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Mystery Ferranti Set

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lotsalines
(@lotsalines)
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I have given the EHT rectifier a bit of a prod about to see what state it's in I do not seem to have EHT after all.
There is more or less no arc between the cap and the valve besides a bit of blue flickering around it when they make gentle contact.

When I do make proper contact with them however the inside of the valve lights up a fantastic purple.
I have heard of people doing this for EHT checking but I had been a little reluctant as it's one of the most "fun" parts of a set.

Was going to order a set for the line stage anyway so i'll crack on with that.
I'll chack the resistors/caps in the area too and see if anything has gone open.

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Topic starter Posted : 06/03/2020 6:21 am
sideband
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Have you measured any voltages yet? You should do first otherwise its called 'poking and hoping' (as we used to say in the trade)! Check your boost volts.....(measure across C81, the boost capacitor) that can tell you a lot. If its correct, then there is a good chance that the line stage is working well. If low or the same as normal HT then it will never work properly until you find out why (no G2 (screen grid) volts to the tube for a start). It could simply be the boost diode. Don't rule out the possibility of faulty valve holders....it happens especially if the set has been stored damp at any time.

I haven't looked at the circuit details....what valves are fitted to the line stage? What EHT rec does it use?

 

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Posted : 06/03/2020 7:34 pm
lotsalines
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The set uses a U26 EHT rectifier, A U191 boost and a 30P4 line output.
I have gone over the "line board" to make sure nothing had shorted/gone open, nothing has.
Resistor values all more than reasonable.
Seeing as it only had 3 plastic/wax caps on it I have changed them so I know they wouldn't be an issue.
I can hear the deflection working for both the horizontal and vertical on the set.
The board has been gently cleaned which was required just to make contact with my meter and the components under a lifetimes suppy of dust and nicotine.
Valve sockets also cleaned with a little contact cleaner.

I now have a replacement U26 which no longer produces the purple glow however I cannot see any glow from the heater inside of it.
I have yet to check continuity across the couple of turns around the LOPT that power it but it will still draw the small 1/8th" arc.
There are a few components tucked behind the large metal plate between the line output gear and the picture tube that I have not checked because I will have to disassemble the set completely to reach.

I do not seem to be getting anything at all across C81 which seems off to me.

Do keep in mind this is the first valve television of any kind I have worked on so expect lots of mistakes and general stupidity on my end.

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Topic starter Posted : 11/03/2020 3:38 pm
freya
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@lotsalines

Can you hear the line frequency changing as you adjust the line hold control.

 If you have nothing on the boost capacitor you wont get anything on the screen.

 

Image (9)

 

 

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Posted : 11/03/2020 6:56 pm
lotsalines
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Yes I can.
I am going to re-measure the cap because it seems odd.
I should mention that I also get nothing from the tuner other than the sound crackling when I turn the dial.
It's like the HT is there but not across the whole set.
I think i'm going to take the chassis out so I can get a proper look at it.

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Topic starter Posted : 11/03/2020 8:49 pm
freya
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@lotsalines

I have uploaded the genuine Ekco sheets for this model, they will be available in the library when Chris gets a chance to stitch a couple of sheets together.

 I'm using references from the genuine ekco sheets (small snippet below), you may have a shorted C29 16uf electrolytic, which could have burnt out the 100 ohm resistor R48. Which would result in no HT to front end.

Image (8)

 

 

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Posted : 11/03/2020 9:06 pm
sideband
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....But can you PLEASE give us some voltages to work from?? You say there is nothing across the boost cap. Please measure the voltage on the screen grid of the 30P4 line output valve. Do you have a nice negative voltage on the control grid? Please also measure the main HT voltage (top end of C81).

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Posted : 11/03/2020 9:32 pm
Till Eulenspiegel
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A quick test to establish if the boost capacitor has gone short circuit:  remove the top cap of the booster diode.  If the EHT comes up then the boost capacitor is S/C.

The line output valve grid should be >40 volts negative when it is receiving the correct line drive.

Check the value of the screen feed resistor of the line output valve.  5.6Kohm in TP1026, might be 4.7Kohm in T1085.

Till Eulenspiegel.

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Posted : 11/03/2020 10:36 pm
helloekco
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Posted by: @lotsalines

I now have a replacement U26 which no longer produces the purple glow however I cannot see any glow from the heater inside of it.

This caught me out - the heater glow on these is very faint. With my set now close to fully working I can only see any kind of glow from the EHT rectifier with the room completely dark. Considering that the heater only runs from a couple of turns of wire wrapped around the LOPT, that gives an idea of the amount of power drawn by the heater.

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Posted : 11/03/2020 11:54 pm
lotsalines
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I would love to give you some voltages but all i'm getting off the set is a big fat nothing.

Checked both grids of the line output as well as the back of C81 and the HT along with the negetive is simply not there.

I checked R48 as well as the screen feed resistor, both of them measure near bang on.
I also discovered that R35 has cracked and gone open so i'll put a new one in.
It looks like it's a HT hunt.

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Topic starter Posted : 12/03/2020 2:56 pm
freya
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@lotsalines

just for clarity are you using trader sheet or genuine component sheet for circuit references

 

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Posted : 12/03/2020 6:46 pm
sideband
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Posted by: @lotsalines

I would love to give you some voltages but all i'm getting off the set is a big fat nothing.

