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questionable mod?

P

Pman62

Member
Nov 20, 2017
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I came across this on a website and, on the surface, looks plausible, but am wondering if it would really work.

"There are a couple general items that you can do to get extra power out of almost any radio. First, replace the stock final with a 2SC1969 transistor. Second, locate the resistor bringing the driver signal to the base of the final, and solder a jumper across it. Third, locate the diode running from the Audio output transformer to the final/driver supply chokes, and solder a jumper across it. These mods generally do not apply to sideband radios, most of which already use a 1969 unit. Some readjustment will be needed, and some radios may be able to be further increased by changing various capacitor values in the final and driver circuits. Experiment CAREFULLY!"

I'm am a little hesitant on this, but I have some old cb's that I could burn up and not care. It's been a LONG time since I did any electronic work and even back when I was active, most of my experience was with stereos and televisions. I do have a 40 mHz dual trace o'scope, but unfortunately, my rf signal generator is presently in need of repair and even then it's not the greatest (Heathkit IG-102). So, any opinions or advice?
 

Butcher mod ,
There was however a well known mod that would prevent over modulation in a 2 or three crystal Cybernet AM board.
Move the final into the driver hole and a 1969 into the final hole, swap two capacitors, improve the current path to the radio and in the radio (chokes replaced or rewound), gets you every bit of 10 watts so the TA7205P rated at just 5.8 watts @ 10% THD can't keep up and voila! no limiter necessary, you haven't got enough power to overmodulate !

In all seriousness it actually did have a few niche uses but was largely just another Secret CB solution in search of a problem. Try it on an old AM base and the power supply would fold up and hide below 10V. If nothing else it would confirm that the mod was working. Fix the lack of current and you didn't have enough heatsink . Fix the heatsink and the power supply and you couldn't completely modulate it without smoking the TA7205P . Replace the TA7205P with a bigger chip and fix the power supply, put an even bigger heatsink on the final, and overwhelm the downstream audio chain. Say fergidaboutit and just supply your own audio on the 2SC1969 collector and smoke the junction because there never was enough heat path in the first place....

Aside from all that Have you actually sourced a 2SC1969/2312SC/1307 recently? Good? Bad ? real? fake? new? used? pixie dust on top ? Just askin :)
 
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kopcicle said:
Butcher mod ,
There was however a well known mod that would prevent over modulation in a 2 or three crystal Cybernet AM board.
Move the final into the driver hole and a 1969 into the final hole, swap two capacitors, improve the current path to the radio and in the radio (chokes replaced or rewound), gets you every bit of 10 watts so the TA7205P rated at just 5.8 watts @ 10% THD can't keep up and voila! no limiter necessary, you haven't got enough power to overmodulate !

In all seriousness it actually did have a few niche uses but was largely just another Secret CB solution in search of a problem. Try it on an old AM base and the power supply would fold up and hide below 10V. If nothing else it would confirm that the mod was working. Fix the lack of current and you didn't have enough heatsink . Fix the heatsink and the power supply and you couldn't completely modulate it without smoking the TA7205P . Replace the TA7205P with a bigger chip and fix the power supply, put an even bigger heatsink on the final, and overwhelm the downstream audio chain. Say fergidaboutit and just supply your own audio on the 2SC1969 collector and smoke the junction because there never was enough heat path in the first place....

Aside from all that Have you actually sourced a 2SC1969/2312SC/1307 recently? Good? Bad ? real? fake? new? used? pixie dust on top ? Just askin :)
Click to expand...

C1969's are real :) and about $3 a pop. I looked up the spec sheet and even compared the load line to a final in one of my old cb's. This got me scratching my head over whether this mod would work or not.
 
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Pman62 said:
C1969's are real :) and about $3 a pop. I looked up the spec sheet and even compared the load line to a final in one of my old cb's. This got me scratching my head over whether this mod would work or not.
Click to expand...
These kind? I think HG came out with MRF477 and 1969 replacement parts!!
https://www.ebay.com/p/2SC1969-Original-Mitsubishi-NPN-Power-Transistor-C1969/1400239472

A 2SC2314 driver transistor is an easy way to increase gain, and if I recall correctly the 2SC2078 has more gain than a 2SC1969, but the 1969 has more "head room" and can produce a little more PEP compared to 2SC2078.

