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Stella ST8617U

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Cathovisor
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Agree with you 100% on that one Frank - it never ceases to amaze me how many people resurrect a 1950s television with relatively little fuss.

 
Posted : 25/03/2020 10:36 am
PYE625
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Whether this is a good idea or not, I don't know, but I have decided to leave the set running so it is just ticking over with reduced mains via my lamp limiter of 2x 200w bulbs in parallel. This I can do for several hours each evening. HT is around 150v, Boost is about 440v. The voltages and width are stable at this reduced level and hopefully, everything will dry out....if indeed moisture is the problem still.

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To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 25/03/2020 5:17 pm
PYE625
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Soon after my last post above, I decided to remove the LOPT and leave it with current passing through it as before. This is through the complete main winding and not the overwind. The overwind has already been dried previously.

Now for the interesting bit.... there are a couple of series 2k2 resistors, R65 and R66 with C49 (100pf) in the middle. This is across the winding D in the trader circuit, and between the top caps of the PL36 and the PY81. One of these resistors was reading 1k8 so I decided to replace them both. You can see the new ones in my rather poor first photo below. The paxolin looks blackened in the photo, but it must be a shadow as in reality, it is perfectly fine.

Now, I have replaced the LOPT into the set and it has been on all afternoon since 3pm. It is still on now, and boost HT and width are normal. I felt the LOPT (set OFF of course !) and it is now only just warm to the touch. Previously, it was very hot and could barely be touched. So, either replacement of the low value resistor has solved the problem or the LOPT has finally dried out. What do you think is the answer?

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To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/03/2020 7:17 pm
PYE625
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Here is a better picture of R65 and R66....

IMG 5593

 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/03/2020 7:29 pm
PYE625
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R65,66 and C49 look to be a snubbing or damping network. I don't fully understand their exact purpose, I have to admit. Someone with a better knowledge may advise. But I must add that the resistor that measured low fell to bits when I removed it and it had been soldered with a big ball of solder right up to the body. It is possible that it had been damaged when originally fitted.

I could be barking up the wrong tree, and the values described above are none too critical. It could simply be that the LOPT has dried out now, but whatever it is, the fault of reduced width and overheating of the LOPT have now vanished.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 27/03/2020 7:51 pm
Nuvistor
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I don’t know their purpose, snubber of some sort, new parts fitted so that’s done.

Frank

 
Posted : 28/03/2020 9:57 am
PYE625
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Now this fault is out of the way, I can concentrate on the more minor things like replacing the rest of the Philips capacitors not directly in the higher voltage sections. Things like AGC decoupling, etc. I would also like to look at the earlier repair of a preset soldered across the height control. Obviously, the height control must have gone open circuit sometime in the distant past.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 28/03/2020 10:47 am
PYE625
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Regarding the height control, it is indeed very high in value. About 20 megs in fact, although at each end stop, there is continuity. This gave me an idea. The vision interference limiter control is 2 megs and would do very nicely as a replacement. The interference limiter is not really used nowadays, so I was thinking that as there is still continuity at each end of the height control pot, it could still be used in the limiter position. So, I swapped the pots over. This was done by desoldering the wires and untwisting the small lugs that secure each control in place. With reference to the trader circuit, you can see that it is R38 and there must still be 2 meg across the pot. I achieved this by fitting a 2.2 meg resistor.

IMG 5594 50

The height control now works fine again, and the vision interfernce limiter does too. I am assuming this is because of the very high impedance via a 10 meg resistor to the suppressor grid of the video output valve. The voltage is still being varied even though the pot is very high value.

IMG 5597

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 28/03/2020 3:45 pm
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PYE625
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Posted by: @nuvistor

 David often suggests running a low current through the LOPTX windings for a few days to dry it out.

I think that is an extremely wise and valid suggestion. Furthermore, I will say it is folly to run up an old LOPT in a set without doing so first. It may well be fine if it is known the set has been stored in perfect conditions, but this is rare. As in this case, it was known that the set had been stored for a very long time in a barn. These are hardly ideal conditions and it is obvious that the humidity will be higher than in a centrally heated house. Again it should be obvious that anything not hermetically sealed will absorb moisture over a period of time. Even a Murphy LOPT apparently sealed in an oil filled can will absorb a degree of moisture. So really, it should come as no surprise that the LOPT in this set has taken a good while to dry out properly.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 28/03/2020 9:11 pm
PYE625
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A fault has occured just as I was sitting in front of the chassis minding my own buisness. The sound just went. Simple as that. Probing on the wiper of the volume control produced a buzz, so the audio power output was ok. I did a quick voltage check and found practically nothing on the PCL83 triode anode. This supply comes down the line from the boost HT that is fed to the height control, and that was fine. I found R80, a 2.7meg anode feed resistor that was competely open circuit. How can a lovely beefy resistor like this fail? Well, it did and it was replaced. I now have sound again !

