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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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19th Mar 2019, 9:53 pm | #1 |
Heptode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
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Sony broadcast monitor degaussing.
Hi all,
I hope this is in the right section, mods please move if necessary. I have a Sony broadcast monitor that I occasionally drag out for testing vcr’s etc. Been renovating a Philips N1700 for a friend and we did a composite video mod on it the other day so out came the monitor. Picture was there but colour blotches all over, was a bit confused at first then it dawned on me. Checked where the monitor has been sitting for the past few months - right next to a pair of bookshelf speakers that have faulty tweeters. Left it until tonight and as I’m lacking a degaussing wand just spent an hour with a powerful permanent magnet and an electric drill. It’s not perfect but just passable for testing purposes. Beware what you keep near CRT’s! Cheers, Kev |
19th Mar 2019, 11:36 pm | #2 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
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Re: Sony broadcast monitor degaussing.
Does its own degassing circuit work? sometimes the thermistor or posistor will go OC during operation leaving the CRT magnetised.
Usually speakers in enclosures do not magnetise the tube to an extent that the internal degassing coils can't deal with.
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19th Mar 2019, 11:46 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
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Re: Sony broadcast monitor degaussing.
You need to allow the thermistor to fully cool down after moving the set.
If you have a pistol type quick warm up soldering iron it will work like a wand. |
20th Mar 2019, 1:44 am | #4 |
Heptode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
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Re: Sony broadcast monitor degaussing.
Could well be a fault with the degaussing circuit then, I’ll take another look at it! Unfortunately I don’t have one of the pistol soldering guns or anything else that can generate a decent magnetic field - the drill was the best thing to hand.
I know a lot of PC CRT monitors had a degauss button or menu option and would have thought broadcasts monitors would too but maybe not, or do I have an oddball one?!? |
20th Mar 2019, 10:11 am | #5 |
Heptode
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 636
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Re: Sony broadcast monitor degaussing.
What model is it? Sony broadcast monitors usually make quite a loud 'thump' when the degauss coils are activated and there's no mistaking they are working - the picture being grossly distorted too when they are working being another clue.
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20th Mar 2019, 11:46 am | #6 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
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Re: Sony broadcast monitor degaussing.
If that's the one I think it is, the degauss energy actually comes from the line output stage IIRC. It can't come from the mains because it has the option to run from batteries. As a result, it's pretty silent in operation.
Then again, the one I'm thinking of has a front panel degauss button. So perhaps this is a different one - it's really hard to be sure from the photo. |
20th Mar 2019, 12:47 pm | #7 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Sony broadcast monitor degaussing.
Just a guess, if it's a PVM-9L2 Elektrotanya has the service manual which gives the details of the degauss driver circuitry.
Lawrence. |
20th Mar 2019, 1:30 pm | #8 | |
Heptode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
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Re: Sony broadcast monitor degaussing.
Quote:
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20th Mar 2019, 1:57 pm | #9 |
Dekatron
Join Date: Jul 2007
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Re: Sony broadcast monitor degaussing.
No, that one has too many buttons...
My first thought was PVM-9042 or 9045, but those have 8 buttons (including a manual degauss) and a grey power switch. This monitor appears to have only 6 buttons, and there is no manual degauss button. Plus the power switch is green. That makes it more likely a PVM-9044: https://www.adverts.ie/film-cameras/...d-case/8879189 All nice monitors, especially the HR versions. EDIT: Looking again, it's not the HR version. So, I'll revise my guess. The standard-res version of the PVM-9044 is the PVM-9041: https://vandiemenbroadcast.co.uk/Sony-PVM9041-Monitor Last edited by mhennessy; 20th Mar 2019 at 2:06 pm. |
20th Mar 2019, 2:36 pm | #10 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Sony broadcast monitor degaussing.
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20th Mar 2019, 6:21 pm | #11 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,191
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Re: Sony broadcast monitor degaussing.
I have a PVM-9044 (lovely little monitor). The service manual (which I think I got from Elektrotanya) shows that the degaussing coil is fed from the 120V DC line (obtained from the line ouput transformer) by a rather odd transistor/thyristor circuit. This is on the 'P board' (the board containing the line output transformer, located under the CRT).
Since the monitor works in other respects the 120V line must be present. So perhaps that transistor and thyristor is a place to start looking. On the other hand I don't remember my monitor making any deguassing 'clonk' at power-on. |
20th Mar 2019, 6:57 pm | #12 |
Dekatron
Join Date: Jul 2007
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Re: Sony broadcast monitor degaussing.
It works by "ringing" the inductance of the degauss coils with a capacitor. The thyristor provides a pulse to start it off after a short delay.
The later 9042/5 use a very similar arrangement, but have an additional relay to allow the process to be started again by the front panel button. That one is fed from 400V derived from the collector of the line output transistor rather than the 120V line. Next time I take one apart, I'll see about capturing the waveform on my DSO. Neat, because there's no need for a posistor, with all their problems. But plenty else to go wrong... |
20th Mar 2019, 7:02 pm | #13 |
Dekatron
Join Date: Jun 2015
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Re: Sony broadcast monitor degaussing.
I thought it had to 'ring' the coils. You need an oscillating, decaying field which is what ringing would produce. And there is no oscillator circuit.
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20th Mar 2019, 7:24 pm | #14 |
Dekatron
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Re: Sony broadcast monitor degaussing.
Could be faulty posistor or even a dry joint on same.
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20th Mar 2019, 7:48 pm | #15 |
Dekatron
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Re: Sony broadcast monitor degaussing.
Ah, but there is no posistor
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20th Mar 2019, 7:49 pm | #16 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
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Re: Sony broadcast monitor degaussing.
Connecting an already-charged capacitor across a coil will produce ringing. If there is a big resistor from HT+ to the junction of L and C, then earthing that point will set it ringing.
..... Are you sure it is a thyristor and not a triac? Or is there a diode back-to-back with it? (There's no need for a way to control the current in the opposite direction to the switch.)
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20th Mar 2019, 8:57 pm | #17 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Sony broadcast monitor degaussing.
If this link works it should take you to the relevant page showing the degauss circuit under discussion:
https://archive.org/details/sony_PVM...nual/page/n155 Lawrence. |
20th Mar 2019, 9:54 pm | #18 |
Heptode
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
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Re: Sony broadcast monitor degaussing.
Hi all,
Model is PVM-9041QM. Thanks to Lawrence for the link to the manual, the degauss circuit does look rather odd. I will investigate further in the future, it’s acceptable enough for now but I agree there must be a fault somewhere. Thanks for everybody’s input on this one Cheers, Kev |
21st Mar 2019, 2:07 am | #19 |
Dekatron
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Re: Sony broadcast monitor degaussing.
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22nd Mar 2019, 12:56 pm | #20 |
Octode
Join Date: Apr 2007
Location: Hampshire, UK.
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Re: Sony broadcast monitor degaussing.
Its not unknown for the scan coils to slip back down the tube neck on these, messing up the purity. Check that the end of the coils is firmly up against the bowl of the tube and that there's no movement there. If the yoke has moved, undo the clamp and push the coils as far forward as they will go, the purity then should sort itself out.
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