Is there another tap you could use for the boost diode, a little further down the winding? That would be like changing from 4th gear (where the poor little PL81 is struggling - its getter flash doesn't look great either) to 3rd gear!
Check the winding resistances and see if you can move the yoke down to the bottom and put the damper cathode on the tap vacated by the yoke....
Or substitute a tougher bottle such as a 6CM5?
The GEC 2040 CTV line transformer is a very simple thing. The primary consists of the connections for the line output valve, the booster diode and the boost capacitor. The secondary winding has two taps.
I returned to the workshop yesterday evening to try out various value resistors for the screen grid resistor for the PL81. The original factory value is 2.2Kohms, a 1.8K resistor in series with the existing resistor had little effect on the cathode current. Next, a 10K resistor was fitted, this reduced the cathode current considerably, the 'scope waveform shows 0.22volts P - P = 220mA cathode current. As the mark space ratio of the waveform is 50:50 the mean value is 110mA, still too high for the PL81. The EHT is down to 6KV, was 8.5KV.
Fit a more powerful line output valve? Unfortunately, there is no provision for an octal or magnoval valveholder. Strange that Philips chose the PL81 for this 110* CRT receiver because the previous 90* sets employed the PL36/25E5. Possibly the efficiency of the 110* line output transformer is much better than the transformer in the previous models? Also, because the 110* CRTs have a 28.5mm neck compared with the 38mm of earlier 90* tubes there will be a useful reduction of demands from the line timebase. The 110* scan coils magnetic fields are closer to the action.
Till Eulenspiegel.
More progress, and good progress at that.
An old BRC tripler was hooked up and the EHT at the CRT final anode connector was found to be 13KV. A little low for this 21" set. Next, the PL81 screen grid resistor was reduced from 10Kohms to 4Kohms, the existing 2.2K resistor + 1.8K. A bright but narrow raster on the screen, EHT now 18KV, perhaps that's a bit high for the AW53-88? The GEC 2040 transformer has a 200pF 8KV capacitor connected to the boost diode cathode to ground, except in this set it isn't. The capacitor was duly connected into the circuit and the result is almost full width, another capacitor in parallel will almost certainly bring about full width.
However, all these initiatives have increased the PL81 cathode current, but not by too much, just over 230mA with a drive waveform with a 20:50 duty cycle. The PL81 will not last long under these operating conditions and will have to be replaced with something a bit more beefy, either the PL36 which was around when the set was made, or better still the PL504. Actually, I have some late production PL36 valves which have the internal appearance of a PL504.
Not a pretty sight but all will be tidied up when the new circuits are working properly.
Till Eulenspiegel.
NEWS FLASH! How about this?
OK, it's not perfect but at least it's showing pictures. Needs more picture width.
Adding another 180pF 8KV capacitor between the boost diode and chassis increases the width of the picture but the trade off is a reduction of EHT, down to 13KV.
Till Eulenspiegel.
Fit a more powerful line output valve? Unfortunately, there is no provision for an octal or magnoval valveholder.
Till Eulenspiegel.
I wonder if a 21B6 would take a bit more cathode current than the PL81......
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, compared with the PL81/21A6 the 21B6 has slightly better ratings. From Frank's tube data website:
Outside France I don't think the Mazda-Belvu 21B6 and the 6.3volt version 6DR6 was used in TV receivers, although the 21B6 was often marketed as a replacement for the PL81.
The obstacle to achieving the results I'm after is the matching of the scanning coils to the secondary of the donor transformer. Sure, the additional capacitor has increased the width the EHT is much reduced. The ideal EHT voltage for the 21" tube is 16KV.
Till Eulenspiegel.
I just noticed that your yoke is series connected. All Philips 110 degree yokes we saw here were parallel connected. What is the DC resistance of the line coils in your yoke?
I may have a more suitable transformer. I picked up some NOS Rola parts about a year or so back, on Ebay. They are all different and intended for a number of 90 degree, late 50s TVs. Would you like me to investigate? I don't have any data on them, just the models of TV they are suitable for. Probably cost about AU$30 to mail one to you.
