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Ekco T344 plus another one  

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PYE625
(@pye625)
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I would be surprised too, perhaps one way to test it would be to temporarily unsolder the connection from the overwind and keep the loose end well away from anything. Then check the boost HT to note any significant difference. Remember that there may be a slight rIse anyway due to there now being no beam current taken by the CRT, as of course the EHT will be absent.

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

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Posted : 01/05/2020 7:55 pm
helloekco
(@helloekco)
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Here is mine:

505C4E89 E5AC 4D64 B313 1023D01928BB
1F65AD3F 3986 4D91 B17F D39B663A5E3E

Apologies if those come out oriented wrongly, I’m posting this from my phone and Apple have strange ideas about image file metadata compared to everyone else it seems.

@pye625 unfortunately disconnecting the overwind looks tricky, because as can be seen the connector is glued into the shroud!

Whilst on the subject of disconnecting things to see the effect on boost HT, I tried disconnecting the first anode of the CRT, leaving nothing but the deflection coils and flyback blanking to the boost HT. This resulted in no EHT, EHT rectifier heater supply, or boost voltage at all! I don’t quite understand that, I’m possibly being dim...

I’ve also tried running with the EHT rectifier top cap disconnected, this results in the boost actually reducing from 465v to 432v. 

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Posted : 01/05/2020 9:42 pm
Nuvistor
(@nuvistor)
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The case is not an original Ekco, perhaps it’s been rebuilt in the past, or is it a substitute transformer?

 

Frank

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Posted : 01/05/2020 10:03 pm
PYE625
(@pye625)
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In that case, you could disconnect the overwind at the other end, where it joins the junction of the 30P4 top cap lead at the LOPT. It would be safer to do this perhaps anyway. Not only will it eliminate suspect leakage on the perspex, it will eliminate the overwind from suspicion too.

Looking at your photo's above, they would seem to show a cloudy area extending close from the U26 top cap connection point towards a screw thread hole within the perspex (right hand side in particular). If there are metal screws in here, then it could be wise to replace them with nylon screws.

 

 

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

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Posted : 01/05/2020 10:18 pm
freya
(@freya)
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I do have some spare lopts with U shaped perspex housings which were the revised versions from 1958. But you would need to know the SA number to match up.

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Posted : 02/05/2020 10:14 am
helloekco liked
helloekco
(@helloekco)
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Posted by: @nuvistor

The case is not an original Ekco, perhaps it’s been rebuilt in the past, or is it a substitute transformer?

 

Thanks, that's interesting to know - so if it wasn't a correct match, that could possibly explain the low boost HT, particularly as EHT itself seems to be probably correct.

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Posted : 02/05/2020 3:01 pm
helloekco
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Posted by: @pye625

In that case, you could disconnect the overwind at the other end, where it joins the junction of the 30P4 top cap lead at the LOPT. It would be safer to do this perhaps anyway. Not only will it eliminate suspect leakage on the perspex, it will eliminate the overwind from suspicion too.

Looking at your photo's above, they would seem to show a cloudy area extending close from the U26 top cap connection point towards a screw thread hole within the perspex (right hand side in particular). If there are metal screws in here, then it could be wise to replace them with nylon screws.

 

 

Thanks, I will try that.

I think, am not 100% certain, that the cloudiness is all some sort of glue. As far as I've seen the screws in the shroud are all nylon.

 

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Posted : 02/05/2020 3:02 pm
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PYE625
(@pye625)
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I must admit, if it were me, I would have had the LOPT completely out of the set for a good careful clean and inspection. If need be, the perspex can be replaced, but it may be easier to drill a couple of large holes either side of the U26 top cap connection. This would help to eliminate any internal tracking of the high voltage towards the outer edges. If indeed that is what is actually happening of course.

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

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Posted : 02/05/2020 5:49 pm
helloekco
(@helloekco)
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@pye625 I removed the LOPT in its frame and unsoldered the connection to the 30P4 top cap as suggested, but as with other things I tried it only lowered the boost voltage if anything (the line timebase would not run judging by the sound of it). It was a real swine to solder back together too 😩 . I think it's probably safe to say there isn't a fault with the LOPT.

Back on the subject of the RF valves, I found in my stock a few more 30L15s and PCF80s and so I tried a few of these and got a slight improvement to the picture. Also, what I did discover was that the voltage at the anode of V1 was pretty low at 115v versus the 165v specified. Remembering what @freya said about the anode resistors getting cooked by incorrectly placed valves, I thought this was probably the cause, and so I managed with great difficulty to replace the resistor without dismantling or removing the tuner box. It was indeed a little high in value, but not all that much, at about 5.8k. When I tested the voltage again after the replacement, it was up slightly at about 125v - obviously still pretty low. I think I have a slightly perceptible improvement in contrast now though, with the changed valves too.

