The BTA-250 could be ordered separately and the rest of the 1-K added later, or the 1-K could be ordered complete. A kit of parts was used to convert the -250 for use as the 1-K exciter. Presented below is the list of mods I noticed after studying the schematic of the 1-K, and looking at the BTA-250L itself. Mentally reversing these should allow the enthusiast to understand the BTA-250L's wiring.
2. Disconnection of the plates of the 828 modulators from the BTA-250 modulation transformer, and connecting them to the 1-K chassis to drive a pair of 833's. The secondary of the BTA-250 modulation transformer remains in the 810 plate circuit. A resistance of 4000-6000 ohms is inserted in the B+ lead to the primary side of the 833 audio driver transformer to supply the 828's with high voltage. This resistance reduces the BTA-250 modulator output from perhaps 300-350 watts to a lower value of 20-40 watts as required for drving a pair of 833's in modulator service.
3. Disuse of the portion of the BTA-250 audio feedback network which is connected to the 828 plates. The audio input transformer return leads are then conected to a somewhat larger voltage divider which is connected to the 833 modulators' plate circuit.
4. AC Power input and interlock wiring is changed. (the 1-K uses 230VAC, whereas the BTA-250 uses 115VAC). There are probably other mods, but these seem to be the most relevant.