Uniden PC 78XL (same as Cobra 29) Noise Blanker Problem

[CollinsMan]

I have a Uniden PC 78XL, and the ANL works extremely well. However, the NB doesn’t do anything at all. At first, I thought that it just wasn’t very effective. However, after testing it with the “hair drier running next to the antenna method ala Lou Franklin’s advice,” I’ve determined that it does not cut out noise pulses at all. (My Uniden Grant's NB suppresses ALL of the noise generated in an identical test.)

Is this a common problem? If so, is there a “fix” of some sort? Any help would be appreciated.

BTW, this is a “virgin” unit that I bought it new from Amazon last year. Is was made in China, unfortunately.

 

[loosecannon]

not sure if this is a "fix", but L1 is the NB/ANL tuning coil.

with the NB on, and the hair dryer next to the antenna producing noise on your radio; you can try adjusting L1 for minimum noise.

now, i have not done this myself, so i cant tell you what other effects this might have on the radio, but it should be pretty safe.

i would make a mark on the can of the orig. position of the slug, and then turn it very slowly to see of its making any difference at all.

i hate to say it, but my guess is that you will see very little difference in the noise level no matter where you set the coil.

best of luck,
loosecannon

 

[Switch Kit]

I can't count how many guys tend to think that NB's don't work on radios , THEY DO !! Just not like so many would think. Shit !! if I put a hair dryer by my antenna IM sure I would pick up all kinds of junk with all anls and nb's in the on slots !! I know the 78s and 29s and those type radio have pretty darn good anl/nb's in them , the 25's 66's have there anl built in , but does have a nb switch , once upon a time ago I thought the nb's on those didn't work !! but I was wrong !! they did work ! it just depended on where I was at and what the interferance was at the time , turned out the nb's on those radios did exactly what they were supposed to do without touching or adjusting a damned thing in the radio. Still don't mean you might have a problem there AC5CH , Good luck to you , just hope you don't go turning things that you don't need to and rip yourself off of good incoming receive , but I guess in your case , unwanted noise is already doing that for you

 

[CollinsMan]

Switchkit,

The "hair dryer test" is valid to see how well a noise blanker works. Noise blankers are supposed to cut out pulse interference from such sources. That is what they are designed to do; they turn off the receiver momentarily when pulse interference is detected. If they don't, you still hear all of the pulse noise.

My Navy surplus Furuno FS-5000 HF marine transceiver passes the test. So does my "el-cheapo" $139.00 SuperStar 3900. So do all of the other CB and HF radios I've tested. (Some Icoms do suck in the NB department, though. However, they do work...) The PC 78XL just doesn't at all. That is the fault of the radio, not an unrealistic expectation on my part.

Don't worry about the radio though. I was a (29E) radio-repairer in the US Army for 14 years, and am fairly unlikely to screw it up...

Thanks for the comments. You are right- I have SEVERE noise problems around here! I live behind a telephone exchange and numerous RFI sources. That's why I am so big into noise reduction.

It is so bad that I have to run an underground receive loop on 160 meters. (Yes, I am serious.) Unfortunately, that method of noise reduction doesn't work very well at CB frequencies...

 

[CollinsMan]

Hi Loosecannon,

Thanks for the advice! I tried the adjustment a few days ago, and couldn't tell any difference. I'm gonna order a service manual from Uniden and troubleshoot the noiseblanker section. Maybe one of the transistors is bad or something.... :?

Oh well, I'll sit it on the shelf for a while. I just bought it to tinker with, anyway... Thanks again!

