In order of importance, most dangerous first:
- Separate adjustable H B+. Put a regulator between the 112V rail and T403 pin 1 so the horizontal output stage can run at about 56V for 15k, about 75V for 24k and 112V for 31k. The main supply regulates off the 112V winding, so it can't do this itself. Without this step the other changes are how you kill Q412.
- Mode detection and switching order. Use a comparator, or a window comparator for three modes, on BA7078 pin 4 (VSEPA, the voltage proportional to H frequency) to drive the relays. The order matters: lower B+ before the frequency drops, raise frequency before B+ goes up. Also give it a safe default for no sync or out-of-range sync, with lowest B+ the safest. A mode change that briefly runs 112V at 15k does the same damage as never switching at all.
- Switched LA7856 oscillator timing. Relay-switch the H-TC network (C407/C406 and R416, or extra caps in parallel) so the free-run frequency sits near 15.7k, 24k or 31.5k and the AFC can lock in each mode. Each mode then needs its own H-HOLD trim.
- Switched S-cap. About 1.6µF for 15k and about 700nF for 24k, against the existing ~400nF. Without it the picture is stable but the linearity is wrong.
- HV converter sync (KA3843B). Retune its Rt/Ct so it free-runs below 15.7k and the sync pulse can pull it up in every mode. Otherwise expect beat bars at 15k and 24k. Check that HV regulation and the protection timing (C4A7/Q4A2) still behave at the lower rates.
- Retrace caps (optional). With the B+ scaled down, a 31k-length retrace at 15k is survivable, but it's short by 15k standards. Changing them would ease timing and blanking, at the cost of more relay contacts carrying high voltage.
- Geometry and width trims per mode. The diode modulator (Q413/L404) and the H-size/H-position pots were calibrated for one mode. Expect per-mode offsets, and possibly H-position needing switching too.
- Vertical check. Vertical stays 50-60Hz, so the LA7838 side should be mostly fine, but confirm V-size and linearity hold with 262 lines per field instead of 525.
- Bring-up on a current-limited supply. Feed the H stage from a bench supply at reduced voltage first, put a scope on Q412's collector peak in each mode, and only then connect it to 112V.
Steps 1 and 2 are a pair: one without the other doesn't make it safe. All the values are first-order scaling off the schematic, so treat them as starting points to measure.
