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TK790H Intercom Issue

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crawler

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May 6, 2017
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Hello,

I have a TK790H with a remote mounted KCH-11 head.

The KCH-11 head is interfaced to a Rugged Radios Intercom system via the 12 pin connector on the back of the head unit, they are mounted immediately next to eachother.

We are having an issue where we cannot get the speakers in our headsets to work if the mic/PTT in the headsets is functional and vice versa.

In order to get the speakers in the headsets to work pins 3 and 6 must be shorted together on the 9 pin connector on the radio, BUT if these are shorted the PTT and mic on the headsets is not functional and we cannot transmit out.

If we disconnect the jumper plug and 'open' pins 3 and 6 the PTT and mic works, but we do not get audio in the speakers of the headsets when receiving but the busy light on the display illuminates.

Is there an internal modification that must be made to the radio for us to get the intercom to work properly or something else that we're missing? We're stumped.

FYI- the 12 pin connector is wired as follows:
Pin 4 - Mic input
Pin 6 Aux input 1 PTT
Pin 10 Speaker
Pin 11 Speaker

Thanks in advance for any input on this.
 

crawler

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May 6, 2017
Messages
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You don't list a ground. How is that made, through the power wires?
Ha. One of the two contacts on the 12 pin that I have listed as "speaker" has two leads terminated to it... though i'm not sure as to what the function of each of the pins on the intercom side of the adapter are.

The com adapter cable in question was removed from an other intercom system. It was mated to a TK790h as well, but that system had the KCH-10 head with the internal speaker.
 

crawler

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May 6, 2017
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...also, the intercom itself is powered and grounded independently of the radio... but currently hooked up to the same 40A power supply.
 

crawler

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May 6, 2017
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Follow up:

-Tried a different intercom, have the same issue.
-Tried using the programming file from another radio with a similar setup, same issue.
-Tried the radio:com interface cable that I have with another TK790H that has a KCH-10 head, NO ISSUE. (Note the second radio that this cable worked with has been "modified" by a race radio shop for use with this intercom system, though I'm not sure what that modification entailed, everything works as intended through the 12 pin molex connector with the radio:com interface cable and that radios KCH-10 head... my radio is brand new out of the box without any modifications, with a KCH-11 head)

I continuity checked the radio:com interface cable and determined that the intercom mic shares a ground with the speakers, hence the two wires at pin 10 of the 12 pin connector.

The PTT ground is interesting though as I figured it was grounded back to the battery through the intercom ground, but it's not. Through the com I have 2.5k ohms to battery ground with the PTT depressed... and from the PTT lead back to the leads going to pin 10 of the 12 pin I have 6 ohms with the button actuated. Sooo the PTT grounds through the same leads as the Mic and the Speakers?? What the heck?

I'm assuming this is where my issue lies as if I bypass the intercom PTT and run my own switch off of Pin 6 (12 pin) to ground everything functions as intended. Alternatively, if i remove "speaker (+)" from pin #11 of the 12 pin connector and place it at pin # 6 of the 9 pin connector (with the 3:6 jumper removed) everything functions correctly as well.

Anyone have any idea why this cable is functioning properly on the "modified" radio and not mine, IE, what has been modified on the radio to allow this intercom to mate to the 12 pin plug on the head unit AND have everything ground through pin 10?

I was hoping to have this all situated by the end of the weekend, but that's not looking so good. I suppose running the intercom speaker wire back to the 9 pin connector on the remote mounted radio base is a work around, but if there is a way to make this all work through the 12 pin connector that anyone knows of please let me know.

Any input is greatly appreciated. Thanks in advance.

*Corrected 12 pin pinout
Pin 4 - Mic input
Pin 6 Aux input 1 PTT
Pin 10 Speaker/Mic/PTT (-)
Pin 11 Speaker (+)
 

ohiohonda

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Mar 24, 2017
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66
This is directly from the manual : "Note : Since the TK-790 uses a BTL audio amplifier, do not
ground the speaker output pin." So the ptt, mic , and speaker should not share the same ground. This is the pin out on the 12 pin connector:

1 IGN I Ignition sense input.
2 SB O Power output after power switch (13.6V or 13.4V±15%).
3 E – Earth.
4 MIC I MIC signal input.
5 ME – MIC earth.
6 AI1 I Auxiliary input 1 (FPU selectable).
7 AI2 I Auxiliary input 2 (FPU selectable).
8 AO1 O Auxiliary output 1 (FPU selectable).
9 AO2 O Auxiliary output 2 (FPU selectable).
10 RS1 O Remote speaker output.
11 RS2 O Remote speaker output.
12 – – Non connection.

I am not sure what modification have been made to the other radio but in order to make your new radio work you would need to use pins 10 and 11 for speaker, 4 and 5 for mic, and 3 and 6 for PTT. Note that since there is not a fixed level audio output from the radio that the volume control on the radio will affect the input into the headset equipment.
 

ramal121

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Dec 5, 2008
Messages
1,744
Location
Sonoma, CA
I did not go through all that you did and look at the schematic, but to me it sounds like the intercom unit is not compatible with the 790 speaker out. It is probably for an unbalanced speaker output (where one side is ground, as in certain portables) and as ohiohonda pointed out the 790 is a balanced speaker where neither side is ground. Also there is a DC bias voltage on the speaker leads and by grounding either will blow the audio PA amp.

What probably saved you is the 6 ohms resistance that kept the current down enough to prevent smoke, but as the DC voltage on the speaker lead was present across this resistance that prevented the PTT line from going fully to ground and hence the no transmit condition.

You'll need to isolate your speaker leads before you connect to the intercom unit. The best way would be a 1:1 isolation transformer. The 790 speaker would feed the primary and on the secondary one lead would go to the speaker hi input of the intercom and the other would go to ground preferably a good radio side ground and not the intercom side that seems to be slightly lifted.

Alternatively you could run the speaker hi input from the I/C through a capacitor (100 uF?) and connect to one side of the 790 speaker and don't connect anything to the other. This will block the DC bias and work against the existing ground to drive headsets with more than enough oomph. This is how I connect Motorola handsets with an unbalanced grounded speaker to a Kenwood radio and it works great.
 
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crawler

Member
Joined
May 6, 2017
Messages
5
Thanks for the replies OhioHonda / Ramal121.

I spoke with the manufacturer of the intercom on Monday, and Ramal is correct on the adding of a transformer to the radio to get it to jive with the com.

Unfortunately a five pin mini xlr input for the interface jack and cable (pinned as I have outlined in one of my previous posts) is the standard for a few different intercom companies in off-road, and after taking a look inside the intercom itself to see what could be done to change this I'm thinking my best bet is to modify or have our local race radio company modify my radio.

Thanks again for the input fellas.
 
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