Hi Nielsen,
Measuring ripple especially those having pk-pk 2-4mv seem very tricky to get because noise created by probes in the air near the test point already seeing noise more than 2mv.
How did you overcome those noise when measuring 2mv ripple noise in PSU?
Just to share:-I also come across 2247A unit will randomly shutdown by itself after maybe 1 hours and seems quite random. ?Sometimes run 4 hours then shutdown. ?All test at same room temperature.Once shutdown, power-on/off switch would not bring up the unit. ?Main socket have to reset then power on the unit seem the only way to bring it back live. ?I given two unit to test, one of them behave like this and I opt for the other unit without this issues. ?I guess it could be due to overheat causes by bad cap and thermal switch kick in. ?
Not sure that is exact behavior under overheat situation.?
From: "nielsentelecom@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Friday, 1 April 2016, 0:17
Subject: [TekScopes] Re: Tek 2247A [SN B029000] Recap and diode part list ?
? Ting,
I looked at your pictures of your 2247 power supply, and it appears identical in all respects to my 2246A scopes pwr supply. I recapped my power supply about a year ago after checking the power rails prior to calibration for ripple. All the outputs with exception of the high voltage DC outputs were bad, and I could not get all voltages back into spec at the same time by the single available adjustment. BTW, my scope is the later version: I think it is a B version, not sure offhand. I had read on this group blog the secondary diodes should also be replaced, so I did that since the time and risk involved in removing the power supply is considerable. I wanted to tell you and the group of some of my experiences and what I did to test the pwr supply out of the scope.
If you should be probing the xistor at the base of the vertical heat sink (the one with the thermal switch), don't let the shield of your probe touch the heat sink. If it does, replace the PWM controller on the supply. I blew mine out. I did have the DUT running thru an isolation xformer. I did replace the 5.1 ohm (x4) resistors that are by the xformer drivers with higher power rated resistors and raised them up for more airflow cooling. The originals tested fine, but the scorching just shouldn't happen. I also replaced the TIP31C drivers also as a precaution. I didn't want to risk the unobtaineum xformer. I did consider using the TIP41 transistors because of the HFE rating was better, but they were capable of higher current and I believe they would have ran cooler, but I didn't need that, and I would rather blow out the drivers than the xformer.
I tested the power supply out of the scope by building a load bank with a proper connector to the multipin connector on the bottom of the PCB. I placed the supply in a box so the PCB was supported by the edges of the box, and the multiresistor load bank was suspended in free air inside the box. I then placed the cooling fan in the box to blow air across the resistors. The fun part was the high voltage CRT cable. I placed it in one of my lineman's high voltage gloves. I then monitored the AC line current going into the power supply while tweaking the voltage adjustment. One of these days I will post the resistor ratings I used for the load bank one of these days.
I know this was risky so everyone can save the lectures if you feel so inclined. But I couldn't think of any other way to load down the supply without placing it in the scope and covering it up with the processor board and risking my ribbon cable connectors everytime I plugged/unplugged them.
If you haven't replaced you diodes, I can give you my P/N's I used.
NielsenTelecom
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