Davis Clark ISOCOM pinouts

[Wayne]

Working on a 172 with an ISOCOM panel mount intercom. There are no jacks
on the rear seats. Anyone have the pinouts for the unit? I've never got to
see the instructions for an intercom and wondered how they are wired so the
PTT only works for the guy that pushes the button. I assume that the mic's
for the rear are completely isolated from the com radios but am not really
sure.

I have seen people use splitters for the headsets, anyone know if the
change in impedance would hurt the little amp chips or are all headsets
paralleled into one channel? I really wish I had my hands on an installation
manual.

Thanks

Wayne
PP-ASEL
63.9 Hours

[MikeM]

Wayne wrote:
>
> Working on a 172 with an ISOCOM panel mount intercom. There are no jacks
> on the rear seats. Anyone have the pinouts for the unit?

Pins labled 1-8 (Right to Left across top of connector)
J-A ( " " " " bottom " " )

7 A/C Mic Key Line
B A/C Phone Bus
D Dimmer
C Dimmer
1 14/28V Power
J Phone Audio Out (my guess: wire all headset jacks in parallel)
6 Pilot Mic Key (my guess: wired to switch in Pilots yoke)
8 CoPilot Mic Key (my guess: wired to switch in CoPilots yoke)
A A/C Ground
F A/C Mic Audio
E Pilot Mic Jack
H CoPilot Mic Jack
2 Pass1 Mic Jack
3 Pass2 Mic Jack
4 Pass3 Mic Jack
5 Pass4 Mic Jack

> I've never got to
> see the instructions for an intercom and wondered how they are wired so the
> PTT only works for the guy that pushes the button.

Two diodes inside the Isocom could do this?

> I assume that the mic's
> for the rear are completely isolated from the com radios but am not really
> sure.

Appears that each mic has its own bias source, but likely the audio from
all mics is summed ahead of the intercom squelch circuit.

> I have seen people use splitters for the headsets, anyone know if the
> change in impedance would hurt the little amp chips or are all headsets
> paralleled into one channel?

My guess is that it can support up to 6 stations...
No splitter required; just wire all four (up to six) headset jacks in
parallel

>I really wish I had my hands on an installation
> manual.

I dont have one.

MikeM
Skylane '1MM
Pacer '00z

[MikeM]

Wayne wrote:
>
> Thank you! I haven't dug the unit out of the panel yet so I hadn't seen
> the labes. Still not sure where the negitive of the headsets and mic's go
> but I imagine that will be obvious when I tear into it. Now I know that they
> are labeled and that all the heatsets are wired the same and that the pilot
> and co-pilot each have their own PTT and mic input. Thank you very much,
> that explains things nicely. Not sure what to think about the diode thing
> though, audio is AC current and and so are a mic's output so a diode would
> make it pulsating DC rather than stopping backflow.


There are two aspects of this switching: Mic and PTT. The PTT "or gate"
is usually implemented with two diodes, wired so that if either Pilot
or CoPilot Key switch is depressed, then the PTT line of the Com
transmitter is pulled to ground.

During PTT, whichever (Pilot or CoPilot) Key switch is depressed causes
the Mic line from the respective side to be connected directly to the
Com transmitter's input. In this way, only audio from the selected
Mic modulates the transmitter, Moreover, during transmit, the Mic bias
is taken from the Com radio (not from inside the intercom like it is
when the PTT is not keyed). Most older intercoms used a reed relay
to acomplish the internal mic line switching (Three states: Intercom-
niether relay on, Pilot Ptt- one relay on, CoPilot Ptt- other relay no)

All audio grounds (including shields) are connected to pin A
(Intercom Power ground). To keep alternator whine and strobe squeal
out of the audio, (especially) the rear seat jack (Mic and Hdphone)
barrels must not be "grounded" to the airframe. Instead, use
plastic sholder washers to keep the jack barrels from touching, and
connect the barrels to either a "ground wire" inside the shield, or
use the shield itself to connect the remote jack barrels to Pin A.
Trust me on this: I know of which I speak....


MikeM (PhD EE)
Fixed dozens of aircraft audio systems...

Skylane '1MM
Pacer '00z

[[email protected]]

I've got a DC ISOCOM in my bird, but have been recently looking to
upgrade. It works well, but has only one squelch setting for all mics,
opens all mics at one (not just the one who's speaking), and doesn't have
music input. I've found that these are usually pretty advanced intercom
features so I'm shopping around.

I recently got the DC ISOCOM working better, now that I managed to
google up the manual for it (very obscure link somewhere, definately not
on DaveClark.com). There are two adjustments behind the two faceplane
philips screws. If you remove them, you can put in a precision regular
about 1.5" and adjust the sidetone level through the left hole, and radio
volume through the right. The nifty thing was that the manual had an
*ACTUAL* schematic in the thing, so now I know how it works/doesn't work.
Thinking about hacking it slightly to add music input, but can't figure
out how to make it mute when the radio comes on.

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[Wayne]

I got your email with the file attached. Thanks again, very much. I saw the
schematic, I knew diodes wouldn't work to isolate an AC mic source. Tgey
just call it an isolation circuit. Sorry I can't help on the auto mute.

Wayne
> wrote in message
...
> I've got a DC ISOCOM in my bird, but have been recently looking to
> upgrade. It works well, but has only one squelch setting for all mics,
> opens all mics at one (not just the one who's speaking), and doesn't have
> music input. I've found that these are usually pretty advanced intercom
> features so I'm shopping around.
>
> I recently got the DC ISOCOM working better, now that I managed to
> google up the manual for it (very obscure link somewhere, definately not
> on DaveClark.com). There are two adjustments behind the two faceplane
> philips screws. If you remove them, you can put in a precision regular
> about 1.5" and adjust the sidetone level through the left hole, and radio
> volume through the right. The nifty thing was that the manual had an
> *ACTUAL* schematic in the thing, so now I know how it works/doesn't work.
> Thinking about hacking it slightly to add music input, but can't figure
> out how to make it mute when the radio comes on.
>
> -Cory
>
> --
> ************************************************** ***********************
> * The prime directive of Linux: *
> * - learn what you don't know, *
> * - teach what you do. *
> * (Just my 20 USm$) *
> ************************************************** ***********************
>