UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 16th Nov 2018, 2:09 am   #1
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Osram Music Magnet 4

Hi
It's my first post here. I have purchased an Osram Music Magnet 4 TRF. The seller sold it as seen. This is my first vintage radio and also the first restoration project. I have obtained the circuit diagrams with the kind help of Bill the moderator here.

My set has two PM12A RF amp, PM2A grid leak detector, PM1LF audio output amp.

I have tested all capacitors, they seem to be ok. Grid leak capacitor 364pF. Grid resistor 2.64MOhms and audio bypass cap 2.7microfarads. It even came with the original grid battery.

I am new to radio electronics but I have been building a quite a few homebrew regenerative radios for a few months and have a good collection of battery radio valves mostly American and European types.

For this radio, I have purchased the Vellman signal tracer/inject and the Cyclon 2v 5AH for the filaments. I have also ordered a new old stock PM12M in case the original ones are no good.

Without the valve tester, I am going to build single valve regen radio on a breadboard to check the RF valves one-by-one. I have made a working audio amplifier for the PM1LF that came with the radio.

I have a few questions here and hope I can get some advice here:

1. The original Gecophone interstage transformer has the secondary open circuit. The primary has DC resistance of 1784 ohms. This is bad news.

I read this is very common. Lucky I have purchased a 1920's Peerless 3.5: 1 interstage transformer imported from USA. They are very hard to find and expensive.

I have taken the PM1LF out of the Osram 4 and hooked it up to the Peerless transformer. It can drive a loudspeaker with very loud volume with the output from my homebrew single valve 6SN7GT regen reflex radio. Now the transformer is wired the OPPOSITE way round like the Osram Music Magnet with the primary low impedance side of the Peerless transformer as input and high impedance side to the audio output (to a Bogen T725 audio matching transformer and to an 8 ohms loud speaker). This is the right way in most books and magazines for homebrew battery set that the transform is supposed to set up the voltage and gain. I am kind of confused why the interstage transformer for the Osram MM4 is wired the opposite way round?

I read someone said it is impedance matching for the anode but I am not convinced because the 3 stage TRF is not much different from the one valve regen receivers I have been building.

Also anyone has tried to rewind open circuit interstage transformer before?

2. I cannot visually find the tickler coil in the third coil set, where is it hidden? Is the top Litz wire coil for MW and the bottom for LW? Why are the interstage Rf transformer coils at right angle to each other?

3. Why the throttle capacitor looks like a differential capacitor in the schematic? Normally a variable capacitor is connected across the tickler coil and to Earth.

I have limited knowledge of vintage radio technology but I am still reading lots of old magazines in 1920-30 to test out very old circuits on breadboards.

Sorry many questions.

Sing
regenfreak is offline  
Old 16th Nov 2018, 10:53 am   #2
HamishBoxer
Dekatron
 
HamishBoxer's Avatar
 
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,923
Default Re: Osram Music Magnet 4

Welcome Sing, yes Ed Dinning on the group can rewind most transformers.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S
HamishBoxer is offline  
Old 16th Nov 2018, 12:21 pm   #3
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Osram Music Magnet 4

An interstage AF transformer in early sets is connected to give voltage step up. This helps with the low gain from the valves and works because the input impedance of the second valve (a megohm or so) is much higher than the output impedance of the first valve (a few kilohms) so even when reduced by a factor of 10 or so by the transformer it still presents a reasonable load.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 16th Nov 2018, 12:28 pm   #4
Mike. Watterson
Heptode
 
Join Date: Dec 2014
Location: Limerick, Ireland.
Posts: 901
Default Re: Osram Music Magnet 4

You can test with a 110 + 100 to 24 + 24, 5W mains transformer. Put the 48V secondary on anode. Try one 110/120 winding and compare with both in series to drive grid.
I've also used a shaver transformer as 2:1 isolated and both 3:1 and 4:1 autotransformer.
Mike. Watterson is offline  
Old 16th Nov 2018, 9:48 pm   #5
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Re: Osram Music Magnet 4

Hi Sing, I have rewound many of these, but due to the fine wire involved and 1000's of turns it is time consuming.
NOTE it is vital that these valves be run with their correct GB voltage. You should not even attempt to alter the GB setting with the HT connected as it can damage the valves and coupling transformers. If you run them with the GB too high it will cause no damage, simply reduced or no sound. Low GB causes damage.
I have a couple of these sets myself and they look good and perform well.