Checked both grids of the line output as well as the back of C81 and the HT along with the negetive is simply not there.

I checked R48 as well as the screen feed resistor, both of them measure near bang on.
I also discovered that R35 has cracked and gone open so i'll put a new one in.
It looks like it's a HT hunt.

...but then you wouldn't hear a line whistle....very puzzled by all this. You cant be drawing any sparks off the EHT rec if there is no HT.....it simply can't happen........Do you have HT anywhere? There are several HT rails but all have a common source....start at the main smoothing capacitor after the rectifier.

Unless something else has failed since you first started working on the set, you wouldn't have had a blue glow inside the old EHT rec either. If you can hear a line whistle, there must be HT somewhere......

 

We are talking DC voltages...make sure your meter is set correctly.....

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Posted : 12/03/2020 8:13 pm
Cathovisor
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I agree with @sideband here - we need voltage measurements if we're to help you. "Poke-and-hope" is not going to fix this.

Only last Friday I was driving this home to some colleagues telling me of a peculiar fault on a piece of broadcast kit; "have you measured the voltages on the power supply?"

"No, because it's doing this and this-"

"Have you measured the voltages on the power supply?!"

 

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Posted : 12/03/2020 9:05 pm
lotsalines
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My fault, i'm a muppet!

I do have voltages after giving the board a proper scrub.
I am measuring in DC.
The boost cap starts off high around 600v on a cold start but rapidly drops down to 248V as the set warms up.
On the screen I have 60V and on the control I have -11V.
I have checked multiple times and those are the values I am getting.

I am using the trader sheet for reference.

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Topic starter Posted : 13/03/2020 10:56 am
sideband
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OK now we are getting somewhere. There are several possibilities here. The boost starts off at 600V....good, but drops down to 248 as the set warms up....not good. 60V on the screen grid of the line valve not good.....the HT is very low. -11V on the control grid also not good. What type of rectifier is used? Is it a selenium type (fairly long with a number of fins) or is it a silicon jobbie? If the latter it's probably OK, if the former it probably isn't. In any case you need to find the rectifier or if it's easier, the main smoothing can and check what voltages you have there. There could be a number of reasons why the HT is low. If a selenium rectifier is fitted, it is likely to be faulty....they go high resistance with age. This means that without much load, they can give near normal HT until the rest of the set starts drawing current and then the HT drops through the floor. If the rectifier is a silicon type it's probably OK in which case there could be something causing the HT to drop......heavy current drawn by one of the valves perhaps or a capacitor that has not been replaced but lets take this a stage at a time. 

Find the rectifier (or smoothing caps) and see what voltage you have at first switch-on. Leave the meter connected while the set warms up and see what happens to the HT. Come back and tell us your findings.

GOLDEN RULE with any fault like this, CHECK THE HT FIRST. Get that right before going any further.

The fact that you get 600V to start with suggests that at first, the line stage is working which might indicate that the rectifier etc is also good but something is causing the HT to drop.....that is what we need to find out.

 

 

 

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Posted : 13/03/2020 3:09 pm
crustytv
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Posted by: @sideband

GOLDEN RULE with any fault like this, CHECK THE HT FIRST. Get that right before going any further.

Indeed, and I remember you and many other ex TV engineers drumming that mantra into to me when I first started "a decade ago"

Which is why on page one of the thread I pointed the OP to the Basic B&W TV Repair Guide. The block diagram showing the logical path to getting a set working, "A" through to "L". "A" being make your start at the PSU.... No or low HT, in turn no or low EHT.

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Posted : 13/03/2020 3:29 pm
lotsalines
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I'm fairy certain that it's silicon as the wire going off to it goes into an incredibly inaccessible small metal can on the underside of the set that would never fit a selenium unless there exists a type that's roughly 1/8" thick.
The mains caps start off at 250v and quickly drop down to 120v.
The process happens almost immediately as the set is plugged in and the voltage stabilizes at 120v as the line whistle frequency stabilizes.

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Topic starter Posted : 15/03/2020 5:25 pm
sideband
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Hmm. Is there a loud hum from the speaker? Just thinking that the smoothing caps may be open circuit....that will cause low HT but will also cause a loud hum. You could try bridging them with new caps. There is also a strong possibility that there is an HT decoupler somewhere that is pulling the HT down. As an experiment, disconnect the top caps of the line output valve and boost diode. Obviously the line stage won't run but see what happens to the HT. If it drops as before then there is a fault elsewhere (possibly as stated above). If it stays up at approximately the right voltage then something in the line stage is pulling the voltage down.

The fact that the voltage drops almost immediately isn't allowing enough time for the line stage to warm up so at this stage I'm thinking more in terms of an HT decoupler somewhere (could be a paper or electrolytic type) that is causing the problem but try the suggestions above first.

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Posted : 15/03/2020 11:10 pm
lotsalines
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I have measured it again with the top caps disconnected and I still have the voltage drop but at 150v instead of 120v.

The drop is slower however and only happens after 10 seconds or so of power applied.

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Topic starter Posted : 16/03/2020 1:32 am
sideband
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Something else is pulling the HT down. Check the main smoothing caps by substitution first and see if you can identify that rectifier...try and take some pictures of it.

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Posted : 16/03/2020 9:45 pm
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