NX-P1300 – no mic audio to TX, bias voltage acting weird after resoldering

[baka23]

Got a second P1300 here that's otherwise fully functional — RX is fine, PTT keys up RF properly (confirmed on a second radio, squelch breaks and shows busy channel) — but zero audio makes it to the transmitted signal. Sounds like it's putting out a dead carrier with nothing on it.


Here's the whole timeline since it's a bit of a mess:


Started off assuming it was just a bad mic capsule, so I swapped in a new one. Didn't fix it. Pulled the new mic back off and set it aside for a bit.


Decided to check what bias voltage should look like, so I measured a known-good P1300 I have on hand — read 2.7-2.9V there. Went back to the problem radio and measured it with the mic off the board, and it was also reading 2.7V. Figured great, board's fine, and soldered the mic back on.


It worked — for a couple seconds — then died. Audio gone again.


Took it back apart, measured voltage with the mic still soldered on: 0V. Desoldered the mic again and measured with it off the board: now it was jumping around, reading somewhere between 1.1V and 2V, not stable at all.


At that point I figured maybe it's a bad solder joint or pad, so I reflowed the pads a few times. Checked voltage again with the mic off: 3.3V. Looked promising, so soldered the mic back on — straight back to 0V again.


Did the same thing again: desoldered, measured 3.1V. Soldered the mic back on: 0V again.


So the pattern I'm seeing is completely consistent — voltage reads fine (anywhere from 2.7V to 3.3V) whenever the mic is off the board, and it collapses to 0V every single time I solder the mic back on, no exceptions.


Checked resistance from the bias pad to ground too, both ways:

• Mic off the board: OL (open), so no dead short to ground on the board itself.
• Mic soldered on: reads about 1.428kΩ, which I believe is just the mic's normal internal FET load, not a short.

My current theory is that the pull-up resistor feeding that bias line (from the supply rail down to the mic pad) has drifted way out of spec or got damaged from all the heat I've put into this board reflowing/resoldering. That would explain why it reads fine with nothing connected (barely any current flowing) but collapses once the mic's normal load is applied and the voltage divider math falls apart.


Does anyone know where that bias resistor would actually be on the board, or have a schematic/board photo they could point me to? I'd like to isolate it and measure it directly, but I'm not sure exactly which component to look for near the mic connector.


Any input at all is welcome — schematics, board pics, "seen this before" reports, anything.

 

[kd4efm]

United radio for repair.

 

[ramal121]

The mic element goes directly to the accessory connector switching contacts. Make sure they are not boogered.

 

[n7maq-1]

The radio is still under factory warranty, I would send it to United Radio as KD4EFM said.

 

[AM909]

@baka23, do you have a speaker-mic and does it TX audio with it?

What they said above – if it's under warranty, let the experts repair/replace it before you do something that could violate the warranty.

If you insist ...

I see 2270 ohms to 3.2V

 

[WB5UOM]

are we really going to ask for warranty repair after soldering and reflowing on the board and removing and reinstalling mic element?

 

[n7maq-1]

are we really going to ask for warranty repair after soldering and reflowing on the board and removing and reinstalling mic element?

If the work is clean there is no reason that you can't.

 

[WB5UOM]

uh no...not from reading 1st post.
But then again, I am known for brutal honesty. I would not do it.
Now, I have attempted what I thought was an easy repair on a in warranty radio..and it didnt go well for me..I sent it in with a note of what I did and said to bill me regular non warranty rate.
They repaired it, included a note in the box that said they appreciated my honesty, and did it at no charge.

 

[n7maq-1]

If you look at the pictures in the first post you can see the pads and they look fine. The mic is not mounted to the board, it has wires that solder to the board.



I have worked as a tech for over twenty five years for a Kenwood dealer and if something is time consuming (The warranty labor rate is not that good) we will send the radio in after we have done work on them that did not repair the issue.

 

[AM909]

uh no...not from reading 1st post.
But then again, I am known for brutal honesty. I would not do it.
Now, I have attempted what I thought was an easy repair on a in warranty radio..and it didnt go well for me..I sent it in with a note of what I did and said to bill me regular non warranty rate.
They repaired it, included a note in the box that said they appreciated my honesty, and did it at no charge.

