Am hum in receive

[nomadradio]

This model was made in 1978 with the uPD858 PLL chip and a uPC1156 audio chip.

The next year it was redesigned with a uPD2816 PLL that locked you out of adding extra channels easily. And the audio power chip was changed to the TA7222P.

A year after that this chip grew a letter "A" after the digits. The TA7222AP. And that's what Uniden continued to use for nearly 30 years.

The 1979 version radio might have the original "non-A" version of the TA7222. I do remember some odd symptoms from those, like a faint hum in the transmit audio.

But it's been too long and I didn't take terribly good notes in 1979. Getting a look at that number is not easy, since the face of the chip with the numbers is turned away from view where it's mounted.

It's not just the miles, it's the years, too. Knowing how old it is could shed some light.

73

 

[MadisonMod]

Before you start tearing into everything, Put the unit into PA mode, hook up an external speaker and see if the hum is still present. If not, that eliminates the power supply as the culprit. If it's still there, you have narrowed it down to the supply or the audio section.

- 399

Okay. I plugged external speaker into the unit and turned unit on. I pushed in the pa button and the meter lights went out . The channel light was still on. I tried speaking through the pa circuit but nothing was heard. I'm assuming the pa circuit is not working. Is there something in the audio section to check ?

 

[Xracerx13]

You would have to hook up the speaker to the PA jack if you wanna hear yourself. You were to plug it into the rear speaker ext. jack and listen for the hum while in PA mode unkeyed.

 

[MadisonMod]

This model was made in 1978 with the uPD858 PLL chip and a uPC1156 audio chip.

The next year it was redesigned with a uPD2816 PLL that locked you out of adding extra channels easily. And the audio power chip was changed to the TA7222P.

A year after that this chip grew a letter "A" after the digits. The TA7222AP. And that's what Uniden continued to use for nearly 30 years.

The 1979 version radio might have the original "non-A" version of the TA7222. I do remember some odd symptoms from those, like a faint hum in the transmit audio.

But it's been too long and I didn't take terribly good notes in 1979. Getting a look at that number is not easy, since the face of the chip with the numbers is turned away from view where it's mounted.

It's not just the miles, it's the years, too. Knowing how old it is could shed some light.

73

Okay. PC-198A8 is the # on the board. The unit does have the PLL D858 chip. The unit looks original inside. All the wax is still untouched, but no coil in l11. Does it need it ? no jumper or clipped parts, and no burn spots on the board. Maybe the audio chip was replaced? It has NEC C1156H (E7303B). The transformer may be replaced at some point = Uniden TF-103. It has a lot of the old yellow bonding glue on some of the board and parts. Should I remove and clean along with recap kit ? It has C2092, final and driver looks like c2n91 ? Most of lettering gone. Not sure. Don't know what to check for on PA.

This model was made in 1978 with the uPD858 PLL chip and a uPC1156 audio chip.

The next year it was redesigned with a uPD2816 PLL that locked you out of adding extra channels easily. And the audio power chip was changed to the TA7222P.

A year after that this chip grew a letter "A" after the digits. The TA7222AP. And that's what Uniden continued to use for nearly 30 years.

The 1979 version radio might have the original "non-A" version of the TA7222. I do remember some odd symptoms from those, like a faint hum in the transmit audio.

But it's been too long and I didn't take terribly good notes in 1979. Getting a look at that number is not easy, since the face of the chip with the numbers is turned away from view where it's mounted.

It's not just the miles, it's the years, too. Knowing how old it is could shed some light.

73

 

[MadisonMod]

You would have to hook up the speaker to the PA jack if you wanna hear yourself. You were to plug it into the rear speaker ext. jack and listen for the hum while in PA mode unkeyed.

Okay. I plugged the external speaker into the PA jack on the back of the radio. I pushed PA buton on and the hum is there.

 

[unit_399]

OK. To remove the on-board power supply from the equation, move the AC/DC switch on the rear panel to DC. Unplug the ac cord from the wall outlet. Connect a 12 volt dc supply to the jack on the rear panel and see if the hum is still there. NO ... then the onboard supply IS the problem.

- 399

 

[MadisonMod]

OK. To remove the on-board power supply from the equation, move the AC/DC switch on the rear panel to DC. Unplug the ac cord from the wall outlet. Connect a 12 volt dc supply to the jack on the rear panel and see if the hum is still there. NO ... then the onboard supply IS the problem.

- 399

Okay. I tried the unit unplugged on 12 dc power supply. Hum is still there

 

[Xracerx13]

Did you check the shield wire in the microphone plug? See if the mic is wired properly. Sounds like shield and RX wire are backwards. what mic are you using?

 

[MadisonMod]

Did you check the shield wire in the microphone plug? See if the mic is wired properly. Sounds like shield and RX wire are backwards. what mic are you using?

I tried stock mike, astatic power mic and a rod talker mic. Volume up or down the hum is there in both positions am receive or pa with all 3 microphones.

 

[MadisonMod]

Is radio connected to antenna or just a power supply? Is that switching or linear PS?
Could you post audio recording of that hum?
Mike

I tried both ways. Antenna plunged in and unplugged. The hum is there both ways

 

[MadisonMod]

Did you check the shield wire in the microphone plug? See if the mic is wired properly. Sounds like shield and RX wire are backwards. what mic are you using?

I found something weird. I tried this with headphones and the external speaker. With the volume nob down all the way so nothing except the hum is heard . I turned the squelch up and the hum gets louder. The more squelch is turned up the louder the hum gets. If I turn power/ volume nob off and back on the hum goes away for 1 to 2 seconds then comes back

 

[TM86]

Probably a dumb question, but what frequency is the hum? Does it sound like you'd expect 60 Hz to, or something else? I ask because you've tried the radio on an external power supply and you still get the hum. Heck, I think about the only thing you haven't tried is dragging the radio out to the car and hooking it up to the battery.

If you have no idea what frequency it is, you can go to this link and try different ones until you can at least get close: https://www.szynalski.com/tone-generator/

 

[Dr. Frankenstein]

As has already been said ..... it's time to recap the radio due to its age.

We haven't been told if the hum is also heard on the receiving end by the people that are listening to the transmitted signal, but at this point, it doesn't make a difference.

No matter who does the work it would be interesting to find out exactly which capacitor(s) caused the hum.

This would involve replacing all the capacitors one by one until the hum goes away, then continuing to replace the rest of the capacitors.

But some people just don't care and want to get the job done ...... with the answer being ......
"Who cares, it's fixed!"

I'm sure we would like to know.......................

73

 

[MadisonMod]

Is radio connected to antenna or just a power supply? Is that switching or linear PS?
Could you post audio recording of that hum?
Mike

115v ac 60hz 250 watt continues regulated powersupply. 13.8 v dc output. I think linear. Brand new out of the box.The radio makes the hum plugged into the wall, also when I plugged it into the power supply. I have antenna plugged into radio. When I unplug antenna, the am static and receive go quiet but hum is still there. https://photos.app.goo.gl/hqhcDxHEFeyRh5jE9 let me know if that link works

 

[MadisonMod]

Probably a dumb question, but what frequency is the hum? Does it sound like you'd expect 60 Hz to, or something else? I ask because you've tried the radio on an external power supply and you still get the hum. Heck, I think about the only thing you haven't tried is dragging the radio out to the car and hooking it up to the battery.

If you have no idea what frequency it is, you can go to this link and try different ones until you can at least get close: https://www.szynalski.com/tone-generator/

Haha. I have not tried the car yet. I used that link.