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Review of the Uniden 78LTD CB radio

loosecannon

Sr. Member
Mar 9, 2006
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4,064
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I recently had a chance to look at one of these as a friend of mine needed the deadkey lowered in order to use it with his amp.

Upon first looks this radio holds true to the classic Uniden styling, which in my opinion is a good thing.

I'm not going to go over all the features of the radio, as all that info can be found elsewhere on the interwebs.

This radio was untouched and in "just left the factory" condition.

The deadkey out of the box was already only 2 watts. PEP swing was about 6 watts.

Upon opening up the radio, the first thing that hits you is the new SMT design of the board.

There are no adjustments to make for receiver sensitivity, and no adjustments for the noise blanker.
(a pic is included at the bottom of this post)

That being said, I found no need to adjust either of these. the receiver in this radio is quite sensitive, and hears down to about .3uv.
the noise blanker does its job well, and easily blanked out my homebrew relay noise source.

the few adjustments allowed are done by VR's. These trimmers are very small, very fragile, and DO NOT have detents for stopping its travel. in other words, they will spin round and round as you turn them.

you can adjust the S-meter, which in this radio did not need adjustment.
you can adjust the squelch range, and it did not seem to need adjustment either.
you can adjust the frequency of the radio, and this was off by 200hz (27.1852 on ch. 19) easily fixed with a slight adjustment.
lastly you can adjust the AMC, but we'll discuss that below when we talk about the transmitter.


Ok, now for the part everyone cares about- the transmitter adjustments.

To start with, this radio uses a 2SC2314 for the driver, and an IRF520 for the final.

I found the Gate of the IRF520 had about 5 volts on it, and with no way to adjust it, i have to assume that's where Uniden wants it.

There is an inductor to adjust for the final output, but there is not one for the driver output.
Yes, there are three metal cans near the driver, but following circuit traces is all but impossible with these new SMT boards, so i left them alone.
yes, these are probably the TX mixer cans, but even if i knew for certain that these were TX mixer cans, there's usually not much to be gained by adjusting them anyway.
I do not recommend turning these cans while looking at a wattmeter no matter how strong the urge.

I was able to adjust the final output inductor using a 1000hz tone, and was able to attain a 3.5 watt deadkey with about 10 watts of PEP swing.

now for the part everyone is going to try to adjust. the AMC trimmer.

i first just had the wattmeter attached and was able to adjust this control up to about a 25 watt swing!

IT'S A LIE!!!

i put the radio on the scope and the pattern looked TERRIBLE! i don't just mean some flat lines forming on your sinewave terrible, im talking about skewed squarewave kind of terrible.

this radio will unleash a barrage of harmonics and trash the likes of which you have rarely seen when this control is maxed with a wattmeter alone.
GUARANTEED!

backed the control down to a nice looking sinewave and ended up with 16 watts PEP.

now, here's the part where i try to convince the purchasers of this radio to either leave the AMC adjustment alone, or at least have a scope around when you adjust it.

I did not get any on air tests of how my voice sounded with the AMC wide open.
maybe some people will think this sound is appealing, maybe it's muffled and worthless, i don't know because i didnt bother to check it.
the reason you don't want to run your 78LTD with the AMC wide open is specifically because this is an SMT chassis.
These little tiny parts just cannot handle being driven to extremes like the old through hole components could.
when the AMC gets jacked up to full open, all these little parts get overloaded with harmonics that stress them out.
when these parts get stressed out they are either going to change in value, or blow, but it will happen eventually.
I fully believe that Uniden knows this, and also knows that many people will turn that AMC control up, and need a new radio in a year or so.

There's no fixing these guys! Uniden will not release any service info or schematics on these boards, so aside from replacing finals and reverse polarity diodes, there's not much that can be done to bring them back to life.
I would certainly never take on in for a repair without knowing for sure that it's one of the big easily replaceable parts at fault.

I also wanted to mention that big resistor near the final. those of you familiar with the cobra 29LTD swing mod might think that this resistor can be changed in value or have a cap paralleled with it in order to decrease the deadkey while keeping the swing high.
NOPE!
my best guess is that this resistor sets the bias level for the gate of the final. changing its value will not increase or decrease your output. I even tried putting a cap across it just to be able to tell those who might try it that it does not have the desired, or any noticeable effect on output.

the only way to lower the deadkey in these radios while maintaining a higher PEP wattage is by adding diodes in series with the diode that goes from the modulation transformer to the driver and final.
I added five diodes in series to achieve a 2 watt deadkey.

final output with a clean pattern on the scope was a 2 watt deadkey with about 10 watts of PEP swing.
yes, this is a bit above 100% modulation, but the pattern was still a nice sinewave that showed minimal signs of flat-topping occurring.
try to increase this PEP number to 12 watts or above, and the pattern just goes nuts.

all in all i found this to be a pretty nice radio that i would enjoy owning. It's well built and all the faceplate controls are connected by ribbon cable instead of the method cobra is now using where the horizontal boards solder directly to the vertical boards. (that method will definitely break with continued vibration)

as with any new radio, the case screws all needed a final tightening, and i use a bit of nail polish to keep them from vibrating loose.

