Help Required with an Osram 4 New Music Magnet

[Richard55]

at the tender age of 63 I have decided to try and get to grips with valve receivers

I have tried to renovate a 1930s set with limited success. I have read the books, looked at U tube and can get no further!

is there anybody within an hours drive of Watford that would be prepared to spend some time showing me the basics and how to fault find etc

[Nuvistor]

I can’t help with tuition but it would be easier for you if you found a late 1950’s early 60’s MW/LW British radio, perhaps an Ekco although virtually any will be ok. They tend to be easily repaired and fault finding on those will give you the knowledge to tackle other sets.
Spares and valves are usually easily available for those sets, try to find one with plenty of space on the chassis so access is easy.

[PJL]

Welcome to the forum. 1930's sets are generally pretty simple to repair. What make/mode is this? Can you post some pictures?

If you can wield a soldering iron and have a DMM I am sure we can help you get it going.

[terrykc]

You say you have tried to renovate it 'with limited success'.

It would be useful to know what you have done to it so far and what, exactly, is/are the fault(s) that need to be fixed.

[Richard55]

Gentlemen

many thanks for taking the time to reply to my post - most appreciated. As this process develops I am sure I will ask many dumb questions so please bear with me!

As in all walks of life I am sure the solution lies in taking a methodical, step by step approach which with your help I am sure will result in a positive outcome.

The radio in question is an Osram 4 New Music Magnet. Fortunately there is quite a lot of information on the net and I have downloaded both the two practical wiring diagrams. (see attached)

The radio was supplied as a complete kit with no soldering necessary - all the wiring is connected from point to point by screw terminals.

On lifting the lid the radio itself, apart from a bit of dust is in remarkable condition looking like it was put together yesterday.

My limited success in renovation has consisted firstly in inspecting all the components - they all seem to be in place with no visible defects or rust. My next step was to inspect underside and check that all the wiring was in place and sound with no obvious shorting etc.

I have made up a HT supply using PP3 batteries tapped at 72V and 117V, LT consists of a few D cells giving 1.5V. I presumed these voltages would be sufficient to get some sort of response from the set. I also applied 4.5V for the grid.

I checked that when the on off switch was operated the current flowed through it.

After power up I double checked that each valve was securely in place - I then discovered that V3 had a problem. It wasnt an H210 as stated in the literature but a Mullard PM something (cant read it fully). I presume this is an acceptable equivalent. The main problem with it was that the glass bulb could rotate around its black base and when gently shaken it made a very impressive rattle. I presume this is now a dud. I am in the process of obtaining replacement.

The final problem I noticed is that next to the V3 tray and holder is a component marked 31 - Fuse Bulb and holder. I presume that when powered up the lamp should be on? it wasn't. I removed the bulb and applied 1.5V to it and it lit up.

So I m now at the head scratching stage and would like some advice

regards
Dick

[Cobaltblue]

Hi Dick

In normal operation the fuse bulb won't light it's to protect the filaments from the nasty HT battery

The filaments will be happier at 2V they were originally designed to run off an accumulator.

A dry battery might be at the very low end of the filament operation.

It would be expecting a high impedence speaker 2,000 ohms or so so if you fit a normal speaker you will need a matching transformer.

But you'rE off to a great start these are lovely sets to play with but it will need a longish aerial

Cheers

Mike T

[PJL]

The valve maybe OK. The glue that secures the envelope to the base dries out and bits fall into the cavity causing the rattle.

Check the continuity of the filaments of all of the valves and also the transformer.

[Ed_Dinning]

Hi Richard, I'll be following your progress with interest as I have 2 of these sets myself and they will work well.
If you have a multimeter, even a cheap one it will help you to test the components.
There are many tests that can be done without applying power.

Likely problems will be leaky capacitors (but there is only one in the NMM I think); poor or corroded connections will be a source of problems and there are likely to be open circuit windings on the A/F transformer (easily replaced or re-wound.)

I look forward to hearing of your progress.

Ed

[Geoff 555]

Hi, I got caught out beautifully by my MM, I tried it out after doing the bad solder joints etc. and was met by complete silence.
I had left the 'cans' off covering the coils/valves, fitting them cured the silence.
What a silly boy was I.
Cheers.
Geoff.

[Aerodyne]

Cyclon 2 volt rechargeable cells are available from RS:
https://uk.rs-online.com/
You will need 2 volts for the filaments. This is essential IMO. Other points have been covered by replies in the thread.
Good luck.
Tony

[Richard55]

Gentlemen

many thanks for all your positive comments and patience, I have already learnt a lot from the help provided and seem to be making progress which bodes well for the future.

Mike and Tony thanks for the tip about the need for 2V - I will sort out a Cyclon rechargeable, have been using 2000 ohms headphones so that should be ok. Also thanks for the tip about the purpose of the fuse bulb holder, makes obvious sense now I have been told!

Geoff I will make sure the cans are in place when trying again

Ed and PJL - I have tested the other valves and have continuity on those so I suppose I can assume they are usable

I have tested the Transformer for continuity and think its ok - I set my MM to the sound setting and got no response. When I set it to the lowest setting and tested across A and + I got a reading of 1.1. I double checked with my very simple meter and got a small deflection on the needle.

