Sync Separator Diode Replacement

[old_coot88]

Which way did you install the diode? Seleniums are often marked with a plus sign(+) on the cathode end. With Si diodes, the banded end is the cathode.

[Jon1967us]

Originally Posted by old_coot88

Which way did you install the diode? Seleniums are often marked with a plus sign(+) on the cathode end. With Si diodes, the banded end is the cathode.

I have the cathode indicating band corresponding to the cathode “+” indicator on the Selenium in the schematic.

In other words, the cathode side of the Si diode is in the same position of where the “+” side would’ve been on the Selenium. I guess I’m aware of “discrepancy” between how Seleniums used to be notated on schematics. Anyway, if I orientate the Silicon the opposite direction, hardly any signal is displayed on screen, which was the situation prior to replacing the bad Selenium diode.

[old_coot88]

Looks like the diode is used as a phase comparator (normally a dual diode is used). Never seen a single diode used there before. The circuit depends on a feedback pulse which would come in via C50 and R60.

If there's no waveforms present at the sync sep and noise cancellor stages, the the signal loss must be somewhere 'upstream' of there. Probably be helpful if you could post a fuller schematic.