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Mystery Ferranti Set

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lotsalines
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I have swapped out the smoothing can and still get the same result.
As for the rectifier i'll have to get back to you on that.
The only way I can access it is by removing the chassis and there's a large screw holding it in that has zero clearence to access it.
Will provide pictures soon.

 
Posted : 19/03/2020 3:45 pm
freya
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I think you may be best disconnecting some sections of the HT to pinpoint what/if  its being dragged down, resistors R48 & R49 is a good start and will just leave the timebases running. if it remains the same once this has been carried out then suspect the rectifier

 
Posted : 19/03/2020 4:34 pm
sideband
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Posted by: @lotsalines

As for the rectifier i'll have to get back to you on that.
The only way I can access it is by removing the chassis and there's a large screw holding it in that has zero clearence to access it.
Will provide pictures soon.

Well if it was fitted then there must be a way to access it....The service sheets that Chris gave you access to has full instructions on how to remove the chassis.

 
Posted : 19/03/2020 10:46 pm
freya
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I can see the mains voltage tap has been changed at some point , its best in the settings

for 230 volt.

The picture also shows the location of the rectifier should you need to access it.

6267 IMG02351
Image (8)22

 

 
Posted : 20/03/2020 11:08 am
lotsalines
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The set may use the internals/chassis of the ekco/ferranti in the trader sheet provided but the housing it is in from what I can gather has a few differences that are giving me trouble getting the set out.
After unscrewing everything holding the set in the housing the internals are loose.

IMG 0237

These side screws secure the top half to the housing.
I should also mention the set has no back cover.

IMG 0238

The top section is part of the chassis, it slides out with the picture tube if you give it a wiggle.

The bottom however, will not slide out nor be lifted out in any way, shape or form. It is held in by a force in the bottom of the set, it can be shifted left or right a little implying there is a screw or something in the middle locking the chassis inside the housing.

IMG 0240

There is a screw in the bottom middle that I believe is the culprit.
There is literally no room whatsoever to get to it. The bottom of the set on the other side has no nut, socket or anything it's connecting to.
It can literally only be accessed by removing the picture tube from what I can gather.

IMG 0239

Unbolting the picture tube is just as easy to do from what I can see.
I worry about doing this as I do not want to accidentally break the tube, and even doing this I still have no direct access to the screws.

IMG 0242

I need to be able to access the area those wires go down into which is on the bottom.

IMG 0243

The flat can behind the yellow caps is where the rectifier is located. I was barely able to change out those caps, there is no way I am getting to any rectifier in there without removing the cabinet.

 
Posted : 20/03/2020 4:55 pm
sideband
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Well the only other thing to do is to try and trace the wires to/from the rectifier, disconnect them and fit a silicon diode in place of it leaving the old selenium rectifier in place but disconnected. You will need a resistor in series with the silicon diode to reduce the HT to the correct level....start at around 100 ohms at 5 watts. A wirewound type is best. You can probably mount a small tagstrip in a convenient place to take the diode and resistor.

You could use a 1N4007 or if you can find a BY127, that is a vintage type and more in keeping with what would have been used back in 'the day'.

 

It looks like the rectifier is a 'contact cooled' selenium type. They fail in much the same way as the older finned type. They have a high internal resistance compared to a modern silicon type which is why you need a resistor in series with the replacement. You may need to adjust the value of the resistor to get the HT to the correct voltage. If you don't fit a resistor, the HT will be much too high.

 

 
Posted : 20/03/2020 6:44 pm
lotsalines
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I have a 100ohm on the way, hopefully the post will still be running on monday.

Don't have many power resistors, would be nice to have a set of them handy.

 
Posted : 21/03/2020 5:40 pm
sideband
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100 ohms is a starter value....it may need increasing or reducing

 
Posted : 21/03/2020 8:24 pm
lotsalines
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I am aware that this is only a starting value.
It would be useful to have a beefy deckade box for a situation like this.

The 100ohm came today and I put it in series with a silicon rectifier diode bridging it over the two sides the rectifer connects to and sadly I am getting the same voltage drop I had with the old rectifier.

I get about a 10V difference changing the line frequency and a breif voltage spike when I change the tuner.
I will try what freya suggested and see how that effects the set.

 
Posted : 27/03/2020 11:48 am
lotsalines
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I have disconnected R48/49 but the problem persists.
It does idle at 160v as opposed to 130v now.

Can't be much left now to drag it down.