Honestly the 1969 mod is for people who want to brag about the fact that their radio does more (bare foot) or who are trying to build a " 'monster' single final radio" and don't care about burning up parts..

As stated by Kopcicle eventually you need more audio than the I.C. can provide to modulate the (increased) carrier. Unless your doing a competition radio, I find it makes more sense to lower the carrier/pep (or leave it stock) and let it all run cool.

There is no point in stressing out the parts to hot rod a daily-use radio, it is not worth it; unless you just wanted to do it for fun.
Some people add capacitance to increase output, you can
add a 68pF ceramic disc capacitor across C46 (270pF) for Cobra 25 LTD
&
add a (30pF-68pF) capacitor across C62 (220pF) for Cobra 29 LTD, to get a little more (D.K.) out of 'em.
 
Last edited:
LeapFrog said:
These kind? I think HG came out with MRF477 and 1969 replacement parts!!
https://www.ebay.com/p/2SC1969-Original-Mitsubishi-NPN-Power-Transistor-C1969/1400239472

A 2SC2314 driver transistor is an easy way to increase gain, and if I recall correctly the 2SC2078 has more gain than a 2SC1969, but the 1969 has more "head room" and can produce a little more PEP compared to 2SC2078.

Honestly the 1969 mod is for people who want to brag about the fact that their radio does more (bare foot) or who are trying to build a " 'monster' single final radio" and don't care about burning up parts..

As stated by Kopcicle eventually you need more audio than the I.C. can provide to modulate the (increased) carrier. Unless your doing a competition radio, I find it makes more sense to lower the carrier/pep (or leave it stock) and let it all run cool.

There is no point in stressing out the parts to hot rod a daily-use radio, it is not worth it; unless you just wanted to do it for fun.
Some people add capacitance to increase output, you can
add a 68pF ceramic disc capacitor across C46 (270pF) for Cobra 25 LTD
&
add a (30pF-68pF) capacitor across C62 (220pF) for Cobra 29 LTD, to get a little more (D.K.) out of 'em.
Click to expand...

I guess I was just itching to do a little circuitry work. I kinda figured that this mod was a waste of time (I still remember a FEW things about amps lol) My soldering skills are still up to snuff, so no practice needed there. I've really only done one IF alignment ever and that was on a high end stereo reciever that some one brought to me to fix. Too bad he didn't tell me from the start that the problem was caused by a nearby (not direct) lightning strike. BOY! that job was a nightmare. It seemed like every time I fixed something, it would work for like five minutes and something else would go out. I felt like a puppy chasing its own tail. I did finally get it fixed, but was mad as hell when the owner fessed up that it was a lightning thing.... Never again will I touch anything burnt by lightning. HAH!
 
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Pman62 said:
I guess I was just itching to do a little circuitry work. I kinda figured that this mod was a waste of time (I still remember a FEW things about amps lol) My soldering skills are still up to snuff, so no practice needed there. I've really only done one IF alignment ever and that was on a high end stereo reciever that some one brought to me to fix. Too bad he didn't tell me from the start that the problem was caused by a nearby (not direct) lightning strike. BOY! that job was a nightmare. It seemed like every time I fixed something, it would work for like five minutes and something else would go out. I felt like a puppy chasing its own tail. I did finally get it fixed, but was mad as hell when the owner fessed up that it was a lightning thing.... Never again will I touch anything burnt by lightning. HAH!
Click to expand...
Yeah, I hear you about the lightening horror story. :confused: What a pain.

The resistor/jumper found from driver to final has been a prime candidate for circuit tweaking, if they have a resistor present from the factory, downsize it by half (100 Ohm to 50 Ohm)
to increase power.

Or add resistance to decrease the power/D.K. (A.K.A. JP36 mod cobra 29)
If you want it to over-modulate you can add a capacitor across the resistor, just stay 220uF or under, else it tends to get really nasty. Can't say that I recommend a mod to over-modulate, but some people want the little radio to "swang" at any cost. :eek:
 
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