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Ekco T344F

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Nuvistor
(@nuvistor)
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R45 has noted earlier was a common problem, check the resistor in series with it, 330 ohm?

The F series use flywheel sync and that problem on the left is folder over of the line scan.   It can occur when the horizontal hold is not set correctly or a fault in the sync circuit. Faults in the Line and scanning circuits can also cause it but I would check sync circuits and adjustments first.

 

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Posted : 09/11/2021 9:02 am
helloekco
(@helloekco)
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Hi @nuvistor - thanks, yes the 330R resistor is fine.

Fold-over, yes of course - I'd forgotten about that. I don't think it's the hold control so I will check over the sync circuits.

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Topic starter Posted : 09/11/2021 11:43 pm
helloekco
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Having got past the initial "get it running" stage and given the above fault, I started looking more closely at the timebase panel. I didn't mention above, but someone had made a small modification in the past to the set, consisting of a couple of extra resistors soldered to the track side of the PCB. I'd previously ignored them, thinking they were just a hack to work around a PCB that had gone bad, however finding that the voltage at the cathode of V17B, the line oscillator stage 1 triode, was way too high I realised that the circuit had in fact been messed around with. I discovered that the modifications consisted of deleting R97 (leaving V17B disconnected at its cathode - hence the high voltage reading) and bypassing V17B with a 100K (I think) resistor from its anode to chassis; and of an 80K resistor from the anode of sync separator V19 to the anode of the line oscillator V17A.

Does anyone have an idea as to why these modifications might have been made? It looks to me as though it's basically bypassing the ratio discriminator for the line sync.

I've currently undone these modifications and the line timebase seems to run way too slowly. The only picture I can obtain like this is doubled up horizontally as though the timebase is running at about half speed (it sounds that way too).

I'm thinking that perhaps the high valued resistors between the sync separator and the line timebase may be high or open circuit? Would this be enough of a problem for someone to basically hack a repair like this?

I've also realised, just as of tonight, that the manufacturer's sheet I have for the T344F timebase is wrong! There must have been a revision to the flywheel sync at some point, and it seems that my set matches the one in the TV Servicing data, given to me by someone in my previous thread on the T344 a while back. It had me completely stumped as to why the horizontal hold on mine was in the cathode circuit of the line oscillator V17A, rather than the anode of V17B.

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Topic starter Posted : 26/11/2021 1:00 am
Nuvistor
(@nuvistor)
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@helloekco 

Were they trying to make it direct sync? The original circuit worked so I would take the set back to the original circuit if it’s not too much work then fault find. See what other members advice is first.

Another member may be able to look at the circuit, I won’t be able until early next week.

 

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Posted : 26/11/2021 9:46 am
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Till Eulenspiegel
(@till)
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Should be a simple task to change to direct sync.   Short circuit the grid of the control valve V20A direct to chassis and connect a small value capacitor, say 10pF, between the anode of the sync separator valve V19 and the anode of V20B.

Some alteration to the values of the anode load resistors of V20A and associated components might be called for in order to correct the line timebase frequency. The line timebase frequency is adjusted by the variable resistor in the cathode circuit of V20B.

Ekco T344F Line Sync

Till Eulenspiegel.

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Posted : 26/11/2021 1:24 pm
freya
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I would revert it back to flywheel sync, much more fun finding out what they did and where.

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Posted : 26/11/2021 1:50 pm
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Till Eulenspiegel
(@till)
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Back in the days of 405 line TV I did prefer in some situations receivers with flywheel sync, not so much for ragged vertical caused by noisy signals but rather to smooth up the picture when receiving ghosted images.  The simple Ekco flywheel system gave excellent results.    

Till Eulenspiegel.

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Posted : 26/11/2021 2:27 pm
helloekco liked
helloekco
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Posted by: @nuvistor

@helloekco 

Were they trying to make it direct sync? The original circuit worked so I would take the set back to the original circuit if it’s not too much work then fault find. See what other members advice is first.

 

Thanks @nuvistor, that was my thought too; the question I have is, why would someone want to ditch the flywheel sync?

