CRT refitted on the chassis. As the attachment shows the tube doesn't display a very bright picture. It's possible the tube might become brighter if the set is left working over a longer period of time. It's a low emission condition but it's not caused by the common problem of having a partially shorted heater.
Till Eulenspiegel.
A great improvement.
Frank
Resistor R91 is connected between the height control and the anode of the frame oscillator valve. This 680Kohm resistor had risen in value to 4.5Megohms.
After replacing the faulty part full picture height is restored. The CRT is holding up but for how long?
Till Eulenspiegel.
Hi Till, When you are driving the moisture out of the LOPTX how many volts / milliamps do you pass through the wingdings. I Take it you connect between the top cap of the boost diode and the output of the EHT overwind? Also how warm do you let it get and how may hours / days do you cook it for? I usually just leave the loptx in the airing cupboard for a week or two before starting work on the set but passing DC through the winding would seem a much more efficient way of doing things.
Dave
Hi Dave, Considering that the set was stored in a damp environment there was no way I'd test the set without making sure there was no moisture in the line output transformer windings.
A bench 0 - 30volt power supply unit was employed to carry out the drying out process. Connection is between the PL36 and EY86 anode connectors. DC resistance is 250 ohms. Power supply adjusted for 25volts. Power dissipated in the transformer windings will be 2.5watts. 30volts will give 3.6watts. So the windings will be quite warm but there is no possiblity of the tar insulation melting. Over the five days when work was being carried out on the chassis the transformer was at the same time allowed to dry out. It's most likely without this warming up procedure the transformer would have failed as soon as full voltages were developed in the windings. Glow discharge would have destroyed the EHT overwind.
Meanwhile I've got the no sound fault to sort out. The loudspeaker speech coil reads 5 ohms but it would seem the transformer secondary is open circuit. I don't think so, more likely it's a fault in the wiring to the various connectors on the chassis.
The cabinet is receiving attention. As the attachment shows it is clamped up after applying glue in the cabinet floor sections.
Till Eulenspiegel.
Thanks for the information. I will certainly try technique out in the future. Mean time I will watch the rest of the restoration on this set with interest.
The secondary of the sound output transformer is OK. The fault was caused by a bad solder joint on Tag Strip 1. In fact it looks as if it had never been soldered in the first place. But resoldering the wire to the tag didn't solve the no sound fault. Sure there is some activity from the loudspeaker when the tuner is rotated and the valves wiggled about in the sockets, but there's still sound.
First, the PCL83 audio amplifier and output valve will be replaced , it's valve that was well known for going faulty. Also, R26 the 2.7megohm anode load resistor of the triode section should be checked, it has the unusually high value because the HT comes from the boosted HT line.
The Stella ST1007 along with later models such as the ST1017 and Philips 17TG100U don't have the usual series diode sound interference limiter. Instead, the anode load resistor of the triode audio amplifier has a very high value and with the 560pF capacitor between the anode and chassis forms a limiter for interference spikes.
Till Eulenspiegel.
Also, R26 the 2.7megohm anode load resistor of the triode section should be checked
The 2.7Megohm anode load resistor R26 was found to be open circuit. The resistor has been replaced and the sound has now been restored but there are still a few minor faults to sort out.
Till Eulenspiegel.
Many of the preset controls at the rear of the chassis have gone high in value, for example the line hold control should be 250Kohms. It's value had risen to almost 1Megohm. This is the reason why the line hold control had such a wide frequency range. To improve matters a 390Kohm resistor has been connected across the track. Line hold adjustment is now much easier.
The attachment shows the excellent definition this set is capable of displaying.
The line structure is noticeable because the camera has captured only one picture field. Only one adjustment of the vision circuits was required, coil S17 the 38.15Mc/s sound trap.
Interlace is good despite having no interlace filter. The frame sync pulse from the sync separator is supplied to the frame oscillator via a differentiation capacitor rather than the usual practise of using an integrating circuit.
Till Eulenspiegel.
Till,
Your photo capturing both fields shows the good interlacing very well.
