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Philips 21TG100U Line timbase fault.  

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irob2345
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Boost cap??

I've seen some of those pink Wima caps that look like polyester but are actually paper.

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Posted : 23/11/2020 10:13 pm
Till Eulenspiegel
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The boost cap has been replaced with a new yellow hi-viz component.

Connected an oscilloscope to the control grid of the PL81 line output valve.  Soon after warm-up a 100V P - P 10Khz waveform is apparent on the 'scope screen but when the boost diode is fully warmed up the amplitude of the drive waveform diminishes down to <20V, but the correct frequency is maintained. PL81 becomes red hot again.

I've found a line transformer that might work in this set.                  Will report my findings later today.

Till Eulenspiegel.

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Topic starter Posted : 24/11/2020 12:28 am
irob2345
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What voltage rating is that boost cap, dare I ask?

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Posted : 24/11/2020 12:50 am
Till Eulenspiegel
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630V

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Topic starter Posted : 24/11/2020 10:39 am
irob2345
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OK, I thought it was worth checking! I assume then it is a "suitable" cap for the role and not one of those tiny metallised foil types.

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Posted : 27/11/2020 10:33 am
Till Eulenspiegel
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It is a tiny yellow capacitor but it will be replaced during the course of the transformer substitution job.  The CTV transformer used a 0.47uF capacitor.

It'll be interesting t see how the ex-CTV transformer works in the Philips 21TG100U.  The theory is that the scanning power for a 90 degree colour TV is about the same as a 110 degree monochrome set.

Till Eulenspiegel.

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Topic starter Posted : 27/11/2020 10:43 am
irob2345
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Good luck with that!

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Posted : 27/11/2020 10:46 pm
Till Eulenspiegel
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The GEC 2040 line output transformer connections have been modified to ease fitment into the Philips TV's line timebase screening box.

The primary and secondary windings are rearranged as to be  accessible from only one side of the transformer assembly.

LOPTx 1

The plan is to wire in the replacement transformer in such a manner that no modifications will be necessary to the main chassis.

Till Eulenspiegel.

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Topic starter Posted : 30/11/2020 8:46 pm
Nuvistor liked
irob2345
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Are valve colour TV LOPTXs somewhat available?

Philips B&W LOPTXs, I found, are now so rare as to warrant investigation of other options. Assuming valve colour TV hor yoke windings are usually parallel connected, I can see how a colour LOPTx and a tripler (still readily available new here for some reason) could work.

Let me know how you get on. My previous "Good luck" response was genuine, not intended to be sarcastic!

A technique I have used with Philips B&W LOPTXs with shorted turns EHT windings (most common failure mechanism) is to cut the winding off the core using a hacksaw or a small angle grinder (only way to do it, the cores are glued together with epoxy) and use a tripler driven from the line output anode. You need to add about 47pf 5kV between the damper cathode and the boost connection to compensate for the lost winding else the EHT is too high and width too low. Adjust the value of the cap so that the stabilised boost ends up around 610 volts for correct width.

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Posted : 30/11/2020 10:43 pm
Till Eulenspiegel
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There's an Ekco dual standard CTV in the workshop which still needs attention.  The 25KV EHT winding on the Philips sourced line output transformer failed and was duly removed.  Unfortunately, you can't get away with simply connecting a tripler to the anode of the PL509 line output valve. A winding to produce an extra 1KV is required, this is no big deal because we are not talking turns per volt, it's better than that as it is volts per turn. Something like six volts per turn. In fact I think it's even better than that.

Till Eulenspiegel.

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Topic starter Posted : 01/12/2020 1:08 am
irob2345
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Yes that's what I would have thought with the B&W transformer too, but it makes more volts that you'd expect.

Did you try it?

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Posted : 01/12/2020 4:20 am
Till Eulenspiegel
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Today, I wired the GEC 2040 line output transformer in the Philips 21TG100U. Well, I suppose it could be said I'm getting some results but as the waveform displayed on the 'scope shows the mark space ratio of the drive to the line output valve is all wrong, the negative going part of the waveform is too short. Something like a ratio of time off to time on should be 40:60. The first part of the scan comes from the reclaimed energy in scan coils.