I definitely intend to put it back to standard. There is a slight horizontal 'smearing' of images where there are blacks and whites adjacent to one another, and the horizontal hold control seems to have become more critical too. It's currently difficult to get straight verticals without any kinks. My thought was, with these modifications, I've no idea whether they're contributing to the problem, and it's going to be harder to fault find with the set not as the manufacturer intended. Besides, it's supposed to be a flywheel set, so that's what I want it to be!

Thanks for the diagram @till - that's the one I have and the one that matches my PCB.

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Topic starter Posted : 27/11/2021 2:28 pm
Till Eulenspiegel
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If an oscilloscope is available check the quality of the video waveform at the grid of the video amplifier valve, there shouldn't be any sync crushing. Also, the shape of the sync pulses at the anode of the sync separator valve. The negative going leading edges should be reasonably sharp, the positive going trailing edges will in these simple circuits have a less rapid rise time. It's the leading edge of the pulse that matters.

Till Eulenspiegel.

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Posted : 27/11/2021 3:12 pm
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freya
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Ekco had problems with the 34 series from launch, a variety of official mods were made to both the flywheel and non versions. Reading the Service sheet news from Ekco to the service departments they list one common fault on the flywheel versions as having one side of the sync transformer having shorted turns causing jitter and raggedness.

I will try and scan the sheets for this detailing all the mods for the line time base section

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Posted : 27/11/2021 3:22 pm
helloekco
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@freya Thanks, that would be very helpful!

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Topic starter Posted : 27/11/2021 3:28 pm
freya
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@helloekco 

Just emailing it all to to Chris.

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Posted : 27/11/2021 3:58 pm
crustytv
(@crustytv)
Vrat Founder Admin

You don't need to e-mail it, there is the upload service, please read the service section sticky. The service has been available since May of 2020.

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Posted : 27/11/2021 4:04 pm
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freya
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Must have missed that bit, just tried it out so ignore if you have already attended to the previously emailed versions.

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Posted : 27/11/2021 5:20 pm
Till Eulenspiegel
(@till)
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Circuit of the IF stages and video amplifier, Ekco T344F. 

Ekco T344F Video ccts

Till Eulenspiegel.

 

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Posted : 27/11/2021 5:48 pm
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crustytv
(@crustytv)
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EKCO service updates provided by Stephen, are now in the Data Library, I converted them to a single PDF.

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Posted : 27/11/2021 9:36 pm
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helloekco
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Thanks for the service updates @freya / @crustytv, those are really interesting. I need to have another look at those (it was last weekend when I skimmed them) but there is actually something in there that might be helpful for my other T344 - it exhibits this problem that I'd assumed might be to do with my digibox, described in those documents as something like "AGC flutter", it seems to be always when a bright static screen such as the end of an advert appears.

I did get a bit further with the T344F; referring to Spreadbury's chapter on flywheel sync I found that the waveform from each discriminator diode appears to be the same magnitude and in sync, but I also found that shorting out the grid to the control triode V20A brings the line frequency back up to speed. There is about -5-6V on the grid normally. I have tried disconnecting the flyback feed into the sync circuit and it appears to make little difference, the grid voltage rising a mere 1V or so to about -4.9V. When disconnected, this flyback feed does have a waveform that is in sync with the horizontal sync at tge discriminator but it does seem to be very small in magnitude, and there is a DC voltage of about -1.5V (if I remember coorrectly, as I didn't take notes).

I am wondering from this whether the fault is in the components around the flyback feed.

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Topic starter Posted : 04/12/2021 5:02 pm
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Till Eulenspiegel
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The fact the line speed returns to the correct frequency when the control valve's grid is shorted to ground indicates an unbalance in the discriminator. The positive going pulse from the line output transformer is integrated by C108 (0.002uF) and the series resistor R82 (330Kohms). The capacitor is connected to the junction of the two diodes, MR2 and 3.  A sawtooth like waveform is present at that point in the circuit. In a normally working receiver the sync pulses will align with the rapid changing part of the sawtooth waveform. The tips of the bi-phase sync pulses should then be on the zero crossing point of the short duration part of the reference waveform.   With no signals supplied to the receiver and the contrast turned fully down to display a blank raster with no noise, note if the line frequency changes with and without the short-circuit to the control valve grid.   

Till Eulenspiegel.

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Posted : 04/12/2021 5:38 pm
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