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Old 18th Nov 2023, 4:04 pm   #1
matthewhouse
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Default Cossor 945

Hello all, I've been away for a long while due to being busy with work and other things, but I'm desperate to get back into vintage electronics. An old work colleague left this at my house on Thursday which was a good excuse to start again. I got right onto replacing the wax caps, set the mains tap and powered via a lamp limiter. Nothing bad happened so I shoved it on the mains.

I could draw a spark of about 3mm off the top cap of the eht rectifier with an insulated screwdriver and after playing with the user controls I had first light.

I found the Aurora and connected it, nothing received, but after cleaning the aerial socket and the tuner contacts I had the test tone OK and something of a test card.

There wasn't much in the way of height so I replaced the resistors in the frame stage that had substantially increased in resistance. This now allowed a full picture but there is foldover.

I'm concerned the LOPT is failing, as when the set has been on for a few minutes the picture size increases and and the image fades away... I need to take some voltage readings before condemning parts though.
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Old 18th Nov 2023, 5:07 pm   #2
mark pirate
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Default Re: Cossor 945

Before condemning the LOPT, change the EHT rectifier valve, I had the same problem with one of my sets, could also be a weak boost diode.


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Old 18th Nov 2023, 5:21 pm   #3
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Default Re: Cossor 945

It's very likely that the line transformer has absorbed moisture over the years and could be destroyed if the set is powered up without thoroughly drying it out first, best done by passing limited DC through it for a few hours. The symptom you describe is a classic sign of this.
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Old 18th Nov 2023, 5:29 pm   #4
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Default Re: Cossor 945

I will do this before proceeding further.
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Old 21st Nov 2023, 9:29 pm   #5
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Default Re: Cossor 945

After ensuring everything was thoroughly warm and dry, there were no improvements. A new EHT rectifier solved the problem though, and the set has been running over an hour without the image fading, the brightness control also now doesn't change the picture size. I don't know why I didn't think of this myself as 2 of the TVs here also had that problem due to aging EHT rectifiers. It's amazing what you forget when you move away from something for such a long time.

I did have one small panic moment, when moving the top cap wire from the rectifier the other end became detached from the LOPT! The wire is restrained by looping through a hole in the paxolin end but that's not much restraint. Anyway I managed to find the end, solder the wire back on and remelt the wax around the joint to hold it again. It's also tied more firmly to the paxolin now... I don't fancy having to rewind the LOPT, though I do want to have a go at one at some point in the future if I find a telly with a bad one in it.

I've replaced 2 resistors in the frame circuit that had increased in value and this has greatly reduced the folding at the top of the picture. I did swap the sync separator and frame output valve which has increased the height a bit, but both valves test as 'due for replacement' on the valve tester. I've got a load of NOS TV valves but none of those so have ordered a pair.

Now the set has been running a while things seem quite stable, no pops, arcs, hisses or other bad noises. I did clean the EHT valve holder and wiring as there was some corona discharge once the good rectifier was fitted.
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Old 22nd Nov 2023, 4:40 pm   #6
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Default Re: Cossor 945

What a difference a couple of new ECL80s made.
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Old 23rd Nov 2023, 2:26 pm   #7
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Default Re: Cossor 945

Yes, the old ECL80s don't have to be too far gone to start causing issues.

Normally when the EHT rectifier valves started to go down in emission back in the day, the blooming effect would be pretty much constant from switch on, possibly improving slightly as the line output stage and the valve itself warmed up and not the other way round as you've just found. What I think was happening in your case was that with the tube being dormant for for so long and now gradually improving over time as the set is run, that with the gradually increasing beam current, the flagging EHT rectifier would be more stressed over time. There would also likely to be the effect of the video output valve improving over time, also adding to the increasing beam current. The transformer would certainly have had some dampness absorbed into its windings (unless it had been kept in a constantly warm dry atmosphere over the last few decades), so it would have been no bad thing to warm it up with a bit of DC through it. It's certainly looking good now.
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Old 23rd Nov 2023, 8:25 pm   #8
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Default Re: Cossor 945

Glad you got it sorted, I did think the EHT rectifier was duff, as I have had the same problem in the past, probably why the set was retired all those years ago!


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Old 24th Nov 2023, 2:28 am   #9
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Default Re: Cossor 945

Thanks for the information, it has been working faultlessly. I say has because it has stopped again.

No sound other than motorboating, no image. There is something of a raster though and most controls still do what they are supposed to

I've measured all the valve voltages and compared them with the service data. All the voltages are close except the grid2 of v1 common IF amp is 180V data says 100V. Some of the connecting wires to L1 and L3 were very close to shorting to the chassis I have spaced them away now but wonder if a short there has caused some damage.

And v8 sync separator grid 1 measures -28V data says very low reading-, and the anode measures 112V data says 80V.

Are these voltage differences likely to be the cause of the issues? I assume the anode voltage is so high is because the grid voltage is so negative and so little current is flowing through the valve?

What is likely to cause such a high negative voltage at the grid? There are a number of diodes linked to this part of the circuit but I haven't tested any of those yet. I'm guessing I should be checking R59 & C55, C23 & R21?

Looking at the circuit again should I be looking at V1 common IF amp section first, as I have both problems with the sound and sync sections?

Any help appreciated especially if I'm going completely in the wrong direction...
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Old 25th Nov 2023, 1:20 am   #10
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Default Re: Cossor 945

I've got the TV working again this evening, which is a relief. A resistor lead had broken off its mounting tag. There are still a few niggling problems that need working through but at least it's basically working again.
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Old 27th Nov 2023, 9:29 am   #11
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Default Re: Cossor 945

One Off Topic post moved to this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=88917
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