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Old 20th Feb 2017, 1:44 pm   #1
prajan
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Default Mullard Radio set model MAS231

Dear friends

I have recently got this set for restoration. It is in a bad condition and will take a lot of time. I will be needing our forum members help during the restoration. The first step I have taken is to clean the set properly and secondly checking the set thoroughly as per the schematic and make it original in case somebody has made some changes. Then I will take up work the changing of the defective capacitors including all the black tar ones and also the main filter capacitor. On physical checking I have notice the following for which I request help.

1. Most of the printing on the dial has erased. I want to get the printing done. I request if someone can send the scan for the dial for this set. I have the scan for the set MAS407 but it is slightly different from MAS231. Dial for Philips 681A and Mullard MAS279 is same as MAS231.

2. As per the schematic the 6.3V voltage given to the dial lamps and EM34 filament is through a separate transformer and filament supply to all the other valves is from the main transformer. This transformer is missing. Were some sets manufactured by Mullard without this transformer? In case "NO" then I will request if someone can give me specification of this transformer? What should be the No-Load secondary voltage? If it is kept at 6.3V then it may drop on Load. Also if somebody has changed the main transformer then I may need the Specification of the main transformer also? I shall be grateful If somebody can help me.

3. I have the schematic including the top and bottom component layout drawings for the set Philips 681A. The top drawing includes the wiring also but the bottom drawing shows only the components and no wiring is included without which it will be very difficult to check if somebody has done some modifications in the original circuit. I shall again be grateful if somebody has this drawing ( The bottom component layout with the interconnections), and he can send it to me.

In the first instance I have noticed the above problems.

Thanks in Advance

With Regards

P.Rajan
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Old 14th Mar 2017, 2:16 pm   #2
prajan
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Default Mullard 12 band radio set MAS231

Dear forum members

I have started the restoration oh this set which is in a very bad condition. The audio amplifier is in a very bad shape. While I was studying the schematic I have few question to ask?
1.As per the schematic the filament winding is S10.
2. This gives the 6.3V AC supply to all the valves except B8 (EM34) and the dial lamps.
3. As per the schematic none of the ends of the filament winding S10 is grounded.
4. But in the radio set it has been done as follows: The two ends of S10 are first connected to pin 2 & 7 of valve B6. Then they are connected to Pin 2 & 7 of valve B5 and from B5 to B4 finally to B3. In the socket for B3 pin no 1,7,8 are interconnected and also connected to pin no 5 which is grounded. So one end of the filament gets grounded at B3.
5.One end of filament supply to pin no 2 for the valves B2 and B7 goes through Coils S103 and S104 respectively. The other terminal i.e.pin 7 for both these valves is locally grounded.
6.Similarly the pin no 7 of valve B1 is also locally grounded.

My Question is why such an arrangement has been done? What is the advantage? If One end of the winding S10 was grounded would it not have served the same purpose?
Secondly what are the benefits of the coils S103 and S104?

Regards

P.Rajan
5.
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Old 14th Mar 2017, 2:46 pm   #3
Boater Sam
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Default Re: Mullard 12 band radio set MAS231

Its all to do with interference and noise suppression I think, the 2 coils are chokes (inductors) and the filaments are running on AC, introduce a phase shift.
Classy set, will be a nice restoration.
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Old 14th Mar 2017, 5:05 pm   #4
ukcol
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Default Re: Mullard 12 band radio set MAS231

I have just downloaded the schematic from radiomusium; a complex set with an RF amplifier, double conversion and push pull output.

The heater arrangement for valves B2 and B7 is for the two frequency changers. The grounding of these two heaters and feeding the heater supply via RF chokes is probably (as has been suggested above) to keep noise out of the two converters and/or address a stability problem. The set will have a lot of gain which is probably why that arrangement was required. The earthing is done at the valve bases rather than at the mains transformer to keep the impedance to chassis as low as possible as seen at the converter stages.

It looks a very nice high specification set, there is a limited amount of information on the schematic and so I hope you have access to the service manual.
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Old 17th Mar 2017, 7:45 am   #5
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Default Re: Mullard 12 band radio set MAS231

As said, the idea is to prevent any unwanted RF from getting into or out of the two frequency changers by way of the capacitive coupling between their cathodes and their heaters.