 
Posted : 27/03/2020 1:07 pm
freya
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So a bit more drastic, what voltage do you get if you disconnect the outward wire from the smoother, so no HT is getting to any of the set. Just the smoother sections connected to the DC from the new rectifier.

I'm beginning to wonder if you have a high resistance double pole on/off switch or something, it shouldn't be this difficult.

 
Posted : 27/03/2020 3:51 pm
sideband
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Posted by: @lotsalines

The 100ohm came today and I put it in series with a silicon rectifier diode bridging it over the two sides the rectifer connects to and sadly I am getting the same voltage drop I had with the old rectifier.

I may have read that wrong....do you mean you are bridging the old rectifier with the new one? You have to disconnect the old one completely.

 
Posted : 27/03/2020 6:27 pm
lotsalines
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I did not word that very well.

The old rectifier is out of circuit with just the new one in.

I disconnected everything from the smoothing circuit and it now stays the same at 320V.

I'm not sure what the idling at 160V was about it does not seem to do that any more.

The resistance of the dual pole is next to nothing, that is working correctly.

 
Posted : 28/03/2020 7:46 am
lotsalines
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I cannot seem to edit the last post so i'll add in updates as extra posts.
I have disconnected connection 7 on the mini line output board and the majority of the voltage drop has disappeared.

 

Mod Note: Posts can be edited by a user for up to 10-mins after the initial posting. This is normal practice on forums.

 
Posted : 29/03/2020 9:30 am
lotsalines
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Thanks for letting me know about the edit time limit.
I've been on forums that do not feature this and then all the users have a whinge about you making a second post in a row.

Been working my way along everything directly connected to connection 7 disconnecting them one at a time, powering the set up and seeing if it affects the voltage drop.
Nothing did however I did find the voltage went up from the usual 160V to 210V with C79 disconnected.
Still along way off the 315V when first powered on but at least it gives a clue I suppose.
All the big waxies in the line output stage have been changed so I cannot see it being a capacitor issue unless it's C73/74 and that's only because they're inacessable.
So far I have only been powering it up long enough to let the valves warm up enough for the set to work but I left it on a little longer and I have noticed that the line output valve gets very hot very quickly.
I recall something overheating if the HT is low from somewhere, cannot remember if it is the boost, LOPT valve or transformer.

 
Posted : 31/03/2020 1:08 pm
freya
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@lotsalines

Does C79 measure short, its 130pF. Can you post a picture so can see what type/make is fitted.

Image (9)

 Would certainly explain why the sets so angry if it is shorted.

 
Posted : 31/03/2020 2:26 pm
sideband
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OK now when you first switch on, all valves are cold....minimal current being drawn....probably just a few milliamps due to leakage and resistance paths. Therefore voltage at initial switch on will be high. As the valves warm up, current starts to flow so voltage goes down.....this is what happens with valve equipment when a selenium or silicon rectifier is used. It is likely the main HT rail will be around 210 volts when the set is working normally.....that's a very typical voltage. I don't have the circuit here, so check what the main HT from the smoothing cap is, I wouldn't frown much at 210V.

The line output valve will overheat if the line drive from the oscilliator is low/non-existent but I think you said you can hear the line whistle. The boost diode will overheat if the boost cap is shorted....I think you have already changed that. 

Next I will look at the circuit!

 

 

 
Posted : 31/03/2020 2:33 pm
sideband
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Posted by: @freya

@lotsalines

Does C79 measure short, its 130pF. Can you post a picture so can see what type/make is fitted.

Image (9)

 Would certainly explain why the sets so angry if it is shorted.

Ahh beaten to it! Yes indeed and C79 is likely to be a pulse-rated capacitor so check what type it is first. In the past we have always said that paper caps are inevitably leaky and should be changed. However when it comes to line output stages, low value pulse ceramics and mica caps can sometimes give problems simply because they have a very hard life. It would certainly kill the line stage if shorted or very leaky.

 
Posted : 31/03/2020 2:43 pm
lotsalines
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I went to test the capacitor and it seemed OK and then I realized something else.

The boost diode has a small coil in line with the wire connecting it which had snapped.
I must have accidently broke it off.
When I resoldered it back on it dropped back down to 160V.
With the cap off it goes back up to 210V.

 
Posted : 31/03/2020 3:53 pm
sideband
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Well......it's possible that the capacitor is breaking down under load....it happens. Is there any info stating what the working voltage is? It's the same with any capacitor....can show low or no leakage on a meter but start applying voltage and the leakage becomes very apparent.

 
Posted : 02/04/2020 12:59 pm
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