Here is the actual resistor followed by readings either side. There should be a bit more than 0.2vdc !

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To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 29/03/2020 4:15 pm
Cathovisor
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A fine multimeter you have there, Sir... ? 

 
Posted : 29/03/2020 5:44 pm
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Till Eulenspiegel
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The self same resistor failed in the Stella ST1007 TV set that appeared in this Forum last year.   The high value anode load resistor and the capacitor shunted across PCL83 triode together function as a noise limiter.

 Till Eulenspiegel.

 
Posted : 29/03/2020 5:55 pm
Till Eulenspiegel
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Posted by: @cathovisor

A fine multimeter you have there, Sir...  

It is indeed. I've been using a similar one for many years now.

 

 
Posted : 29/03/2020 5:57 pm
Cathovisor
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I have the identical model to Andrew; I must prune the unruly population of AVOmeters I have to just one good model 7, to save me having to think when using old service sheets prepared using one. It would be nice to bring my much-loved Amprobe analogue meter back into service too; it was my very first multimeter, a Christmas present from my parents when I was a teenager.

One of these, in fact. https://www.recycledgoods.com/amprobe-instrument-am-2a-ac-dc-multimeter-voltmeter/

 
Posted : 29/03/2020 6:05 pm
Katie Bush
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@till At about £350 a piece, a very fine meter indeed - Certainly makes my 117 look like a kid's toy! . . . I still use my AVO 8 Mk.V, or will when I get active again, and correct me if I'm wrong, but didn't you give me an old AVO a couple of years ago? Someone did, with leads that have turned to goo over the years - could be a 7 or early 8, but without looking it out, I'm not sure.

Must admit, I like to use a meter as close to that recommended by the set makers, then you should get close what the makers determine to be correct, V & A, in live circuits.

 
Posted : 29/03/2020 8:10 pm
PYE625
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The under chassis looks a bit different now since I first started. Black capacitors have been replaced by red. The only Mullard mustard colour cap I used was for C25, a 0.18 uf just visible below the black sound output transformer on the left. I was lucky with the main smoothing can by having a suitable replacement that had lugs to fit the cut-out in the chassis. Only two holes had to be drilled for the clamp.

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To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 30/03/2020 6:37 pm
PYE625
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A minor fault that was solved this evening was slightly ragged verticals. It was rather similar to the effect of a weak signal, even though the signal is strong. I had just accepted this as a characteristic, but seeing a screen photo of David's Stella ST1007 with an almost identical chassis, made me realise there may be a fault with mine.

The first thing I planned to do was to check valve voltages in the stages of video amp, sync separator, line clipper and oscillator. Then if all was ok, get the 'scope out to have a look at the sync pulses.

One voltage that seemed very wrong was the screen voltage on the sync separator, an ECL80 V6b on the Trader diagram. The anode voltage was a fair bit higher than it should be too. R44 is meant to be 100k and the screen voltage in the region of 26 volts. I was getting only 3 volts. R44 turned out to have gone high to the tune of nearly 500k. I replaced it. This brought the screen and anode voltage to near normal. Not only that, the ragged verticals have now gone. I can assume that this would cause the line sync pulses to have been weaker than normal.

Below is the circuit, the resistor R44, then a before and after screen shot that if you look carefully, you can make out the slight line tearing in the first picture of the test card. (Last photo looks like no interlace, but is due to speed of the camera shutter).

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To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 31/03/2020 7:52 pm
Katie Bush
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I wonder, back in the day, how many people would have noticed, or even bothered about, those ragged verticals. I remember seeing sets with pictures like that, in my granddad's workshop. It seems a pity, but as always, "You only repair what the customer wants repaired!" - "You can advise them, but they rarely ever heed the advice".

I guess this where we mirror the car restorers, in as much as most restored cars are better now than they ever were when in regular use. Most people would put up with a below par performance for the sake of saving a shilling!

 
Posted : 31/03/2020 8:43 pm
PYE625
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I agree completely. If I had not seen another set to compare it to, I would not have given it a thought to be quite honest. On a normal programme, it was hardly noticeable anyway.

But it is good to have things like this to fault-find and see the effects. I bet there are many resistors that are out of spec., but until a fault becomes apparent, it is hardly worth worrying about. Besides, some need to be a very long way out before a symptom shows itself.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 31/03/2020 9:11 pm
Jayceebee
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As this set has flywheel sync slight tearing as you say Marion could go unnoticed by the customer, no so with direct sync. With some sets even the very slightest tearing producing awful sounds from the LOPT similar to arcing.

Great write up by the way Andrew.

John.

 
Posted : 31/03/2020 10:21 pm
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