TK-790H No Modulation

[W0DAY]

So I’ve come to confess my shortcomings. 😂.

Last week I ordered a RMC-9C desk mic to rewire for my TK-790H since the RMC-9B are like finding gold bullion.
When I rewired it and tested, I got “low audio” reports and some reports of a “buzzing” when transmitting.

I ordered a new mic to try and rule out and now I have a full 110w carrier but no modulation whatsoever.

Tried 2 seperate Kenwood OEM Mic heads and an Amazon knockoff head. All were the same.

Have I just trashed my radio and turned it into a scanner!? 😂

Open to ideas.

 

[AM909]

You mean KMC-9C, right? Is there 13.8V on the SB (switched B+) pin 1 at the top of the round mic connector on the radio when it's on? Did the stock KMC-27B or C mobile mic work?

 

[W0DAY]

You mean KMC-9C, right? Is there 13.8V on the SB (switched B+) pin 1 at the top of the round mic connector on the radio when it's on? Did the stock KMC-27B or C mobile mic work?

Yes… KMC is what I meant.

And yes, the stock KMC-27b worked before trying the modified desk mic.

I will have to check on the voltage. Not at the house at the moment.

 

[AM909]

Hmmm. That radio is before (and after) my time, so I'm not directly familiar with it, but burnout of SB to the mic connector is a common problem across Kenwood models. The KMC-27B looks like a straight dynamic mic, and so would work without that SB. The -27/27A have noise cancelling that needs it, and the -9/9B/9C are all amplified and so would need it.

 

[W0DAY]

Hmmm. That radio is before (and after) my time, so I'm not directly familiar with it, but burnout of SB to the mic connector is a common problem across Kenwood models. The KMC-27B looks like a straight dynamic mic, and so would work without that SB. The -27/27A have noise cancelling that needs it, and the -9/9B/9C are all amplified and so would need it.

It does have 13v at the mic.
I tried a KMC-27 that I had,
I bought a new KMC-28 DTMF mic that lights up and works otherwise, just no modulation.

I’ve tried 2 factory mic cables an a knockoff Amazon cable with no changes.

 

[mmckenna]

If it helps, here's the pinout for the KMC-9B:




Here's the pinout for the KMC-9C:

 

[mmckenna]

And I have to ask:

Is this a used radio you purchased and did it ever sound clean? There can be a lot of causes for the buzz in the audio, and that may take some troubleshooting.

There are also a lot of settings that can impact audio, and if something got changed, that may have caused this.

 

[W0DAY]

And I have to ask:

Is this a used radio you purchased and did it ever sound clean? There can be a lot of causes for the buzz in the audio, and that may take some troubleshooting.

There are also a lot of settings that can impact audio, and if something got changed, that may have caused this.

Yes. Radio has been functioning properly for over a year with the KMC-27 mic. I tried making the adaptor for the KMC-9C and after that was when the noise started.
I tried changing a few pins and then just waited until I got the new mic in. (KMC-28)

After the new mic, I tried the KMC27 head and the KMC-28 head and neither would give me any audio.

 

[buddrousa]

Now that no mic works it makes me wander if power from pin 1 was put back into the radio on the audio circuit?

 

[W0DAY]

Now that no mic works it makes me wander if power from pin 1 was put back into the radio on the audio circuit?

It very well could have been. Me being dumb, I didn’t look up the pinouts when trying to make the adaptor cable, I just tried to match up the tiny wires. 😬😂

 

[n7maq-1]

You might try hooking a mic up to the rear DB25, you will need to address R640 and R641. The service manual is up on repeater-builder.com.

Jim

 

[AM909]

There is actually a physical mic sensitivity pot, VR501, on the TX/RX board. Is it maybe dirty?

If you don't have an audio signal generator to inject audio, if I'm reading it right, it looks like the junction of the top of VR501 and R518 should measure 400-600 ohms to ground and straight through to the MIC input on J1 pin 4.

 

[W0DAY]

You might try hooking a mic up to the rear DB25, you will need to address R640 and R641. The service manual is up on repeater-builder.com.

Jim

I’ll look into that process.

 

[W0DAY]

UPDATE

In doing some testing, I hooked up and powered up the Pyramid vehicle repeater that used to be on this radio and when I key that up through the DB25 connector, I get full modulation.

So…

Do you think I maybe shorted something in the remote head via the janky microphone?

Worth ordering a used remote head to try?

 

[W0DAY]

@AM909 @n7maq-1 @mmckenna

 

[mmckenna]

So…

Do you think I maybe shorted something in the remote head via the janky microphone?

Worth ordering a used remote head to try?

There's not much in the control head that has to do with the microphone, other than the connector. It's possible that there is a damaged trace and replacing the control head would fix it, but I'd not gamble on that.

The paths for the control head mic and the rear DB25 mic input are separate in the RF deck, up to a point where they are combined/switched. It is entirely possible you damaged that, or some other component along the way.

Honestly:
Unless you have the skills, tools, parts and eyesight to do component level repairs on these radios, you might come out ahead by just buying a new RF deck, since that is likely where the issue is. If you can get a new control head cheap/free, you can give that a try, but not much in there that would screw up the mic function other than a burnt trace, which is unlikely (but possible).

Gotta find your own comfort level repairing old radio like this. Make sure you consider the risks/costs/time/labor and the benefit of just grabbing a new one off e-Bay. I personally find that my time and sanity is worth more than trying to fix a 25 year old radio.

 

[W0DAY]

There's not much in the control head that has to do with the microphone, other than the connector. It's possible that there is a damaged trace and replacing the control head would fix it, but I'd not gamble on that.

The paths for the control head mic and the rear DB25 mic input are separate in the RF deck, up to a point where they are combined/switched. It is entirely possible you damaged that, or some other component along the way.

Honestly:
Unless you have the skills, tools, parts and eyesight to do component level repairs on these radios, you might come out ahead by just buying a new RF deck, since that is likely where the issue is. If you can get a new control head cheap/free, you can give that a try, but not much in there that would screw up the mic function other than a burnt trace, which is unlikely (but possible).

Gotta find your own comfort level repairing old radio like this. Make sure you consider the risks/costs/time/labor and the benefit of just grabbing a new one off e-Bay. I personally find that my time and sanity is worth more than trying to fix a 25 year old radio.

That makes complete sense. I did find a cheap head for $25 to try. When it comes In I’ll give it a shot.

If no luck, I’ll see about wiring up a mic on the db25 plug since that even obviously works and I have a db25 and a mic.

Thanks for the help!