Thanks John, I do enjoy documenting it as best as I can here.
To understand the black art of electronics is to understand witchcraft. Andrew.
The electronics side of things seem to be pretty much wrapped up now, unless another fault occurs. The next step is to attend to the rather tired cabinet, but I will await warmer weather for this.
I am extremely happy with the set and it can give a very good picture indeed. A very capable design from Mr Philips.
To understand the black art of electronics is to understand witchcraft. Andrew.
These excellent sets show just how good 405 line pictures can be. There's been a fair bit of discussion on the other two forums about the wisdom of restarting television in 1946 with the pre-war TV standard.
The Stella ST8617 displays pictures with perfect interlace which is achieved without any frame sync pulse shaping circuits. Instead of the usual integrator the frame sync pulse is supplied to the oscillator through a differentiator capacitor.
The video DC component is maintained right up to the CRT cathode.
The feedback between the cathodes of the video amplifier and the final IF amplifier correct any short comings in the mean level AGC.
Many of the circuits in the ST8617 were carried over to the later 110 degree CRT models.
Till Eulenspiegel.
Another minor fault solved. This time, it involved the AGC, or rather the apparent lack of it. All this time I have been using an 18db attenuator on the output of the aurora. Other sets were fine with only a 6db attenuator, but this set produced bad sound on vision and even vision on sound, with a strong signal.
I thought I would measure the AGC line and found it always remained at about 0v regardless of signal strength. According to the data in the Trader sheet, there is a delayed AGC produced by using the suppressor grid of V6 (ECL80) as a diode. Even though the contrast control is working fine with a negative voltage from the grid of V6, the AGC voltage at the junction of R49 and 50 remained at zero volts. I would expect this to be able to become more negative as the signal strength increased. R43, 48 and 49 all proved ok and there were no shorts on the AGC line, so it could only mean that the voltage was being held in the positive direction by the suppressor grid of V6. I tried another ECL80 and the fault was cured. The AGC line now becomes more negative according to signal strength, typically down to about -5v with a strong signal. The result is that the set now behaves itself with varying levels of signal input.
So, why should an apparently good ECL80 give problems here? Well it can only, I assume, boil down to internal leakage resistance within the valve. The "faulty" valve clearly has a few megohms of resistance at the suppressor grid (pin7) to various other electrodes. This internal resistance must be sufficient to hinder the pulling down of the AGC voltage. A new ECL80 does not have any measurable leakage resistance, so the delayed AGC (via the fairly high value resistors) can now work.
I will keep the "faulty" ECL80 as I am sure it will work fine in another less critical circuit, e.g. as an audio amplifier.
To understand the black art of electronics is to understand witchcraft. Andrew.
The next step is to attend to the rather tired cabinet, but I will await warmer weather for this.
Did I say that?
Yes well, the warmer weather is here but I am not in the mood.
Next excuse?
Errrm....oh yes, there is an R in the month. ?
To understand the black art of electronics is to understand witchcraft. Andrew.
- It will be cold on Monday, the varnish won’t dry. ?
Frank
Possibly the worst excuses I've ever heard!
...Although, saying that, I have used the first one myself, on more than one occasion!
Regards,
Lloyd
Haha, true, but I really need to be in the right mood for cabinet re-work.
To understand the black art of electronics is to understand witchcraft. Andrew.
No more excuses, I think it is high time to attend to the cabinet.
The gold paint around the inner front edge is perhaps the only area in good condition and I will mask this off carefully before I apply the stripper. The cabinet, as mentioned previously, is in a poor cosmetic condition but is sound otherwise. I can see no other option but to remove what is left of the battered and worn finish. Then to clean with white spirit to remove all traces of stripper. It then may or may not need wood dye, if so, only the darker border around the front will be attempted.
It is hardly Chippendale furniture, and was made to a price to perform a basic function. But it will be good to make it presentable again.
To understand the black art of electronics is to understand witchcraft. Andrew.
One thing I will not be doing however, is to convert the cabinet into a fish tank or drinks cabinet. I simply cannot see the attraction in this despicable practise....
To understand the black art of electronics is to understand witchcraft. Andrew.
The cabinet fittings, Stella badge and the implosion guard glass etc have all been removed and the cabinet stripped of the old finish. I have maganed to maintain the gold painted inlay around the glass area, but have removed the dark brown painted border around it. I may well repaint it in the same way as it was done before, but it looks quite nice without it.
As it is now, freshly cleaned with white spirit to remove all traces of the stripper.
To understand the black art of electronics is to understand witchcraft. Andrew.
Goodness me, that looks amazing already.
John.
Thanks John, I must admit it has come up better than expected. Most of the scratches were only into the original finish and the veneer is unscathed.
The first application of Liberon finishing oil has now been applied.
To understand the black art of electronics is to understand witchcraft. Andrew.
The Stella badge itself was next to receive some attention as it would seem to be made of brass and simply dunked in brown paint. Most of which flaked off as I rubbed it. I cleaned it up and sprayed it with black paint. When dry, I rubbed lightly to reveal the letters and then applied a coat of laquer. Although nothing like it's original look, it has taken on a rather classy style if I may say so myself. ?
Quite what it will look like on the set remains to be seen. If it looks naff, I can easily return it to a plain brown colour.
To understand the black art of electronics is to understand witchcraft. Andrew.
Well, it's different.
To understand the black art of electronics is to understand witchcraft. Andrew.
Oh John.... well I'm going to get a paracetomol as I have a banging headache from the fumes of paint stripper and white spirit.
I tell you what, why not have a pint of Stella? ?
To understand the black art of electronics is to understand witchcraft. Andrew.
The lettering on the Stella badge reminds me of how it was on a tape recorder I once had (the on-speaker badge) but that was flat, and red on grey. This looks very stylish, and, whereas-I like to see things as they originally were, I don't mind the odd improvement.
"Yes, a bit of wet string may get you a good TV signal here on four channels, but you'll have to dry it out to get Channel 5!"
The cabinet has come up lovely! The Stella badge looks good too 🙂
Regards,
Lloyd