In the pic below you will notice that i added an electrolytic to the power jacks for some additional filtering and headroom, and i also added a ferrite bead to the power wire.
I do not have enough experience with this chassis to know whether or not they are actually needed, but they certainly cannot hurt, and may keep the user from experiencing a problem or two in a certain installation.

hope this review was informative.
I give this radio a solid thumbs up!
LC
20170322_203316.jpg
 

Thanks LC.

Is is still basically the same schematic as a 78? Sure it is hard to tell. Was wondering if they just used the same schematic but converted the chassis to a double sided board with SMD's?

Either way, I agree with you; the thru-hole part radios still are on top. Easier to fix and trace out, so long as parts are still available.

Amazing it didn't pop the mosfet with that high gate voltage after you peaked it - tho.
 
I recently had a chance to look at one of these as a friend of mine needed the deadkey lowered in order to use it with his amp.

Upon first looks this radio holds true to the classic Uniden styling, which in my opinion is a good thing.

I'm not going to go over all the features of the radio, as all that info can be found elsewhere on the interwebs.

This radio was untouched and in "just left the factory" condition.

The deadkey out of the box was already only 2 watts. PEP swing was about 6 watts.

Upon opening up the radio, the first thing that hits you is the new SMT design of the board.

There are no adjustments to make for receiver sensitivity, and no adjustments for the noise blanker.
(a pic is included at the bottom of this post)

That being said, I found no need to adjust either of these. the receiver in this radio is quite sensitive, and hears down to about .3uv.
the noise blanker does its job well, and easily blanked out my homebrew relay noise source.

the few adjustments allowed are done by VR's. These trimmers are very small, very fragile, and DO NOT have detents for stopping its travel. in other words, they will spin round and round as you turn them.

you can adjust the S-meter, which in this radio did not need adjustment.
you can adjust the squelch range, and it did not seem to need adjustment either.
you can adjust the frequency of the radio, and this was off by 200hz (27.1852 on ch. 19) easily fixed with a slight adjustment.
lastly you can adjust the AMC, but we'll discuss that below when we talk about the transmitter.


Ok, now for the part everyone cares about- the transmitter adjustments.

To start with, this radio uses a 2SC2314 for the driver, and an IRF520 for the final.

I found the Gate of the IRF520 had about 5 volts on it, and with no way to adjust it, i have to assume that's where Uniden wants it.

There is an inductor to adjust for the final output, but there is not one for the driver output.
Yes, there are three metal cans near the driver, but following circuit traces is all but impossible with these new SMT boards, so i left them alone.
yes, these are probably the TX mixer cans, but even if i knew for certain that these were TX mixer cans, there's usually not much to be gained by adjusting them anyway.
I do not recommend turning these cans while looking at a wattmeter no matter how strong the urge.

I was able to adjust the final output inductor using a 1000hz tone, and was able to attain a 3.5 watt deadkey with about 10 watts of PEP swing.

now for the part everyone is going to try to adjust. the AMC trimmer.

i first just had the wattmeter attached and was able to adjust this control up to about a 25 watt swing!

IT'S A LIE!!!

i put the radio on the scope and the pattern looked TERRIBLE! i don't just mean some flat lines forming on your sinewave terrible, im talking about skewed squarewave kind of terrible.

this radio will unleash a barrage of harmonics and trash the likes of which you have rarely seen when this control is maxed with a wattmeter alone.
GUARANTEED!

backed the control down to a nice looking sinewave and ended up with 16 watts PEP.

now, here's the part where i try to convince the purchasers of this radio to either leave the AMC adjustment alone, or at least have a scope around when you adjust it.

I did not get any on air tests of how my voice sounded with the AMC wide open.
maybe some people will think this sound is appealing, maybe it's muffled and worthless, i don't know because i didnt bother to check it.
the reason you don't want to run your 78LTD with the AMC wide open is specifically because this is an SMT chassis.
These little tiny parts just cannot handle being driven to extremes like the old through hole components could.
when the AMC gets jacked up to full open, all these little parts get overloaded with harmonics that stress them out.
when these parts get stressed out they are either going to change in value, or blow, but it will happen eventually.
I fully believe that Uniden knows this, and also knows that many people will turn that AMC control up, and need a new radio in a year or so.

There's no fixing these guys! Uniden will not release any service info or schematics on these boards, so aside from replacing finals and reverse polarity diodes, there's not much that can be done to bring them back to life.
I would certainly never take on in for a repair without knowing for sure that it's one of the big easily replaceable parts at fault.

