Uniden 980SSB no longer swings!

[ExitThirteen]

Keep in mind that the 980 uses a mosfet, and mosfets are biased on a set voltage, while bipolar transistors are current based and biased with a small amount of current. 50-100ma of current on a transistor isn't enough to heat it up, but apply constant 3.5-4V to a mosfet and it will heat up.

When a 980 is keyed, that bias voltage is applied to the mosfet regardless of whether on not the radio is modulated in AM or SSB modes. The same is true for an older radio with bipolar transistors and current, but the low current will not heat up a transistor like 3.5-4V will heat up a mosfet. Different breed of cats in the new radios these days.


~Cheers~

 

[GnG8d]

Since you can have potential without current, but you can't have current without potential I'm not so sure I follow. It takes current to create heat. Whether setting bias with voltage or current, it accomplishes the same thing in the end.

 

[ExitThirteen]

You do have a point 359, what I have noticed is that with bipolar transistors you're limiting your bias current with an adjustment, whereas with mosfets, there is no current limiting, that being said, a mosfet in a particular circuit can potentially pull whatever amount of current it can, based on the whatever the bias voltage is at the gate.

Admittedly, I haven't studied it to a large degree as of yet, but in an RF application, a mosfet can require more current draw at the gate to function, so there could potentially be more current present at the gate of the mosfet if it is "turned on", or in this case, the radio being keyed up for a long period of time.

Reason I bring this up is because thru my applications of mosfets in CB's, I have found that if I key up on SSB unmodulated for long periods of time, the mosfet still heats up, where a transistor does not. It's merely an observation.


~Cheers~

 

[BikerTrash]

ExitThirteen,

I didn't know before what you just taught me about Mosfets vs transistors in your latest reply. That's enlightening to me since I carefully removed the covers from my radio and just did a visual inspection of the board and all its solder joints, tiny pots, chips, and other components. I knew I wasn't likely at all to be able to repair anything in there but I was curious to see if anything would be visibly discolored or fried. Nothing appeared abnormal EXCEPT for board discoloration around all three of the Mosfet solder joints. Those solder joints are larger than the others so I thought, perhaps, the discoloration was because it took more heat to solder those Mosfet legs to the board. Now, I wonder if the Mosfet heated up enough that it caused those tiny orange margins around its connections. Anyway, though the radio seems to be working like before, how much do you reckon the integrity of that Mosfet has been compromised?

Come Saturday morning I'm taking the radio to the CB shop to have it checked out.

 

[ExitThirteen]

Do you talk primarily on AM? If you do, there is a possibility that the AM pass regulator transistor has gone fault. A bad AM pass regulator would exhibit the symptoms you're describing of high deadkey with no modulated output. It's Q206 on the board (labeled A1725), on the right side of the chassis.

Now I wouldn't think that the radio being keyed up on SSB for a long period of time with little or no modulation would damage the AM pass regulator, but in some radios, the SSB output is also regulated by this transistor. It's a possible starting point. Since the radio still deadkeys good wattage, I wouldn't suspect a fault with the final.

(Edit) I have noted (scrolling back in posts) that the radio seems to be working again... while I don't think the final took any damage, I would have the shop check solder joints and have the AM pass regulator looked at. The radio should be adjusted for a deadkey of about 3W or so and swing up from there. Good luck!


~Cheers~

 

[Lil'Yeshua]

Don't forget,there may have been seat noises,loud music,loud talking,maybe
road noises if the mic is sensitive enough.

I remember an incident to where a couple of employees who worked for one of my company's customers who accidentally sat on their company's vehicle radio mic. The two employees have a interesting conversation until another employee chased them down and told them that their radio was transmitting.

Must of been a Motorola. op:

 

[222DBFL]

I would suggest getting a Better watt meter if the astatic one you have is the small one that the truck stops sell. They are not the most accurate and reliable meters, I had one and it was a not a very reliable meter. If you have the large one, I believe they are built a lot better. Had a small one myself is why I ask, as I for rid of mine and got a radio shack meter. The radio shack meters aren't the best, but for. 40 dollars, they are pretty accurate. Anyway, like posted, was the radio on and turned up, were people talking, was your mic gain set high, windows down making road noise, all of these could result in some modulation and over a long period of time may have resulted in some damage to the radio. I would send the radio out to a good tech like DTB, Doug's custom cb's, bells cb shop, or clays cb shop. These are all good techs that can make that radio work well and give it a good alignment and tune as well as setting the dead key. If you plan on running an amp, have them set the dead key to 2 watts, if not then get it set at 3-4 watts as suggested. If anything, you can get the radio working correctly and know it's right when it comes back. Good luck and hope you get it sorted out, tell your bro that his driving privileges have been suspended due to damage of communications systems lol. God bless.

 

[Lawnmowerman427]

I own a couple of these radios and the most wattage I see out of either of them on AM is 5 watts and no matter what I do they won't do any forward swing at all. Is their another pot in the radio that needs adjusting to get the forward swing? I know about the ones for the AM dead key, AM modulation and the SSB modulation. Is their another one that needs adjusted to get the wattage for the forward swing?

 

[camaro1]

I own a couple of these radios and the most wattage I see out of either of them on AM is 5 watts and no matter what I do they won't do any forward swing at all. Is their another pot in the radio that needs adjusting to get the forward swing? I know about the ones for the AM dead key, AM modulation and the SSB modulation. Is their another one that needs adjusted to get the wattage for the forward swing?

are you using a p.e.p. meter or average,, you need a pep meter(a true peak / hold meter) to see the swing

 

[RadioGuy772]

A couple of things...