 

[Switch Kit]

very good dude ! like I said , I've had my share of antenna generated noise over the years , so having a decent transceiver surely is a plus. Over the years with certain cars ,different radios used would usually help me with noise problems . From 148s/Grants/29s and so on , IM personally not a fan of any of those radios anl/nb sections , all though they are better then many. SSB is one thing with those radios but AM seems to be more sensitive when it comes to noise and I prefer AM over most. I found that somewhat older type Uniden 25s 66s with there built in anl have been best with the car I own today , I once thought that the NB didn't work in those types of radio from certain standing points , but it turned out that I was wrong , They did work and they worked very well !! it just depended on where I was at the time. With my experiance and the radios that I mention above with the 3 stage switches (anl / anl/nb / off) always had a strange way about them , on 29s I've used over the years along with the Grant/148 boards concerning AM , they would almost appear to act like RF gains when used , in turn making it harder for me to hear folks futher away......yes , they would knock out the noise fairly well , but also knock out someone further away in my ear holes. I can honestly understand the trade off when it comes to that. I honestly don't know why that I feel the chassies of 25s to have better built in anl along with it's NB ? They just seem to be top notch when it has come to unwanted antenna noise in many of my vehicals over the years with AM only in mind , SSB has always been a much cleaner where noise is concerned with those radios I mentioned. Good luck to you

 

[bushwacker]

You can't compare a Furuno ssb marine radio phone to a Uniden 68...... huge difference in quality.

Even your superstar.... thats a dual conversion reciever.

Better reciever..... better signal to noise ratio.... the better the noise blanker will work.

BUT... regardless, you should hear SOME drop in noise. Just don't expect alot from that radio.
With your Furuno, its possible it has dsp noise blanking.

I seem to recall someone making a noise pulse generator out of a relay..... basically you get the relay oscillating and opening and closing to generate the "pulse noise" and use that to adjust the noise blanker. Its supposed to work better for setting the NB than a vucuum or hair dryer.

Anyhow... even with the squelch open and NOT using a noise generator you should hear a drop in noise when you turn the NB on and the radio is mounted in a mobile.

Don't expect alot.... its just a uniden 68... regardless, get it exactly how you want... thats what its all about.

Take care

 

[bushwacker]

It is so bad that I have to run an underground receive loop on 160 meters. (Yes, I am serious.) Unfortunately, that method of noise reduction doesn't work very well at CB frequencies...

Sounds like you have some real bad noise in your area... unfortunatly if the noise is that bad I would'nt count on a noise blanker in a Uniden am radio to do too much.

The only real way to cut alot of that down is to go with processing..... I'm actually surprised you havent tried it yet.
A dsp would most likly make decent amount of difference in your situation.
Even the cheap dsp's do a surprisingly decent job.

Good luck and take care.

 

[CollinsMan]

Thanks for the responses - I always like to learn about the point of view that others have.

Yep, the Uniden definitely can't compare to a Furuno. I really love the Furuno, and that is what I use on HF most of the time. It does have IF level DSP processing, and will knock out just about any type of interference. However, it doesn't sound nearly as good in the AM receive department as a CB due to the narrow AM filter. However, SSB sounds EXCELLENT when using the 6 Kc filter. Boy, some of the guys really love you when you transmit with that filter on SSB...

My radio is a PC 78XL, though. It does have a dual conversion receiver. However, I could buy about 130 PC 78XLs for what that Furuno cost the Navy. I just enjoy seeing how well I can get an inexpensive CB to perform. I've had really good luck with my experiments so far.

Bushwhacker, I'm gonna have to build one of those relay-type NB testers. It probably is better than a hair dryer. I get a charge out of watching my wife shake her head when I'm using the hair dryer that way, though!

I run a Heil audio-level DSP along with my Furuno's IF level DSP, and it does a really great job on most bands. However 160 meters is a special case, and requires the underground receive loop. The loop is 5 feet underground (to the top), and sits in the middle of a concrete "box." I use a sump pump with automatic water sensor to get rid of any water that might seep into it. With the loop, my noise level is about S2 on 160M most of the time. I have a rotator on it, and can null out even more static that way. This makes for really nice listening.

