Uniden 78ltw audio conversion

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Goldfinger
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Uniden 78ltw audio conversion

Post by Goldfinger »

Doing a 78 should sound about like 2TX240's Cobra conversion. On a Uniden 78ltw you would change C68 from a 0.1 uF electrolytic to a 1.0 uF tantalum. then change C37 a .033 uF to a .1 uF ceramic disc or mylar. Be careful not to make the value too big as the audio transformer only responds well down to about 100 hz and if you go too low it will start to sound choppy from transformer saturation. Just dropping them down from 300 to 100 hz makes them sound fuller for sure. You can also change C48, the cap between the audio chip output and the transformer from a 470 to a 1000 uF.

The 29 LTD schematic is similar for reference:


http://www.cbtricks.com/radios/cobra/29 ... sm_sch.jpg
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Re: Uniden 78ltw audio conversion

Post by 33_n_d_byu »

Goldfinger wrote:Doing a 78 should sound about like 2TX240's Cobra conversion. On a Uniden 78ltw you would change C68 from a 0.1 uF electrolytic to a 1.0 uF tantalum. then change C37 a .033 uF to a .1 uF ceramic disc or mylar. Be careful not to make the value too big as the audio transformer only responds well down to about 100 hz and if you go too low it will start to sound choppy from transformer saturation. Just dropping them down from 300 to 100 hz makes them sound fuller for sure. You can also change C48, the cap between the audio chip output and the transformer from a 470 to a 1000 uF.

The 29 LTD schematic is similar for reference:


http://www.cbtricks.com/radios/cobra/29 ... sm_sch.jpg
what would it take to bypass all of the audio chain, including the transformer and just feed the final direct? or is that possible on a solid state radio? i would like for the radio to get its carrier the normal way. but use a pa amp and a matching audio transformer to feed audio straight into the final.
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Post by Reverend Bow »

33,

I am looking into doing that as well.

That is basically what Motormouth Maul is doing.

As Goldfinger has stated in another post, most of the Export rigs, and the 148 do not use an Audio transformer in the TX audio chain.

It can't be that hard... we can figure this out. ;)
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Post by 33_n_d_byu »

yea i mean that would be the thing to do for anyone with a rack. i cant explain how much i love having my black face fixed up like that. you can do as many watts as your carrier and tube will alow by just turning up the volume. its the best thing in the world. i wish i had time to look into it and try to figure it out but i am knee deep trying to get my johson finished up and working on a few amplifiers i have over here.

but if someone could tell me the impedance needed and where to put it i would ******* love to run my rack into a cobra 148 or cobra 25, 29 the best way in my opinion, and that is right to the final.

and what would be cool its simple to have variable carrier in those radios so you would be all fixed up and can you image how good a 148 with direct fed hi-fi audio would sound on side band?
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Post by Goldfinger »

You could isolate the collector feed to the driver and final and insert the secondary of an audio transfomer in series with the 12 volt feed. Problem with that would be finding a suitible high fidelity transformer with a low impedance primary/secondary. Most of the audio transfomers currently available are high impedance on either the primary or secondary for connection to vacuum tubes. Having one designed and manufactured is possible, but the cost would be prohibitive. One solution would be to use 2 identical transformers wired back to back with the high impedance windings wired together, then using the remaining 4/8/16 ohm windings that are left on each transformer. using one to drive a high fidelety audio amp into (with the rack gear feeding it) and the one on the other transformer wired in series with the 12 volt feed to the finals. You could experiment also trying the 4 then 8 then 16 ohm taps to see which one matches the best. A Hammond 1615 would be a suitable choice.

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On the schematic above you would connect the "P's" together only. Blue to blue, brown to brown. the "Sg's" and "B+" would be not connected to anything and insulated with heatshrink.

To vary the carrier, the usual TIP-120 transistor variable power circuit connected to the driver transistor could still be used, wired just like you normally would on the collector feed.
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Post by 33_n_d_byu »

what impedance would the plastic radio be looking for? i need an 8 ohm primary i know that and have it fed with a pa amp. now once i know the secondary impedance i will need, like you said i can run another transformer after the first one it to lower the impedance if that is needed.

then what would we be doing just taking 12 volts from anywhere in the radio and go to one leg of the secondary or wold the 12 volts need to come from a specific place.

then after that the other leg secondary would perferably go to the final and where would that connection be made.

1. what impedance is the radio's final looking for. is that an answerable question or does that take some investigation.

2. where do we need to get the unmodulated power from to go to the secondary of the hi-fi audio transformer.

3 where does the modulated power from the hifi transformers secondary need to be tied in so that it goes to the final and not all of that nut crunching audio circuitry in that plastic radio.

:roll:
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Post by Goldfinger »

I typed for 20 minutes straight last night answering your question, and when I hit enter the forum was gone, along with my post. what a pain! :roll: I'll make this one shorter.... lol

1. The stock transfomer is usually a little higher in impedance on the secondary to step up the peak to peak voltage of the audio signal, and the primary is designed for the 4 ohm output of the car stereo type audio chip in the radio. So I would say try the 8 or 16 ohm tap on the secondary side of the transfomer setup and see which one sounds smoother. On the input side, prolly stick to the 8 ohm tap. It wouldn't necessarily hurt the performance trying the 4 ohm output tap, but since the transformation ratio would be stepped down, it will require a little more drive from the external audio amp to get to 100% mod. It still might be worth a try though as the "tone" may change a little on each tap.