We usually take care of the easy stuff when we can (and have the parts) but leave any work involving more than about an 8-pin critter to someone with better eyes, hands, and tools. It's a careful balancing act between the mundane unchargeable administrative hassle of shipping and installation, initial diagnosis and work, low re-imbursement rates, and (dare I say) ego bruising by needing someone else to help.

 

[WB5UOM]

I dont bother getting reimbursed for simple warranty work...
but, really there isn't much of it anyway...KENWOOD makes a pretty dang good product...better make money on the sale,cuz there will not be hardly any repairs.

 

[fontanei]

Got a second P1300 here that's otherwise fully functional — RX is fine, PTT keys up RF properly (confirmed on a second radio, squelch breaks and shows busy channel) — but zero audio makes it to the transmitted signal. Sounds like it's putting out a dead carrier with nothing on it.


Here's the whole timeline since it's a bit of a mess:


Started off assuming it was just a bad mic capsule, so I swapped in a new one. Didn't fix it. Pulled the new mic back off and set it aside for a bit.


Decided to check what bias voltage should look like, so I measured a known-good P1300 I have on hand — read 2.7-2.9V there. Went back to the problem radio and measured it with the mic off the board, and it was also reading 2.7V. Figured great, board's fine, and soldered the mic back on.


It worked — for a couple seconds — then died. Audio gone again.


Took it back apart, measured voltage with the mic still soldered on: 0V. Desoldered the mic again and measured with it off the board: now it was jumping around, reading somewhere between 1.1V and 2V, not stable at all.


At that point I figured maybe it's a bad solder joint or pad, so I reflowed the pads a few times. Checked voltage again with the mic off: 3.3V. Looked promising, so soldered the mic back on — straight back to 0V again.


Did the same thing again: desoldered, measured 3.1V. Soldered the mic back on: 0V again.


So the pattern I'm seeing is completely consistent — voltage reads fine (anywhere from 2.7V to 3.3V) whenever the mic is off the board, and it collapses to 0V every single time I solder the mic back on, no exceptions.


Checked resistance from the bias pad to ground too, both ways:

• Mic off the board: OL (open), so no dead short to ground on the board itself.
• Mic soldered on: reads about 1.428kΩ, which I believe is just the mic's normal internal FET load, not a short.

My current theory is that the pull-up resistor feeding that bias line (from the supply rail down to the mic pad) has drifted way out of spec or got damaged from all the heat I've put into this board reflowing/resoldering. That would explain why it reads fine with nothing connected (barely any current flowing) but collapses once the mic's normal load is applied and the voltage divider math falls apart.


Does anyone know where that bias resistor would actually be on the board, or have a schematic/board photo they could point me to? I'd like to isolate it and measure it directly, but I'm not sure exactly which component to look for near the mic connector.


Any input at all is welcome — schematics, board pics, "seen this before" reports, anything.

Can you show us your soldering station? 🤣🤣🤣

Although I laughed in the above question, I am curious on the tip size you used. And the heat setting of the station if it has such setting.

You've to be careful when using a solder iron. (Not that you weren't)

Do the trades still have continuity?

 

[baka23]

Can you show us your soldering station? 🤣🤣🤣

Although I laughed in the above question, I am curious on the tip size you used. And the heat setting of the station if it has such setting.

You've to be careful when using a solder iron. (Not that you weren't)

Do the trades still have continuity?

Ok so yes I finally got around to working on this radio again, so first things first yes, I'm using the smallest tip size I have (I don't want to overheat anything I don't need too) at 380 degrees C, I did end up fixing it; ramal121's post mentioning that the mic's positive pad trace leads to the mic jack ports, I reflowed the solder joints on the headphone jacks and it started getting voltage again at the mic pads, so I soldered the mic back and now it's a stable 2.5v with the mic soldered, so it fixed itself somehow by reflowing the jack solder joints, as it works now, so I'm happy about that. All of you guys talking about the warranty, there really is no point of going to warranty, they take forever and I have no way of providing proof of purchase, as I'm just an IT guy. I've been just repairing all of my NX340s and NX1300s. Thanks for all the help you guys, I'll come back when another radio is bound to trip me up.

 

[baka23]

@baka23, do you have a speaker-mic and does it TX audio with it?

What they said above – if it's under warranty, let the experts repair/replace it before you do something that could violate the warranty.

If you insist ...

I see 2270 ohms to 3.2V

Even though I didn't end up using this, I appreciate you giving this, this is very useful info and hard to find at least for me, Thanks.