| Re: Reviewing a cheap CRT degaussing wand [message #404132 is a reply to message #404116] |
Tue, 12 January 2021 20:48   |
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Originally posted by: Trevor Wilson
On 13/01/2021 7:48 am, Rayner Lucas wrote:
> Hi all. This post is a review of the cheap "green stick" CRT degaussing
> wands, as I haven't found much discussion of them and some other
> repairers of vintage monitors may find it useful.
>
> CRT degaussing tools seem to be hard to find these days. Occasional used
> ones come up for sale, and there are still some to be found in the US.
> However, in the UK they're near-unobtainable. The main source seems to
> be eBay sellers in China, all of whom are selling the same type: a wand-
> style degausser in a green plastic shell. So, having a couple of CRTs
> with purity problems, I bought one to see if it would do any good.
>
> The wand cost about 15 GBP and arrived within a couple of weeks. There
> was no documentation included, leaving me with just the specs given in
> the eBay listing, which read:
> -Relative magnetic field: 70MT
> -Load current: 1A
> -Working hours: 20 seconds
> -Power: 220V
> -Specifications: About 31*31*200(mm)
>
> The outer shell is some soft semi-translucent plastic (polythene?), and
> feels extremely cheap. A momentary switch pokes out of the top of the
> casing (a momentary switch is good, as it stops me accidentally leaving
> the coil energised).
>
> The wand came with a moulded two-prong plug, which I had to cut off to
> fit a UK plug. And... I have never seen mains cable that thin before. It
> is, at least, double insulated, but the conductors are at most 28AWG and
> possibly even thinner (it's hard to measure stranded cable, but the
> diameter is somewhere around 0.25mm to 0.35mm). The strain relief clamp
> in the UK plug wouldn't even hold the cable until I wrapped some extra
> plastic around it. Oof. Looking at the ampacity ratings on the Wikipedia
> article for American Wire Gauge, that cable must be very close to, if
> not exceeding, its recommended current rating. It feels worryingly
> plausible that someone, somewhere made the calculation "it's fine, if
> they push the button for too long the coil will burn out before the
> cable insulation melts".
>
> However, the tool does what it's supposed to and noticeably reduced the
> blotches visible on the CRT display. I used the standard technique of
> powering the coil from a couple of metres away, bringing it up to the
> CRT face, circling it around a couple of times, then smoothly backing
> away two or three metres before switching off again. I definitely
> recommend sticking to the stated maximum of 20 seconds continuous
> operation and letting the wand cool fully before using it again. The
> heat seems to take a few seconds to conduct to the outside of the
> casing, so it's not until after you've switched it off that you feel how
> warm it's really getting.
>
> I popped the end cap off the casing to take a look inside, but haven't
> disassembled it further. Strain relief is just a knot in the mains
> cable. I don't see any current limiting apart from the coil itself. The
> coil is wrapped around a core of steel plates, and seems to have some
> more plastic insulation around it. The non-business end of the coil
> seems to have some copper mesh shielding. Hooking the whole thing up to
> a component tester, coil resistance measures around 140 ohms, with an
> inductance of 320mH.
>
> In summary,
>
> Pros:
> - Cheap.
> - Does what it's supposed to.
>
> Cons:
> - Not particularly sturdy.
> - Probably not the safest thing ever, use with caution.
>
> If there were better-quality tools available, I would definitely buy
> those instead. But there weren't, and this one did at least provide the
> functionality I needed.
>
> HTH,
> Rayner
>
**I have always been an audio tech. I have always avoided TV work where
possible. However, back in the day, I would frequently perform a CRT
degauss, using my Han-D-Mag head demagnetiser. I found that it could
deal with any degaussing requirement.
https://www.ebay.com.au/itm/Tape-head-demagnetiser-demagneti zer-Han-D-Mag-220-240V-/271234117484
Geez, they're expensive nowadays. I still have mine.
--
Trevor Wilson
www.rageaudio.com.au
--
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