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Power seleniums can be replaced by modern silicone diodes, just check the current rating of the original (usually listed in sam's) against the new one (replace with the same or higher) and make sure the PIV is a good bit higher than what it appears the circuit can produce.
I suspect M3 is the Horiz AFC (I don't have the schematic) those are usually good, but I've encountered a dud or two....The symptom of a bad H AFC diode is soild Vert sync, and horizontal can be made to almost sync, but will at best roll slowly either direction without locking. The AFC diode is three leg because they put 2 diodes in the same case and tied two legs together internally.
If the H AFC diodes are on the board and the board is a royal pain to change parts on (like many claim the predicta boards are) then it may be wise to replace the H AFC diode pack as preventative maintenance.
You can use 1N4007s for H AFC. Some claim you need fast shottkey diodes, but in my experience the 4007s work fine in the sets I've needed to do AFC diode swaps on.
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