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B&W TV Stella ST1011U line output transformer substitution.

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Till Eulenspiegel
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Pictures of the Stella ST1011U. Except for cabinet presentation the set is electrically identical to the Philips 21TG100U and like the Philips set which received a line output transformer from the later 210 series the intention is to fit a transformer from the Style70 series in the Stella. 

Stella ST1011
Stella ST1011 2

It was planned to fit a BRC jellypot transformer in this set. However, the owner of the set retained the line output transformer from a Philips 17TG171U Style 70 set.

Stella ST1011 Style70 LOPT

 Much better to fit parts from the same firm that made the set.  Some time ago I did attempt to fit a jellypot in a Philips 19TG108U, the transformer substitution wasn't satisfactory and was abandoned. 

Till Eulenspiegel.

 

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Topic starter Posted : 30/08/2021 10:50 am
LSmith, ntscuser and Lloyd liked
Lloyd
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Good luck! Those style 70 LOPT’s are not very good, my own style 70 set has eaten 2 already...

Regards,

 Lloyd 

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Posted : 30/08/2021 11:08 am
Till Eulenspiegel
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Hi Lloyd,  over the weekend the transformer was warmed up by passing a small current through the windings.  I understand the Style70 set was working when it was scrapped. 

Till Eulenspiegel. 

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Topic starter Posted : 30/08/2021 11:26 am
Till Eulenspiegel
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Circuit diagram of the 19TG170 "Style70" series line output stage.

Stella ST1011 Style70 LOPT cct

 Component layout of the line output stage components.

Stella ST1011 Style70 LOPT 2

I can report the donor transformer windings are nice and warm.

Till Eulenspiegel.

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Topic starter Posted : 30/08/2021 5:15 pm
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Till Eulenspiegel
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Today, the faulty line transformer was removed from the line output stage sub-chassis and the Style70 transformer fitted in position ready to be wired into the circuit. The original EHT rectifier valveholder will be retained. The replacement transformer employs the DY86/7 which requires a single turn heater winding.

Like the Philips 21TG100U line transformer substitution the width control will take the form of of a chain four 75ohm resistors. 

Stella ST1011 Style70 LOPT 3
Stella ST1011 Style70 LOPT 4

Till Eulenspiegel.

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Topic starter Posted : 07/09/2021 12:02 am
Bobhowe
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Hi David will this set give out a good picture like the Deep 17 when done kind regards Bob

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Posted : 07/09/2021 1:23 pm
Till Eulenspiegel
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Bad news, the replacement transformer lasted for a few minutes, insulation failure between the primary and secondary windings.  Looks like it's going to be a Thorn jellypot transformer substitution after all.  The attachment shows the circuit diagram of the line output stage in early production Philips 17TG100U models.  The line output transformer is a much simpler autotransformer compared with the later production transformers which have a split secondary windings.

Stella ST1011 LTB early

 by adopting this circuit it should be possible to employ the jellypot transformer without too many alterations to the Philips circuits.

Till Eulenspiegel.

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Topic starter Posted : 09/09/2021 10:58 am
Bobhowe
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Hi was ad Davids shop yesterday and seen this wonderful  engineered set looking forward to seeing it  working kind regards Bob

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Posted : 09/09/2021 11:58 am
Lloyd
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Oh no! That’s how the first LOPT failed in my style 70, the second one’s over wind failed, so I unbolted the 2 and made 1 good LOPT out of them! Last time I checked it was still working, but that was a good few years ago now!

 Regards 

Lloyd 

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Posted : 09/09/2021 1:31 pm
Till Eulenspiegel
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In terms of picture and construction quality these superb 405 line Philips TVs are let down by the reliability of of the line output transformers, and in the 17TG100U series the scanning coils weren't much better.  Bob was asking between the Philips sets under discussion in this topic and the RGD Deep17, which was the most reliable? The Deep17 was the clear winner. 

The attachment shows the connection to the octal plug and socket on the line output transformer sub-chassis.

 

Stella ST1011 Style70 LOPT connections

Till Eulenspiegel. 

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Topic starter Posted : 09/09/2021 3:07 pm
Bobhowe
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Hi David like the construction of this set must have been one of the easiest sets to work on ? kind regards Bob 

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Posted : 09/09/2021 6:26 pm
Till Eulenspiegel
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Hi Bob, very easy sets to work on, especially on house Service calls. swing-out chassis for valve replacements and easy access to the component side of the chassis. 

More glasnost. Now to the Stella set I'm presently working on. The jellypot transformer is wired into circuit, everything checked before switching on.   After allowing the boost diode time to warm up the result is a tiny spark at the tripler connector and a PL81 glowing red hot again.   But I'm not going to give up at this stage. Still have the line oscillator unit I made up for the Philips 21TG100U. Now to try it out in the Stella, at least there will be a full -100 volts P - P drive for the PL81.

