Hello all,
I have a defective Haidenhain display unit here. PSU is a SMPS with > 7 V output and a linear post regulator. If I feed the linear regulator with 7 V from a lab PSU, the unit works. Current draw is around 450 mA. So putting in a 7+ V module is a fallback.
I think I reasonably well understand how these supplies work, but I have never repaired one. So here we go. I have connected a HV lab supply, set to 200 V, to the input (makes me feel better than unlimited power variac). Unit is labeled as 100-240 V, so I assume 200 V DC to be fine. The design looks very German - the original Siemens&Halske logo on many components. Controller TDA4605-3 stamped "Austria". I think I have never seen this. Date code on other chips is around 97.
Controller datasheet:
https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/1135/TDA4605-3.pdfI removed the clip on heatsinks and also unsoldered the switch FET (BUK 446), which was fine according to my transistor tester.
The startup supply resistor chain looks pretty burnt up, some resistors have even cracks, but resistance value is OK for all of them.
Overall behaviour: VCC capacitor charges, controller starts at about 12 V, switches for around 40 ms, then stops. This repeats itself indefinitely.
To me, all switching and secondary side waveforms make somehow sense, no short or otherwise (obviously) dead component.
I attached a scope plot of this behaviour, it shows the VCC pin 6 of the controller. What I find strange is that at some point the primary feedback/bias winding should power the controller, however switching seems to stop before that happens.
Also strange is that there is a high 4Vpp ripple on the voltage once the controller switches, as the controller supply is bypassed with a 470 uF capacitor. What looks like some greyish goo having leaked out of the capacitor is present on the output cap as well, is rock hard and seems some epoxy glue.
Any ideas? Thanks!