Volting a Cobra 148 GTL final?

[AudioShockwav]

While I've seen it pushed to do around 1250w,

I have seen 4 2sc2879 do over a KW, but the amp was running class C, and supply voltage was in excess of 20 volts, it is not a good idea, and is considered bad RF practice.
Now.
We have stated our opinion on the subject of what/how/when/why or something else....., lets get BACK to the Topic, that is about a 148 GTL.
I am asking nice nice, because I am such a good guy :headbang
Lets all have a Happy Hump Day :blush:


73
Jeff






My friend HH would have locked the thread, banned half the members, and given the other 25% a vacation by now.

 

[eagle1911]

^Very true, this thread is about a 148, and really at the power levels achieved by the final in that radio (or even an upgraded/modded final PA circuit) any spurious emission will be small enough in intensity that it shouldn't be overly harmful. It's mostly the bigger stuff like amplifiers and souped-up import radios that can be really problematic in terms of potential interference.

 

[Dark Star]

smackdown, they are talking about something else.
its not like volting an amp where you supply it with more voltage from the power supply, its an internal mod where the final transistor gets the full supply voltage on its collector instead of about 6 volts.


there is absolutely nothing wrong with volting the final in the 142/148 style chassis.
it does not affect SSB at all. the voltage takes a different path in AM than it does in SSB, and the final already gets the full supply voltage in SSB in stock form.

it will not make your radio sound like crap unless it already does.

it will not stress your radio out unless you keep the deadkey higher than about 4 or 5 watts.

it will actually make the AM modulator transistor run cooler because now it only has to modulate the driver.

what kind of power gains are we talking about? on average, the difference between about 17 watts PEP to about 25 watts PEP.

not really a noticeable gain, but it can wake a sluggish amplifier up.
again, we're not even talking about a .5 S unit increase, so the answer is no; its not really needed, unless you just want to increase your PEP for the fun of it.

i dont run my radios with that mod, but its just because it doesnt really do all that much, but i have done it in the past, and had no problems with SSB or anything else.

i also agree that a good alignment/peaking of the TX coils and cutting R131 is all these radios need to sound amazing.
LC

Hi loosecannon, I know this an old thread but what would you consider the most effective way to get 12v over to tp7? I know this only gives about and additional 10w pep out of the 148 but that would make a considerable difference in driving my Texas Star 500V. After a proper peak and alignment I'm seeing about 19w forward. If I was running barefoot that extra 10w wouldn't mean anything but it will make a big difference in driving this amp I think.

 

[nomadradio]

The foil pad at the center leg of the AM modulator transistor is where we put it. The bias-test wire for the final transistor should reach it okay.

And there is a short wire jumper on the component side connected to this foil pad. Lap-soldering the end of the final bias-test wire there works just fine.

73

 

[Dark Star]

The foil pad at the center leg of the AM modulator transistor is where we put it. The bias-test wire for the final transistor should reach it okay.

And there is a short wire jumper on the component side connected to this foil pad. Lap-soldering the end of the final bias-test wire there works just fine.

73

So this is a 2001 Chinese model. Where is the AM modulator transistor located on this radio? Do I just clip the test lead and then essentially make a connection between the center leg of the transistor to the pin that the test lead connects to. Please forgive my inexperience with this one.

 

[999]

Hi loosecannon, I know this an old thread but what would you consider the most effective way to get 12v over to tp7? I know this only gives about and additional 10w pep out of the 148 but that would make a considerable difference in driving my Texas Star 500V. After a proper peak and alignment I'm seeing about 19w forward. If I was running barefoot that extra 10w wouldn't mean anything but it will make a big difference in driving this amp I think.

