Stella ST1011U line output transformer substitution. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 12th Oct 2021, 12:27 pm

The Stella ST1011U is electrically identical to the Philips 21TG100U. These are possibly the best 405 line receivers ever made and well worth spending some time on restoration work.
The line transformer has failed and a replacement is almost impossible to find.
I recently performed a line transformer substitution on a Philips 21TG100U using a Pye 368 line transformer, the same type of transformer was used in the Philips 210 series.
For the Stella it was decided to to take a chance with a Thorn 1500 jellypot transformer.
In 2014 I attempted to use a BRC 1400 line output transformer in a Philips 19TG108U, this wasn't a successful transformer substitution. The set is still in existence and using all the lessons learnt will be applied to the Stella transformer job. In due course the 19" set will be returned to the workbench.
We know that the Philips scanning coils are a not match for the 210 series line transformer and it is the same for the Thorn/BRC transformers. Fortunately the Stella has a replacement scan coil assembly which was found to be a good match for the 1500 jellypot. The line oscillator in the Philips circuit uses a positive feedback pulse from the line transformer to initiate the flyback part of the scanning waveform. This arrangement doesn't work with the jellypot transformer so the circuit around V611 was rewired to become a cross coupled multi-vibrator. The circuit chosen is from the Alba T655.
The line transformer was removed from the sub chassis, the linearity coil retained and used in the modified circuit. Picture width is by means of a chain three 75ohm resistors which supply HT to the anode of the boost diode.
EHT is provided by a doubler which supplies 16.5KV to the CRT final anode.
The flyback tuning capacitor between the cathode of the boost diode and chassis is now 250pF. The value was chosen to provide the excess of picture width needed for the 5 : 4 aspect ratio of the AW53-88 CRT.
The attachments show the results. the horizontal linearity isn't perfect but for normal viewing the results are entirely satisfactory.
Also circuits showing the modifications.

DFWB.

Heatercathodeshort · 12th Oct 2021, 1:58 pm

That is a very good conversion David, probably a lot better than the original. I have a 21" Philips 21TG100U from the museum, an early one with the continuous primary winding.
I have a scrap late model 20" Thorn 1500 that will donate the Jellypot, doubler and the scan coils so it has a chance! Regards, John.

FERNSEH · 12th Oct 2021, 8:13 pm

Hi John,
The early 21TG100U timebase lends itself to a much easier line transformer substitution than the later version with the split LOPTx winding.
The octal plug and socket used to connect up the line output sub-chassis to the main chassis assembly is retained for easy removal of the unit but after modification is not electrically interchangeable with the original sub-chassis.
Also retained is the Philips line linearity coil which is connected to the boost HT pin on the jellypot and via the octal plug and socket to the red wire scanning coils connection. The scan coil black lead is connected via the plug and socket to the tap on the jellypot primary winding.

DFWB.

bobhowe · 12th Oct 2021, 9:19 pm

Quote:

Originally Posted by FERNSEH

The Stella ST1011U is electrically identical to the Philips 21TG100U. These are possibly the best 405 line receivers ever made and well worth spending some time on restoration work.
The line transformer has failed and a replacement is almost impossible to find.
I recently performed a line transformer substitution on a Philips 21TG100U using a Pye 368 line transformer, the same type of transformer was used in the Philips 210 series.
For the Stella it was decided to to take a chance with a Thorn 1500 jellypot transformer.
In 2014 I attempted to use a BRC 1400 line output transformer in a Philips 19TG108U, this wasn't a successful transformer substitution. The set is still in existence and using all the lessons learnt will be applied to the Stella transformer job. In due course the 19" set will be returned to the workbench.
We know that the Philips scanning coils are a not match for the 210 series line transformer and it is the same for the Thorn/BRC transformers. Fortunately the Stella has a replacement scan coil assembly which was found to be a good match for the 1500 jellypot. The line oscillator in the Philips circuit uses a positive feedback pulse from the line transformer to initiate the flyback part of the scanning waveform. This arrangement doesn't work with the jellypot transformer so the circuit around V611 was rewired to become a cross coupled multi-vibrator. The circuit chosen is from the Alba T655.
The line transformer was removed from the sub chassis, the linearity coil retained and used in the modified circuit. Picture width is by means of a chain three 75ohm resistors which supply HT to the anode of the boost diode.
EHT is provided by a doubler which supplies 16.5KV to the CRT final anode.
The flyback tuning capacitor between the cathode of the boost diode and chassis is now 250pF. The value was chosen to provide the excess of picture width needed for the 5 : 4 aspect ratio of the AW53-88 CRT.
The attachments show the results. the horizontal linearity isn't perfect but for normal viewing the results are entirely satisfactory.
Also circuits showing the modifications.

DFWB.

Hi David good result nice set another set bought back to life kind regards Bob

bobhowe · 12th Oct 2021, 9:23 pm

Quote:

Originally Posted by Heatercathodeshort

That is a very good conversion David, probably a lot better than the original. I have a 21" Philips 21TG100U from the museum, an early one with the continuous primary winding.
I have a scrap late model 20" Thorn 1500 that will donate the Jellypot, doubler and the scan coils so it has a chance! Regards, John.

