Uniden PRO500D - Bad Audio Chip or Bad Caps???

[KE0SBX]

Hi all,

I have a Uniden PRO500D CB radio I picked up for a real steal. I turned it on and there is no sound on receive, no matter the antenna connection, no matter the mic connection. I've checked all possible solutions (Is the mic plugged in? Is the antenna plugged in? etc...) and nothing. I've also noticed that the nearby 7808 regulator gets STUPID HOT on TX and refuses to cool down. This radio uses the long defunct TDA1905 audio amp chip. Everything else is fine - channel displays light up, radio keys, S meter lights work...but no audio

The radio dead keys at 4 watts solid, so no issues with the driver or final. However, even if I try talking, I can't get any modulation out of it in addition to the lack of RX. So no TX or RX audio is coming to or from this radio period. One other thing I checked was the speaker, and it too tested okay.

In doing some testing, I found that the output pin on the TDA1905 is indeed shorted to ground. I suspect that this is the issue but I wanted to see if my gut feeling was right.

Am I correct to assume that the chip has gone bad or is there another variable afoot here such as the electrolytic caps? Visually I didn't see anything wrong with the capacitors - they all looked fine to me, no bulging, no brown goo/corrosion, etc., and PCB traces are on par too.

For reference, I've included the schematic here:

Let me know what you all think and if there's any other troubleshooting I should do! Thanks!

 

[niceguy71]

Hi all,

I have a Uniden PRO500D CB radio I picked up for a real steal.

someone made a real steal alright!

that radio was ??? what $59.99 when it was new.... and it wasn't even worth that!

any part you have to put into it, is spending too much money and time.... just my opinion

lot of great radios out there right now... it's the greatest time ever to be in the Market for a CB

if you watch this video... this brand new $69.99 radio dead keys over 10 watts and peaks over 17 watts.... it also has NRC which is a total game changer in the radio world ( it cuts out most of the background noise so you can hear people that are very distant or have very low power.... meaning.... if you can hear people without all that static, crackling, popping, hiss you can talk to them at much greater range )

you can buy it here Radioddity 606 PRO at Amazon $69.99

why would anyone waste their time with a Uniden 500????

again just my opinion... I'll never understand why people go to yard sales or flea markets and buy complete obsolete junk for $10 then spend 50 bucks to get it working and all they wind up with......is still a piece of junk

 

[KE0SBX]

Please excuse my forwardness but that was a terrible answer. I know it's only opinion but please understand where I'm coming from. I just asked a simple troubleshooting question.

I actually beg to differ on the Unidens, even the newer PRO series. I think they're far from junk actually, at least in my experience with the Uniden radios and electronics I own and have owned. This radio actually still works too even after 40 years too. It just needs to have a chip replaced. No need to go buy a new radio just because. I also own many 23 channel CBs. Again, useless, but still I learn from working on them. In fact, I'm in the process of converting one to 10 meters as a project. Why? Because I want to, because I feel like it.

Also, some people fix these radios that you call "junk" just to get some practice in with electronics, like me. If it was $5, it was $5. If I screw up, oh well. I also learn from fixing them, studying the schematics, etc. Also, this CB is WELL DOCUMENTED on CBTricks.com. It shows every adjustment point, every voltage reading...you get the idea. And this is a simple radio too. I don't need all the bells and whistles.

I prefer not to "golden screwdriver" CBs to push them past the legal limit either. Everyone knows that these radios nowadays and made within the past 10 or 20 years are Made in China - not the greatest quality and sure to fail in 10 or so years. Unfortunately, you could even look at Cobra - Made in China.

For that matter, get your ham ticket and buy a big three rig: Icom, Yaesu, Kenwood. Made in Japan - quite the difference if you ask me than these knock off Chinese companies. I know many people who push their CBs beyond 4 watts - sometimes around 200 or even 300 watts - and I think it's just childish and stupid. Yeah 4W AM or 12W SSB isn't a lot but it's also an incentive to get your ham ticket if you don't have it. Or look at it this way: It might not be a lot but I made a DX contact on 15 meters with Italy on just 10W SSB. And that was with a compromise limited-space end fed random wire antenna.

Sure I sound stupid for being legal beagle and uptight and stingy but in my defense I'm a ham...and sure, while I might not get caught pushing above 4 watts, the minute I lose my license and equipment because I got caught from a neighbor's complaint or at the wrong place at the wrong time, I'll feel like a sorry son of a gun for ever attempting a stunt like that. I know the FCC has much better things to focus on and has abandoned CB but I don't want to take the risk of losing my stuff over a simple rule.

Besides, why would I want to spend $70 on a radio I'm never going to use? I have at least 7 or 8 CB radios already that I could pull off the shelf and use.

I don't know if you're a ham @niceguy71 but if you are, you seem like a "sad ham." Not trying to start a fight or anything, but I just asked a simple question if the audio chip was bad, not what new radio I SHOULD buy and why the radio I'm working on is .

I hope you have a nice day today and can find some good in it. 73s

 

[WSAC829]

Please excuse my forwardness but that was a terrible answer.

Agree. That's what he does though, and if you disagree with him, or correct him, he will ignore you like he did me. He’s also the reason a few newer members deactivated their accounts. So next time, save your fingers and don’t bother replying to him.

