Low Audio with ID-O-MATIC III and two GM300's

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Low Audio with ID-O-MATIC III and two GM300's

Postby KE4NYVJR » Fri Nov 01, 2013 5:52 am

I have a crummy repeater I built up using two GM300's, strapped together in the most simple form to create a repeater. It has been running for over a year without a problem, good audio too. However, I wanted to add a tail, ID and courtesy tone, so I picked up one of the new ID-O-MATIC III kits. Kit went together fine. I got everything wired up and I have good COR detect/keyup and I do have the courtesy tone (kind of low, but works), but my transmit audio is barely audible. I did some searching and came across this document on modding the ID-O-MATIC II for use with GM's: http://www.repeater-builder.com/motorol ... k-rev2.pdf

However, it refers to modding the II that also has an on-board LM386 op amp. They did away with the op amp on the III, so this document is sort of useless to me. I did print out both schematics and tried to make a comparison, but since the op amp is no longer used, it's a whole different audio pass scheme (from what I can tell).

I did a quick test by leaving the ID-O-MATIC III tied to the COR ground and power and removed the RX audio and TX Audio lines. Tied them back together (like the old configuration) and I got my good loud audio back. The audio is obviously being attenuated in the ID-O-MATIC III.

Just wondering if anyone else has tackled this yet. We might have a used GR300 coming soon and I would like to pull it's RICK out and replace it with another III, if I can get it working correctly.

Thanks!
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Re: Low Audio with ID-O-MATIC III and two GM300's

Postby Jim202 » Fri Nov 01, 2013 12:44 pm

KE4NYVJR wrote:I have a crummy repeater I built up using two GM300's, strapped together in the most simple form to create a repeater. It has been running for over a year without a problem, good audio too. However, I wanted to add a tail, ID and courtesy tone, so I picked up one of the new ID-O-MATIC III kits. Kit went together fine. I got everything wired up and I have good COR detect/keyup and I do have the courtesy tone (kind of low, but works), but my transmit audio is barely audible. I did some searching and came across this document on modding the ID-O-MATIC II for use with GM's: http://www.repeater-builder.com/motorol ... k-rev2.pdf

However, it refers to modding the II that also has an on-board LM386 op amp. They did away with the op amp on the III, so this document is sort of useless to me. I did print out both schematics and tried to make a comparison, but since the op amp is no longer used, it's a whole different audio pass scheme (from what I can tell).

I did a quick test by leaving the ID-O-MATIC III tied to the COR ground and power and removed the RX audio and TX Audio lines. Tied them back together (like the old configuration) and I got my good loud audio back. The audio is obviously being attenuated in the ID-O-MATIC III.

Just wondering if anyone else has tackled this yet. We might have a used GR300 coming soon and I would like to pull it's RICK out and replace it with another III, if I can get it working correctly.

Thanks!



Without a diagram of how your ID unit is, I can only take a guess at the problem.

On just about all of the Motorola radios, there is a bias voltage on both the RX audio and the mic high line. You don't want to ground these or run them through a transformer that has one side of the winding grounded. it is always best to use at least a 1.0 uf cap in series with the lines. Too large of a cap will cause all the highs to go away.

You may even have to add a series resistor (5 to 50K depending on the circuit being used) on the mic line to keep from loading down the circuit.

Good luck on your efforts.

Jim
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Re: Low Audio with ID-O-MATIC III and two GM300's

Postby Will » Fri Nov 01, 2013 12:57 pm

[quote="KE4NYVJR"] I did some searching and came across this document on modding the ID-O-MATIC II for use with GM's: http://www.repeater-builder.com/motorol ... k-rev2.pdf

That is our company's document that was supposed to be removed from the site. It has some errors in it.

In the interim, Wire the RX Radio's audio directly thru a 2.2 K resistor and a 2.2uF capacitor in series to the TX radio's mic input, capacitor's + towards the TX radio mic input.
The ID-O-Matic pin 11 thru a 6.8 K resistor to the TX radio mic input.

It is sad that the audio IC was eliminated from the ID0Matic III design.
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Re: Low Audio with ID-O-MATIC III and two GM300's

Postby KE4NYVJR » Sat Nov 02, 2013 8:11 am

Will wrote:In the interim, Wire the RX Radio's audio directly thru a 2.2 K resistor and a 2.2uF capacitor in series to the TX radio's mic input, capacitor's + towards the TX radio mic input.
The ID-O-Matic pin 11 thru a 6.8 K resistor to the TX radio mic input.


Thanks for the tip. This morning I built up this little board to do exactly what you suggested:

Image

However, I didn't have a 2.2uF cap, so I had to use two 1uF caps in parallel. I am hoping that I won't get some kind of harmonics by passing audio through two parallel caps opposed to a single cap of the correct value.
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Re: Low Audio with ID-O-MATIC III and two GM300's

Postby Will » Sat Nov 02, 2013 12:17 pm

Jason,

Great idea, that will work.
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