Back in 2014 I tried a jellypot transformer in a Philips 19TG108U. This is a set that employs a variant of the  17TG100U chassis and uses same type of line output transformer.   A separate line oscillator was employed in order to make the line timebase work. In due course the ECL80 line sync clipper and part oscillator was rewired to become a multivibrator.

The link to the 19TG108U project:  Philips 19TG108U Not a success story. - UK Vintage Radio Repair and Restoration Discussion Forum (vintage-radio.net)

Till Eulenspiegel.

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Topic starter Posted : 10/09/2021 5:09 pm
Bobhowe
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What shame i remember that post on the Philips 19TG108U could the original transformer be rewound ? kind regards Bob 

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Posted : 10/09/2021 8:09 pm
Till Eulenspiegel
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Hi Bob, it might be possible to rewind the 17TG170U line transformer primary windings.  However, better news. Since my last post the line oscillator module has been fitted, the ECL80 sync clipper and part line oscillator valve removed and the the heater of the ECC82 multi-vibrator wired into the heater chain. 130volts of drive applied to the PL81 grid. Duty cycle set to 100 micro seconds, ratio 40:60. Result? a healthy spark at the EHT connection of the jellypot. Tripler connected up to the transformer and now a fearsome 21KV measured at the final anode of the CRT.   A very bright raster on the screen proving the tube is excellent. Like the Philips 19TG108U a different set of scan coils will be required because the Philips coils are a bad match for the Thorn transformer. Then, after all this good news something else happened, the tripler unit failed. That familiar smell associated with selenium rectifiers. These will be replaced with silicon diodes.

The attachment shows the line oscillator module.  Circuit employs an ECC82 as a cathode coupled  multivibrator. Copied from the Bush TV105.

Stella ST1011 LTB Osc Module

Till Eulenspiegel.

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Topic starter Posted : 10/09/2021 8:59 pm
Bobhowe
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Hi David glad the tube is good hope to see some pictures of it first light when you have sorted the silicon diodes kind regards Bob 

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Posted : 11/09/2021 2:44 pm
Till Eulenspiegel
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Five miniature high voltage silicon diodes will replace the selenium rectifier "sticks" in the Thorn tripler unit.

Stella ST1011 Tripler

Till Eulenspiegel.

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Topic starter Posted : 11/09/2021 10:18 pm
crustytv
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Those little silicon diodes look remarkably like BY8410 EHT diodes, are they? If they are, unlike the selenium, they are super efficient you're going to get way more than the required 20kV. You're also going to have your work cut out taming that ladder! plenty of flash-over potential, I assume it's the open type tray.

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Posted : 11/09/2021 11:08 pm
Bobhowe
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Posted by: @crustytv

Those little silicon diodes look remarkably like BY8410 EHT diodes, are they? If they are, unlike the selenium, they are super efficient you're going to get way more than the required 20kV. You're also going to have your work cut out taming that ladder! plenty of flash-over potential, I assume it's the open type tray.

Hi it would interesting to see what voltage is produced from the diodes kind regards Bob 

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Posted : 12/09/2021 6:13 pm
Till Eulenspiegel
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Hi Chris,  for the Stella ST1011 the maximum EHT should be no more than 16KV, we've got to consider the well being of the AW53-88 CRT, any figure higher than 16KV might cause internal flashover and possible damage to the gun.  So tomorrow steps must be taken to reduce the EHT and increase the picture width.  First, insert a chain of four 75ohm resistors to reduce the applied HT voltage to the anode of the boost diode.  Like the recent Philips 21TG100U line transformer substitution project the resistors are mounted on the width control tags. Second, fit the scanning coils that were used in the Philips 19TG108U jellypot transformer substitution, that should increase the width.  The circuits around the ECL80 line part line oscillator and sync pulse clipper will be rewired to become a multi-vibrator, and like the 19TG108U job the line oscillator circuit of the Alba T655 will be adopted. Just two additional capacitors required, 47pF and 500pF. 

Hi Bob, as Chris states those little EHT diodes are much more efficient than the selenium diodes in the Thorn EHT multiplier unit. At this stage of the project we don't want any increase of EHT voltage, 20KV is just too much for the CRT.

I've a Bush TV22 which has a miniature silicon EHT rectifier used in lieu of the EY51 valve rectifier. The insulation of the EY51 heater winding on the line output transformer had broke down and the silicon diode was an easy fix. Proves the diodes are good for 7KV.

Till Eulenspiegel.     

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Topic starter Posted : 12/09/2021 10:57 pm
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Bobhowe
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Hi David 21KV thats colour TV EHT as Chris says how are tame that ladder kind regards Bob 

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Posted : 13/09/2021 12:40 pm
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