Author Topic: (Solved) Old CRT circuit repair  (Read 15315 times)

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Offline SpidersWeb

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Re: Old CRT circuit repair
« Reply #25 on: July 04, 2014, 08:23:00 am »
Any chance it's been replaced before?  I was looking at the glue around it a few days ago which seemed to suggest it was.  That is to say it almost looks like the factory glue outlined a different diameter capacitor.  The soldering looked factory so I didn't mention it, but the fact that there's an identical one at Jaycar gives pause.

I think it has been replaced, what I'm unsure of is if it was installed faulty or got damaged in circuit (poking around the web suggests to me it's not exactly a quality part).
There isn't much point grabbing that exact model again tomorrow and installing it if it doesn't fit the bill.

I know it was mentioned I can use a pair of 22uF Low ESR caps joined at the hip, but since I can get a 10uF greencap (poly?) - does anyone know if that would be ideal?
 

Offline mij59

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Re: Old CRT circuit repair
« Reply #26 on: July 04, 2014, 09:02:33 am »

I know it was mentioned I can use a pair of 22uF Low ESR caps joined at the hip, but since I can get a 10uF greencap (poly?) - does anyone know if that would be ideal?
[/quote]

The capacitor has to withstand high frequency an high current, the capacitors will start to conduct, heat up, and blow to pieces.
You might try a low loss metal film capacitor.
 

Online vk6zgo

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Re: Old CRT circuit repair
« Reply #27 on: July 04, 2014, 11:37:46 am »
A Greencap should do the job OK.
I used a "stack" of them in a "Boost HT" circuit to replace an NP Electro.---maybe a bit less current,but only a "bit"!
Greencaps are used extensively throughout Industry,& they are pretty tough!
 

Offline SpidersWeb

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Re: Old CRT circuit repair
« Reply #28 on: July 05, 2014, 07:03:05 am »
Cheers guys.

The 10uF 'greencap' I found was $11 in store, and enormous, so I did the normal thing and grabbed a 10uF NP electrolytic - the model in store was of a different brand to the one I just pulled (C505) so I thought it was worth a shot. I did grab a couple of 22uF Low ESR caps just in case someone thought it was wise to try out (something like 50c each) if the NP didn't pan out.
If none of them can survive, I'll order a better part and wait.

I compared another mono monitor I have open, found it's equivilent C505. It uses a 4.7uF NP electrolytic (I know it's original becasue I unboxed it two weeks ago) - made in the same year, and it's 4.7uF cap is as big as the circle drawn on my PCB.  I'm not sure if this is just "technology improving" or if it was larger for an engineering reason (like heat). Further confirms C505 was replaced - will be interesting to see how it's similarly sized replacement deals with it.

For the 2.2uF caps, I found the originals were marked +/- 5% and still in their value range, so I plan to reuse them to start with because I think the replacements I grabbed are possibly worse (and I could just be creating a new issue by fitting them).

Turns out my $1 scope works (Advance Instruments OS250TV - 10Mhz dual channel CRO), so grabbed a set of 1x/10x probes - calibrated it using the built in square wave - and I should be able to post more interesting/informative results if replacing C505 doesn't do the trick. (absolutely amazed it works)

Still no ESR meter, but I'll take care of that soon.
Busy weekend but will try and make time to get it powered up tonight/tomorrow.
« Last Edit: July 05, 2014, 07:05:37 am by SpidersWeb »
 

Offline SpidersWeb

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Re: Old CRT circuit repair
« Reply #29 on: July 07, 2014, 09:01:48 am »
Woo! I just powered up to a perfect display. The display is bright, steady and in focus.

C505 was the only component that tested as faulty. The replacement looks tiny, and I'm not sure how long it will last, but the fault has been found :)

Edit: of course after posting this, I went back to use it again, and now the power supply wont start up  :palm:
Got a dull green light, then it shutdown, now it does nothing. Resistance shows 0 ohms initially but increases to 12 ohms (I guess as caps etc charge up). Will check fuse next, then test PSU on it's own.

Edit2: was the fuse
« Last Edit: July 07, 2014, 09:41:33 am by SpidersWeb »
 

Online vk6zgo

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Re: Old CRT circuit repair
« Reply #30 on: July 07, 2014, 12:58:04 pm »
If the replacement is tiny,it will last around 30minutes before the magic smoke comes out.
Even back in the mid '90s,the lousy,lossy NPs were much smaller than the low ESR ones.

Have a look at my posting (#20).
« Last Edit: July 07, 2014, 01:00:55 pm by vk6zgo »
 

Offline SpidersWeb

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Re: Old CRT circuit repair
« Reply #31 on: July 07, 2014, 07:52:05 pm »
Ah yes your post #20 covers it. Thanks, it's weird that I missed that point.

Jaycar only stock one model by the looks, the tiny one, ESR not advertised. The other place I grab components locally is RS NZ - but for the life of me I can't see how I find NP caps on their site. Mouser etc charge a large shipping fee, but I'll work it out and get something sorted. It's a lot more promising now I've seen the clear display.
 

Offline SpidersWeb

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Re: Old CRT circuit repair
« Reply #32 on: July 23, 2014, 09:30:33 am »
So I decided to wait for delivery, and order a part specifically rated for this purpose. I got two sets from different suppliers, and just as well, because one set was 21uF despite the 10uF label (I tested 4 of them). The lower quality models I got with +/- 20% tolerance however, were a nice 10.45uF. I fitted one along with a new fuse, and tada:



New man-sized capacitor - designed for "horizontal deflection circuit" and apparently 9A of ripple current. Has been running for over an hour now with no explosions and/or fire storms.



Thanks to everyone who posted. I learned a LOT about how these monitors work in the process.
 

Online tautech

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Re: Old CRT circuit repair
« Reply #33 on: July 23, 2014, 09:39:49 am »
Nice screenshot. Well done.  :-+
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