Replacing the Band Switch on a Pilot Radiogram
Written by Bryce Ringwood   
I guess we all make mistakes, but this one turned out to be very destructive. The radio was (and happily still is) a Pilot Radiogram, with four nondescript wires hanging out of the chassis. Two of the wires were for the mains connection and two were for the antenna and earth.
 
Sadly, someone chose the wrong pair. But then, because the radio didn't respond .. they turned the band change switch. The owner of the set blamed "kids". I think today's kids are far more savvy. I blame "adults".
 
This simple act burned out all the antenna coils and the antenna wafer on the switch. The antenna coil on the ferrite rod was missing - presumably someone brushed the charred remains away. 
Burned Out Wafer Swich
 
The contacts have melted and the rotor is cracked and burned
 
I contemplated the situation, then offered to install 4 antenna wires. The owner said " that would be ridiculous "(which it would) so I gave her a quote for a new switch (replacing the single burned out wafer wasn't an option, as the offending wafer was the "middlest" one.) Fortunately I had a switch kit. Note that you can purchase switch kits from electronic suppliers, such as RS electronics and Mantech. Each wafer costs well over R100-00 before you begin, so that the new switch can cost well over R1000-00.
 
There is an additional complication, in that the original switch was a custom-made item with front and back wafer contacts. These special wafers had to be simulated by adding additional wafers, where the original switch had five wafers, the new one had to have seven.
 
Original band switch
Original Band Switch
 
Since this won't be the first or last time a band-change switch needs replacement, here's one way of going about installing a replacement:
 
  1. Try to get a circuit diagram for the set, then study the action of the switch. If no circuit diagram is available - trace the circuit.
  2. Take out each antenna coil in turn. The break will often be at the point where the antenna coil is soldered to the tag on the former. In the case of this radio, a portion of each coil had melted away, like a blown fuse.
  3. Unwind the antenna coil counting the turns and rewind it with the same gauge of wire. Be sure to tin the ends before soldering - very tricky with enamelled wire.
  4. I wound 30 turns of fine wire at the earthy end of the ferrite rod winding. One end was soldered to earth, the other to the antenna tag on the wafer switch temporarily. Replace at step 11 and connect to the correct tag on the antenna wafer.
  5. Check that the radio works on all bands by attaching the antenna to each antenna coil in turn. Also check that there is continuity between the hot end of the antenna coil and ground. If there's a problem, its probably soldering that wretched enamelled wire.
  6. Have you drawn out the circuit and understood how it works ? This is your last chance to check it. 
  7. In this case, the wafer switch could be removed by removing the rods holding the spacers and extracting the main shaft through a slot in the chassis. The only way to remove the rods was to drill two holes at the rear of the chassis, and draw them out backwards.
  8. Snip off the wafer connections.
  9. Remove the old assembly, including wafers.
  10. Now study the new switch and wafers. The rods will be at a different pitch and orientation, so new holes for the rods will have to be drilled - in this case the entire width of the chassis. I used a special extra long 2mm drill to drill pilot holes, and finished with a 3.2 mm long drill from each side of the chassis.
  11. The antenna circuits in the Pilot were separated from the local oscillator circuits with a screen. These were reassembled first. Any wires that were too short were replaced in their entirety. Solder the deepest connections first. Put any extra components on the wafers first. 
  12. CHECK YOUR WORK!
  13. Each wafer in the local oscillator section was wired up, followed by the gramophone connection.
  14. After a final CHECK - the set was switched on. 
  15. The set will now work with an antenna. Check and re-align each band in turn - the rewiring will have disturbed the alignment by a small amount.
 
The final jobs were to check the AGC, refit the volume control/on off switch and fit a decent 3 core mains lead and plug. New Band Swich
New Band Switch
 
The turntable that came with the set had a modern stereo cartridge with no stylus. I used a new BSR stereo-compatible mono cartridge. This is a very loud radio, whether on GRAM or MW/SW. The sensitivity is about what you would expect from an ECH42 front-end, nevertheless there are plenty of stations to be heard on each of the shortwave bands.
 
The valve line-up is EZ41 (Rectifier), EL41(Output), EBC41 (Detector and AGC) , EF41 (IF Amplifier), ECH42 (Mixer/Osc) and an EBC41 doing something unknown. The magic-eye is the expensive EM34.
 
(To be edited)
 
 
 
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