RCA 1-K and BTA-250K AM Broadcast Transmitter Schematic

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A few words are in order before perusing the schematic for BTA-250K or BTA-250L details. Chiefest among facts is that the schematic is actually of the RCA 1-K Standard Broadcast Transmitter, which uses the BTA-250K as the 'exciter' to drive a 1KW RF PA and attendant modulator.

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.

1. Removal of one of the 810 final amp tubes. Only one is needed to drive the 833's in the KW stage.

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.

Here's the RCA-1K schematic 5700x2300 GIF image size, 0.5 MB download.
NOTE: some web browsers don't want to display a picture this size. Might be best to download it.