Tech Tips on the 831-G2
by James Thorusen
In addition to the tips below, My
suggestion (assuming the parts are still available from Continental) is to
replace the plate tuning capacitance plate with a loading capacitance
plate. These are the plates that move in and out under motor drive to tune and
load the PA. The loading capacitor plate is larger in area than the tuning
capacitor plate. I found that making this switch allowed more tuning range for
plate tuning and reduced the amount of tinkering with the shelf position for
coarse PA tuning, which can be a real nuisance if it has to be repeated numerous
times to get "in the ball park".
James K. Thorusen, Chief Engineer Central Coast Electronics
by Gray Haertig
(If you have additional tips that you would like to share on the 831-G2, or any other transmitter, please send them to us.
The 831-G2 was a 27.5 kW FM transmitter manufactured by Rockwell-Collins in the late 1970s.
1) Mount two real iron vane voltmeters in power supply cabinet to measure PA and IPA filament voltages (with selector switch) and line-to-line and line-to-neutral AC mains voltages (again with a selector switch). Square-D makes an industrial switch that will do all the proper switching gyrations for the mains meter. Make sure you connect the filament voltage samples right at the fil transformers.
2) Throw away the filament voltage regulator card and bypass the line voltage terminals. Install a constant voltage transformer in the middle of the middle cabinet to supply the filament voltage transformers. Move the PA filament voltage variac to the front panel. (It should be wired after the sola transformer). Decouple the IPA and PA filament adjusts so they can be adjusted independently. (As they come from the factory, the IPA filament adjuster (a line side pot) is hooked up after the PA filament variac.)
3) Replace the gating cards with the new version. Recap the electrolytics in the old ones and keep for emergencies.
4) Install an isolator at the RF input to the IPA.
5) Install the exciter in an outside rack located on the LEFT end of the transmitter. Things located on the right end tend to get a lot of induced hum.
6) Check all the remote control latching relays and various control relays. Percussive testing is appropriate.
7) Install a motorized 3 phase 240V delta connected variac before the screen voltage transformer. This allows you to fine tune the plate voltage to screen voltage ratio for best synchronous AM or least group delay or best efficiency. Whatever floats your boat.
8) Tap the transformers so the XMTR will just make 105% to 110% wide open. Things are nicer when the gating cards don't have to do much.
9) Throw away the old phase loss circuit and replace with one of those modern solid state plug in over voltage, under voltage, and phase rotation monitors.
10) DOCUMENT ALL YOUR MODIFICATIONS, in case I have fix this thing after your boss finds out what you REALLY do when you say you're going to the transmitter. <g>
11) Stand back and enjoy what I think is the best high power FM tube transmitter on the market, which you have made even better.
This information comes courtesy of Gray Haertig, in Portland, Oregon