts (using the Avo 7). This includes the loading effect of the meter; it's equivalent to putting a shunt of 200 kilohms from anode to chassis. This will draw additional current of around 0.34mA through the series resistor chain of R6 and R7, hence the voltage at the valve anode is actually reduced to 68 volts by the action of measuring it. In reality, the true voltage at the anode is likely to be nearer 90 volts, and that is what the valve needs to see in order to give the correct operating conditions.

The real issue is that say, for example, R7 has risen in value from its correct 22k to 50k - a not uncommon occurrence. The true voltage at the valve anode will then be around 69 volts, at which point the valve may struggle to start or maintain oscillation. If you now measure the voltage with a DMM having a 10 megohms input impedance, you'll read almost exactly 68 volts; you'll look at the valve voltage table and conclude that everything is perfectly OK. However you'd have missed the fault entirely, and will end up scratching your head and going round in circles! An Avo 7 will register around 61 volts, and your senses will be alerted as the voltage is below the expected value. Hopefully this would encourage you to measure the value of R7, and hey presto, you will have found the fault. The long scale length of the Avometer enables the voltage to be 'read' accurately even at small pointer deflections, and this is still one of their major benefits.

The loading effect of a conventional analogue multimeter can actually be a useful 'tool' when fault-finding. However it is important to take note of the loading when comparing actual measurements with stated values, as in the above example.

If you are thinking of buying an Avo 7, I have several restored examples available - drop me a PM.