I also wanted to mention that big resistor near the final. those of you familiar with the cobra 29LTD swing mod might think that this resistor can be changed in value or have a cap paralleled with it in order to decrease the deadkey while keeping the swing high.
NOPE!
my best guess is that this resistor sets the bias level for the gate of the final. changing its value will not increase or decrease your output. I even tried putting a cap across it just to be able to tell those who might try it that it does not have the desired, or any noticeable effect on output.

the only way to lower the deadkey in these radios while maintaining a higher PEP wattage is by adding diodes in series with the diode that goes from the modulation transformer to the driver and final.
I added five diodes in series to achieve a 2 watt deadkey.

final output with a clean pattern on the scope was a 2 watt deadkey with about 10 watts of PEP swing.
yes, this is a bit above 100% modulation, but the pattern was still a nice sinewave that showed minimal signs of flat-topping occurring.
try to increase this PEP number to 12 watts or above, and the pattern just goes nuts.

all in all i found this to be a pretty nice radio that i would enjoy owning. It's well built and all the faceplate controls are connected by ribbon cable instead of the method cobra is now using where the horizontal boards solder directly to the vertical boards. (that method will definitely break with continued vibration)

as with any new radio, the case screws all needed a final tightening, and i use a bit of nail polish to keep them from vibrating loose.

In the pic below you will notice that i added an electrolytic to the power jacks for some additional filtering and headroom, and i also added a ferrite bead to the power wire.
I do not have enough experience with this chassis to know whether or not they are actually needed, but they certainly cannot hurt, and may keep the user from experiencing a problem or two in a certain installation.

hope this review was informative.
I give this radio a solid thumbs up!
LC
View attachment 20478
Thanks for the review/summary LC
 
glad you guys liked it.

Robb, i honestly cannot tell as the traces are just too small to trace, they also jump from one side of the board to the other and then back again. just too much to trace through.
I can tell you that the PLL is all different and is a CPU.

My guess is that it's a new design based largely on the old designs.

Sonoma, i did not try that as i would never want to replace a SMT component with a leaded one. also too hard to find the resistor since i can accomplish the same thing with the diodes.

The big resistor in the back near the final is NOT for this purpose. read the review and i mention in it how i tried the things others will always try.
LC
 
I will have to take time and write the mod up for this radio doing the swing mod for the new radios. you do not change any parts, just add a resister and cap to a large part that is not smt.
 
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which part Sonoma?
LC
At one Leg of the driver, right after L206, So raise the coil up at an angle and tie one end of the "Super-Talk" Variable in the hole closer to the final, and the other end to the now "floating"/raised up L206 (the end by the driver staying in the board obviously)
[photo="medium"]4632[/photo] .
I wonder if this would work, I'd have to see if it's the same location on the 29 LTD style radios.
EDIT: Looks like it is
cobra29_driver.gif
.
image sourced from web search, hosted here.
 
Last edited:
on the 880 and 980 you lift the center leg of the driver and put the cap and resister on the middle leg. it should be the same on this radio. worth a trial to see if it is the same basic board as the 880.
 
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leapfrog, your idea might work regarding L206, but you want to put the resistor and cap on the power supply side of that coil, not the driver side of it. your wording kind of made it sound like you were going to put them on the driver side.

as for your other image, yes that is the general idea and it can be applied to just about any 40 channel AM radio that uses a modulation transformer.
I have at least one post from a few years ago on here where i explain the concept behind that mod to the point where it should allow someone to apply it to another radio.

sonoma, that is what i figured. I don't think i could stand to do something like that to someone else's radio though. not when i can accomplish the same thing using the anti-spiking diode.

take a look at the left hand side of the modulation transformer in the pic.
that black square is the five diodes in series covered by heat shrink tubing.
so much neater than that "dead bug" style of adding in the components.

I have always liked the diode method better.
if anyone wants to read more about it, you can find info here:
http://www.cbtricks.com/pub/secret_cb/vol_29/graphics/secret_cb_vol_29_pg56_57.pdf

thanks to all for the interest in the review.
LC
 
leapfrog, your idea might work regarding L206, but you want to put the resistor and cap on the power supply side of that coil, not the driver side of it. your wording kind of made it sound like you were going to put them on the driver side.
Thanks to all for the interest in the review.
LC
So looking in the 78 LTD the end of L206 that is towards the center, I should have worded it better I agree.

Thanks Once Again, Great Review!
73
-LeapFrog
 
Last edited:
No receiver adjustment is a deal breaker for me.

But, I suspect this is in the future for all CBs. I'll bet 5¢ that the FCC is clamping down on type acceptance and wants as few adjustments as possible to cut down on modifications. I'll bet another 5¢ that Uniden's refusal to release schematics goes hand in hand with that.

And they did have to get special approval so they could market that wireless mic, which is technically remote operation of a CB station. I suspect they'll be the FCC's bitch for a while in return.
 
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I did the variable power in the 880 and the 505XL. Works great, same layout and I also experienced similar results as you adjusting various caps . These radio even though its a 520 will not do any more then what you received them as. Havent finished the receive side but transmit sounds and looks good on the scope. Definately needs a power mic. Good radios, just dont expect old school numbers on these radios.
 
on the 880 and 980 you lift the center leg of the driver and put the cap and resister on the middle leg. it should be the same on this radio. worth a trial to see if it is the same basic board as the 880.

I'm assuming that you would put the positive leg of the cap/resistor on the middle leg of the driver transistor?
 
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I wouldn't do it there at the collector of the driver, the coil prior is there to strip any RF from getting into your supply. I do it at the coil. Works good on all the new radios.
 
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