Some alteration of the values in the line oscillator circuit might improve matters but I remember when a Thorn jellypot transformer was tried in a Philips 19TG108U a different line oscillator circuit was used.

Nevertheless, there is a healthy spark present on the EHT winding, quite flame like it's about 10KV or so.

A real hook-up job.

LOPTx 2

 The waveform at the control grid of the PL81:

LOPTx wfm

Till Eulenspiegel.

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Topic starter Posted : 03/12/2020 5:51 pm
Till Eulenspiegel
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More waveforms. The grid of the line oscillator V611a which is the triode section of an ECL80. The triode connected pentode section V611b serves as a sync pulse clipper.

LOPTx wfm 2

After inserting a 100pF capacitor between the anode of V611a and chassis the negative going part of the waveform is widened but the amplitude is reduced. >140V P - P.

LOPTx wfm 3

Till Eulenspiegel.

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Topic starter Posted : 04/12/2020 12:02 am
Till Eulenspiegel
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Posted by: @till

Some alteration of the values in the line oscillator circuit might improve matters but I remember when a Thorn jellypot transformer was tried in a Philips 19TG108U a different line oscillator circuit was used.

The circuit around V611 was rewired to resemble the line oscillator circuit of the Alba T909. Easy enough to do as the pentode section is already triode connected.

Alba T909

Till Eulenspiegel.

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Topic starter Posted : 05/12/2020 12:42 pm
Till Eulenspiegel
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Yesterdays work:  With the intent to measure the actual current flow in the line output valve a 1ohm resistor is inserted between the PL81's cathode and chassis.   Oscilloscope connected across the test resistor. Simple calculations have determined the mean current through the valve is about 180mA, too high for a PL81. Bear in mind the GEC 2040 transformer was designed to work with the PL509 which is designed to operate with a much higher cathode current than the PL81.

It is essential that some means of reducing the current in the PL81 will have to be found.

Philips 21TG100U wfms 1
Philips 21TG100U wfms 2

 Waveforms across the 1ohm resistor.

 The EHT delivered to the CRT final anode without a doubler is 9KV.

Philips 21TG100U 9KV

Even with only 9KV there is a bright raster on the screen.

Till Eulenspiegel.

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Topic starter Posted : 09/12/2020 10:41 am
irob2345
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I think it comes down to tuning the transformer for a current null in the PL81, as you would with a linearity control. You should be able to do this by varying the 100pf capacitor. That may reduce the EHT but a tripler will take care of that.

Do you have a large style radio tuning gang? Or a high voltage cap substitution box?

The aim is to tune for the 3rd or the 5th harmonic, the theory tells us.

If you are still pushing the PL81 too hard, you could add VDR-style regulation or increase the screen resistor.

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Posted : 09/12/2020 11:21 am
Till Eulenspiegel
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Certainly increasing the value of the screen grid resistor will be considered.   I do like the idea of of using a large tuning capacitor to adjust the transformer tuning, be it the third or fifth harmonic.

The truth is there's nothing particularly academic about this transformer substitution, just a matter of fiddling about with components and their values until the desired result is achieved.

Till Eulenspiegel.

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Topic starter Posted : 09/12/2020 12:36 pm
Till Eulenspiegel
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PL81 in distress!

Philips 21TG100U PL81

 Slightly modified drive waveform.

Philips 21TG100U wfms 3

 Till Eulenspiegel.

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Topic starter Posted : 09/12/2020 6:23 pm
Katie Bush
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@till Yikes! A glowing anode!!

I've heard of these valves getting hot enough to melt the glass, but by the look of that one, it looks like the valve would expire before it got the chance to melt the glass.

What's plan to calm it down?

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Posted : 09/12/2020 6:56 pm
Till Eulenspiegel
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Hi Marion,   maybe an increase in the value of the screen grid resistor might cool things down. Something to try out later today.  But the real cause of the over current problem is that the line transformer was designed for the great big PL509.

Till Eulenspiegel.

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Topic starter Posted : 09/12/2020 7:47 pm
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