Uniden PC 78 LTW Issue

[KPN28379]

New guy here. Been lurking a few days after buying a new Cobra 29 after 20years of being off the air, and so far this forum has been a great help. After reading through the threads here, I managed to throw together a 1/4 wave ground plane in a few hours that throws quite loud for several miles with SWR a little over 1 before tuning. Thanks

Now I have an issue I'd like to throw at you folks.

I have a Uniden PC 78 LTW that belongs to my brother in law. I'm taking a stab at getting it straightened out, but apparently I've pretty well forgotten most of what I used to know.

The radio is supposedly new and came out of a storage unit in the original box, but was not in the plastic, so it obviously had been at the very least hooked up once.

I set it up in the BIL's truck, and all seemed good till we keyed it up.

No meter swing at all. No meter movement through keying, switching to SWR cal.

Screaming feedback through the speaker until we reduced the mic gain, then it sounds like a loud talkback, but the radio is not equipped with talkback.

You can key up, hear yourself through the speaker, but it does not look like anything is going out according to the meter. I have'nt had a chance to run a check using my Cobra since I took his Uniden and am bench working it using my Cobra antenna-power supply setup, but I did get a faint request to stop keying up while I was fooling with it. Not sure if they were referring to me.

At any rate, what could cause no meter swing, and the transmit to blare from the speaker?

I am suspecting someone modded it, but wont be sure until I find a clear diagram of the internals.

 

[KPN28379]

Update for anyone considering answering. PLaying around got some meter action going. Radio is getting out.

Pretty certain someone modded it and added talkback.

SWR is almost 4, while the same antenna setup with my Cobra shows a little over 1.

Oh well.

 

[loosecannon]

sounds like the S meter is acting up from sitting in the storage unit or where ever it sat.

you need to use an external swr/wattmeter to know whats really going on with it, and make sure you are correctly re-calibrating the meter each time you check the SWR.

as for the talkback, look on the solder side (green side) of the PC board right where the EXT SP and PA SP jacks are.

do you see a big resistor there that looks like it was put in by someone other than the manufacturer?

if so, thats your talkback resistor.

just remove it to get rid of the talkback.
LC

 

[KPN28379]

"do you see a big resistor there that looks like it was put in by someone other than the manufacturer?

if so, thats your talkback resistor just remove it to get rid of the talkback.

Thanks, that's mostly what I was looking for now. There is indeed a resistor where you describe. I was pretty sure it wasnt supposed to be there, but if I aint 100% certain, I don't pull out the snips


Now that I am fairly sure someone modded it, who knows what the heck else they did and if they did it right eh?

 

[KPN28379]

Took the soldering iron to it and pulled the resistor. Working normally now.

While I realize the radio meter is'nt entirely accurate, I don't see why there should be that huge a difference in SWR readings with the same antenna setup between radios. If the built in meter is THAT bad, it's useless and no sense even having one on a radio.

I guess I'm wondering that since this radio was obviously modded, are there popular mods that will raise SWR if not done well? If I can get hold of a good meter I will, but till then I gotta use what I have.

 

[mackmobile43]

Took the soldering iron to it and pulled the resistor. Working normally now.

While I realize the radio meter is'nt entirely accurate, I don't see why there should be that huge a difference in SWR readings with the same antenna setup between radios. If the built in meter is THAT bad, it's useless and no sense even having one on a radio.

I guess I'm wondering that since this radio was obviously modded, are there popular mods that will raise SWR if not done well? If I can get hold of a good meter I will, but till then I gotta use what I have.

Most modded radios have had the coils spread inside and will change the radios' output tuning to not a true 50 ohms impedance and this may be what you're experiencing.

 

[loosecannon]

just reassure us that you know what you are doing when it comes to checking SWR, how to calibrate the internal SWR meter correctly, and interpret the SWR reading.

no offense intended, its just that many of us have seen it so many times where there was no real issue beyond incorrect use of the function.
LC

 

[KPN28379]

Understand completely. I took some time to read the threads and saw it for myself.

I calibrate the SWR for 1 while keyed, then check the SWR reading while still keyed, then recalibrate for 20, then check, then again on 40.

 

[loosecannon]

in that case, mack's suggestion sounds about right.

look around the back of the radio near the antenna connector for copper coils that look like they've been spread open.

if you find any, squeeze them back to normal.

the last owner might have also tried to adjust the TVI trap (L11) in an attempt to get more power out of the radio. (BIIIG no no)

it should be covered with wax, and if you see that someone scraped that wax off in order to get to the ferrite adjustment slug in the coil, thats pretty much a dead giveaway that someone who didnt have a clue was messing with it.
LC