I did a little further digging with the RC96, and discovered the following...
The SELECT 1 line which controls the CD4053 (U9)RX audio gate control pin behaves as follows: When the controller is in carrier access modes (COP07, 08, or 09), the pin never changes state with or without a COS and/or PL signal present. It is always low, allowing audio to pass. When the controller is in PL access mode (COP10), the SELECT 1 line becomes high when no signal is present, thus blocking rx audio from passing. When only an active COS signal is presented, it remains high. When only an active PL signal is applied, it still remains high, and no audio passes. However, when both COS and PL active signals are applied, the SELECT 1 line becomes low, and audio is allowed to pass. The line remains low even when COS then dissappears, and stays low until the PL signal ceases. Basicly, both COS and PL are needed to open the audio gate, but only PL is needed to keep it open. Fortunately, there is an easy fix. I simply bent up pin 9 of the 4053 IC (U9) so that the pin is removed from the socket. Conveniently, there is also a board solder/feedthru connection for pin 9 (SELECT 1) just to the right of the IC socket. I simply soldered in a 1N4xxxx series diode, anode connecting to the aforementioned thru-board hole tied to pin 9, and the cathode to pin 9 of the 4053 IC itself. Next, I installed the cathode of a second diode to pin 9 as well. The anode of this second diode connects to the COS signal coming out of the ULN2804 input buffer IC (U5) at pin 18 (this is also the same signal appearing on pin 3 of the optional ADM connector location). The two diodes are simply used to fully isolate the SELECT 1 and COS signals from each other. Now, when the controller is in any of the three COS modes, the SELECT 1 signal has no effect, but the newly connected COS signal switches the 4053 audio gate to track with COS. When the controller is in PL mode, the SELECT 1 signal controls the audio gate so that audio only passes when both COS and PL are present. Since this mode is already working properly via the SELECT 1 signal to track COS (along with PL), the newly installed COS signal is simply "paralled" with the original SELECT 1 signal, and effectively has no bearing on the control at that point. Now, the controller will effectively mute rx audio properly no matter which receiver mode is selected (COS or PL). I successfully tested the RC96 on a Micor receiver feeding approx. 1Vpp into the controller with absolutely no blowby whatsoever. Full muting. This mod will work on the RC85 as well. With the simple installation of two $0.05 didoes, the ACC controllers will now accept unmuted rx audio without a problem. Eric KE2D --- In Repeater-Builder@yahoogroups.com, "Jeff DePolo" <j...@...> wrote: > > > Has anyone ever figured out why the RC96 does NOT mute RX audio when > > the receiver is inactive? > > > > Looking at the schematic it appears that the rx audio mux switch pin > > of IC9 (4053) should, in theory (according to the schematic), control > > the audio gating. The switch control pin is tied to the "select 1" > > line, which is driven by the cpu (indirectly). > > If I remember right, both the RC85 and RC96 behave this way. As you said, > it is likely that they never implemented the control in software. The > RC85/96, unlike many other controllers, let hardware logic do a lot of the > grunt work to un-burden the processor, as is evidenced by the way it does > audio gating. I did a modification at the delay board, possibily similar to > what you did but I don't remember exactly, to properly mute when feeding it > unsquelched audio. > > --- Jeff WN3A >
