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Have I blown up my scope??
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I was tinkering with an RF control circuit and couldn't get it to work. So I dug out my Tek 2465 scope that hasn't been turned on in about ten years to do some signal tracing. I plugged it into the handiest power outlet which happened to be my computer surge suppressor power strip. Then I turned the scope on, all seemed normal, I probed the scope's square wave voltage calibrator, it looked pretty good, I wanted to optimize the square wave with the probe adjustment, but didn't have a tuning tool handy. So I'm thinking to myself, I'll just put the probe ground alligator clip to ground to see if the square wave cleans up a bit. Around the time I placed the ground clip to the outer shell of the channel input BNC, I heard loud pops and saw lots of smoke. I didn't notice any sparking at the ground clip. I immediately unplugged the scope.!!
So my first question, did I cause this malfunction by placing the scope probe ground clip to ground or was it just an unfortunate coincidence???? My second question, is the following a sensible approach: 1. Take the covers off and do a power-off visual inspection 2. Replace any visually suspect components 3. Then with cover off, power up and check for smoke. 4. Replace any suspect components 5. Then with power on and isolation transformer connected, do voltage checks I have some electronics background but no experience with troubleshooting switch mode power supplies. Anyone recommend any books, web sites, youtube videos that cover troubleshooting / repair of Tek 2465 SMPS??? |
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I'd chalk it up to coincidence.
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The power supply boards have snubber networks consisting of C1018, R1018, C1016 and R1016 that are across the mains. Those caps are known to blow out and smoke like you describe - some more violently than others. If you remove the covers and then the inner, top cover, the PS boards are visible and those caps are near the rear of the scope. Those would be my first suspects. Thanks, Barry - N4BUQ ----- Original Message -----
From: "ted webb 1942" <ted.webb.1942@...> I was tinkering with an RF control circuit and couldn't get it to work. So I |
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As a note:? the smell of those capacitors is unique, and lingers.? They are not really needed for operation, but keep the scope from affecting other equipment on the same power line.? They are more prone to fail on 240 volt circuits than 110.? They are moderately easy to replace, and there are many threads explaining the problem and the solution.
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The capacitors are designed to fail in such a manner that they do not short out (otherwise, the AC line is shorted). the "10 years" is significant, since the parts can fail due to moisture in the air getting into the parts through small cracks. Harvey On 2/23/2022 9:27 PM, n4buq wrote:
I'd chalk it up to coincidence. |
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Yes, it was just a coincidence. The probe ground clip is the same potential as the chassis. Now, if you happened to be poking around in a line voltage or power circuit, it would be a different story.
Since the scope had been sitting unused for a long time, a number of things could have happened some time after powering it up. I think what the others referred to sounds like the infamous Rifa or Wima brand epoxy-potted paper X-caps often used in the AC line EMC filtering of this vintage equipment. These are well known to be a problem, shorting out and melting down, making noxious fumes, and quite a mess sometimes. The good news is that although they tend to self-destruct, they usually do no further damage except for blowing the line fuse, and making a mess of the surrounding area. If you open it up and do careful inspection of the line side of the PS (with power cord unplugged), you should be able to spot the problem - you'll know it when you see it. Having equipment with these caps sitting for a long time, especially in damp conditions, or being used in a "high voltage (line 240 V) country" tends to aggravate the situation. Ed |
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You are probably the coincidental recipient of the dreaded RIFA capacitor failure. "RIFA" is a brand of capacitors found in the RF filter section of the power supply input.
I'm not going to go into it, but search the group for threads. There is a ton of information on it here on the group. You'll learn symptoms, causes, fixes, see photos, etc. Don't despair - it's not rocket science. Probably a good time to consider recapping your PSU. That's not too hard either, though a bit tedious. I don't want to provide false hope either, but I agree with the other folks...probably a total coincidence, and the "power on after 10 years" part was an unavoidable part of this kind of failure. Just my opinion |
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On 24/02/2022 04:17, Ed Breya via groups.io wrote:
Yes, it was just a coincidence. The probe ground clip is the same potential as the chassis. Now, if you happened to be poking around in a line voltage or power circuit, it would be a different story.I'm just replacing the RIFA caps in a 2445 PSU It's tempting to do a re-cap on the basis that "I might as well do it while it's out of the 'scope", especially as these PSU's are not the easiest to extract from the chassis and disassemble to work on but I am not sure I would rush into a blanket replacement of the existing caps unless you know that there is a problem. Be careful that you get the right caps - the originals are (or should be, according to the service manual) Sprague (now Vishay) 672D, low ESR, 4000h@105¡ãC But you might not need to replace them - partly out of curiosity I pulled the 10?F caps in the 87V circuit as those are under most voltage stress and they measure just fine, capacitance in tolerance and with an ESR of ~ 2.2¦¸ @ 120Hz compared with a spec of 10¦¸ - the only one I might replace, if I can find a suitable candidate is C1130 (the 87V voltage doubler) as it looks like a previous repair swapped it with a general purpose 85¡ãC Nichicon. -- Paul |
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Gentlemen,
I thank each and everyone of you for your prompt and helpful replies. Although I come from a career in electronics I don't have a lot of experience with component level troubleshooting. I appreciate you taking the time for some "rookie hand holding" it is very much appreciated. As indicated I haven't used the scope in about ten years, but I was having trouble getting a encoder/decoder circuit and thought I might take a look at the wave forms with the scope. When the popping and smoke occurred I was extremely disappointed. However, I'm excited about getting into the troubleshooting and repair, I have already downloaded the service manual and ready to remove covers for a visual. The scope is in a rack mount front panel, but will getting torx drivers tomorrow to remove the panel. Assuming the local flooding subsides! I live in Australia now, but from Texas. Most of my career was spent with Texas Instruments in Field Service, later with Boeing Australia. Have been retired since 2010, turning 80 later in the year. I can still remember those Tek LA545s! Thank you again for your help. Ted |
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Earlier I wrote...
I should have known better, I suppose, than to tempt fate :) The 10?F 100V caps looked and measured OK and I left them in situ, the two 180?F caps for the +/- 15V unregulated lines had high ESR (strictly one ended up with infinite ESR as one of its leads fell out when removing it from the PCB), the 250?F caps decoupling the 5V rails measured OK but the bottom red plug was bulging a lot (normal??) so I swapped those, all of the 100?F 25V caps measured absolutely fine as well but I could see evidence of electrolyte leakage around the rubber bung so they all got replaced. New caps were Nichicon UPW/UPM/UPJ on the inverter board and general purpose Nichicons (UKL's) on the regulator board - apart from C1130 where I used a "high ripple current" part. So much for "if it ain't broke, don't fix it". The scary bit will be reassembling it and hoping that it does not go "bang". -- Paul |
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