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1969 Ekco CT104

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Mar 27, 2016 1:21 am

There was another miniature line output, the Marconi-Osram N339. This valve is not an equivalent to the PL81. However, it's possible that the 21B6 was also relabeled as an N339.
From the National valve Museum: http://www.r-type.org/exhib/abh0093.htm

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by nuvistor » Sun Mar 27, 2016 9:55 am

Hi Marion,
I have seen line output valves with the glass sucked in due to no drive, occasionally seen them melt right through. All for the sake of a few extra fuses in the sets, well the sound was still working and we wanted to finish( insert program), when asked why they did not turn off.
Frank

 
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Re: 1969 Ekco CT104

Post by Focus Diode » Sun Mar 27, 2016 10:04 am

Till Eulenspiegel wrote: Something we might consider for a future project will be to replace the 25KV winding for one which produces 8KV. The 8KV pulse then can be supplied to a tripler. The shunt stabiliser tube is retained.
The modified line TB circuit will resemble that of the Baird 700 series.


Getting this thread back on topic this is a brilliant idea.
Brian

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Mar 27, 2016 11:38 am

Considering the idea of fitting a tripler to any colour TV which employs a 25KV EHT winding on the LOPTx. We know that the peak pulse present at the anode of the line output valve is in the order of 6KV, this is insufficient to supply a five diode tripler. An additional 2KV overwind is required.
If my memory serves me well I believe the 3.15 volt heater of the GY501 EHT rectifier is supplied from a single turn winding on the transformer. Now doesn't that equate to volts per turn rather than turns per volt? So I'd reckon it will be an easy matter to add an addition 2KV winding to the transformer.

It would seem the Baird TV designers had the foresight to realise a 25KV EHT winding would be a future source of trouble. The Baird 700 series doesn't seem to have too much trouble in the EHT department, or does it?

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by Doz » Sun Mar 27, 2016 12:56 pm

Focus 2 wrote:
Till Eulenspiegel wrote: Something we might consider for a future project will be to replace the 25KV winding for one which produces 8KV. The 8KV pulse then can be supplied to a tripler. The shunt stabiliser tube is retained.
The modified line TB circuit will resemble that of the Baird 700 series.


Getting this thread back on topic this is a brilliant idea.
Brian


Would a tripler take kindly to being shunted?

 
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Re: 1969 Ekco CT104

Post by crustytv » Sun Mar 27, 2016 1:14 pm

Doz wrote:Would a tripler take kindly to being shunted?

The Baird 700 tripler does, could make one the same, see here doesn't even need to be potted.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Mar 27, 2016 1:21 pm

Good point, but the Baird 700 series gets away with using BY140 silicon diodes in the EHT cascade. Also worth thinking about is that the Pye 691 and the GEC 2028 employ a voltage dependant resistor across the EHT supply and this device will provide a quasi constant load. Later Bush and Murphy CTVs use a quadrupler to supply the EHT.
Worth remembering the shunt stabiliser is there to provide a constant load across the EHT supply. With careful choice of components the shunt tube should track the varying current demands of the CRT.
When the CRT is drawing only a small current the shunt tube draws all that is required to take the maximum of 1.2mA from the EHT supply. Conversely, when the CRT is fully conducting the shunt tube will be near to cut-off.

It gets forgotten that the demands on the EHT supply is over 25watts!

Till Eulenspiegel.

 
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Re: Ekco CT104

Post by PYE625 » Tue Mar 29, 2016 9:26 pm

Till Eulenspiegel wrote:...and here it is. The mighty line timebase and EHT module.

Till Eulenspiegel.


Seeing that picture has me drooling !!
Wish my CT72 had the same line stage.... :bba

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Tue Mar 29, 2016 11:06 pm

There is no doubt the EHT "tower" is an impressive looking affair and when it's working as it was designed to do so the EHT regulation is first class.
I will give the idea of a EHT voltage tripler serious consideration, using this device will improve reliability. The shunt stabiliser valve will be retained. Bush and Baird employed the tripler and shunt stabiliser arrangement to good effect so there is no reason the same system should not be employed in the Ekco CT104. Nevertheless the single standard CT72 is a very good receiver and thanks to the tidied up interconnection cableforms a much easier receiver to service.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Apr 03, 2016 3:51 pm

I'm definitely going down the tripler and shunt stabiliser route.

I have here a line transformer for the GEC 2028. This set along with the Baird 700 series was one of the first sets to employ an EHT voltage tripler. Not forgetting the BRC 2000 of course.

The 8KV winding is layer wound and appears to have 77turns, I have to determine if there are two or three winding layers, It looks like two. Nevertheless, this confirms the volts per turn theory.
The peak voltage present at the anode of the PL509 must be measured, I'd guess it is 6 or >6KV, hence the reason for so few turns on the tripler winding.
Another transformer designed to work with a tripler is the one used in the Pye 691. I do have one somewhere in the shop so when it is found I'll examine it to find out how many turns exist on the EHT winding.

Looking at the GEC 2028 line transformer it would appear there is not all that many turns on the main windings.
Such transformers can be rewound or reproduced. Don't throw away those old transformers, we'll need the high efficiency cores for any rewinds.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by nuvistor » Sun Apr 03, 2016 3:55 pm

If you have the 691 transformer, from memory it is a very similar physical fit albeit 625 only, the specification may be a good fit as well.