At any rate, i can’t help with this specific radio issue of yours, but i totally get buying stuff cheap, or picking it up from off the curb and trying to fix it. I’ve been doing that since i was a kid and i still find it quite enjoyable.

 

[prcguy]

TDA1905s are available no problem and not very expensive. If you think its bad just replace it.

 

[niceguy71]

No offense, taken.. great thoughtful reply.

My responses are never very eloquent... it's just lost in the way of my written words.. if we were face to face I would come across much better

Believe me I get the enjoyment of working on something no one else sees as worth while... I play with Hand held CBS. The new ones are far superior to the old ones... and I waste my time and money to learn it would have been cheaper to buy a new one..... but I enjoy it

I ment no disrespect. I was honestly trying to save you from wasting time and money.... I see guys get a deal on an old piece of junk... and then spend lots of money and weeks and weeks trying to get it to work they get frustrated and leave the hobby.... then they finally learn they could have gotten a much better radio for less money and agitation

I was really thinking you were trying to repair it to have a great CB, I was only pointing out it will never be as great As the new radios... without the NRC they can never compete

Sadly I keep seeing guys want to have a really good cb system... they buy a $49 dollar Uniden pro 505 or 510 and a 3 foot tall antenna.... then can't figure out. Why they can only talk for a couple miles or never get anyone

I now realize that's not you. Your just enjoying the hobby

I keep trying to help the new guys in my area... and they keep reading 20,000 reviews on Amazon saying these little antennas and $49 radios are great so the get them and come and ask my local Facebook cb group for help..... no real help for that setup. most of the reviews on Amazon Are from first time buyers...if you read them they say it looks great it was easy to install.... I have not made a contact yet but its great????

So i get a little frustrated seeing people wasting their money and time then scream CB is dead

No ham license for me. . Some day...I have never used any frequency other than the CB band as I respect the law.
Witth that said I try to get a radio with 10 to 20 watts on AM as it's the only way to talk to others in the hobby.. the 4 watts are lucky to make it 5 miles and most times there is no one within 5 miles... but if I can make it 10 miles I always find others to talk to and enjoy the hobby... I'll never get that whole Amp thing as that would only work to talk to others with an Amp

Good luck with the radio

 

[kc2asb]

I actually beg to differ on the Unidens, even the newer PRO series. I think they're far from junk actually, at least in my experience with the Uniden radios and electronics I own and have owned. This radio actually still works too even after 40 years too. It just needs to have a chip replaced. No need to go buy a new radio just because. I also own many 23 channel CBs. Again, useless, but still I learn from working on them. In fact, I'm in the process of converting one to 10 meters as a project. Why? Because I want to, because I feel like it.

None of the Unidens I have purchased, be it a CB or a scanner, have been junk. My first SSB rig was a PC-122 in 1989. Definitely entry-level and basic, but not junk. Made a lot of contacts on it, back when I was restricted by the folks to an indoor groundplane antenna. And 23 channel radios are not useless, IMHO.

Also, some people fix these radios that you call "junk" just to get some practice in with electronics, like me. If it was $5, it was $5.

Exactly right, and unlike most current radios with surface mount components, they can be repaired by knowledgeable hobbyists. That is the beauty of 70's and 80's vintage rigs.

 

[KE0SBX]

Thanks everyone for your replies! I'll go ahead and replace the chip and the regulator, see what happens! I've got both chips on the way. Wish me luck!

 

[KE0SBX]

Chips are here but I have a problem...

How the heck do I desolder this giant PITA ground plane? I've tried solder wick, desoldering pins... any ideas?

This is also the reason I bought a DIP 16 IC socket lol

 

[tramd]

Make sure the tip of the solder iron is clean and tinned with fresh solder. Use a damp sponge or rag to wipe it clean, then tin it. A dry iron will not melt solder, it just cooks everything. The iron also has to have enough heat to do the job. A 25 or 30 watt iron may not be enough to do it but should be. When the iron touches the part to be unsoldered or soldered it starts to lose heat fast as it has to heat the lead of the part, the solder pad and the solder all at the same time. Sometimes you can add a little solder to the solder on the board to get it to melt. Nothing melts solder better or faster than hot solder. That's why you need to keep the tip of the iron tinned with solder. It's good that you are using a socket, so no soldering ever has to be done at those points again and the new IC won't be damaged from the heat.

 

[slowmover]

For us that ain’t that terribly interested in DIY, here’s procedure and result.

You take a look inside, and . . . .

De-solder is another procedure.

 

[KE0SBX]

Well after a little under an hour, I got both the 7808 and 1905 chips replaced.

And there was sound!!! REALLY loud sound at that. I'm kinda surprised this little simple radio could put out such loud receive audio.

There is still bad news...I'm still lacking in TX audio. So I'll have to do some digging around another day and figure out what is going on.

But hey, it works now! Partially...

Not bad for about $10 if you ask me

And for what it's worth, if you ever replace ICs, do yourself a favor and add an IC socket if the chip doesn't have one already. You'll be glad you did!

 

[kc2asb]

Thanks for the update, and great work! Hope you get the TX audio sorted out. For $10 and the satisfaction of getting a radio back to working condition, you did quite well.

 

[K3YGX]

Check the mic too.....I had one with no xmit audio and it was a def mic......
I like to do that same thing but with guitars......(electric) replacing pickups and
internal parts. I fix them up and give them away.