Peter
Hi Peter, achieving perfect interlace has always been a problem with 405 line TV. The story goes that Alan Blumlein wouldn't countenance such refinements such as equalising pulses. It should be up to good receiver design to achieve perfect interlacing.
it's interesting that some set designers could get away with very simple frame sync circuits and yet still achieve good interlace, the Stella ST1007 for example.
At the time I was ready to sign off the Dynatron TV27A "Fulmar" the interlace was perfect, but then it was discovered that the Aurora was sending equalising pulses with the frame sync. When the converter was switched back to the basic 405 sync pulse train the set displayed almost no interlace at all. It was discovered that the frame sync circuits utilised the trailing edge of the frame sync pulse and for some reason or other this arrangement didn't work well with the Aurora sync pulse, and yet the syncs look perfect on the oscilloscope. Something had to be done in order that the set will work on a standard system A signal. An interlace circuit similar to that use in the Pye FV1 and Pilot CV34 was employed. The results are perfect and good interlace is possible irrespective if the EQ pulses are present or not.
Till Eulenspiegel.
A new steel band has been made for the CRT.
Till Eulenspiegel.
Making good progress "Till"! Just a couple of points. You say the sound is nothing special, doesn't this set actually have two speakers in the set up? I thought it might be a bit better therefore. Also, what about those push buttons on the front, initially I thought they were sound tone adjustments but apparently they affect the picture, do they actually make any difference?
Hi Derren, The chassis is now reinstalled in the cabinet so now the front loudspeaker is reconnected which does improve the sound quality. The two loudspeakers are connected in series. Those buttons do make an effect on the picture quality. The "soft" button simply introduces a degree of peak white clipping from the interference limiter. There's a pre-set control under the chassis to adjust the clipping level. The "crisp" button alters the frequency response of the video amplifier. The sound "tone" button is a simple top cut setting. At the top is the on-off button. The switch had failed a long time ago and was replaced with choc block connector.
The rear pre-set controls still need occasional adjustment, the contrast control is the worst.
Till Eulenspiegel.
Very nice I've wondered why Philips/Stella made this as a "left handed" set, when for a long time most televisions had the controls at the right of the screen.
Hi Richard, the Stella ST1007 was developed from the 1957 models Philips 1768U and Stella ST8617. These receivers had quite deep cabinets so it was possible to fit the control unit well away from the CRT bulb. The ST1007 was introduced in late 1959 the same year the new slimline models with 110 degree CRTs were in the shops. The St1007 cabinet is much slimmer than it's predecessors but with still having the left side control unit and 90 degree CRT the cabinet had to be extended to accommodate the large tuner and controls unit.
The restoration job on this set was signed off today but we're still on the lookout for a good AW43-80 CRT.
Till Eulenspiegel.
The final task was to sort out the video "soft" button. The soft setting is actually an alternative interference limiter control adjustment. When the "soft" button Sw4 is depressed the rear control is switched out and the under chassis slider pre-set control R41 can be adjusted to set the desired amount of peak white clipping. The pre-set control had gone open circuit and was causing almost complete loss of picture. It has been replaced by three resistors which can provide three soft picture settings.
Till Eulenspiegel.
When one Considers the simple circuits employed in these receivers it's often remarked how Philips were able to produce such high quality 405 pictures. But there are quite a few circuit design subtleties to take into consideration. The 1768U series have a mean level vision AGC system. To compensate for variations in the AGC voltage due to picture content a clever and cheap auxiliary AGC system is employed.
Video Amplifier Circuit:
" V8 cathode is coupled via R48 to V7 cathode, providing a form of mean-level AGC. An increase in mean picture-level produces a greater D.C. positive voltage on the video grid, and raises the cathode voltage. This rise is appied to V7 cathode and reduces the gain of this stage. The reverse occurs when V8 cathode falls less positive. C42 is included to prevent video degeneration, allowing only slow voltage changes to be fed back."
There is a direct connection between the video amplifier anode to the cathode of the picture tube, thus the video DC component is maintained without the need for DC restoration. These circuits were carried forward into the 17TG100U and ST1017 110 degree CRT models.
