Funny discoleration on oscilloscope CRT

[poot36]

I am thinking of getting a non functional oscilloscope to repair but I am worried that the CRT may be damaged.  Here is the link to the auction with the pictures of the unit: http://www.ebay.ca/itm/ISO-TECH-ISR640-Oscilloscope-40MHz-as-is-/201564330050?hash=item2eee2b9842:g:leIAAOSwu1VW6y0m as you can see there is a blackish discoloration on the bottom of the CRT.  Does this mean the CRT is bad or is it just something trapped between the plastic protective cover and the CRT glass?  I can pick this scope up locally so no shipping charges but am wondering if this is not the best scope to learn how to repair as I can not seem to find any service manuals for it?  Thanks for any advice you can give me.

[DTJ]

Hard to tell from the photos.

The power switch is below and just to left. Could the PSU be just behind the front panel and the mark is from a catastrophically failed PSU?


I'd be wary, its pretty old and you might even pick up a working unit for a similar price.

[edpalmer42]

If it's local go look at it.  Take a bright flashlight and a good magnifying glass or magnified visor.  Look really closely at the stain and the CRT around it.  If the stain in inside the CRT you should be able to tell.  Pick the unit up and carefully tip it up, down, and around while listening carefully for any tinkling glass sounds - or worse.  In the second picture there's a tan colored mark on top of the stain.  Is it just dust or is the plastic shattered?

It doesn't look promising, but since it's local you can take a good look.  There's a slim chance you can get a repairable unit - but only a slim chance.

Ed

[Ian.M]

Sniff it! If it smells of burnt electronics, and the stain is smoke damage, its probably B.E.R. and should only be considered as an exterior parts and controls donor for an identical model that's badly beat up but basically working.

[poot36]

Thanks for the replys. I will see if I can go and have a look at it within the week.  Scopes in Canada are expensive!  There is some clown on kijiji trying to sell a BK precision 10 Mhz scope (no probes single channel) for $60 locally and I am not sure it is working either!  Got any ideas for getting a cheap ~20Mhz scope in Calgary?

[Paul Moir]

My only suggestion is to also keep an eye on the government & university auction sites, which aren't on most people's radar.  Don't forget to check both the federal and provincial government auctions.  Nothing jumping out at me right now, but there are a couple of Teks (2225 and 7104) out in Ottawa on the federal site.

[edpalmer42]

Thanks for the replys. I will see if I can go and have a look at it within the week.  Scopes in Canada are expensive!  There is some clown on kijiji trying to sell a BK precision 10 Mhz scope (no probes single channel) for $60 locally and I am not sure it is working either!  Got any ideas for getting a cheap ~20Mhz scope in Calgary?

You could always search ebay for scopes within 50 Km of your postal code.

Ed

[poot36]

Thanks for the suggestions.  I have found a Tektronix 475 for $275 including probes and is showing working locally on Kijiji.  It is more scope then I need right now but the price per Mhz is killer.  It will be a plan B if I do not decide to go with the broken scope.

[Paul Moir]

Check it out on all the volts/div settings on both channels using the built in calibrator.  Sometimes the attenuators are a little flakey.  It's an easy fix but can be a bargaining point. 
While I love my Tek, a far more capable (but much lower build quality) DS1054Z is "only" $558 + $20 from a Canadian Rigol distributor.

[poot36]

Well I decided to go for the Tek.  Got it for $250.  It has 2 P6100 100Mhz probes.  It is missing the probes pouch, the front cover and the fan filter but other than that it works and the attenuators both work just fine.  The mounts that the power cord wrap around are solid plastic and are starting to degrade, there is a V/div light out, and the previous owner tried to clean the push buttons with Windex causing the lettering to wipe off and then used a thin felt marker to try and rewrite the labels!  Gaa!  At least the rest of the scope looks very clean.  Would using printed stick on lables be a good solution to the missing button label problem?

Already I have used the scope to find some caps on a computer monitor that are starting to go bad even though they are not leaking but there is about 0.4V of ripple on the 24V rail to the CCFL backlight driver board.  Is this to much ripple?  I ask because the monitor will work for a random amount of time and then the backlight will just turn off, there is no high voltage hissing before it turns off and all of the tubes are still glowing white.  The power on hour counter is below 1000 hours (if it is to be believed) so I hope it is just the caps.  The monitor is from around 2012 or so made by HP and is 24" and has a ambient light sensor in it.

[tggzzz]

Well I decided to go for the Tek.  ... and the previous owner tried to clean the push buttons with Windex causing the lettering to wipe off and then used a thin felt marker to try and rewrite the labels!  Gaa!  ... Would using printed stick on lables be a good solution to the missing button label problem?

For my 485 I used a Dymo Letratag, which produces very similar glyphs at a similar line spacing. If you use the transparent adhesive tape with black writing, you should be able to avoid "changing the colour" of the buttons.

The Letratag is also useful for labelling your drawers and components, particularly since it can produce small characters in two lines.

[poot36]

I have a Dymo Labelwriter 430 would that work?  I would need opaque labels to cover the permanent marker on the current buttons and the Labelwriter has white labels in it.

[tggzzz]

I have a Dymo Labelwriter 430 would that work?  I would need opaque labels to cover the permanent marker on the current buttons and the Labelwriter has white labels in it.

No idea. Suck it and see!

[poot36]

The labeler worked just fine.  Now I just need to find a air filter for the fan!