Hi John the conversion as you say is a lot better than the original hope to see a thread about your Philips 21TG100U soon kind regards Bob

FERNSEH · 12th Oct 2021, 9:44 pm

The Thorn jellypot line transformer is noted for it's reliability so hopefully that will be the end to line transformer failures.

DFWB.

bobhowe · 12th Oct 2021, 9:59 pm

Quote:

Originally Posted by FERNSEH

The Thorn jellypot line transformer is noted for it's reliability so hopefully that will be the end to line transformer failures.

DFWB.

I second that kind regards Bob

FERNSEH · 13th Oct 2021, 12:15 pm

The position of the jellypot transformer permits the use of alternative EHT trays.
A one ohm resistor is inserted between the cathode of the PL81 cathode and chassis. The waveform shows the valve is cut-off during the conduction of the efficiency diode. The mean cathode current is 130mA but it seems the PL81 can cope with the demands of this circuit.
There is provision for a larger line output valve, PL36, PL502, PL504 etc.

DFWB.

bobhowe · 13th Oct 2021, 8:39 pm

Hi David good job you saved some jellypot transformers kind regards Bob

12th Oct 2021, 12:27 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Stella ST1011U line output transformer substitution. The Stella ST1011U is electrically identical to the Philips 21TG100U. These are possibly the best 405 line receivers ever made and well worth spending some time on restoration work. The line transformer has failed and a replacement is almost impossible to find. I recently performed a line transformer substitution on a Philips 21TG100U using a Pye 368 line transformer, the same type of transformer was used in the Philips 210 series. For the Stella it was decided to to take a chance with a Thorn 1500 jellypot transformer. In 2014 I attempted to use a BRC 1400 line output transformer in a Philips 19TG108U, this wasn't a successful transformer substitution. The set is still in existence and using all the lessons learnt will be applied to the Stella transformer job. In due course the 19" set will be returned to the workbench. We know that the Philips scanning coils are a not match for the 210 series line transformer and it is the same for the Thorn/BRC transformers. Fortunately the Stella has a replacement scan coil assembly which was found to be a good match for the 1500 jellypot. The line oscillator in the Philips circuit uses a positive feedback pulse from the line transformer to initiate the flyback part of the scanning waveform. This arrangement doesn't work with the jellypot transformer so the circuit around V611 was rewired to become a cross coupled multi-vibrator. The circuit chosen is from the Alba T655. The line transformer was removed from the sub chassis, the linearity coil retained and used in the modified circuit. Picture width is by means of a chain three 75ohm resistors which supply HT to the anode of the boost diode. EHT is provided by a doubler which supplies 16.5KV to the CRT final anode. The flyback tuning capacitor between the cathode of the boost diode and chassis is now 250pF. The value was chosen to provide the excess of picture width needed for the 5 : 4 aspect ratio of the AW53-88 CRT. The attachments show the results. the horizontal linearity isn't perfect but for normal viewing the results are entirely satisfactory. Also circuits showing the modifications. DFWB. Attached Thumbnails Last edited by FERNSEH; 12th Oct 2021 at 12:36 pm.

12th Oct 2021, 1:58 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Stella ST1011U line output transformer substitution. That is a very good conversion David, probably a lot better than the original. I have a 21" Philips 21TG100U from the museum, an early one with the continuous primary winding. I have a scrap late model 20" Thorn 1500 that will donate the Jellypot, doubler and the scan coils so it has a chance! Regards, John.

12th Oct 2021, 8:13 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Stella ST1011U line output transformer substitution. Hi John, The early 21TG100U timebase lends itself to a much easier line transformer substitution than the later version with the split LOPTx winding. The octal plug and socket used to connect up the line output sub-chassis to the main chassis assembly is retained for easy removal of the unit but after modification is not electrically interchangeable with the original sub-chassis. Also retained is the Philips line linearity coil which is connected to the boost HT pin on the jellypot and via the octal plug and socket to the red wire scanning coils connection. The scan coil black lead is connected via the plug and socket to the tap on the jellypot primary winding. DFWB. Attached Thumbnails Last edited by FERNSEH; 12th Oct 2021 at 8:33 pm.

12th Oct 2021, 9:44 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Stella ST1011U line output transformer substitution. The Thorn jellypot line transformer is noted for it's reliability so hopefully that will be the end to line transformer failures. DFWB.

13th Oct 2021, 12:15 pm	#8
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Stella ST1011U line output transformer substitution. The position of the jellypot transformer permits the use of alternative EHT trays. A one ohm resistor is inserted between the cathode of the PL81 cathode and chassis. The waveform shows the valve is cut-off during the conduction of the efficiency diode. The mean cathode current is 130mA but it seems the PL81 can cope with the demands of this circuit. There is provision for a larger line output valve, PL36, PL502, PL504 etc. DFWB. Attached Thumbnails

13th Oct 2021, 8:39 pm	#9
bobhowe Heptode Join Date: Jun 2005 Location: Sunderland, Tyne and Wear, UK. Posts: 661	Re: Stella ST1011U line output transformer substitution. Hi David good job you saved some jellypot transformers kind regards Bob __________________ Kind regards, Bob.