Tips for improving the Ranger and other transmitters that utilize the 6AL5 as a bias rectifier. Over the years many Johnson have traveled the bench of D-Lab. I have had a few with melted filament wiring. Its a real mess since the wiring is wrapped in a harness, the wires fuse together. Resulting in a lengthy repair. Tracing the short source normally ends up being the dial lamps. Either the center post shorts as shown in this video or the right angle type swings down and shorts on the keyer platform. The 6vac circuit is not fused and of course normally see high current, so will not blow the AC plug fuses. Bad situation. Closely inspect your lamps, insulate as needed. Second issue, the negative bias system. Its very common to have Rangers come in with high modulation idle current. You would think, just readjust the current resistor tap below the chassis and off you go. Well if you look closer at the circuit you will see that the negative bias also plays a part in modulator ma's. The 6AL5 is the rectifier, C90A/B filters it, then goes thru a voltage divider (2-4.7K resistors). The center of that divider goes to the grids of the audio tubes (Modulators) If that voltage drifts over time, the mod current will follow. A symptom of this failure is increasing mod current over time, you readjust the 20K resistor tap, then it drifts again. If you are experiencing this, I have the fix. Replace the 6AL5 with 2-1N4007's, install a fresh filter cap (33uf@160v, Positive is ground!) Replace the 4.7K resistors with 1 watt type. Readjust your mod current per the transmitter manual.
I have serviced many Johnson transmitters over the years. If you are experiencing a strange problem with one, drop me a line.
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