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Old 14th Jun 2009, 6:19 pm   #21
Boom
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Default Re: The M23 restoration - Any ideas?

AH!!!!!! Think I might be onto something. The cathode resistor of the MSB4 should be 300 Ohms but measures 500+. Strapping a 470 Ohm across it is making a difference but I'm still not sure that I have the correct circuit. As Paul said it might be an RS2 that I have and not an M23 at all but what the differences are I don't know.

Do these resistors change value? Its a 'sticky' one with end caps on.

<Hunts around for a 300 Ohm resistor and hasn't got one naturally!>
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Old 14th Jun 2009, 11:06 pm   #22
PJL
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Default Re: The M23 restoration - Any ideas?

Have you measured the PM24A anode current?
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Old 20th Jun 2009, 7:40 pm   #23
Boom
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Default Re: The M23 restoration - Any ideas?

I never did find out what model it is. It's not an M23 or an RS2 but....

The answer to my distortion problem was bizzare and solved after spending a few hours on the valve museum site today.

The output valve as fitted is a PM24A. The valve that SHOULD be in it is a PM24 (with no suffix!)

The site points out that the letters 'PM24' indicate a series of widely differing pentode output valves and not a valve itself.

The PM24A needs -22.5V bias and the PM24 which should be in it needs some -14V bias which is what I have!

I eventually spotted an extremely faint imprint on the chassis screen with a layer of rust on it which clearly reads 'PM24'!

Biasing the PM24A to -20V for now with a small voltage doubling kludge gives distortion free sound.

The original PM24 is rated at .5W while the PM24A is rated at 2.4W so maybe someone has shoved the A in some years back in an attempt to get the sound louder?

My only problem now is sourcing a PM24.

Last edited by Boom; 20th Jun 2009 at 7:50 pm.
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Old 22nd Jun 2009, 11:42 am   #24
PWH
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Default Re: The M23 restoration - Any ideas?

Hi are you sure its an M23? The M23 used a moving coil speaker with output transformer. The RS2 had the moving iron speaker. Of course it could have been fitted in the past?
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Old 22nd Jun 2009, 12:17 pm   #25
Boom
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Default Re: The M23 restoration - Any ideas?

No I am not at all sure that it is an M23. It LOOKS like one but the circuit is only similar not the same. It also isn't an RS2.

My one uses a moving iron 'speaker. It all looks totally original.

the clue as to what was causing the distortion was that one of the many circuit variations that I found used a variable pot' for the bias which was probably so that several different valves could be used as the output valve. Checking at the valve museum then brought the whole solution together (That I had the wrong valve fitted).

This one also uses a much smaller mains transformer which on reflection would be all that it needs at .5 Watt output instead of 3.

Here's a picture of my one. Note the mains transformer under the rekky' and the AF driver transfomer missing from the left. It is under the chassis here as shown in picture 2.

Just a thought. Is it possible that I have an RS2 in an M23 cabinet?
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Old 22nd Jun 2009, 2:25 pm   #26
PWH
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Default Re: The M23 restoration - Any ideas?

HI. definitely an RS2. I have one of each RS2 & M23

The M23 has M23 stamped on various parts of the chassis including the shroud over the tuning gang (absent in the RS2.) The M23 has its interstage transformer above the chassis next to and behind the valves and the choke is at right angles to the mains transformer.

With regards to the case, have a look at the M23 & RS2 on my website you will see the tiny difference. Whilst yours is in bits, cuold you note for me the speaker make if possible as I need one for my RS2 (& an intervalve transformer if anyone has one)

So It looks like you have the wrong output valve in your set. There is a Cossor equivalent to the PM24. If I find it I'll post the number here.
Regards
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Old 22nd Jun 2009, 3:27 pm   #27
Boom
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Default Re: The M23 restoration - Any ideas?

No idea on make. Says Reg Des 743916.

What is the link to your web page?

Dave
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Old 22nd Jun 2009, 5:36 pm   #28
PWH
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Default Re: The M23 restoration - Any ideas?

www.vintage-radio-resources.com. if you click on the PWH at the top left of this post a drop down pops up which takes you there. Regards
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Old 22nd Jun 2009, 5:45 pm   #29
Boom
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Default Re: The M23 restoration - Any ideas?

'The clue is the on off switch'. Mine is towards the rear so that answers that!
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