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Old 4th Nov 2019, 1:59 pm   #1
ben
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Default GRUNDIG C8200 CUC60 sparkgap noise

Just dragged out this set which has not seen use for about twelve years, in the (vain) hope of using it in an event this week!

Unfortunately, on powerup there is what sounds like continuous low sparking at the sparkgap. I have replaced tired looking caps in the SMPS as a precaution - no joy - and have measured the 'A' line (PSU HT out) which is rock steady at 150v.

I have also cleaned around the CRT and the neck PCB, which was grotty. I suppose some grime in the socket might be to blame?

There is a full raster so would I be right in thinking the tripler is likely Ok? I have only ever had them fail s/c. Would appreciate any pointers before digging into the EHT stage.
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Old 4th Nov 2019, 2:50 pm   #2
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Hi
I think this uses a CRT base with integral focus spark gap. If that's the case then it's probably got green corrosion inside. You can try cleaning, but a look in your spares box would be a better bet. I'd not be inclined to suspect the tripler, but check if there's a high value resistor south of the focus pot.
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Old 4th Nov 2019, 3:19 pm   #3
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Thanks Glyn. Will desolder the tube socket and see what lurks within!

I have just changed the line o/p snubber capacitor (FKP type, 0.01Uf) for a newer MMKP type but no change in the symptoms, so I will probably put the original back.
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Old 4th Nov 2019, 5:53 pm   #4
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

An open circuit somewhere to the aquadag or tube frame might cause discharge as well.
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Old 4th Nov 2019, 10:49 pm   #5
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Now you may be onto something there Maarten. There was a loose spring which was responsible for tensioning the metal strap around the rear of the CRT. I did replace it where I thought it came from, but possibly not correctly. Will look again.
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Old 5th Nov 2019, 2:30 am   #6
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Just spent a few more hours on the set. The aquadag ground strap is a tight fit and the connection from it to the neck pcb is good.

Dismantled and thoroughly cleaned the base socket. What a wonderful design that is - it appears to be ceramic with three bolts holding it all together. I have never seen on so well made. Clearly meant to be disassembled and repaired if necessary. There were some specs of dirt in there near some of the pins but the actual sparkgap area was clean. No corrosion.

Sadly, that had no effect the fizzing still goes on and I am starting to wonder if there might be a problem in the tube.
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Old 5th Nov 2019, 11:00 am   #7
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Tried applying heat from a hairdryer to the line o/p stage and also replaced the FKP original snubber cap - no joy. I vaguely recall having this exact problem with a 14" Sanyo set of similar (early 80s ) vintage about 20 years ago, and am racking my brains as to what the cause was.

Here is a brief video
showing the symptoms. As you can see there are horizontal bands of defocused interference.

I attach diagram of the line stage in case it jogs anyone's memory!
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Old 5th Nov 2019, 11:12 am   #8
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Well, it's definitely to do with the focus. As you can't see into the ceramic base, it's just possible the discharge is happening beghind the tube base insulator. Carefully prise it off and have a look there. Clean it then (again carefully!) put the CRT base back witout the insulator in place. Ive had them do this and appear innocent and this will prove it. If no good then it's tme to turn the lights out and look in the CRT neck which may well confirm your worst fears.
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Old 5th Nov 2019, 1:03 pm   #9
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

I think the procedure for HV shorts in the tube, is to ground the electrodes and then turn on.
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Old 5th Nov 2019, 1:31 pm   #10
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

I attempted to remove the tube base insulator and whilst it did waggle slightly, was resistant to coming off. So for the moment I let it be as I don't want to break the tube! I may try again later with a little warmth to try and coax it off.

There does not appear to be any sparking within the tube, but will do a more thorough test tonight. I will also try and locate a spare K536 tripler on the off chance there is some kind of problem in the focus ladder behind the symptoms. I see Donberg have this one and this one.
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Old 5th Nov 2019, 1:47 pm   #11
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

That's a good way to go - there's not much else left!
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Old 5th Nov 2019, 2:37 pm   #12
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Desoldered the tripler.
It's a siemens TVK 196-27, with B92945 - K7154 - M243 printed below.

Diemen
equivalent for the basic 196-27 appears to be HRT227BP, but listed below there is a TVK19627M243 (which my eagle eye notes is the last number of the original part ref). That is given as HRT 235BP.

Since the schematics do not pop up when I click them (probably as the items are discontinued ) I cannot verify which would be right. I will ask my local supplier, who often has old stock, if he has something suitable...

Incidentally, would anyone care to hazard a guess as to the symptoms if C537 (see diagram in a previous post above) were to fail in some way?
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Old 6th Nov 2019, 2:55 am   #13
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

My man at the component place managed to come up with an HRT 225BP for me. There is a difference between the 227 (the correct part) in that the resistor in series with the focus pot is 295 MOhm instead of 253 MOhm (schematics attached). Presumably this means that the focus pot might not have enough travel. I might give it a try, unless anyone advises against it.

Thanks for the help and responses so far.
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Old 6th Nov 2019, 11:35 am   #14
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Go for it - what's the worst that could happen?
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Old 6th Nov 2019, 12:21 pm   #15
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

Hi Glyn, Fitted it and on powerup there was no more hissing, so that I suppose proves the other tripler's guilt!
As I suspected, the focus looks a bit poor. I have not been able to check that fully with a video source as there seems to be a problem with the tuner. On a snowy raster it didn't look too sharp. Will continue checking later on.

There also seems to be a slight smell of ozone now, but that might just be me.
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Old 7th Nov 2019, 12:40 am   #16
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

I hope you don't smell of ozone, Ben!
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Old 7th Nov 2019, 2:37 am   #17
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Default Re: GRUNDIG C8200 CUC60 sparkgap noise

More like eau de fluxite!

Messed about with the tuner and got a picture. Slightly under focused with the pot at the end of its travel. Not great, but usable for what I need the set for this weekend, and in the longer term I can keep my eyes out for the correct tripler. I found three later Grundig ones in a junk box, one of which I may be able to modify.

There does seem to be a slight whiff of ozone coming off it but no sign of any arcing or other distress. Will keep under observation.
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