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A dusty relic arrives....
Topic Rating: +4 Topic Rating: +4 (4 votes) 
July 9, 2017
9:45 am
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PYE625
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With reference to the Trader service sheet 770, the screen voltage on V3 begins to fall gradually after the set is switched on. When it has dropped by about 30 volts, the audio distortion is so bad it is almost impossible to discern what is being broadcast.

C24 is a screen de-coupler for V3 and lifting one end of this capacitor results in correct voltage at the screen (approx.90 volts) and normal distortion free radio sound. On Gram, the grid of this valve is used as the anode and therefore the voltage is not so critical, so leakage in C24 is not quite so noticeable as far as audio effects are concerned.

Obviously the leakage within C24 is sufficient to upset conditions enough to affect the radio operation though. It will have to be replaced.

Andrew.                                                                                   

                         

July 9, 2017
11:19 am
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PYE625
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Remember me going on about trying to re-create a Philips black tar capacitor earlier in this thread?

Well I had a go using a small plastic tube, a new capacitor, some black wax and a hot soldering iron !

All was great until the plastic tube started to melt and the wax leaked out. I tried again with a lower temperature and the results were better. I think practise will improve things and a small thin cardboard tube would be better so it can be pulled away more easily from the hardened and cooled wax.

rsz_img_3034.jpgImage Enlarger

Old original capacitor, new one and plastic tube.

rsz_img_3035.jpgImage Enlarger

The black wax I used.

rsz_img_3036.jpgImage Enlarger

Disaster as the plastic tube melts !!

rsz_img_3037.jpgImage Enlarger

Eventually, a half decent result. To get the ends shaped similar to the original, I sliced around the edge of the wax at an angle with a Stanley knife.

Andrew.                                                                                   

                         

July 9, 2017
11:52 am
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PYE625
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After going through all that performance, even though the screen voltage is now normal on V3, guess what....

The distortion comes back after a short period again !!   doh_gif

Not quite out of the woods yet then. But how odd that the distortion goes when the capacitor is not in circuit??

Andrew.                                                                                   

                         

July 9, 2017
12:46 pm
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Cathovisor
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What surprises me is that the set works with V3's SG decoupler out of circuit and doesn't go unstable. I take it that S27 is closing properly on radio?

To know yet to think that one does not know is best; not to know yet to think that one knows will lead to difficulty. Lao Tzu
July 9, 2017
12:59 pm
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PYE625
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Interestingly, although the screen voltage for V2 this time, gradually creeps down to a very low level of 30 volts or so, it was not capacitor leakage in the decoupling capacitor C13.

It was a duff V2, ECH35. Changing this valve has cured it completely. It's screen voltage now sits at about 75 volts as it should and remains so.

So, even though capacitors may be a bit leaky, other faults cannot be ruled out.

Andrew.                                                                                   

                         

July 9, 2017
1:01 pm
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Cathovisor
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This was the next bit I was coming to - 'gassy' valves. First time I met it was in a Murphy A170 where the 6P25 glowed a nice shade of blue internally after a while, with accompanying distortion.

To know yet to think that one does not know is best; not to know yet to think that one knows will lead to difficulty. Lao Tzu
July 9, 2017
1:05 pm
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PYE625
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Cathovisor said
What surprises me is that the set works with V3's SG decoupler out of circuit and doesn't go unstable. I take it that S27 is closing properly on radio?  

Yes, it worked fine with the SG decoupler out of circuit. Mind you, I remained tuned to Radio 4 LW at all times and it may have become unstable if tuning through other bands.

The oddest thing for me was that with V3 SG cap removed, the distortion went away. It shows that interaction can occur at RF/IF frequencies I guess. 

V2 was the culprit in this case.

Anyway, that's enough for today. It's too hot to be faffing about with molten wax and making black capacitors !!

Andrew.                                                                                   

                         

July 9, 2017
7:37 pm
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PYE625
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Hi, well I re-fitted the magic eye and you would think the eye was closed as it is so dark, only just visible with cupped hands around it.

