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Old 14th Sep 2018, 5:42 pm   #41
Richard55
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Default Re: Help Required with an Osram 4 New Music Magnet

Tony, Ed, Lawrence and PJL

many thanks for the helpful advice - I didn't realise that the top cap for two of the valves was the anode so would have gone and measured the wrong thing!

Now you have all set me on the right path I will spend an hour or so in the shed tomorrow measuring away to my hearts content and will report back

I feel I am getting somewhere as after every post I feel I have learnt something - I then go and look at the wiring diagram and it seems to be becoming clearer

So once again many thanks - I know my questions must sound daft to you but O Level physics doesn't seem to help too much

regards
Dick
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Old 16th Sep 2018, 10:29 am   #42
terrykc
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Default Re: Help Required with an Osram 4 New Music Magnet

Only two things I learned in O level Physics that have any real significance that I can think of off the top of my head are:

Energy can neither be created nor destroyed

and

When the temperature is constant, the potential difference across the ends of a resistance is proportional to the current flowing through it.

Of course, I'd known Ohm's law for years before I encountered it at school, except for those first five words, and I'm sure there are a lot of folk who think they know Ohm's law but have never encountered those five words.

Ignore them at your peril!
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Old 16th Sep 2018, 10:40 am   #43
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Default Re: Help Required with an Osram 4 New Music Magnet

"When the temperature is constant" Yep, was in my City & Guilds stuff/training.

Lawrence.
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Old 16th Sep 2018, 11:16 am   #44
Aerodyne
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Default Re: Help Required with an Osram 4 New Music Magnet

Mine too. In practice, many a resistor will be found to have 'cooked' - its resistive value changed by the heating action over a protracted period of heavy current, resulting in lowering of resistance which leads to ... its a circular path to destruction. This applies to carbon-based resistors in the main. Wire-wound resistors are more stable and are used for heavy current situations.

Tony
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Old 16th Sep 2018, 5:14 pm   #45
Richard55
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Default Re: Help Required with an Osram 4 New Music Magnet

Hi Guys

I do remember Ohms law and basically electricity can only flow in a circle so check all your wiring

I going to have to stop this project after tomorrow for a few weeks to concentrate on a DIT project my wife has lined up!

I rechecked the Voltages at V1 and V2 measuring them correctly this time - 100v each

I checked the current flow as recommended earlier and found that V1, V2 and V4 showed no readings at all, presumably this means that those three valves are not conducting. Does this mean they need replacing?

Or is there another explanation / step?

regards
Dick
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Old 16th Sep 2018, 5:30 pm   #46
terrykc
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Default Re: Help Required with an Osram 4 New Music Magnet

Quote:
Originally Posted by Aerodyne View Post
... many a resistor will be found to have 'cooked' - its resistive value changed by the heating action ... This applies to carbon-based resistors in the main.
Tony, it might have escaped your notice but carbon composition resistors weren't around when Georg Ohm did his experiments and wrote his law in 1827!
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Old 16th Sep 2018, 5:50 pm   #47
terrykc
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Default Re: Help Required with an Osram 4 New Music Magnet

Quote:
Originally Posted by Richard55 View Post
I checked the current flow as recommended earlier and found that V1, V2 and V4 showed no readings at all, presumably this means that those three valves are not conducting. Does this mean they need replacing?
I think that for three valves out of four to fail catastrophically like that would be very, very bad luck indeed!

However, there is a mystery here. You've measured V1 & V2 anode voltages and they are both 100V. You previously measured V4 anode voltage and that was 100V also. However, these are all fed from the HT rail, V1 & V2 via very low coil winding resistances, so almost a short circuit, yet you say the HT supply you've arranged is 117V, so where have the other 17 volts gone to?

I think you need to recheck what you have done, starting with the PSU.
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Old 16th Sep 2018, 5:54 pm   #48
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Default Re: Help Required with an Osram 4 New Music Magnet

Quote:
Originally Posted by Richard55 View Post
I checked the current flow as recommended earlier and found that V1, V2 and V4 showed no readings at all, presumably this means that those three valves are not conducting. Does this mean they need replacing?
Dick
In a previous post you said you had checked the filament of each valve and you had continuity? They should have measured just a few ohms and it can be confusing as a DMM will measure Meg ohms just from the continuity of your hands. It's worth checking these again.

If you heard something in the headphones then V4 must be conducting. Did you wire the meter in series with the headphones? Have you checked the headphones have continuity? They should measure 2-4 KOhms.

It would probably be a good idea to show a picture of exactly what you are doing to measure V1 anode current. The meter must be on DC Millamps as the current will only be a few milliamps.
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Old 17th Sep 2018, 12:12 pm   #49
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Default Re: Help Required with an Osram 4 New Music Magnet

Terry, come to think of it, neither was I.

Joking aside, I mentioned the behaviour of carbon based resistors in practice. They continue to obey Ohm's Law even to destruction.

Might also be worth pointing out (in case there's anyone not aware of it, though I know that on this forum that is unlikely) the tendency for high value resistors to go higher with age, even when 'fallow' and un-powered for many years, perhaps up in someone's loft. An odd phenomenon but one borne out by experience.

Richard, are you absolutely sure you have adequate filament voltage?

Tony
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Old 17th Sep 2018, 12:57 pm   #50
terrykc
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Default Re: Help Required with an Osram 4 New Music Magnet

Quote:
Originally Posted by Aerodyne View Post
... the tendency for high value resistors to go higher with age ...
Yes, dinned into me at an early age: low resistors go low, high resistors go high ...
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Old 17th Sep 2018, 3:44 pm   #51
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Default Re: Help Required with an Osram 4 New Music Magnet

Dick is new to this game and he is going to struggle with the large numbers of off-topic posts. Lets stay focused on giving advice on how to get this set working (which has no resistors at all).
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Old 17th Sep 2018, 4:18 pm   #52
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Default Re: Help Required with an Osram 4 New Music Magnet

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Originally Posted by PJL View Post
Dick is new to this game and he is going to struggle with the large numbers of off-topic posts. Lets stay focused on giving advice on how to get this set working (which has no resistors at all).
There's a resistor in the schematic I'm looking at, and one appears to be in the layout but not marked as such, if it's gone very high, performance will be adversely affected.

Lawrence.
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Old 17th Sep 2018, 6:23 pm   #53
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Default Re: Help Required with an Osram 4 New Music Magnet

Dick, the resistor mentioned by Lawrence is shown in the picture. Anything up to 2Mohm should be OK.
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