开云体育

Hello,

some time ago I have acquired TDS744A in unknown condition. Whet it arrived it didn't turn on. I have checked main fuse and it was blown. I have dismantled the scope and found that power supply had catastrophic failure, specifically in section of standby power supply. Both NTCs exploded and created complete mess. I was checking what else was bad, BU508A power transistor had B-C short, CR10, CR11 diodes and VR1 zener shorted. I have cleaned all dirt and swapped blown components, including VR4 and VR5 TVS diodes and famous C17 capacitor. However power supply was still not working. Then I have been checking T3 transformer and found that it is unfortunately broken. It has inter-winding short on primary winding (using my Bob Parker ring tester).

I borrowed similar scope from my friend who was kind to allow me to temporarily desolder T3 transformer from his scope and use it in mine. After the swap my power supply started to work without any problem. This confirmed that really T3 is a culprit. I have also measured inductance, good transformer had 18mH on primary winding, bad one had just 1mH.

I would like to ask your help if anyone has power supply for parts and would be willing to desolder and send T3 transformer (I will cover all costs). It has label 22900937 REV M? (it is small 3x3x3cm cube). I have been checking pictures on internet and the same transformer was used in several different Tektronix TDS scope power supply models, not just in TDS744A.


Thank you very much.


Vladimir

Hi,
not what you want to hear, but have you considered re-winding it ? Either on the same or a similar former. If you're very lucky, the primary winding might be the outer one, or a separate former ? (can't find my photos ATM). If you don't feel like counting turns, and if you still have your friend's T3 you could determine the turns ratio. Not a magnetics pro here but I assume with turns ratio, inductance, operating frequency and guestimating the current loads (should be fairly low) you can probably get pretty close ?

Chris

Hello Chris,
I am afraid there is no reasonable way how to rewind the transformer. It is very small (just 3x3x3cm), has 6 independent windings and broken one is closest to the core. It is also completely potted by some glue so probably the only way how to disassemble is by acetone and it will likely dissolve everything except ferrite core and wires.

Here is the photo of that transformer: /g/TekScopes/photo/300091/3876209?p=Created%2C%2C%2C20%2C2%2C0%2C0 , see the size comparison with RJ45 plug.

If I am not able to find the replacement, I am considering alternative way to build separate small power supply to be put inside of scope. It needs to provide 12V and galvanically separated +/-15V.

Vladimir