2. You would connect one end of the winding the the switched (output) side of the power switch to minimize voltage drop.

3. In the radio, there is a black 1 amp silicon isolation diode on the output of the stock mod transfomer in series with the feed going to the driver/final collectors. You would lift the end connecting it to the transfomer (disconnecting it from the stock mod transfomer) and that is where you would connect the other secondary lead of the new transfomer.

A word of caution: Since the external transfomer/amp combination most likely has much more audio drive capability, make sure you have a compressor/limiter inline and bring the audio up slow until you reach 100-110% on the scope. then do your EQ and final brick wall limiting adjustment's until you get it how you want it. 8)
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Post by 33_n_d_byu »

the same stuff happend to me when i typed that big as question hahaha

i hit enter and it woudnt connect but i hit the back button and copy and pasted my questions to a text file.

but anyways well dam man i'm gona have to check on this stuff i would love to hear how it sounds.

last question for a while i promis till i do some testing. coudnt i get a 1 to 1 transformer and just feed 8 ohm out to the 8 ohm radio? or would it be hard to find a hi-fi 1-1 transformer or well i dont know what i'm talking about but you get the idea. is there a cheaper transformer i could use that is just 1to1 or there about instead of buying another 6.6k to 8 ohm hammond for 60 bucks?

and if all goes well with this mod what would be the limiting factor in bandwith that the plastic radio could pass
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Post by 33_n_d_byu »

on the cobra 29 matic ya got on top of this page would it be d8? and do we need to by pass the amc? i see where it is feeding that i think** and if so please tell me how hahahaha

if i was smart i would be dangerous
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Post by Goldfinger »

Yep D8 be the one. It prolly would be a good idea to remove C73 to keep the new mod audio totally isolated from the radio's audio circuitry. As far as 1 to 1 transformers, ya finding a high fidelity one at 8 ohms in and out would prolly be tough to find. That's why I suggested running 2 of those hammond transformers back to back. they are 40 bucks apiece, so it would cost you 80 bucks, but most people would have already spent a few hundred+ on rack gear and an amp, so what's another 80 bucks... lol.
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Post by Reverend Bow »

I have a Question about inserting the Transmit audio that way, by lifting the isolation diode.

If you insert the transformer there and disconnect the stock audio transformer, wouldn't you lose you receive audio feeding the speaker?
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Post by Goldfinger »

Nope, the secondary winding feeds the speaker and it isn't connected to the modulation circuit.
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Post by Reverend Bow »

OK.. And anther question..

Isn't there a way to do this without audio transformers?

most solid state audio stuff these days are a transformerless design. Since the idea is to isolate the stock audio transformer from the transmit audio chain, we should be able to direct inject the audio after the stock transformer.
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Post by 33_n_d_byu »

you might as well make this a sticky thread or what ever you call it and identify the diodes to be lifted in all uniden and cobra radios. this right here is some good info.

cobra 25
cobra 29 d8
cobra 148
uniden 68
uniden 78
uniden grant
you get the idea



this is what sets the wwpdx apart from the rest of the forums good info and no bull **** i love it
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Post by Reverend Bow »

33_n_d_byu wrote:
does the galaxy 959 use a transformer. i enjoyed that radio
Nope, not according to the schematics:
http://www.cbtricks.com/radios/galaxy/d ... 59_sch.pdf

It seems most of the Export Am/SSB radios do not use a transformer.
this is what sets the wwpdx apart from the rest of the forums good info and no bull **** i love it
Agreed! Most other forums Ideas of "Audio Modification" is clip the limiter circuit out, run your echo maxed and a "Modulator" circuit... :roll:
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Post by Goldfinger »

You could, but then you would have to design a Class A amplifier section with low distortion around a suitable set of driver/output transistors both wired up with DC coupling so they would be able to run with a DC offset to create the adjustable carrier level for AM operation. This kind of project would probably be beyond the scope of most radio operators and would cause alot of frustration for all but the most seasoned electronics people. It took motormouthmaul months of research and development to come up with his "mauldulator" and the price tag for one of those is more than 5 times the cost of 2 small Hammond audio transformers. I only brought this idea to the forum because I thought it might be more feasable to understand for a person with a more modest electronics background.
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Post by Reverend Bow »

It is actually a very cool idea, using the transformers.

I want to look into this method further for my Base setup.

Keep them Ideas flowing Goldfinger, I do believe you have been down this road before :)
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Post by 521 Houston »

Yeah, I gotta agree with Smiley on this... If you all don't mind, let's start another topic for the audio transformer mods and the other types of radios, and keep this thread on the pc78 family so future readers can easily follow the string if they should choose to try the mods you are describing.

Otherwise the discussions on the audio transformer mods could be lost in this thread and overlooked by someone who isn't looking for PC78 info. It's not that we are not interested in the direction of the other discussion, just that it would be better suited in it's own thread to keep all the info seperated and easy to read and follow.

Thread is unlocked to continue the PC78 family discussion.
Please start a new topic for other models.

Thanks for the understanding and new topics!


-----------------------------------------------------------------------------------
Oh, and 33.....
33_n_d_byu wrote:you might as well make this a sticky thread or what ever you call it and identify the diodes to be lifted in all uniden and cobra radios. this right here is some good info.

cobra 25
cobra 29 d8
cobra 148
uniden 68
uniden 78
uniden grant
you get the idea
Start a topic, post the info in an orderly manner, and we'll be happy to do just that!

Thanks!
Rest In Peace, My Child, My Friend!
Luckmeister (Lucky) - March 04, 2007 @ 11:59 AM
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