Ed
Ed_Dinning is online now  
Old 17th Nov 2018, 1:40 am   #6
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

Hi Many thanks for the replies and suggestions.

Ed, thanks for the warning about GB voltage. I have actually downloaded all the valve characteristic curves for the same reasons. I will test each of the valves on a simple regen circuit on their own before i test all of them together so I wont do something stupid.

It will be a tall order for me to wire very fine wires, my eyesight is poor and I don't have steady hands so there will be lots of cursing and wire breaking.....I have almost damaged the RF interstage coil connection wires which are finer than hairs during the cleaning of the dust INSIDE the coils with surgical spirit and a soft make-up brush. I think the tickler coil is those a few extra turns of Litz wires ( about 50 micohenries) in the coil set number 3.

I still don't understand the wirings of the 3 set of TRF inductively coupled coils. I have looked at the Osram Music Magnet 3; its interstage TRF coils are three concentric tubes instead of two 90 degrees coils in Magnet 3.

The main transformer and shaver transformer ideas are certainly interesting if I can get hold of them. Instead of doing the rewinding, I wonder if the choke-coupling or Bogen T725 (which I have) autotransformer coupling may work?

http://makearadio.com/misc-stuff/regennotes.php


I think the R-C coupling is lossy.
regenfreak is offline  
Old 17th Nov 2018, 1:53 am   #7
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

Ed, you are right. The PM2A audio output valve would pump out 20ma anode current at zero grid voltage with an anode voltage of 125V. The maximum anode current is 10ma. So 20ma is enough to burn out both the valve and the ultra fine primary of the interstage transformer.
regenfreak is offline  
Old 17th Nov 2018, 12:14 pm   #8
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

After much head scratching, I think the MW and LW coils are aligned at right angle to each because the wave switch is centered tapped. When it is closed, it bypasses the LW coil but the LW coil is still connected in series with the tuning variable cap. The 90 degree alignment allow decoupling magnetically. When it is opened, the two coils are still in series and even they are at 90 degrees, they still provide enough inductance for LW tuning. The same way works in a variometer.

I still dont know what the short interspace wirings in the top coil are for. The internal wirings of the coils are confusing.
regenfreak is offline  
Old 17th Nov 2018, 12:27 pm   #9
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

The by-passed coil would cause RF loss if both coils are aligned. The same way is like a tesla coil, I would have to make the ground strike rail an open loop, otherwise it sucks out of all the output power when eddy current is induced.
regenfreak is offline  
Old 17th Nov 2018, 9:05 pm   #10
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Osram Music Magnet 4

Quote:
Originally Posted by regenfreak View Post

I think the R-C coupling is lossy.
Not lossy as such, but you will lose the X3 or so voltage gain that the transformer gives.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 17th Nov 2018, 9:41 pm   #11
greenstar
Octode
 
greenstar's Avatar
 
Join Date: Jul 2011
Location: Leicester, Leics. UK.
Posts: 1,681
Default Re: Osram Music Magnet 4

I am not an expert but I believe open circuits of intervalve transformers are often found before the windings start. Its' worth making sure connections from terminals are made, and also checking to the point where that wire connects to the start of the winding, which may involve removing the outer paper sleeve. Might be tricky if its in pitch.
Also, there was a method of repairing open turns by discharging a capacitor through the transformer - a short burst which can arc across open turns and weld across adjoining windings. Others will know more about this but it may be worth trying.
The attached file is from 'The Horn', thanks to the American Radio History site.
Attached Files
File Type: pdf AF Transformer repair.pdf (349.0 KB, 81 views)

Last edited by greenstar; 17th Nov 2018 at 9:47 pm.
greenstar is offline  
Old 18th Nov 2018, 2:11 am   #12
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

Thanks for the suggestions and pdf file. I am not the adventurous type when it comes to repairing.

