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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 16th Oct 2020, 5:31 pm   #1
Apfelmus
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Default Could that picture tube be OK?

Hi all,

Today I've been shown a projector TV-set from the 50ies (one of the Saba units with the Schmitt-Optics). The guy said it belonged to the owner who he bought the house from, and served as a decoration object. Some accessories are missing, like the keys and the remote control.

Since this guy is not an expert he boldly turned the set on and showed me how all (or most) tubes are glowing. Fortunately, no smoke or bad smell.

I could not find a cathode's glow in the neck of the picture tube. Looking through the optics all was dark. When he switched off the set there was a short flash of white light coming through the optics...

Could that mean the cathode and the picture tube is probably OK? No high voltage, but maybe some residues from other voltages that make this effect during swith off?

Any thoughts welcome!

cheers
Martin
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Old 16th Oct 2020, 5:46 pm   #2
beery
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Default Re: Could that picture tube be OK?

Hi Martin,
If it is a mullard MW6-2 crt then it might be quite hard to see the heater glow with the crt in the optical unit.
Open circuited heaters in CRTs are quite rare anyway.
With a projection set, a low emission CRT is more of a worry.
However if it is a mullard MW6-2 then at least they are still fairly common.

As for the symptom. Well assuming the CRT is actually working a bit, the lack of raster could be due to a scan protection circuit biasing the CRT off to prevent a line or spot from being burnt on to the screen. This could explain the white flashes on switch off and the CRT biasing changes. This would also suggest that EHT is present.

Cheers
Andy
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Old 16th Oct 2020, 6:04 pm   #3
Apfelmus
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Default Re: Could that picture tube be OK?

Hi Andy,

no its not a Mullard. Its a Valvo MW6-2, according to radiomuseum.org.
I have no idea how common these were...

cheers
Martin
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Old 16th Oct 2020, 6:27 pm   #4
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Default Re: Could that picture tube be OK?

Valvo like Mullard were Philips companies, the Valvo MW6-2 were possibly made by Philips Eindhoven, possibly the Mullard ones as well.
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Old 16th Oct 2020, 6:28 pm   #5
Maarten
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Default Re: Could that picture tube be OK?

Valvo = Philips
Mullard = Philips

Regardless of the brand on the label, they were sourced from the factory or factories that had the production capacity to make them at that moment in time. Small print indicates where a tube was made, just as with radio valves.

The high voltage unit might have also been made by Philips.

I would guess there are some deflection problems, likely related to paper capacitors being electrically leaky or short circuit and electrolytic capacitors being dried out or worn.
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Old 16th Oct 2020, 10:43 pm   #6
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Default Re: Could that picture tube be OK?

It's very important to go through the deflection circuits, protection and EHT unit and replace wax capacitors and wildly out of tolerance resistors. A fault in the protection and a time base can ruin the CRT.
Also leaky caps in the EHT unit can ruin the oil filled transformer taking out the internal caps due to incorrect oscillator frequency (1khz) or waveform shape.
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Old 17th Oct 2020, 6:43 am   #7
Apfelmus
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Default Re: Could that picture tube be OK?

I have some experience with TV sets. My question here is if the set I'm offered might have a functional CRT. The rest I can manage...

I was restoring an est-german TV-set some months ago (the Rembrandt). The story is here - in german, alas:
https://www.wumpus-gollum-forum.de/f...=46&thread=117

cheers
Martin
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Old 17th Oct 2020, 6:51 am   #8
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Default Re: Could that picture tube be OK?

Quote:
Originally Posted by murphyv310 View Post
...due to incorrect oscillator frequency (1khz) or waveform shape.
That reminds me, I actually noticed a whining noise some seconds after the vendor switched on the set. Could have been 1kHz or thereabouts. It was only audible for some seconds, changing pitch (frequency up) then vanished.

That might indicate there is indeed some EHT, but maybe not the full 25kV.

chees
Martin
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Old 17th Oct 2020, 7:12 am   #9
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Default Re: Could that picture tube be OK?

Quote:
Originally Posted by Apfelmus View Post
I My question here is if the set I'm offered might have a functional CRT.
cheers
Martin
Unfortunately that question can’t be answered, it’s always one of the unknowns until you can test the CRT on a CRT tester or get the rest of the set working to a point were you can see a picture of some form.
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Old 17th Oct 2020, 9:59 am   #10
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Default Re: Could that picture tube be OK?

https://www.vintage-radio.net/forum/...d.php?t=120026

You may find the link of help. The test recommended by Mullard/Philips was quite a simple one. Switch on, allow the receiver to warm up then switch off.

A decaying 'blob' should be seen on the CRT face as the EHT fades. This usually indicates that the protection circuit has come into operation due to timebase failure, usually the frame.

You have to have at least a third of the scan to override the protection circuit and leaky caps as suggested will cause this.

The screen is very easily burned with just a seconds flash or line/frame collapse.

The whine you heard is the ringing choke 25kv EHT unit. It sounds like a groan as it warms up. Hope you get it working.

If you operate the MW6-2 outside it's optical unit, keep the brightness down to a very low level, just enough to see what is happening. The face of the CRT can emit low level X Rays if handled carelessly but perfectly safe if the precautions are followed. If you need to remove the EHT connection, warm the glass cup insulator first. I have seen them snapped off! John.
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Old 21st Oct 2020, 1:25 pm   #11
Apfelmus
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Default Re: Could that picture tube be OK?

Thank you all for your help.

Its always a hard battle with my more reasonable self to decide whether to put such a box in my house. In this case I think I will not go further... the price asked by the seller is too high anyway. The cabinet has some flaws (missing parts), remote control is missing and all the controls are totally stiff. Not that big of an issue each, but all in all it means a lot of work to get it in shape.

And its hard to put on my bench...

cheers
Martin
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