View Single Post
  #5   Report Post  
Old January 13th 21, 02:48 AM posted to rec.games.video.arcade,sci.electronics.repair
Trevor Wilson[_4_] Trevor Wilson[_4_] is offline
external usenet poster
 
First recorded activity by DIYBanter: Jun 2009
Posts: 177
Default Reviewing a cheap CRT degaussing wand

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-hea...-/271234117484

Geez, they're expensive nowadays. I still have mine.

--
Trevor Wilson
www.rageaudio.com.au

--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus