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Mullard MAS407/15 All Wave

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Anonymous
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Arrived in one piece.

Seems to have some work done?

 
Posted : 20/09/2013 1:38 pm
malcscott
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Any pics ??

 
Posted : 20/09/2013 1:58 pm
Anonymous
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Arrived in one piece.

Seems to have some work done?

That will probably be me. But what did I do, I can't remember, and I did not take any pics.
Mike

 
Posted : 20/09/2013 2:00 pm
Anonymous
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Front view as received

Rear view with a couple of EL33

I repaired the wave change knob

The spring clip missing. I replaced it with cut up coffee tin of course!

I made a new mains cable

There is a brown hunts on transformer and a load of black tar Philips caps underneath.

A mysterious resistor connected only at one end (see red tape near square box bottom left).

A couple of new caps (the EL33 grid coupling perhaps?).

Some new resistors.

Probably it needs more caps replaced. I've never seen good tar caps.

The Wave change was a bit stiff at the middle. It uses two sets of wave band switches driven by slots in a moving bar each switch disengages as the other connects so only one at a time operates for part of the range.

Loads of Philips style wire drives :)

Will I check and replace more caps or "just plug it in" (TM) if the EL33 grid voltages are zero?

(No cat)

 
Posted : 20/09/2013 2:18 pm
Anonymous
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Where is on off?

Plus DISASTER. I set it on top of my two EL33 from the Pye 39 JH/E prior to double checking the voltages on the sockets.
:ccf
Crunch
:ccb
:aai

Arrrrgh!

 
Posted : 20/09/2013 3:29 pm
Refugee
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The Phillips style rectifier position can also be above the mains transformer.

 
Posted : 20/09/2013 3:34 pm
Refugee
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Where is on off?

Plus DISASTER. I set it on top of my two EL33 from the Pye 39 JH/E prior to double checking the voltages on the sockets.
:ccf
Crunch
:ccb
:aai

Arrrrgh!

What a nasty mistake :ccb

 
Posted : 20/09/2013 3:35 pm
turretslug
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Oh no! :ccf :ccb. It's got to get better from now on :qq1

Colin

 
Posted : 20/09/2013 4:35 pm
Anonymous
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I figured out the on/off

The switch is on the shaft in front of pot, and had jammed. A squirt of WD40 and tap with screw driver, turn control fully anti clock-wise and then clockwise it clicked and now turns on/off.

The tuning scale wasn't working which gave a sinking feeling. I HAVE successfully done some more insane Philips Drive mechanisms. Those that have been there know the evil that lurks. Anyway only the small friction disc on tuning shaft had "hopped" off the larger disc used to drive it all (!).

AZ31 not lighting but all the other (remaining :ccg ) valves lighting. No pin 7 so I assume it's 2 & 8 for filament. Will check voltage and filament.

 
Posted : 20/09/2013 5:36 pm
Anonymous
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AZ31 filament OK
4V AC on socket.
AZ31 (nor an alternate AZ31) doesn't light up.
!

The socket seemed loose, so from behind with a mapping pin I bent in the four used socket sides. AZ31 now lights :)
Now the Magic Eye has a dim glow.

 
Posted : 20/09/2013 6:21 pm
Anonymous
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Hi Michael
It must have had a few jars on its journey, the tuning drive was engaged before I remember trying it, but it does look like it could easily disengage.
I probably only changed the audio coupling caps, it was working when I last tried it after fitting 2 EL33's.
That was about April 2012, I remember there was plenty of volume.
Were your valves on the bench where you put it down, that's awful. :aai
I had all the wave change drive apart and cleaned it up, I am wondering if it might be better if I had not used grease and just applied thin oil. It was better after I cleaned and greased it but it still left my hand feeling soar after clicking through all the wave bands and back again. :)
Is that a scuff on the front bottom right or a reflection.
Mike

 
Posted : 20/09/2013 9:23 pm
Anonymous
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Just dirt on the Front.

Yes, I mysteriously set the radio on the valves and then looked for them to plug in :ccg

You're going to hate this.

You CAN replace an EL33 with a 6L6, but not vice versa. The bias with no o/p tubes is -4.1V (31mA assuming 150 is correct) due to the HT current of the rest of circuit. As HT current increases via resistor between chassis and transformer (bias point) the HT voltage will drop and grid bias more negative. So the circuit is self biasing.

When people talk about -20V on a 6L6 that is at maximum power and thus a high g2 and anode voltage and pure class B. At lower g2 (it's about 230V with EL33 in according to Philips 681A and 270V with empty socket) the g1 bias can be less for same Anode current. About -9.7V is about 65mA HT assuming 150 Ohms, so total for the 2 x 6L6 is 65 -31 = 34mA. Or 17mA per valve. Which is fine for Push Pull. I have pair 6L6 also now in the Pye 39 JH/E to replace the broken EL33 pair.

So the circuit is self biasing with an extra negative offset from HT current of rest of set (a good idea for push pull) and 6L6 can replace EL33, but not the other way round (power & voltage ratings exceeded).

Did you give it worm tablets? No matter it will get a dose.

It does look like only 3 caps changed, the two grids and a cap beside the "floating" resistor with red tape. Almost certainly more will need replaced. Working on all bands but RF gain is low, that will be leaky caps on g2 pins.

Plugging in 2 x 6L6 the bias rises to -9.7 to -10V, this suggests an extra 15mA

 
Posted : 20/09/2013 11:24 pm
Anonymous
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Come to think of it I think I did give it a brush down with Rentokil, and I seem to remember plugging some holes with bees wax. (the Rentokil I have is in a can left over from when my Dad had his house loft treated back in 1962 to 67 I wonder if it goes of. It still has that same old pungent smell. )

 
Posted : 20/09/2013 11:38 pm
Anonymous
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I seriously doubt it goes off. Interestingly I left a jam jar with some left over Ronseal "Total Wood Preservative" (and kills woodworm) and all the flies & wasps in the outhouse workshop went for it. (separate from radio shack/Electronics workshop). A lot of bodies.

 
Posted : 21/09/2013 12:05 am
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