The line output substitution project restarts today. Results so far, the picture is bright even with the low EHT of 13KV but we really need 16KV for the 21" CRT. The PL81 line output valve is clearly being over run. The PL81A has identical electrical characteristics and was designed as a more rugged valve for use in transportable TV receivers. The 21B6 isn't too common this side of the Channel. The octal base PL36 or the more exotic PL300 are being considered. The latter was developed by Mazda France and was used in early sixties colour TV receivers. Not forgetting the PL504, still plenty of those about.
Till Eulenspiegel.
I guess the good old PL36 would be the more common choice and in keeping with the set. Another thing to bear in mind is any additional voltage drop along the heater chain of course, but that can easily be catered for I would imagine.
To understand the black art of electronics is to understand witchcraft. Andrew.
Hi Andrew, the truth is even a PL36 will be having a tough time in this set. The 1ohm resistor in the cathode circuit of the line output valve is used to monitor the current through the PL81 and the 'scope shows 300mV for 60microseconds of the total line duration of 98.8microseconds. Under these conditions even the PL36 will be over-run. I'm going to abandon the idea of using the GEC 2040 line output transformer and look for an alternative transformer for this set. A jellypot was tried out in the 19TG108U. The 19TG108 was introduced in 1961 and has similar chassis to the 21TG100U. CRT apart, the main difference is the metal sub-assemblies are replaced with printed circuit boards.
Till Eulenspiegel.
Things are never as easy as they would sometimes first appear....
Perhaps it is best to try to select a more suited transformer.
To understand the black art of electronics is to understand witchcraft. Andrew.
News Flash! Found a Philips 210 mono TV line output transformer today. Good chance it might work in the 21TG100U. Add a width stabilising loop or simply reduce the width and EHT with series resistors to the anode of the boost diode. That's how they did it in the very early versions of the Pye model 11. Ekco did the same.
Till Eulenspiegel.
Go the extra mile and fit stabilisation if you can. Makes for a much nicer performing TV.
Go the extra mile and fit stabilisation if you can. Makes for a much nicer performing TV.
Stabilising circuit employed in mid to late 50s Peto-Scott TVs.
In some sets the thermionic diode was replaced by an OA81 germanium diode. The Peto-Scott TV1422 was possibly one of the earliest TV sets to have a stabilised line timebase.
Later designs employed a voltage dependent resistor to rectify the waveform from the line output transformer. The Ferguson 546T was released in late 1959 employed a Mullard VV111 VDR.
Till Eulenspiegel
Interesting circuit of the Peto-Scott. Using a diode section of the versatile triple diode and triode P/E/UABC80 valve usually found in AM/FM radio set's of the era.
To understand the black art of electronics is to understand witchcraft. Andrew.
Other than it's usual purpose as an FM radio demodulator Peto-Scott used the PABC80 for many other applications in their TV sets.
Details of the Philips 21TG210 line output transformer. Connected up for 405 only operation this should be a fine replacement line OP transformer for the 21TG100U.
Till Eulenspiegel.
Fitted the 210 line transformer today. To be honest the results are not much better than the GEC 2040 transformer. The cathode current of the PL81 during the 60microsecond conduction time is down to 200mA so at least the valve isn't glowing red hot, but it is still being overrun. Plenty EHT, a scary 24KV! The picture has only just enough width and only just enough is not good enough. It is becoming clear that the problem is matching the existing scanning coils to the substitution transformers. For the time being I'll keep faith with this transformer, maybe search for a different set of scanning coils.
Till Eulenspiegel.
One of the problems I'm experiencing with this set is achieving good line synchronising. It is possible to lock the picture into the centre of the screen but the verticals are wobbly. However, if the line hold control is adjusted to show the sync pulse about the centre of the screen the verticals are perfect. The fault has to be something to do with the feedback pulse from the line output transformer. To initiate the flyback the grid of the line oscillator valve receives from a pulse supplied from the line output transformer. In the Philips 21TG100 the pulse comes from the boost capacitor as it does in the earlier Stella ST8617U and Philips 1786U models.
Till Eulenspiegel.
Still having problems with the line output transformer substitution. Under consideration is a separate line oscillator circuit. The line oscillator from the Bush TV105 is very simple and was used to good effect in the Dynatron TV27 "Fulmar" TV set.
The Peto-Scott TV1726 of 1958 employs a variation of the stabilised line TB circuit shown in a previous post.
Till Eulenspiegel.