The capacitor C5 appeared to give no reading whatsoever - although I am not convinced I have tested it correctly. On my MM the ends of the capacitor have to be put into two holes - obviously I cant do that so I put two wires into the holes and then touched them onto the terminals on C5

All comments and the next step will be most welcome

regards
Dick

[Aerodyne]

Presumably you disconnected one end of C5 to avoid any parallel false readings. This must be a paper dielectric capacitor - or at least a non-electrolytic type - so its value will be in the uF range. These will generally read open-circuit (possibly with a very slight and millisecond in time needle-twitch when testing with an analogue meter). Such a twitch would suggest a good component, as in truth might a zero reading, despite zero also indicating an open-circuit component. Any resistance reading whatsoever across the capacitor terminals means there is an internal leak. If that's the case, the component should be replaced (or stuffed with a modern equivalent).

From what you've said so far I am not sure whether or not you have obtained any form of response from the radio. If not, power up and touch the grid pin of the output valve with the tip of a screwdriver. You should hear crackling in the headphones, probably low in volume. If that's so, your output stage is working, not necessarily well, but working, and your attention should be directed to other stages.

I'll be away for a few days so will be unable to add more to this thread for the time being.

Tony

[terrykc]

Originally Posted by Richard55

The capacitor C5 appeared to give no reading whatsoever - although I am not convinced I have tested it correctly.

No reading is good! To add to what Tony said, if you look at the capacitor symbol you can see it represents two plates with nothing in between them. The 'nothing' bit is the dielectric - which can simply be air. Obviously no current from your meter can pass through the gap, so there will be no reading.

The dielectric can be just about anything, so long as it is an insulator: paper, mica, ceramic and, in more modern capacitors, polyester, polypropylene, etc.

As for the needle kick that Tony referred to, I don't think you'll see it with low value capacitors unless your meter is extremely sensitive but it would be caused by the initial inrush of current as the capacitor charges.

Larger capacities require larger and more expensive capacitors but a solution exists in the form of the electrolytic capacitor. This has a different symbol where one of the solid bars is replace by an empty rectangular outline. In these, the dielectric is a conductive paste (the electrolyte). In manufacture, the capacitor is 'formed' - a process which generates a microscopically thin gas layer between it and the adjacent plate. An important difference is that these capacitors are polarity conscious - the plate indicated by the oblong box must always be connected to a positive voltage relative to the other plate.

The higher capacity make a needle kick easier to see and you will also see it as a very high resistance. This is normal for an electrolytic as a small current always flows through it, whereas for a paper, etc. type it would indicate a failure.

[Richard55]

Terry and Tony thanks for the information about capacitors - another useful lesson for me.

Can I take it that my readings I obtained from the transformer would indicate that it is OK?

I am waiting for the new valve and a Cyclon 2V battery. Hopefully once these are obtained the next step will be to power up the radio and see what happens.

Unless anybody else has some useful tips or tests I can carry out in the meantime?

All advice most welcome.

Regards, Dick.

[Ed_Dinning]

Hi Dick, the meter test of a cap is a general indication only as the meter only supplies about 9V, in use the cap will have up to 100v across it, so may be breaking down in operation. But hopefully not badly.

Ed

[Richard55]

Gentlemen

I took some time over the weekend to study the circuit and internal working of the Osram 4 to see if I could gain more insight into it - my Grade E O level physics (circa 1970) didn't seem to help much!

I promised at the start some dumb questions so here are a couple. Could anybody explain in simple terms the following:

1] I presume that an HT of 120V is the requirement to drive the valves to obtain their optimum performance. Why therefore is there a second HT of 70V?

2] The said physics o level understood a transformer to be a device to either increase or decrease the supplied voltage from a given input. What is the purpose of the transformer in this radio - is it a different type of transformer - ie audio frequency?

All your combined wisdom is most welcome, plus any other helpful comments!

regards
Dick

[ms660]

The higher HT supply supplies the anodes the lower HT supply supplies the screen grids, you can trace that on the schematic:

http://www.electrojumble.org/DATA/Mu..._4_with_AC.pdf

The transformer is an inter-valve AF transformer between the detector valves output and the input to the audio output valve, it's a voltage step up transformer, I don't know what the step up ratio is off hand for the one in your receiver but something like 1:3 is not unusual. It steps up the small audio signal voltage at the detectors anode to a larger voltage that's suitable to drive the audio output valve into providing sufficient output power.

Lawrence.

[Ed_Dinning]

Hi Dick, the transformer also serves to block the high anode DC voltage on one set of windings (going to the anode) and allow a smaller DC grid bias voltage through to the grid circuit to set the correct working conditions for the next valve.

Ed

[Richard55]

Ed and Laurence

thanks for you answers - things now slowly beginning to make more sense!

The transformer is as you say a Gecophone Intervalve 3:1

many thanks
Dick

[Aerodyne]

FYI, the lower HT supply - to the screen grids - is sourced from the main HT supply via voltage dropping resistors with later receivers, obviating the need for a second supply. Very early receivers, such as yours, and many early home-build designs as offered by, among others, Practical Wireless, Popular Wireless and Amateur Wireless magazines in the 1930s, often kept the design and construction simpler by using a secondary lower voltage. HT batteries were tapped at suitable voltage levels for that purpose. The same applies to grid biasing, where early use of 9 volt grid bias (GB) batteries was supplanted by auto-biasing circuitry.

Tony