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8th Feb 2009, 9:46 pm | #61 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Steve and Trevor and everyone.
Thank you for your kind encouragement (and Tas by Text Message). The scope has seen a lot of use with this set over the last couple of days Steve, and I know the problem is in the subcarrier regenerator circuit. A nice big healthy looking 4.43MHz chroma signal is reaching the chroma demodulators, but there is only a small amount (compared with that which the waveforms on the schematic suggest) of regenerated 4.43MHz reaching the 2 demodulators. There is a colour-killer Trevor, and it has been completely disabled by turning the manual control right down to one end - the result of this is that there is still no colour whatsoever on the screen - not even the slightest hint. The colour-killer is definitely not operating though, as it biases off the chroma to the demodulators etc. Now, surely there can't be too much that can go wrong with a simple APC loop, especially considering that there are only 20-ish components in the whole thing, but I'm struggling to get my head round what waveforms should be on what parts of the circuit (in a very fine-detail kind of way). The burst is being correctly gated and is being fed into the loop (formed of a "Pierce Oscillator" circuit with the 4.43MHz crystal, the 47pF cap, the "F" section of the PCF82, and what-have-you, and then what I guess could be called a "Steering" circuit with the "C" section of the same valve being used to alter the phase). However, whilst I'm sure all the clever Ekco designers new exactly how this should work and what voltages, waveforms etc. would cause what effects, I, being made of lesser stuff, have much less of an idea. Originally, before the valve holder was smashed, I'm sure I remember seeing signs of colour (rainbow stripes, but at least a start). Unfortunately, after replacing the valve holder, the PCF82, and the 47pF capacitor, I am seeing absolutely nothing. I replaced the 47pF cap with a 50pF of the same type (the original got broken in the transplant) and I'm assuming that 50pF is near-enough the 47pF original (but maybe I'm wrong), and I have checked and checked and checked that all the wiring is as it was before. (I took photos and drew diagrams before the holder was removed). The crystal is oscillating, and there is approx 60V of 4.43MHz at the Anode on the PCF82 "F" section, although the DC level (measured with a 10M impedance meter) is 350V (instead of 270V as shown on the schematic). Is this discrepancy due to the valve not conducting enough (as I assume)? And if it isn't why not when all the component values are fine and the valve is fine too (having been replaced twice just in case). Once again, my lack of valve-knowledge lets me down badly here. As mentioned before, the schematic for the modified PAL section is different to how the set is layed out, but only very subtly. And if I were a betting man, I would say that the diagram was wrong. (See the links below for the original schematic and the version that I have modified.) The modified version is also very much more like the original NTSC circuit. For the difference between the two PAL diagrams, look where the crystal is connected. I think I've waffled on enough about this for tonight, and I'll have another feckless prod about tomorrow evening and see if have more luck then. Thanks for all your assistance chaps. Kind regards. From Mike. Original PAL decoder diagram: http://www.oldtechnology.net/misc/Ek...r_Original.jpg Modified (by Mike) PAL decoder diagram: (As the set is wired now.) http://www.oldtechnology.net/misc/Ek...r_Modified.jpg Ekco CT100 (625 Line NTSC Version) Schematic: http://www.oldtechnology.net/misc/ek...0schematic.jpg
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G7TRF Last edited by Mikey405; 8th Feb 2009 at 10:08 pm. |
8th Feb 2009, 10:03 pm | #62 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
One thing I haven't yet tried is to see whether the APC loop is hard at one end (so to speak) and trying in vain to pull the phase in and never actually managing it. This can be tuned to reach a null-point and is detailed in the manual for the NTSC version. I will check that out tomorrow if I get a bit of time.
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8th Feb 2009, 10:09 pm | #63 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Mike.
60v on the anode on the pentode of the PCF82 seems low but 350vdc on the anode seems high you could be right about the conduction of the valve, crystals go duff as well and Iv'e seen then osc ok but give low o/p. The circuit looks a bit odd at first glance I'll get a look tomorrow. Trevor
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8th Feb 2009, 10:12 pm | #64 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Trevor.
Thanks for your reply. What I meant was, with my dodgy English, that there is 60-ish volts of AC on the Pentode section although the overall DC level (as measured by my DMM) is 350-ish volts. I've also changed the crystal... Thanks Trevor. Kind regards. From Mike.
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8th Feb 2009, 10:38 pm | #65 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Just one more thing...
On all the schematics, the PCF82 is V18. I guess that was fairly obvious on the decoder diagram, but not quite so obvious with the big schematic. Thanks everyone.
