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Author Topic: HTX-10: hum in transmit audio  (Read 13534 times)

Posts: 4

« on: December 15, 2011, 07:57:41 AM »

Hello!  My HTX-10 has hum in the transmit audio...any suggestions on how to cure this?  Here's a bit more info... (my apologies if this is posted int he wrong area - this is my first posting in eham).

I purchased this rig via ebay a few years back. Unit seemed to transmit fine (checked with watt meter and freq counter), but with the bands dead, I couldn't make any contacts.  With the new sunspot cycle I pulled out the unit and made a few contacts on SSB with a dipole, but all said my signal was difficult to hear, like there was a strong hum.  I then programmed my scanner for 10m and I heard the hum they spoke of.  It's evident on AM, FM, SSB, etc.  I removed the mic and made the unit transmit by grounding the PTT pin - hum still present.  I then changed from using an AC power supply to the car with a mag mount ant - hum still present.

I suspect the audio is being caused internally, injected into the audio circuit...perhaps a bad cap?  I'm very rusty with troubleshooting and don't spend as much time with electronics as I would like (funny how a wife, teenagers, etc. vie for my time as well).  I've searched online and have not found anything that relates to the symptoms I described.  I do have some basic test equipment (multimeter, freq counter, 15 MHz scope), but I'm not certain where to begin. 

Any help would certainly be appreciated.  Thanks in advance!!

Posts: 3227

« Reply #1 on: December 17, 2011, 12:51:32 PM »


I have the same radio, with the much the same problem you report.  I think you'll find that if you touch your hand to the aluminum rear panel while transmitting, you will find that the hum goes away.

When I find the cause of the problem, and the solution, I will try to remember to post it here.

Steve KZ1X/4


Posts: 3227

« Reply #2 on: December 17, 2011, 02:09:43 PM »

Assuming you have the needed skills and tools:

1)  Remove the three small Phillips head screws from the microphone back cover
2)  Locate the black wire connecting to the electret element
3)  Using a fine gauge rosin core solder and a suitable temperature controlled iron, reflow this solder connection. 

Use suitable care; reflow the joint only.  Do not add excessive solder, allow the wire to lift from the pad, or create a short to the adjacent connection.

Even though the joint looks OK, mine wasn't, and other similar repairs have been noted on line elsewhere.

Please post if this fixes your problem.

If you do not have the needed skills or equipment, perhaps you know someone local to do it for you.  Or, send me the mic and $5 for return shipping and I will fix it.


Posts: 4

« Reply #3 on: February 17, 2012, 01:15:36 PM »

Thanks to everyone for your replies.  Sorry to take so long to reply to the thread--Business activities have precluded my ability to tinker with ham radio (how in the world could that happen?--haha)
I printed the service manual and made a bit of headway with the radio, so you could say we're still humming along [humor].  Seriously though, the hum appears to be ~120 hz (measured via the audio output of a scanner), occurs with and without the microphone attached, and is present on the "audio" mike pin (#6 I believe) *before the PTT been depressed*  Pushing the PTT makes the audio disappear from the the mic pin and sends it the transmit audio amp.
A 200 mw amp attached to the mic reproduces sound clearly through the amp.
Touching my hand to the rear of the radio case seems to have no effect on the audio.
I'm suspecting a poor solder joint somewhere in the radio or a bad part (cap?).  I'm still plugging along with this in 30-60 minute spurts of time that I can carve out.
Any other ideas and possible solutions are welcome and appreciated.  I'll post the "cure" when and if I find it.

Posts: 4380

« Reply #4 on: February 17, 2012, 03:16:29 PM »

120 HZ usually comes via the power supply.  Likely a bad filter capacitor.  Something that is supposed to be grounded may not be.  Also could be a bad solder joint.  Hum that gets into the audio and is 60 HZ  is picked up from nearby lamps, other electronic stuff or bad ground.

Good Luck eventually you'll find it.


Posts: 5917

« Reply #5 on: February 17, 2012, 06:36:52 PM »

It hums when operated from 12 volts in the car? With no AC powered supply?

Posts: 4

« Reply #6 on: February 19, 2012, 12:22:20 PM »

Yes, it hums even when used in the car.  I was hoping I could isolate it to the AC power supply, but that's not the case. 
I suspect it's in the mic audio/amp section of the board, isolating it has proven difficult.
-chris, kc5bks

Posts: 2

« Reply #7 on: July 18, 2012, 08:35:36 AM »

I picked up one of these htx-10 rigs on eBay
And I have the same oscillation problem when transmitting
Mostly on ssb. am and fm are fine. I am able to monitor
My output signal on a kenwood th-f6a
And if I don't overdrive the mic gain the transmit
Does not go into oscillation. Try the mike gain at 11 o'clock
Position but no more than 12 o'clock when
Running am and fm. Otherwise the radio works
Really well. It puts out a solid 30 watts on all modes.
I am thinking that the rig does not like running
On older car battery that has less than 12 volts.
Hope this helps someone. 73. De Steve

