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| Digi-Key Sold Me 7 Year Old Capacitors |
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| Smoky:
Brumby, I monitored the output voltages of the SMPS as I was increasing the incoming AC voltage on the isolation transformer. The SMPS was producing correct output voltages long before I reached 120 VAC incoming. The only other lead exiting the SMPS main board that I didn't check was the output wire going to the CRT board. The display is crisp and sharp at less than 50% intensity: |
| pigrew:
The LGX datasheet states: "After storing the capacitors under no load at 105°C for 1000 hours and then performing voltage treatment based on JIS C 5101-4 clause 4.1 at 20°C, they shall meet the requirements listed at right." So, apparently it's fine, but you may need to look up your JIS standard. How 105 degrees and 1000 hours translates to years at standard temperature and pressure, I don't know. There's an older forum post on this topic, too. It links to a document from UCC describing how to use capacitors. It states that the major degradation will be solderability and leakage current increase. It suggests to power the capacitor through a 1kohm resistor at its rated voltage for 30 to 60 minutes in order to reform the Al2O3, restoring its rated leakage current. Don't worry so much about their age if they are still solderable, but reform the caps if you feel the need. My feeling is that caps from since 2007 are pretty good in terms of reliability (see capacitor plague). |
| Brumby:
--- Quote from: Smoky on August 30, 2019, 12:54:16 am ---Brumby, I monitored the output voltages of the SMPS as I was increasing the incoming AC voltage on the isolation transformer. The SMPS was producing correct output voltages long before I reached 120 VAC incoming. --- End quote --- THIS WAS EXACTLY MY POINT! An SMPS will do its damndest to achieve that. That is their goal in life. The important question is: What current was being drawn on the input side of the SMPS so it could produce that output voltage? With the input voltage lower than the typical operating level, the current MUST be higher - and if the voltage is significantly lower, the current will be significantly higher. Edit: For example: If we have a design for an SMPS that will typically run at 1A input current and we put 5A diodes up front and then provide a voltage where the SMPS tries to pull in 15A of current, then what do you expect could happen to those diodes? Similar questions can be asked on every component on the primary side. |
| Smoky:
Well Brumby, All I can add is that I'm almost positive that the Tenma isolation transformer would have blown its 3 Amp fuse if the Tektronix power supply demanded more current than 3A at 120 VAC. ...and no loads were applied to the Tektronix SMPT while I was ramping up the AC voltage on my bench outside of the oscilloscope. None of the PCB's were connected or fed by the main ribbon cable connector, it was disconnected. Oops, maybe I shouldn't have left out this fact :) ...and another side note to BsFeeChannel and FloobyDust, I have yet to scan the Leader LBO-302 oscilloscope schematics. I hope to return to that repair adventure very soon. Please hang in there! |
| edpalmer42:
If you're using relatively low voltage capacitors, you probably don't need to reform them. It seems like they'll never degrade enough to cause a problem. With capacitors for tube devices, you *must* reform them. Line voltage is in the grey zone. The low input voltage issue that I'm talking about isn't related to the current drawn, although that could be an issue. It's simply that the input voltage isn't high enough to allow the control circuitry to wake up properly. Any startup delay doesn't work right so the output might try to enable itself when the oscillator isn't functioning properly. So, for example, the switching transistors might turn on hard and stay on. Not every circuit, not even every power-up. Maybe new designs aren't vulnerable to this. But I typically work on old supplies. For me, one occurance is one too many so I don't ramp up the input voltage on switching supplies. YMMV. I'm glad that Smoky didn't have any problems. Ed |
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