1: If the radio was truely "tuned up" by someone who made it do that high of a carrier... whoops!

2: Sounds like if it's getting that hot, the gate voltage is way too high maybe in an attempt to get more power out of it... again, whoops!

3: Something I am surprised no one else commented on. You said it had a carrier of 10 watts on one end of the band, and 6 on the other. Even if your watt meter is grossly inaccurate, and also taking into account you weren't using a dummy load... IT STILL seems like who ever worked on this had the TX bandpass section way out of whack. Probably monkey'd with it attempting to get more power out but didn't understand how to balance it out or align it correctly. Again --whoops!

There could be a multitude of things going on, but these are the "Red flags" that stood out to me based purely on reading the thread.

 

[ND88]

What a shame. a perfectly good radio ruined by a golden screwdriver.

 

[w9cll]

What a shame. a perfectly good radio ruined by a golden screwdriver.

The joys of CB

 

[vswr]

You do have a point 359, what I have noticed is that with bipolar transistors you're limiting your bias current with an adjustment, whereas with mosfets, there is no current limiting, that being said, a mosfet in a particular circuit can potentially pull whatever amount of current it can, based on the whatever the bias voltage is at the gate.

Admittedly, I haven't studied it to a large degree as of yet, but in an RF application, a mosfet can require more current draw at the gate to function, so there could potentially be more current present at the gate of the mosfet if it is "turned on", or in this case, the radio being keyed up for a long period of time.

Reason I bring this up is because thru my applications of mosfets in CB's, I have found that if I key up on SSB unmodulated for long periods of time, the mosfet still heats up, where a transistor does not. It's merely an observation.


~Cheers~

I was reading a post from NZ stating that he turned down the gate bias voltage on his 980 and it solved his problems. If there is a way to turn down that gate voltage it would be nice to know. My 980 ebay find needed a new driver transistor and the final. After going through a cacophony of chinese fakeness, I finally got a real mosfet and installed it only to find that the 10 ohm resistor that feeds the mosfet was blown. I replaced it with a 1/8th watt and I got rf out. I put it on ssb and as soon as I added audio the resistor blew. I replaced it with a 1/4 watt and it again smoked out. Bumped it up to a full 1 watt 10 ohm and that smoked too. I bought a known good 980 to use as a base unit for voltages and values of these surface mounts and did a comparison only to find both radios had the same voltage values yet the known good radios 10 ohm doesn't even get warm. Wish I knew how to fix this problem. I find that its generally a good not great radio.

 

[brandon7861]

I was reading a post from NZ stating that he turned down the gate bias voltage on his 980 and it solved his problems. If there is a way to turn down that gate voltage it would be nice to know. My 980 ebay find needed a new driver transistor and the final. After going through a cacophony of chinese fakeness, I finally got a real mosfet and installed it only to find that the 10 ohm resistor that feeds the mosfet was blown. I replaced it with a 1/8th watt and I got rf out. I put it on ssb and as soon as I added audio the resistor blew. I replaced it with a 1/4 watt and it again smoked out. Bumped it up to a full 1 watt 10 ohm and that smoked too. I bought a known good 980 to use as a base unit for voltages and values of these surface mounts and did a comparison only to find both radios had the same voltage values yet the known good radios 10 ohm doesn't even get warm. Wish I knew how to fix this problem. I find that its generally a good not great radio.

Mosfets should be biased by drain current, not gate voltage. Using the gate voltage to set bias was a technique in a few service manuals, and ever since, that incorrect procedure is believed to be proper by many.

The problem with using gate voltage is that the drain current varies wildly with small changes in gate voltage, and mosfets with the same part number don't always have the same drain current for a given gate voltage. You may be smoking your resistor because what you think is the proper gate voltage is resulting in the transistor being partially turned on all the time.

 

[Woody-202]

I switched my radio to AM and quickly checked the SWR while it was still hot. Why I did, I don't know. I can't bring it to the local CB shop until saturday. I will post the results from that visit soon.

Here's a surprise update: I'm out of state for Christmas and only have my mobile (the fried Uniden 980 in question). I drove to an open area on a hill and asked for a radio check on AM 17. It was after dark and the CB waves were very quiet. Lo and behold, a knowledgeable radio/electronics geek with a 4 watt base and Sirio Gainmaster answers. Thank God. He reported that my radio sounded crystal clear, sufficiently loud, and I was pushing the far end of his signal strength meter, "9 to 30+", from 3 miles away. We switched over to LSB 1 (this is what my radio was transmitting on when the trouble struck) and reception on my end went from 30+, loud and clear to barely audible, weak, and crackly. We switched to USB and LSB 18 and 38 and my reception was strong and clear again. Then, we switched back to LSB 1 and my reception was as good as on the other sideband channels and it stayed clear. LSB 1 on my end had mysteriously cleared up. He reported that he was receiving my radio clearly, strongly, and consistently the whole time. I was pleasantly surprised by this but I wonder what the CB shop is going to find when they check it out.

If the time comes to replace this radio, which 11 meter AM/SSB mobile do y'all recommend to replace it?

If you meant a "legal" 40ch. am/ssb mobile, I choose the President McKinley II FCC model. You'll get AM-FM-LSB-USB, for about $219 on Amazon. If you don't care about FM, the President McKinley is still available in limited quantities ( currently $139 on Amazon at this time). Too many good things about it to list, but it's smaller sized CB with the mic jack and speaker on the front panel.