Posts: 2

« Reply #8 on: July 18, 2012, 01:52:59 PM »

This radio I have found from experience likes
To have a regulated power supply. If you run this rig mobile
And you are parked, start engine before transmitting.
Back off the mike gain to 9:00 for ssb transmit and 12:00 for am
And fm. This seems to work for me. You can see when
The radio starts oscillating when the modulation meter
Is full pegged when not transmitting. It does not
Appear that there is a fix from my reasearch on the net.
Anyone else have info on this issue with
The htx-10?    Steve

Posts: 1649

« Reply #9 on: July 20, 2012, 08:54:37 AM »

 I replaced the original ceramic type mic and installed the Radio Shack dynamic microphone catalog# 21-1172 it works great and sounds clear with a balance of highs and lows both on fm and ssb. Note* I did not include the up/down feature in this solution*
If you consider this alternative use caution note* that pin 6 carries a dc voltage to power the ceramic cartrige not needed for a dynamic type replacement. Pin 6 can be "opened" to not transfer the voltage via the connection.
1-Audio    (wht)
2-receive  (blk)
4-Ground  (blu)=Channel Down,2.2K ohm to pin 3=Channel up.
5-Ground and switch commom (yellow and copper sheild)
6-= +dc voltage(brown)
  Re-arrange wires on the replacement Dynamic mic plug catalog# 21-1172.
The plug is numbered 1-6.
    5-copper sheild
Mine works fine 73!


Pin 1-Pair the blu and white
pin 2-Black


Posts: 5863

« Reply #10 on: July 21, 2012, 08:28:34 AM »

I've owned quite a few rigs from Radio Shack--both ham and CB.  One common problem with them that I've run into seems to be bad solder joints.  A visual inspection of the circuit board may tell you where there are solder joints that are poor, either from insufficient solder being used or where the solder temperature wasn't high enough to properly flow the solder into the joints.

One CB I got years ago was sputtering when it was turned on.  Since I purchased it on Saturday and needed it for Monday, I opened it up just to check it--and found that there were several solder joints that were bad.  I resoldered those and the radio worked just fine after the fix.

I suggest checking the board and resolder any bad looking connections--or if you don't feel comfortable doing so, have another ham who is do it for you.  Just re-flow the solder or add just a bit, but be careful not to cross connect adjacent solder points.  Chances are that for a six pack of beer--or not even that--you'll have the hum gone.  73!
« Last Edit: July 21, 2012, 08:30:53 AM by K1CJS » Logged

Posts: 4

« Reply #11 on: March 25, 2013, 07:39:19 PM »

A big thank you to everyone who provided repair suggestions for my rig!  Your suggestions were all very appreciated!!

 I finally found the culprit--L404 was open so I soldered a wire around it.  Here's a link to the repair if someone needs more details  Thanks to KJ4LTC for his suggestion on this (posted here

Prior to a proper diagnosis I did use the "shotgun approach" and made several repairs hoping that one would do the trick: I re-heated all the solder joints as suggested by K1CJS and KZ1X, and replaced the mic element as suggested by W5WSS.  Then again, one of these joints could have been bad as well, so all the work was well worth it.

The big delay for me was building a linear power supply so I could work on the radio.  The computer power supply I was using would not maintain the proper voltage to properly diagnose the problem, so I purchased a automotive battery charger from Harbor Freight for the transformer and the diodes, and then added a LM317 regulator to it along with some filter caps and control transistors--now I have a regulated adjustable supply, and it only took a year!

Once again, thanks to everyone for all of your suggestions and assistance.
-Chris, KC5BKS 

Posts: 54

« Reply #12 on: April 23, 2013, 09:23:03 AM »


Another power supply option is 1 or two(parallel connected) car or marine/RV batteries and a Battery Tender ($40).  Then you've got power even when the mains aint workin, and the Battery Tender makes no noise.  Just be sure you batteries aren't in living quarters.

Posts: 276

« Reply #13 on: May 18, 2013, 08:04:41 PM »

Thanks for this post, for I had some hum reports with only HF rig, the HTX-10. I installed ferrite on both negative & positive leads, a clamp on ferrite on the microphone cord right where it comes out of the connector, and put a 4700 uF cap across my SEC 20 amp power supply DC output, and a clamp on ferrite on the AC cord to the power supply.

So, after read this, I checked quickly with VOM, and saw that the 12 volts is connected to pin 6 of the microphone. So, L404 is not open in my rig, I hope L404 is also not shorted, that might be harder to detect with a VOM.  Wink

Then, I saw the comments from  KJ6N about the mic gain. Oops.  Embarrassed

So, I'm now waiting for a good 10 meter day to try again. If I still get bad reports, then I will suspect the microphone itself.
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