Switchkit, I really like AM, too. It definitely sounds nice! I've had a couple of Cobra 25, and they always do an excellent job at noise reduction. I might trade this PC 78XL for one of them if I get tired of fooling with it.

I really enjoy tinkering with CBs, but it's no big deal to me if this one doesn't meet my expectations. I heard good things about the unit, and just wanted to try it. I listen with a Uniden Grant LT 99 percent of the time, anyway, and it is as close to perfect as I think I'll ever get in a CB radio - especially after adding the 2SC2999 in the front end, and schottky diodes in the NB and AM detector.

Thanks again for the comments; y'all have a great week!

 

[loosecannon]

the noise generating relay thing is just a 9 volt battery wired in such a way as to cause the coil to oscillate.

not sure how its done, but i know its just a battery and a relay.

AC5CH,
have you tried the 2999 and the schottky diodes on your PC78xl?
i bet that would make a difference in the NB performance.

i know it did in my PC68xl.
good luck,
loosecannon

 

[bushwacker]

AC5CH..... go to the cb tricks website an check out the members section. I believe I seen it at one of the members websites.
Its easy to build.... you just have to get the relay opening and closing at a fairly fast frequency.

Its more than likly just a diode, switching transistor, battery some caps and an RC circuit. The rc circuit should set the frequency.

I'm at the shop right now..... I'm sure I have a a schematic to make one of these in some old cb book. So if you can't find it at any of the members website I'll send it to you.

 

[loosecannon]

its in "the cb hackers guide". by kevin ross.
i have it at home, and totally forgot to look at it last night.
loosecannon

 

[CollinsMan]

Loosecannon,

Man, I wish I had kept my copy of the "CB Hackers Guide," now! I'm gonna have to pick up a new copy next time I run up on a good deal.

I've got a bunch of Schottky diodes and 2SC2999 transistors on the way to me from Westgate and Mouser. When they get in, I'll definitely utilize them in the PC 78XL. It couldn't hurt!

I've got a Uniden Madison and MFJ-1026 "Deluxe Interference & Noise Canceler" device on the way from Canada (Ebay find). When it gets in, I'm gonna hook the MFJ unit up and see if it helps any. It is supposed to take a receive antenna and your regular TX/RX antenna and make them into a "steerable array." We'll see on that one...

Bushwacker,

I'll check out CB Tricks in a little. I'd really like to build that circuit. (I'm sure my wife would appreciate it if I'd quit taking her hair dryer off to the radio room, too! )

If I can't find the schematic, I'll give you a shout - Thanks!

Thanks again for the help and comments; I always learn a lot from y'all!

 

[CollinsMan]

I got in some Schottky diodes from Mouser this evening, and replaced the AM detector diode, NB diodes, and ANL diode - WOW, what a difference that made!

My S9+ line noise is now S1 with the NB on! I guess that is what the radio needed. The AM audio seems to have a "smoother" sound to it as do the other radios I've done this mod on.

I haven't got the 2sc2999 transistors in yet, so I'm not able to do that mod at this time.

Anyway, the diode switch did make a world of difference!

 

[Switch Kit]

sure glad I no longer have noise to deal with concerning any radio station , thank God for good radios that made life easier on me concerning any noise generated by any source at any given time. I personally wouldn't deal with noise for very long , I find it to be very nerve racking and would work on giving me a headach most of the time. I know a guy who has dealt with major noise interfearance for 17 years on all his stations used during this time (Some were surely better then others) but living under high tension wires is what he has had to deal with all these years , always so much better for him when the sun goes down. He has learned how to deal with it for all those years , I doubt I would have lasted more then a few days let alone 17 years !! during the day light hours anyway. I love the cb radio hobby , but there are a few things that I can truly live without , antenna generated noise ! bleed over on my neighbors and the complaining that goes with it ! I think it's great that others can take and have the time to change things around for the better when it comes to there equipment ,in many cases , it may not be worth it to me , but as long as it is worth it to you , that's all that really matters.