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Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
To point out the very, very obvious that nobody seems to have said yet, are there any electrolytic capacitors? If so, replace them, they've probably dried out.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
brandon3055 wrote ...
Except for one I found for auction on ebay which was previously owned by a now deceased electrician. Its a bit of a gamble because the seller dosnt know anything about it other then the screen shows a green dot when you turn it on but if the bids don't go to high it may be worth the risk.
This is a good way to buy them if you just want a basic 'scope. The sellers are normally genuine, and don't know what they are selling so you can get a really good deal. It is always a gamble though, it's up to you to decide if the risk is worthwhile. I generally don't part with more than I can afford to lose, and I'm normally pleasantly surprised, although one I bought stopped working shortly after I bought it. It was only around £20, and I've a couple of other usable ones that cost me the same so I'm happy, and I may fix the other one one day
It is worth changing the electrolytics if everything else checks out ok. All the old capacitors could be at fault, but they are difficult to test without at least disconnecting one terminal.
Yea I will try to get it but there are still 2 days left and the bids have already jumped up to $61 and the max I really want to spend on it is 70 (+ the $30 postage)
I did remove and test a few of the caps because I know they tend to dry out after a while but the few a measured seemed fine although the values I measured on a few of them were a fair bit higher then the rated values. The 47uf caps read about 52uf but I just assumed the manufacturing tolerances were higher when they were made. Or could that be a sign of failure? (I just assumed a failed cap would read lower or short circuit) I will just bye a bunch of replacements today when I get the ic's and see if that helps.
I wasn't able to find any bad transistors however I just started tracing the circuit on paper and in the process I made a significant breakthrough.
This is what I have traced so far. Note the BC547 transistor on the bottom right.
Now look what happens when I short the collector and the base of that transistor.
BINGO! The transistor itself was fine but when I discovered this it only took a few more minutes to figure out that the problem was the 115k resistor attached to its base. Now that I found it its actually very obvious that the resistor is fried I just thought it was a different type of resistor.
Well now that that's finally fixed I think its time to blow off some steam!
I would like to thank everyone here for your help with this! If it wasn't for you guys I probably would have given up on day 1. Just a bit annoyed it took me so long to find a dead resistor. Especially one that showed obvious signs of overheating.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Yay! I just repaired my 'scope (10MHz CS10 Calkek) today. The CRT was indeed bad but there were also two bad 2200uF 50V capacitors and I've added a fan to supplement the cooling so it can't happen again.
Now to make a clamp so the tube can't short out/etc and prevent it touching the PCB. Reckon Polymorph wrapped around the mumetal shield will do the job as this is well away from the hot filament (measured <30C after an hour at 3" along tube)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
brandon3055 wrote ...
I would like to thank everyone here for your help with this! If it wasn't for you guys I probably would have given up on day 1. Just a bit annoyed it took me so long to find a dead resistor. Especially one that showed obvious signs of overheating.
Yay scope! Glad to have been one of the teddy bears
How long *should* you take to find a dead resistor? Diagnoses are much more obvious with 20/20 hindsight.
How long *should* you take to find a dead resistor? Diagnoses are much more obvious with 20/20 hindsight.
True. I guess I was just expecting it to me something much more catastrophic and irreparable then a resistor. On a side note I noticed when I was adjusting the trace rotation that if I move the trace all the way to the top or bottom of the screen using the vertical adjustment it seems to bow in a bit at the ends. Chances are its always been like that and I haven't noticed it and I don't think I would ever notice it in normal use. Just wanted to check if this is a common easy to fix issue. And if not I will just leave it because I am not spending any more time on this.
Maby. or maby some of the caps I replaced were low ESR I just replaced them all with standard electrolytics. Is there a way to tell if a cap is low ESR? because all I know is some were blue and some were black. Its also possible I bumped one of the many trimmers because they are very large and easy to adjust considering they are just a large plastic wheel that look like they are meant to be adjustable by hand.
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CRT ocilloscope issue
