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Stella ST8617U  

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PYE625
(@pye625)
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I had trouble with getting the ion trap magnet to grip the neck of the CRT. Looking at the below first picture will reveal why. I sanded down the inner edge of the metal bracket and applied araldite to the broken portion of the flexible grip and inside the protecive sleeve. Hopefully, it will hold ok.

IMG 5576 50
IMG 5577 50

 

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

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Posted : 21/03/2020 9:26 am
PYE625
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On a more serious note, there may be problems with the LOPT. Whilst I have had the overwind with 25vdc flowing through it over the past 6 days, it has dried quite nicely. In use in the set, it remains cool and EHT is approximately normal all the time. However, the width begins to creep in after 10 mins or so until about half an inch gap is at both sides of the picture. Now, the set has had a couple of hours use, but intermittantly. Never been on for longer than a few mins each time.

Upon examination of the LOPT after about 15 mins use, the main winding is very hot. I suspect the insulation to the earthed core is not good, so I am passing a voltage through the entire winding (about 38 ohms in total) in the hope that any moisture here may be driven out, if indeed this is the problem. I intend to leave the current flowing for a few days.

As can be seen from the below pictures, the overwind is quite seperate from the main windings so these have always been cool during the time I was drying out the overwind.

IMG 5578 50
IMG 5579 50
IMG 5580 50

 

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

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Posted : 21/03/2020 4:17 pm
PYE625
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I have used the Taylor 130 to test the insulation and can detect no leakage at 450v. This of course does not mean that there is no leakage at much higher voltages. I will give the process of warming the windings a chance and see what happens. If it makes no difference, then I will need to begin checking to see if any other conditions are changing, such as the line drive waveform for instance. I must admit to jumping to the conclusion that it is leakage in the LOPT causing the problem, but I could well be wrong.

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

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Posted : 21/03/2020 9:14 pm
Till Eulenspiegel
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It may not be insulation failure that is causing your transformer characteristics to change over a given time.  Consider the possibility that the permeability of the ferrite core might be degraded with temperature rise. The condition is well known.

https://www.mag-inc.com/Design/Design-Guides/Designing-with-Magnetic-Cores-at-High-Temperatures

Till Eulenspiegel.

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Posted : 22/03/2020 9:56 am
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PYE625
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There are minor circuit changes (with reference to the Trader circuit in the library) as I mentioned earlier in the use of a PCF80 instead of an ECL80 for V7.

Aside from the base connection changes, R54 becomes 150k and is connected to the screen feed of the PL36, R59 is 180k and connected direct to HT instead of boost HT, and the grid resistor R60 of the PL36 is 2m2.

In this case, that 2m2 has risen to 3.5 megs. The PL36 data specifies 2m2 as the maximum limit for a grid resistor for line output applications only. I think it will be wise to replace this resistor as it is now rather high in value.

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

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Posted : 22/03/2020 11:44 am
PYE625
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In addition to R60 being high, R54 and 59 were also quite a bit higher than they should be. The effect of replacing these three resistors has brought the line hold lock quite a bit away from the control end stop. (The line hold control and R57 checked out ok).

When switched on from cold, the boost HT is near normal at approx. 565v. As the set warms, the boost will begin to slowly fall along with the width. Interestingly, it is now taking about 45 mins whereas it was around 10-15 mins before. Furthermore, the width only comes in about a quarter of an inch either side now. I didn't check the boost voltage before, so cannot say what it was when the width was less. Main HT is pretty much constant.

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

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Posted : 22/03/2020 8:39 pm
PYE625
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Bit by bit I am able to leave the set on for longer before the boost HT drops. The width is coming in less and takes longer to do so too.

I really don't quite understand the mechanism behind this, but at a guess I would say either the LOPT or the scan coils are slowly giving up moisture or somehow leakage is becoming less. Any thoughts on the matter anyone? 🤫 

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

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Posted : 23/03/2020 9:49 pm
Nuvistor
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I think your assumption is more than likely the correct one. David often suggests running a low current through the LOPTX windings for a few days to dry it out.

 

 

Frank

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Posted : 25/03/2020 9:44 am
Cathovisor
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@nuvistor

Andrew's been doing that, Frank. With pictures to prove the point. 

 

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Posted : 25/03/2020 9:48 am
Nuvistor
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@cathovisor

oh dear, missed that. Some LOPTX’S were not that reliable 50-60 years ago, it’s a marvel that many still work after being in damp storage for many years.

 

 

Frank

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Posted : 25/03/2020 10:04 am
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.

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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.

IMG 5585 1 50
IMG 5583 50
IMG 5584 50
IMG 5582 50

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

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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?

IMG 5588 50
IMG 5589 50
IMG 5587 50
IMG 5591 50

 

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

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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.

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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.

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Posted : 27/03/2020 7:51 pm
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