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8th Feb 2009, 10:39 pm | #66 | |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Quote:
I fully understood you, 60v P-P is low for the type of circuit and 350 v DC on the anode would not seem correct is the sine wave ok or clipped and have you checked the frequency? Trevor
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8th Feb 2009, 10:45 pm | #67 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Trevor.
The sine wave isn't clipped, and I guess I need to give my little-used frequency counter a bit of an outing tomorrow as I haven't checked the frequency yet. I'll let you know tomorrow. Thanks Trevor. Kind regards. From Mike.
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8th Feb 2009, 11:00 pm | #68 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Mike.
I must admit I would like a go at the set, too bad your so far away. I'll keep the brain on till tomorrow, bet you I don't get a wink of sleep tonight!! Cheers Trevor
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9th Feb 2009, 1:02 am | #69 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Firstly, try a new PCF82.
Check C130, C127, R138, R135, R137, C131, R134, C126. Get the DC conditions right and then we'll see about the AC side if it does not come in by itself. I'm too far away too, such is life. Cheers, Steve P.
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If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...? |
9th Feb 2009, 11:16 am | #70 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi Mike,
Fascinating thread and great work so far! As regards the decoder, Trevor is right and I would guess the subcarrier at V18b anode should be large (almost HT) the fault resulting in a correspondingly low subcarrier feed to the synchronous demodulators V301/302 i.e. a lot less than the 25v P-P shown. Has C128 been changed? as any lack of decoupling would reduce the signal. If so can the subcarrier be "peaked" by adjusting the anode coil of T302, perhaps the new valveholder has slightly different stray capacitance resulting in a tweak being necessary in this department. You could try disconnecting pin 1 of L34's can (C125) to remove the influence of the loop and see if the circuit "sings" happily by itself. Of course this would result in unlocked "running colour" but may help restore subcarrier amplitude. Great to read everyone's thoughts on this one and look forward to seeing the final result. Regards Eric. |
9th Feb 2009, 2:20 pm | #71 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Thanks Trevor, Steve and Eric. Thanks to all your suggestions I'm feeling much less gloomy and raring to go again. Trevor, you were mostly right about not sleeping - I did sleep but I was dreaming about the blasted thing. I think the mind just gets completely jaded after spending so many hours staring at the same set and, to coin an awful phrase, you end up not being able to see the wood for the trees. Thanks everyone for your advice and psychiatric therapy. Where do I send the cheque?
Kind regards. From Mike.
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10th Feb 2009, 5:26 pm | #72 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Well, a bit of a breakthrough at last. After spending the day fault finding and replacing a whole army of out-of-tolerance (or otherwise faulty) bits-and-bobs, I now have a fairly stable colour picture. I can't be sure, but this set seems to have been a bit of a dump for duff-valves - I've had to replace 5 so far, and they've often been peculiar brands too.
I still have work to do on the decoder to sort out the PAL switching etc. (Notice the peculiar purple "switching" line on the right of the picture, and the bad Hanover blinds). The level of 4.43 at the anode of PCF82 is still too low (giving weak colour) and consequently the DC level on the anode is way too high as well. The frame is still cramped at the top, there are no signals through the RF/IF section, and there is still a bit of an occasional "twitch" on the line. Anyway, thank you again everyone for all your kind comments and suggestions. Apologies for the rather ropey picture. Kind regards. From Mike.
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10th Feb 2009, 5:57 pm | #73 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Well done on the patience front!
I wonder why this set has so many bad valves, did it get used to death or was it put together using oddments? Dom |
10th Feb 2009, 6:16 pm | #74 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
There are two caps inside L34. They are C124 and C125. Change these and try turning L34.
Cheers, Steve P.
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10th Feb 2009, 6:53 pm | #75 | ||
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Re: Ekco CT101 is alive (Actually a CT100(M))
Quote:
Quote:
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10th Feb 2009, 9:33 pm | #76 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
That's excellent progress Mike. I can't wait for the next stage
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11th Feb 2009, 1:20 am | #77 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Mike,
Starting to look really good - most exciting!! A+ for persistence! Trevor. |
11th Feb 2009, 9:48 am | #78 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
You don't mean persistence of the phosphors on the screen do you.......I'll get me coat!
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11th Feb 2009, 2:31 pm | #79 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hello Mike,
Well done old bean, this is great progress, I hope you are feeling quite pleased with yourself as I certainly would be. I wonder what's wrong with the tuner/IF stages Phantom twiddler maybe! Keep up the good work! Cheers Lee
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11th Feb 2009, 5:00 pm | #80 |
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Re: Ekco CT101 is alive (Actually a CT100(M))
Hi.
Good work, I can see you getting a full time job fixing all valve Colour TV's Cheers Trevor
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