wrong with this?
To eliminate the possibility of excessive loading I removed the bridge rectifiers. The circuit still squegged.
With all defective components already replaced, could the problem be in the magnetics? No spare for the main transformer T1 (potted in a metal case, custom made and now obsolete), but T2 was several turns of magnet wire on a toroid core. Was there a shorted turn? Was the core somehow magnetized and saturating from the previous faults? Took it off the PCB and measured the inductance with the following circuit of Figure 3:
Figure 3 Measuring transformer inductance with a scope and square wave function generator. For details see W2AEW's YouTube video .
Figure 4 The response of the DUT transformer With C = 1nF the inductance calculates to 21.3µH using the formula L = 1/[(2πf)2C].
Figure 4 looks not too bad but is quite damped even with all windings unloaded. Shorting the other half of the primary winding caused the inductance to decrease drastically, proving that there were no shorted turns.
Because of the low voltage involved it was unlikely that the transformer was breaking down under actual operating conditions, and the toroid core was not electrically conductive so had not been zapped by a high voltage (lightening strike) surge. But there was still the question of possible core magnetization or an invisible crack, and since I had a few toroid cores of about the same size in the junkbox I wound a few with the same turns count. The original core material type was unknown, but one of my jerry-rigged attempts came close, see Figure 5.
Figure 5 Still 1µs/div, the Amidon FT-50A-77 toroid material has higher inductance (85.9µH) but is otherwise suitable.
Unfortunately, the squegging continued, so I put the original T2 back onto the PCB and threw the others back into the junkbox for future use.
Pulled the main transformer T1 off the PCB and