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Old 20th Nov 2017, 1:49 pm   #1
prajan
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Default Mullard Radio set MAS231

I have recently restored the radio set Mullard MAS231.The cabinet has been restored. I am only waiting for the Dial to be printed and then I will post the photos of the complete restoration.
But I am facing one problem in this set. The original volume control in the set as per the schematic is 500K type 49.500.11.0.( with switch). Since the volume control was defective and the original value not available I replaced it by the volume control 49.470.36.0 ( 650K + 200K) and I also fitted the original switch. When I turn the volume control to switch ON the set I get good volume as during switch ON the volume control moves some 15 degrees after the switch becomes ON thereby giving some volume. IT does not become zero. My question is that is the volume control 49.500.19.0 ( with switch) different from the volume control 49.470.36.0 ( without switch)? Is the difference only in the additional switch or the volume control itself is different? How I can solve this problem? Should I shift the switch to the tone control so that the problem could be solved? Doing this will amount to modification in the original schematic which I least want to do.
Kindly advise. Also I find the bass less. Can this be increased?

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P.Rajan
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Old 20th Nov 2017, 5:03 pm   #2
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Default Re: Mullard Radio set MAS231

Hello P Rajan

You will never get any variable control pot to be exact value, in a lot of cases there will be always a plus or minus percentage of the true value. the value you quote is as near as damb it, for example if you take an original value of 50k for a pot and you replace it with 47k that is near to it you will get, most modern controls available these days will be pretty close to what you want. The bass can be increased simply by putting a slightly larger capacitor in the tone circuit.

Hope this helps

Ken
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Old 20th Nov 2017, 5:12 pm   #3
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Default Re: Mullard Radio set MAS231

If I've read it correctly I think it's the position of the wiper on the track immediately after the switch is switched to on that the OP is meaning, eg: when switched is turned to on, the wiper is already partway round the track, a matter of offset I would assume.

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Old 20th Nov 2017, 9:00 pm   #4
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Default Re: Mullard Radio set MAS231

The description of the replacement vol control is 650K +200K; Does this mean it is a 650K pot with a tapping point at 200K, as used on many continental volume controls with a feedback for loudness control?
If so, simply locate then ignore the 200K tap, and just use the upper and lower connections for feed and grond, with the output taken from wiper, which I assume is how the original was wired. The difference between 500K and 650K is probably irrelevant in this case.
Les.
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Old 20th Nov 2017, 11:04 pm   #5
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Default Re: Mullard Radio set MAS231

Well this is strange. Usually you always need to rotate the on/off volume control by about 15 degrees to switch the set on, the volume control wiper would be at the earth end of the resistance and then turning the control further would increase volume. Notwithstanding how lucky you are getting hold of a Phillps tapped (loudness) volume control it would follow the same logic. I wonder if you have wired the control properly. Just put your ohmeter on the wiper which, following normal practise, will be middle of the 3 connections and see that it goes to zero when tuning the control. The tapped connection
sticking out from the edge of the volume control should be left alone just now. Once you have sorted the volume control out, you could include a simple loudness circuit aka Phillips. However, Phillips radios are usually well designed and other members experience of a similar chassis will be helpful in offering advice
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Old 20th Nov 2017, 11:21 pm   #6
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Default Re: Mullard Radio set MAS231

Quote:
Originally Posted by simpsons View Post
Well this is strange. Usually you always need to rotate the on/off volume control by about 15 degrees to switch the set on, the volume control wiper would be at the earth end of the resistance.
Just for info, I don’t think it’s the case here but there was a range of moulded track volume controls used by RBM in the their TV sets and also sold by RS that would not silence the audio completely, very low but still there.

I never had one go noisy but the switch occasionally failed.
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Old 22nd Nov 2017, 10:40 am   #7
prajan
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Default Re: Mullard Radio set MAS231

Dear forum members

Thanks for the reply.

The volume control pot has a value 650K +200K. The total value is 850K. I have not used the tap at 200K. So I have used it as a simple pot of 850K. Before switch 'ON' the resistance between the min point and the variable is nearly zero. As the volume control is moved to just switch 'ON' the resistance between the min point and the variable is 40K ohms. This is the reason I get a good volume at the just switch 'ON' Point. May be in the volume control with switch the track starts after around 15 degrees thereby giving zero volume at the switch 'ON' Position. If I shift the switch to the tone control pot ( both are same) then may be this problem could be solved but causing change in the original schematic.

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P.Rajan
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Old 22nd Nov 2017, 8:12 pm   #8
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Default Re: Mullard Radio set MAS231

I wonder what the effect of adding a resistor from the 200k tap to ground would be?
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Old 23rd Nov 2017, 9:02 am   #9
prajan
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Default Re: Mullard Radio set MAS231

Dear forum members

Majority of the radio used volume control where the switch was an integral part and so the zero point came into the circuit after the switch was made 'ON'. and hence this problem was not encountered. In many of the radio set manufactured by Philips in 1940's and early 50's the switch used to be mounted separately as in the radio sets BX760X, BX660X, BX761A, BX560A etc. and therefore this problem was never faced and normal volume control was used and the zero used to start from the min. point. But There were some sets where the switch was mounted separately but on the same shaft and it was not an integral part of the volume control like in sets BX735X. BX745X,BX645X and also the Philips U.K. 681A and Mullard MAS231 and may be some others also. So I think in such cases the volume control was different and the zero point would start from the point which would be after the switch was made 'ON' i.e. after approx. 15 degrees. So I feel when the normal volume control without switch is used in place of the volume control which was used with switch this problem is bound to occur.

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P.Rajan
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