Hello everyone.
I've only just joined, so this is my first post and typically I'm looking for help.
I'm an absolute newbie when it comes to the use of oscilloscopes. I'm a life-long self-learner and thought it was time to educate myself. Have some basic knowledge, but not a lot.
Anyway, I did what many before me have done and bought a used scope off eBay for not too much. Did some homework and waited until I scored a Tektronix 2235A. The seller said that apart from powering up, he hadn't done any tests. Caveat Emptor!
Scope arrived the other day and it did power up and looked like it might be a winner. Then, about 15 minutes later it blew the main fuse, a 1.25A 240V. I'm in Australia (240v here). I let things cool and tried another fuse; no good, blew again. Only had a 1.5A left, so tried that.....bang.
Doing some homework I've found that they're susceptible to power supply problems particularly if they've been sitting around unused for a long time, like I suspect this one has.
I've sourced the manuals (including service and schematics) but need some advice on where to begin and if there's a common easy fix.
Any help would be greatly appreciated. As I said, I'm a newb' so simple explanations in words no more than two syllables would help.😀
Cheers,
Wayne (wjcoll)
I've only just joined, so this is my first post and typically I'm looking for help.
I'm an absolute newbie when it comes to the use of oscilloscopes. I'm a life-long self-learner and thought it was time to educate myself. Have some basic knowledge, but not a lot.
Anyway, I did what many before me have done and bought a used scope off eBay for not too much. Did some homework and waited until I scored a Tektronix 2235A. The seller said that apart from powering up, he hadn't done any tests. Caveat Emptor!
Scope arrived the other day and it did power up and looked like it might be a winner. Then, about 15 minutes later it blew the main fuse, a 1.25A 240V. I'm in Australia (240v here). I let things cool and tried another fuse; no good, blew again. Only had a 1.5A left, so tried that.....bang.
Doing some homework I've found that they're susceptible to power supply problems particularly if they've been sitting around unused for a long time, like I suspect this one has.
I've sourced the manuals (including service and schematics) but need some advice on where to begin and if there's a common easy fix.
Any help would be greatly appreciated. As I said, I'm a newb' so simple explanations in words no more than two syllables would help.😀
Cheers,
Wayne (wjcoll)
Welcome to the forums. Also look for blown X type mains rated cap around the power switch / mains entry area. They are usually blocky plastic with several safety standards printed on them.
Hi again guys.
Salas, thanks very much. You were right on the money. When digging a little deeper I found a 0.068uF X2 cap which had totally died. I also found a blown thermal resistor as well.
Have ordered the parts from Mouser and we'll see how we go.
Thanks for the help.
Cheers,
Wayne
Salas, thanks very much. You were right on the money. When digging a little deeper I found a 0.068uF X2 cap which had totally died. I also found a blown thermal resistor as well.
Have ordered the parts from Mouser and we'll see how we go.
Thanks for the help.
Cheers,
Wayne
If the 2235 is anything like the 2465B that I have, you may look into changing the electrolytics in the switchmode supply as well. In my 2465B, all the 'lytics were dried out. Some of them wet themselves and made a mess on the PCB.
~Tom
~Tom
Yeah, you did good so far. A simple but careful visual inspection of all the parts in the power supply is the first place to look. I agree that the caps are best replaced with new ones as well. Good luck!
Jackinnj, Thanks for the tip. I'm trying to join the group. Seems their membership approval is as fussy as they are active. But I'll persist.
UPDATE
Hi again all,
Well it took a while but all the bits and pieces I was chasing finally arrived. I replaced the dead 0.068uF cap (C904) and the thermistor (RT901) , then tentatively push the power switch. Another blown fuse !!!!!!!!!! Exactly the same condition exists. I'm suspecting whatever took out the first two components didn't stop there. These two were easy to spot with some help from advice here as they were physically damaged. So where to from here? Any suggestions would be greatly appreciated. Feeling a bit deflated at the moment. Murphy's law dictates it could never be as simple as I first thought.
Thanks again,
Wayne
Hi again all,
Well it took a while but all the bits and pieces I was chasing finally arrived. I replaced the dead 0.068uF cap (C904) and the thermistor (RT901) , then tentatively push the power switch. Another blown fuse !!!!!!!!!! Exactly the same condition exists. I'm suspecting whatever took out the first two components didn't stop there. These two were easy to spot with some help from advice here as they were physically damaged. So where to from here? Any suggestions would be greatly appreciated. Feeling a bit deflated at the moment. Murphy's law dictates it could never be as simple as I first thought.
Thanks again,
Wayne
Did it take out the replaced components again though? Any tantalum caps around that fail shorting, a blown power semi...
Hi Salas,
No, the components I replaced don't appear to have been affected. The fuse blows only on powering up, not when mains is switched on so I can assume it's after the switch, although that doesn't exclude much from the possibilities. I'll do as you suggest and check the tantalum caps in the area.
Many thanks,
Wayne
No, the components I replaced don't appear to have been affected. The fuse blows only on powering up, not when mains is switched on so I can assume it's after the switch, although that doesn't exclude much from the possibilities. I'll do as you suggest and check the tantalum caps in the area.
Many thanks,
Wayne
Thanks Salas,
I don't have an ESR meter but I was intending to get one, so now is obviously the right time.
I've done a very close visual inspection of the power supply region and there's no hints (visually) of any of the lytics having age or stress problems. That being said, it mean nothing as we know they can die anomalously. It may end up being a case of replacing caps down the line from the switch until I strike it lucky.
Cheers,
Wayne
I don't have an ESR meter but I was intending to get one, so now is obviously the right time.
I've done a very close visual inspection of the power supply region and there's no hints (visually) of any of the lytics having age or stress problems. That being said, it mean nothing as we know they can die anomalously. It may end up being a case of replacing caps down the line from the switch until I strike it lucky.
Cheers,
Wayne
Put a lamp (40W/60W) in series with 240V supply.
This will prevent the fuse blowing and give you time to measure voltages, and diagnose what's crook.
Dan.
This will prevent the fuse blowing and give you time to measure voltages, and diagnose what's crook.
Dan.
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