Collins Aerospace Museum

Jun-18 ···
233D Transmitter Restoration Project

Some progress this week!
Rod brought in switch wafers that appear to be exactly what we need to replace the sections on the channel select switch. He had thought we would need open up the shaft hole, but it turns out that the holes are the same shape and size and the mounting hole spacing is correct. Indexing is also the same. Only difference is the blade on the rotor part may be a little narrower on the new sections, but that should not be a problem.

Jules has started preparing the sections for replacement. For now, we will only replace the two rear sections because they both have badly corroded contacts that have been difficult/impossible to restore. One of the these sections is critical to the Autotune process and the other is used to establish suppressor grid voltage for the oscillator (837) stage.

Interesting thing about the suppressor grid voltage selection: the voltage divider and switch section are not shown on the schematics or the parts list, but are discussed in the tuneup procedure. Other interesting aspect is that it appears only a small, specific range of frequencies can be used for each of the ten channels. This is discussed to some extent in the documentation.

Other thing is that there is a 35pf (aka mmfd) trimmer on the oscillator grid circuit that isn’t shown on the schematic or parts list, but is discussed in the tune up procedure. The purpose is to allow precise setting of the crystal frequency. Problem is that this will affect all 10 channels, so correction for one channel could make the other nine even further off frequency. Have to believe that the intention was that the transmitter would most likely be used primarily on one channel rather than ten. It appears this trimmer was a “factory installed” option.

Jules plans to get a good start on the switch replacement next week. Good chance that we will be able to put the exciter back in the rack after the holiday weekend. We still may have a chance to get a couple hundred watts (CW) out by the end of July. Larry has the 40-meter dipole installed on the roof, so we should be good-to-go (after we make sure we aren’t causing an EMI disaster!)