In addition to interference and regeneration mechanisms, microphony may have been a concern, that is problems caused by acoustic feedback from the loudspeaker(s) within the cabinet to the valves. Valve element vibration could modulate oscillator frequency in the oscillators of the frequency changer stages. For example, cathode-to-heater capacitance could vary with vibration. To minimize any deleterious effects arising from this, having the cathode and heater of each frequency changer valve tied to a very local RF earth would be desirable. Furthermore, the other side of the heater winding would need to be kept at essentially the same RF point, which the choke and capacitor network would do. The majority of any residual RF voltage on the heater line entering the valve is then distributed across the choke, which has a high RF impedance, with very little left across the heater itself, which has a relatively low RF impedance. Thus with both the cathode and the heater at the same RF earth potential, capacity variations between the two that result from vibration have minimal effect. I think that this was more a concern at VHF than HF, but Philips/Mullard may have been taking no chances, and the ECH35 might have been more microphonic than later valves such as the ECH81. Having one side of the heater supply earthed its the most critical points then removed the need to earth one side at the transformer. In fact so doing might have introduced loops through which unwanted RF currents could circulate.

As I understand it, the MAS231 was the Mullard version of the Philips 681A. Using B1, etc. rather than V1, etc., to designate the valves was certainly a Philips characteristic, quite rational given that “buizen” is the Dutch word for valve/tube. I think it [the MAS231] was mentioned in this thread amongst others: https://www.vintage-radio.net/forum/...d.php?t=111407.

Cheers,
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Old 17th Mar 2017, 8:27 am   #6
Radio Wrangler
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Default Re: Mullard 12 band radio set MAS231

To go with the heater chokes you will probably also find some RF decoupling capacitors from the heater lines to chassis.

Mixer stages produce image responses and other spurious responses where harmonics of the RF input and of the LO frequency mix together and can produce 'birdies' as you tune across a band.

The combinations of these things get much when you have two mixer stages on the go, so each mixer and its LO needs to be screened and what goes in and out of the screen needs to be carefully managed. Leakage of a little of the LO from one stage into the other will usually create a number of fake signals, and on a set with wide coverage, it will tune across several.

It's still an issue today. One of the most high-falutin amateur radio transceivers, the Elecraft K3 runs part of its frequency synthesiser, a direct digital synthesiser, DDS, within the 80 metre band in order to make use of cheap US TV crystals in an associated filter. The screening isn't perfect and there is a strong-ish 'tweet' at one point in the band where the DDS frequency and the receiver tuned frequency pass each other. Because the HF amateur bands were cunningly created in a harmonic series, there are also smaller spurii in other bands elated to this, but these are small enough to be difficult to find. It was quite interesting to unravel this and stumble across a cost/performance compromise someone once made.

David
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Old 17th Mar 2017, 10:35 am   #7
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Default Re: Mullard 12 band radio set MAS231

It wouldn't surprise me if a few different heater connection variations had been tried during development of a complex set like this, and the one that resulted in fewest spurious signals was settled on for production. After all, heater wiring represents a link between all the valves with a degree of capacitive coupling to each, or "asking for trouble", as some would say. One well-known professional receiver relies on just wiring proximity to couple its BFO signal into its detector, so it's no surprise that heater wiring could result in unwanted behaviour between stages. I've seen this technique of connecting one side of the heater supply to chassis at the frequency-changer stage with twisted pair run around elsewhere in other sets, which makes sense to keep the possibility of spurious coupling down.

Good luck with the radio, it sounds as if it was a rather good one in its time, and there probably aren't many around still!
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Old 17th Mar 2017, 2:19 pm   #8
prajan
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Default Re: Mullard 12 band radio set MAS231

Dear members
Thank you very much for the reply.

Regards
P.rajan
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Old 17th Mar 2017, 9:27 pm   #9
crackle
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Default Re: Mullard 12 band radio set MAS231

This set looks very similar to the Philips 681, http://www.radiomuseum.org/r/philips_681a681.html
Service information here http://www.service-data.com/product....25/5911/m14625

Also similar to the Mullard MAS407 http://www.radiomuseum.org/r/mullard_mas40715.html which was a nice set.

Mike
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Old 17th Mar 2017, 11:31 pm   #10
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Default Re: Mullard 12 band radio set MAS231

Wireless World for 1949 August included a test report for the Philips 681A; see http://www.americanradiohistory.com/...ld-1949-08.pdf, page 289ff. Included was an explanation of the double conversion circuitry as used on the bandspread shortwave ranges.


Cheers,
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Old 18th Mar 2017, 1:25 pm   #11
ukcol
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Default Re: Mullard 12 band radio set MAS231

The WW article also mentions an interesting aspect of the audio stages; a little trick I haven't seen before.
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