EM34's are at a mad mad price on places such as ebay, so it will have to remain dark.

Andrew.                                                                                   

                         

July 9, 2017
8:12 pm
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Chris
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How about setting yourself a challenge, why not try this solid state project. Looks rather good and convincing too.

July 9, 2017
8:15 pm
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nuvistor
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Another possibility is, but requires the base rewiring or the new valve will be damaged 

http://www.radiomuseum.org/for....._6e5c.html

Frank

July 9, 2017
8:22 pm
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Also I wonder if an EM34 can be bopped like a CRT to increase emission

July 9, 2017
8:27 pm
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PYE625
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Chris said
How about setting yourself a challenge, why not try this solid state project. Looks rather good and convincing too.  

Now that IS an interesting idea !!

Andrew.                                                                                   

                         

July 9, 2017
8:33 pm
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PYE625
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Chris said
Also I wonder if an EM34 can be bopped like a CRT to increase emission  

I don't think that will work as it is not cathode emission that fades, but rather the actual surface deterioration on the target area of the valve. Often, a brighter area of green can be seen at the edges if the eye is driven outside the normal area of display.

I stand to be corrected, naturally.

Andrew.                                                                                   

                         

July 9, 2017
8:42 pm
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nuvistor
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I always understood it was the target area that deteriorates but if it's that bad you have nothing to loose by trying a higher heater voltage has we did with a boost transformer for CRT 's. Will need to disconnect the heater wires, dont want the other valves boosted.

I must admit when I was in the trade spares were easy to obtain and if a magic eye was dim I just replaced them and never give it a thought whether it was emmision or target. Change the valve, check the circuit for faults that could damage the new valve and move it out ready for the next repair.

Frank

July 9, 2017
9:11 pm
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PYE625
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nuvistor said
Another possibility is, but requires the base rewiring or the new valve will be damaged 

http://www.radiomuseum.org/for.....c.html  

This is the route I'm going down, I just bought four from ebay for £40 including postage from Ukraine.

They used to be dirt cheap, but not any more !! 

Andrew.                                                                                   

                         

July 9, 2017
9:22 pm
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nuvistor
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Four, stocking up then, I was thinking you may get one.fright

Frank

July 9, 2017
9:30 pm
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PYE625
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nuvistor said
Four, stocking up then, I was thinking you may get one.fright  

Lol, well I have a number if items that use the EM34 plus the Mazda 6M2, so they will be a useful set of spares.

Andrew.                                                                                   

                         

July 23, 2017
12:45 pm
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PYE625
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Hi, it is a good job I ordered four of these Russian 6E5C because two were physically too long to fit in the holder assembly on the tuning scale. Clearly, production variations are no where near as good as say Mullard used to be. Also, there is no ability to rotate the valve within its support on this set, so you cannot level up the eye.

The results are acceptable although the body of the valve needed to be painted to prevent light from the dial lamps interfering with the eye glow. I used gold as I had no red spray paint.

The minor modifications to the valve base connections are simple enough and can be reversed if I ever obtain a nice load of good Mullard EM34's in the future  grin_gif

rsz_img_3065.jpgImage Enlarger

rsz_img_3064.jpgImage Enlarger
rsz_img_3066.jpgImage Enlarger

Andrew.                                                                                   

                         

July 23, 2017
1:13 pm
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Cathovisor
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Oh, when I think back to all those lovely brand new 'Valvo' badged EM34s that used to be on a shelf in Colomor's when it was down the Goldhawk Road back in the 80s... waa_gif

They had gold metallising too.

To know yet to think that one does not know is best; not to know yet to think that one knows will lead to difficulty. Lao Tzu
July 23, 2017
1:18 pm
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PYE625
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You missed a "golden" opportunity there Mike. They are worth their weight in gold now.  grin_gif

Andrew.                                                                                   

                         

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