Dear forum members,
This is my first post on this forum. I wanted to share my experience in fixing the TDS3014 power supply.
The power supply schematics traced Jay_Diddy_B helped me a lot in fixing my 3014.
The problem was in the mains flyback converter, which couldn't properly regulate its output voltage (was jumping from 19V to 23V up/down) causing the scope to do sporradic things, crashing SW, hanging, rebooting, etc.
The root cause of the failure was in the voltage reference/shunt regulator TL431. It was heating up like crazy leaving black spot of the burned PCB under it.
I've checked the other components, they seem to be in spec. After the replacement of TL431, the new regulator also heats up to 70 degrees. The output voltage with and w/o the load and the ripple figure are fine. I'm running the power supply from 240V AC mains. The scope runs like new again.
But now I'm questioning whether the problem is really fixed.
Is it normal that TL431 heats up so much?
I haven't seen it being glowing red on the thermal scan images kindly provided in this forum. Actually TL431 almost dissipates 400mW! It had left the same kind of burns on the PCB like the snubber network resistors and the load resistance.
If this is not a normal operation, IMHO, I can only assume that the optocoupler has degraded with the time and doesn't have the same Current Transfer Ratio (if the CTR would fall low). Hence TL431 is supposed to put more current through IR LED in order to keep the output voltage on target, which pushes the TL431 operating mode beyond the spec.
For the moment I'm waiting for the CNY17-3 optocoupler to arrive, then I'll try replacing it.
However, if such a heavy operating mode in standby is normal, IMHO, the TDS3000 power supply is prone to fail with the time.
I'm just wondering, is it only my scope or are TDS3000 having a glowing red reference IC when running from 240V AC?
Thank you!
All the Best,
Maxim