I have some setback with the power output valve Mullard PM2A. I cannot make it driving a speaker using the Peerless 3:1 interstage via a Bogen output audio transformer even I set the correct negative grid bias. It may have low emission. I am stuck without a replacement for the PM2A at the moment. I dont see it often on ebay.

The PM1HF is well working with the 3:1 Peerless interstage transformer even it is meant for the detector, but it would be much weaker output than the PM2A.
regenfreak is offline  
Old 18th Nov 2018, 11:31 am   #13
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Osram Music Magnet 4

Have you got a multimeter?

PM2A emission is easily checked by measuring its anode current and actual anode voltage at a known grid bias voltage and comparing with its datasheet.

to measure anode current just use a 1k resistor in series with the anode and use 1V=1mA.

It won't rattle the windows- even on top form and perfectly matched you'll only get a few tenths of a watt of audio.
Attached Files
File Type: pdf pm2a.pdf (172.6 KB, 62 views)
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 18th Nov 2018, 2:57 pm   #14
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

Chris thanks i will try that. I was too scare to drop the grid bias below -4.5v in case i blew my snowfake and expensive interstage transformer. May i ask can why cant i measure anode current with multimeter in series?
regenfreak is offline  
Old 18th Nov 2018, 7:19 pm   #15
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Osram Music Magnet 4

You can, but then unless you have two meters it's difficult to make the voltage readings under exactly the same conditions as the current reading. And you need to remember to change range from current to voltage to avoid possible magic smoke escape!
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 18th Nov 2018, 8:11 pm   #16
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

I have tried both methods. They actually give slightly different readings on the anode current from an American triode Type 30 (I have quite a few of them lying around, they are great battery valves)

1k shunt resistor 9.7mA

direct ammeter 8.78mA

Interesting the direct method distorts the sound while the shunt method doesn't.

I have tested the PM2A at low anode voltage 75V with zero grid bias. It works but the sound is weak. It does not matter if I change the tappings of the Bogen matching output transformer which is connected to the high impedance side the interstage transformer.

Here the anode current measurements for PM2A using 1K resistor; the emission is a bit low compared with the graphs but it does not explain why the sound is low:

74V anode 6mA GB 0V
90V anode 8mA GB 0V

-------------------
82V anode 7,1mA GB -5V
90V anode 8.2mA GB -5V
98V anode 9.45mA GB -5V

The sound output is feeble while the RF triode both Type 30 and PM1HF produces much louder sound (they use zero grid bias).
The maximum current for PM2A is 10mA so I must miss out something here

I have replaced it with space charge Type 49 double grid tetrode running at 12V and it probably is louder than the PM2A!

Also Ed has suggested I use 1 Megohms for the grid bias. He is right again, it would not work without the 1 Megohms resistor with higher voltage negative GB

Last edited by regenfreak; 18th Nov 2018 at 8:15 pm. Reason: typo
regenfreak is offline  
Old 18th Nov 2018, 8:41 pm   #17
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

My bad, the process of swapping tubes around actually detuned my homebrew regen. Now it is quite loud. The PM2A is usable.
regenfreak is offline  
Old 18th Nov 2018, 9:02 pm   #18
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

The 1M grid resistor is necessary if a negative bias is used. Without it, the sound is cut off even there is large anode current flowing.

For a low emission valve, I can run the anode at 100V with zero grid bias with an anode current of 9.5mA just below the maximum limit 10mA quote in the graph.

Last edited by regenfreak; 18th Nov 2018 at 9:04 pm. Reason: typo
regenfreak is offline  
Old 18th Nov 2018, 9:08 pm   #19
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: Osram Music Magnet 4

In the original schematic, no grid resistor is used for the audio output valve
Attached Files
File Type: pdf Music_Magnet_4_with_AC.pdf (827.3 KB, 100 views)
regenfreak is offline  
Old 18th Nov 2018, 9:14 pm   #20
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Osram Music Magnet 4

Quote:
Originally Posted by regenfreak View Post
In the original schematic, no grid resistor is used for the audio output valve
It doesn't need one, but a regen detector will.

Lawrence.
ms660 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:00 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.