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Thank's to J M Noeding la8ak@online.no May 2000here is a circuit showing differences for Drake 1-A
Since I posted the circuit to K4XL only, I should have given you some more notes about 1-A circuit diagram, but the circuit is available on bama site. It seems that RL Drake soon adapted a well known technique of choosing less expensive parts, therefore already in 1958 the last IF transformer (T5) was replaced by a simple coil (L7). This discrete coil stays in the 2-A and 2-B (as L12) for feeding the 6BE6 product detector, but again in 2-C (R-4.. and SPR-4) I believe it is back to a transformer - with link to feed the product detector diodes. (Cant find the 2-C circuit now, but have studied it thoroughly). Instead of tapping the coil, resistive tapping is used, but in 2-A and 2-B, full IF signal (5V peak-to-peak for 2-B) is fed to the AGC amplifier valve, but the resistive divider is now used for 6BE6. it may seem that 12AU7 runs with substantially higher IF signal than 6BE6. IF signal output may be stronger, believe it is so for R-4C. I once modified my first 2-B and used a spare 50kHz IF output transformer from SPR-4 (or RR-1), and tested the R-4B AGC and product detector. LA6OP Sindre Torp says the difference between the two models of 1-A is that one has x-tal calibrator, while the first hasn't, but I can't see any reference to such unit on any receiver circuits. I wouldn't be very much surprised if the so-called "passband tuner" was constructed in 1957, and was later adapted for different selectivity ranges, first two positions in 1960 and three positions in 1961. I have made a table, see attached file, should inform about different valves used, not sure if everybody understands it. Just as I should post it I discovered a fault, had to edit in "powerpoint" and convert to gif file, hi. 73 |