Frank

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Apr 03, 2016 4:49 pm

Hi Frank,
the transformer tag panels are very similar but some study of the circuits of single and dual standard versions is called for before any attempt is made to modify the line transformer in the CT104. It's one of those jobs which must work first time. I think the 691 transformer EHT winding will be the one to copy. Like the GEC transformer there will not be many turns on the 8KV winding.
To find the peak volts present at the anode of the PL509 all that is required is a BY140 type of diode, a 0.001 HV capacitor and the EHT meter. Judging by the length of the spark at the anode (4mm) I reckon it'll be in the order of 6KV. Many dual standard CTV sourced the focus voltage from the anode of the PL509, one set that comes to mind is the horrible Decca CTV25, the colour TV I love to hate.
Actually, the single standard CS2500 wasn't too bad.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by malcscott » Sun Apr 03, 2016 5:16 pm

Hi David, i have full w,shop manual for this set if it is of any help to you, Malc.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sun Apr 03, 2016 7:16 pm

Hi Malc,
I reckon I'll need the full manual to sort this set out. I'm presently using the information in the 1969/70 Radio and Television servicing book. After the line timebase is working properly on both line standards there is still that fault in the IF panel and whatever else that might need attending to.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by malcscott » Mon Apr 04, 2016 10:55 am

Hi David, on chapter 6.8 of the Mullard book on PAL colour tv are the specification for the windings for hybrid loptx. All of the winding info etc, Malc.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Mon Apr 04, 2016 6:47 pm

Hi Malc,
the Mullard "PAL colour TV..." book is proving to be an invaluable asset for our CTV restoration projects. The attachment shows the line transformers used in the GEC 2040 and Pye 697 single standard hybrid colour TVs.
The 8KV EHT winding on the Pye transformer appears to consist of two layers of 65 turns each. The GEC EHT winding might well consist of only one single layer .
Interesting to note is that the 25KV winding of the Mullard transformer is 1680 turns wound at 30 turns per layer. Don't fancy making one of those somehow.

Till Eulenspiegel.
Attachments
000_0038.jpg

 
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Re: 1969 Ekco CT104

Post by davidh1041 » Wed Apr 06, 2016 5:54 pm

Keith has fitted a single standard LOPT into his CT104, see the write up here:-
http://www.vintage-radio.net/forum/show ... p?t=107863

Hope this is ok Mods?

thanks David

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Wed Apr 06, 2016 8:39 pm

Way back in the late 1970s, (1979 I seem to remember) I replaced the burned out dual standard line output transformer in my GEC 2028 with the single standard line OP transformer from the 2040 series. The modifications were quite extensive involving removal of the system switches. It's an action I regret and it is my intention to return the set back to it's original dual standard specification. A dual standard LOPTx has been found and now I'm looking for the system switches to return the set to 405 and 625 operation.

The Ekco CT104: Today, I made a start making the 8KV winding for the line OP transformer. A former is required for the new winding. The Ferroxcube core of the transformer is 19mm diameter, what can we use for the job? Easy, a piece of plastic plumbing pipe and there was some lengths of the stuff dumped at the back of the shop.
The new coil former is 31mm long and the windings will consist of two layers of 65 turns, the same as the single standard 691 line OP transformer.

The direction of the new winding will have to be determined. The extra 2KV has to augment the 6KV pulse present on the anode of the PL509, the wrong way round and we get a 4KV pulse.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Fri Apr 08, 2016 10:34 pm

Well here goes. The EHT tower is now on the bench. The first task is to remove the EHT overwind then connect up a tripler which will be connected direct to the anode of the PL509. Measure the EHT voltage available and then we can determine how many turns are required on the new 8KV winding. Interesting to note that the single standard line OP transformer has more turns per winding compared with the DS version.
According to provisional service manual the 5KV supply for the focus control comes from the cathode of the PY500 booster diode.
So it follows the peak volts present at the anode of the will be more than that figure.

Till Eulenspiegel.
Attachments
000_0044.jpg

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sat Apr 09, 2016 12:09 pm

I reckon it might well be possible to test this EHT unit in a stand alone manner. Everything that is needed for it to operate is contained in the assembly, the power supply and the line oscillator are present. The scanning coils will have to be connected up because these have an important function in the operation of the line output stage. Energy stored in the coils contributes to the flyback pulse.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sat Apr 09, 2016 12:46 pm

How about doing this? it's gonna be a big job disconnecting and dismantling the transformer so why not just hacksaw the EHT winding off. A least doing this will determine if that if it is the problem with the line output stage. If it isn't, too bad, I've ruined an EHT winding.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by nuvistor » Sat Apr 09, 2016 12:56 pm

Seeing that many LOPTX faults are the overwind I reckon it is a good calculated risk. Would a Dremel be safer than a hacksaw though, less danger of damaging other items.
Frank

 
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Re: 1969 Ekco CT104

Post by Focus Diode » Sat Apr 09, 2016 1:08 pm

Seconded! On looking at it myself it it's a long laborious job with risks of damaging other areas. I previously thought trying my own unknown overwind would be a good idea but on examination I note cracks which I insulated using Araldite. Even if it turned out to be OK it wouldn't be for long.
We're confident the fault is in the overwind. We need to liberate the primary ensure it's Ok then on with the restoration.
Cheers
Brian
Attachments
Edited_ImageEasyImageEditor_20160409_50.jpg

 
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Re: 1969 Ekco CT104

Post by Till Eulenspiegel » Sat Apr 09, 2016 1:27 pm

Well I've done it. Interesting to observe the method of how the EHT winding was wound. It is made in a similar manner as suggested in the Mullard book PAL COLOUR TV... That is, "1680 turns,40 s w g (0122mm) Sims QSE, wound at 30turns/layer"

I don't fancy making one of those somehow.

Till Eulenspiegel.

 
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Re: 1969 Ekco CT104

Post by Focus Diode » Sat Apr 09, 2016 1:44 pm

Here's the overwind. Beautifully insuled, look at those windings! No apparent fault to note but it takes just one of those ultra fine wires to short to render it useless.

Brian
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