There are several mistakes that can be made in deciding how to configure a cockpit to cockpit intercom for the Stearman. At Chino, we have made all of them, sometimes twice.
The first was to listen to the marketing hype and conclude that the intercom performance would just have to be better by buying and installing one of the separate “Intercoms” on the market, instead of using the I/C built into the Com unit. For three Stearmans we selected a widely advertised unit that was excellent for “High noise environment and had push-to-talk capability”. We lived with those units for about a year and experienced poor low level intercom performance and equally poor audio reception from the com unit receiver. We had contacted engineering for both the King and the Terra previously and were told by both that we were wasting out money to add an intercom to the one that was already built into the unit. Yeah, right!! What did they know!!
We had a lot of technical support from the intercom supplier and even took the field engineer for a ride in the Stearman to “hear for himself”. He reviewed the wiring hookup, adjusted the volume on the Com unit, the intercom and the headset in several different combinations. Finally he stated “that is about as good as it is going to get”!!
We pulled the intercoms from all three Stearmans and rewired them to utilize the built-in capability of the King and the Terra, using David Clark headsets. At the same time, we added four relays in the setup to incorporate the following functions:
The Mikes on the pilot and copilot headsets are isolated and only one is hot at a time. This cuts the wind and engine noise picked up by the mikes in half, and improves the audio output from the one mike
- on line.
- Some com units don’t like having mikes of different impedances on line at the same time, and don’t perform well when they are.
- Since this relay system isolates the two mikes you can use mismatched headsets with no problem.
- The I/C push-to-talk switch in either cockpit feeds mike output directly into the I/C audio of the com unit and volume can be controlled with the volume on the com, or on a small separate panel with the Terra.
- The transmit PTT switches route the audio into the com X/M terminal, and also keys the transmit switch.
- The pilot X/M PTT will override the Copilot X/M function.
Intercom Relay Configuration for the King 97 and 197
The performance of the reception on the com unit and the volume and clarity on the intercom improved to an amazing extent on all three Stearmans.
On the Terra units we added a small I/C control panel from Terra that just fit in the space where the other intercom used to be. This panel provides volume control, switching from VOX to PTT, and squelch control. However, it stays on PTT with squelch set at the minimum. The Terra wiring is a bit different, but uses the same principle.
Trying to use voice activation (VOX) instead of push-to-talk, in a Stearman is another trap to avoid.
For the system shown in the wiring diagram, you need four double pole-double throw relays. They should be high quality, hermitically sealed relays. Stay away from the relays with unsealed plastic covers. You can put commutating diodes across the relay coils and reduce any clicking noise in the radio. We didn’t and don’t notice any. Don’t underestimate the quality of the intercom built into the King KY-97 and 197A. (96 and 196 if you have a 24 volt system.) You can’t beat it or even match it in Push-to-talk mode.
(This will also work for any other com that has a separate I/C audio input). See Mac McCauley for a testimonial on this subject, if you have an hour or so to spare.)