Photo 03 05 2020, 22 55 35

This picture of the test card was taken with the brightness turned down to suit the light in the room, and at that level the contrast looks acceptable but still not great. When watching actual TV, it's a bit tiring on the eye:

Photo 03 05 2020, 22 57 08

And looking at the grey scale test card on the Hedghog it can be seen that the black isn't very:

Photo 03 05 2020, 22 54 12

It is possible to turn the brightness up so that that white is very bright now, since I rejuvenated the CRT, but then the black is mid-grey.

I decided it was past time that I got the scope out to see what's going on, and hooked up to the CRT cathode showing the above test card, this is what I got (excuse my scope's poor definition):

Photo 03 05 2020, 23 16 49

Now, this is on AC at a scale of 5v/cm, so it looks as though the video waveform is nowhere near as large in magnitude as it ought to be (about 30V to its peak I believe?). It looks to me as though the sync pulse part of that is about correct at around 10v but that it is the video part that's lacking. This was the largest magnitude I could get the video waveform by adjusting the contrast control, any higher and it appears to stay the same or start to compress whilst the pulse waveform grows in magnitude.

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Posted : 04/05/2020 10:44 pm
PYE625
(@pye625)
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Yes, I can see what you mean about the contrast and the video waveform. It almost looks as if the interference limiter is clipping the video.... as if the white spot limiter control is turned all the way up.

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

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Posted : 05/05/2020 6:49 am
Nuvistor
(@nuvistor)
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Do you get the same waveform out of the detector or is that correct?

Frank

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Posted : 05/05/2020 7:16 am
PYE625
(@pye625)
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I don't know whether this will be of any help, but as a quick rough guide, I have made some measurements on my T370. I am using a Hedghog on CH9 with a -6db attenuator in the RF output.

With a low contrast setting (just before loss of sync), the video output measured at the CRT itself is around 20v, increasing to 80v with contrast set to maximum (last photo). I am using a x10 probe with the 'scope set to 2v per division AC.

IMG 5660 50
IMG 5662 50
IMG 5661 50

I can take some measurements at the video detector diode if you wish too, to maybe give a rough idea of the level expected?

 

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

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Posted : 05/05/2020 8:03 pm
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PYE625
(@pye625)
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The video voltage at the detector is a factor of 10 times less than that at the CRT, and inverted of course. Contrast set to maximum.

IMG 5663 50

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

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Posted : 05/05/2020 8:44 pm
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Jayceebee
(@jayceebee)
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Does this set have a vision interference limiter control? If it does rotate it end to end and see if it has any effect.

John.

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Posted : 05/05/2020 9:11 pm
crustytv
(@crustytv)
Vrat Founder Admin

It does John, data is in the library if you want to download it. There's ERT, Trader and Manufacturers. 👍 

vi1

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Posted : 05/05/2020 9:16 pm
PYE625
(@pye625)
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I tried to see the effect of the limiter on mine, but it is very subtle only slightly reducing the peak whites (as it is meant to of course).

Now, if the fault does turn out to be in the final video stage, then V9 (6D2/EB91) would be a possible suspect along with V8 and associated circuitry of course. But looking at the circuit of V9A, I wonder what the effect would be of a partial heater to cathode short for instance, or C65 being leaky.

Before guessing too much though, let's see what the actual waveform coming from the detector is like on the T344.

 

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

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Posted : 05/05/2020 9:23 pm
Jayceebee
(@jayceebee)
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I did see it in the library Chris but haven’t got access to my laptop and my works iPad which I’m using at the moment is blocking downloads. Looks like severe clipping of the video to me.

John.

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Posted : 05/05/2020 9:27 pm
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helloekco
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@pye625 Thanks, yes - that's immensely helpful!

@nuvistor Here is the waveform at the detector, the first one with the contrast just above where the sync is lost and the second with the contrast on maximum:

Photo 05 05 2020, 22 33 25
Photo 05 05 2020, 22 33 49

Those were taken with the scope on 0.5v/cm and the probe still on 1x, so at 10x the deflection of the ones at the CRT.

I had a play with the interference limiter and it makes almost no difference across its range - right at the maximum setting it very slightly reduces the peak of the wave at the CRT, but it's barely perceptible.

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Posted : 05/05/2020 11:14 pm
helloekco
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...These are taken at the CRT on 5v/cm, the first one with minimum contrast before breakup, the second one with maximum:

Photo 05 05 2020, 22 38 01
Photo 05 05 2020, 22 37 38
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Posted : 05/05/2020 11:21 pm
Nuvistor
(@nuvistor)
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The output from the detector is flattening the +ve part of the video when pushed to deliver a larger output, causing the video to be crushed but the sync pulses left intact. Possible causes, video detector diode, or clipping in the grid cathode circuit. I presume the IF amp voltage are correct.

I would check the detector diode, see what others think before delving into the IF can.

Edit. Check the G1 components in the video output valve first, perhaps easier to get at than the diode. 

Frank

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Posted : 06/05/2020 6:54 am
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