It really won't and you'll just end up pushing your 1969 to failure that much sooner.
No matter what extra you might see out of the DX500, nobody will ever notice it on the receiving end. You need to quadruple your output to get 1 S-unit on the receiving end.
Let me put it like this....If you are seeing 400 watts peak out of your DX500 and the person that you're talking to is receiving you at S7 you would need to up that power to 1600 watts just to get to S8.
Conversely if you dropped down to 100 watts the receiver would still have you at S6.
So my advise to you is don't hack up your radio to try and get a few more watts out of it to try and get maybe 80 or 90 watts (if you're lucky) on the top end when it won't make any difference.
Run your equipment as is if it sounds good and let it loaf along and run cool. It will last you years longer and you won't be out so much money in mods and repairs.
I've been doing this for over 35 years and I have the same DX667V that has never blown up since 1996. I have talked all over the world with it and its better than any Timex, that's for sure.

 

[Dark Star]

It really won't and you'll just end up pushing your 1969 to failure that much sooner.
No matter what extra you might see out of the DX500, nobody will ever notice it on the receiving end. You need to quadruple your output to get 1 S-unit on the receiving end.
Let me put it like this....If you are seeing 400 watts peak out of your DX500 and the person that you're talking to is receiving you at S7 you would need to up that power to 1600 watts just to get to S8.
Conversely if you dropped down to 100 watts the receiver would still have you at S6.
So my advise to you is don't hack up your radio to try and get a few more watts out of it to try and get maybe 80 or 90 watts (if you're lucky) on the top end when it won't make any difference.
Run your equipment as is if it sounds good and let it loaf along and run cool. It will last you years longer and you won't be out so much money in mods and repairs.
I've been doing this for over 35 years and I have the same DX667V that has never blown up since 1996. I have talked all over the world with it and its better than any Timex, that's for sure.

I do understand the concept of four times the power is equal to one s-unit on the recieve end. I would just like to be able to fully drive my amp. I didn't think that supplying 12 volts to a 1969 would be harmful in any way. Radios other than the 148 deliver 12 volts to the 1969 by design don't they? I do get your point though about it probably not making a substantial difference on the recieve end.

 

[999]

I do understand the concept of four times the power is equal to one s-unit on the recieve end. I would just like to be able to fully drive my amp. I didn't think that supplying 12 volts to a 1969 would be harmful in any way. Radios other than the 148 deliver 12 volts to the 1969 by design don't they? I do get your point though about it probably not making a substantial difference on the recieve end.

It's more about the longevity of the final at this point because, like Toshiba transistors, they are not being made anymore. I probably wouldn't have said anything if they were, but replacements are getting hard to come by.

 

[Shockwave]

Anyone who applies just 12 volts of DC to the final in this radio, has totally ignored the need to high level modulate two stages in order to provide good AM audio. This modification has been copied wrong so many times it's ridiculous.

It started as a "Double Modulator" which copied the modulation method used in the Cobra 29 LTD, for the final in the 148. However, people left out the AF choke with a DC blocking cap and any ability to apply modulated 12 volts to that final. The end result is a "half modulator" having much more carrier with noticeably less modulation.

If you're going to do this mod, run the 12 volts through an AF choke. The old Radio Shack 10 amp alternator whine filter use to be perfect for this job. Apply AF from the audio output pin of the TA-7222AP, through a 1000uf cap, to the side of the choke that feeds the final. The difference between this original 1985 modification and what it has been watered down to today, is dramatic.

You bet this places a lot more strain on the 2SC1969 final. Thinking it's no worse than the 12 volts used in the 29LTD, ignores the different impedance these two radios run their final transistors into. For the 148 to make the same 4 watt carrier on 6 volts, means its operating the final into a lower impedance matching circuit, in order to get the same output from half the voltage.

Double the voltage to that final into the same impedance and you've also doubled the current, which raised the heat dissipation and output power, by a factor of about four times! When I came up with this modification, I had just started working for Motorola and noticed they had an excellent stud mounted final that could be repurposed for use in the 148GTL.

I wish I could remember the number of this final but it was the one used in the old Motorola low band 25 watt Moxy radios. Being stud mounted, you could just enlarge the heatsink hole and mount it in place with some short, low inductance leads. The MRF455A, worked almost as good, with slightly less gain than the low band part.

 

[1iwilly]

Shockwave,
here are pics of what your talking about this year i remove it and went with a different mod keeping the amc limiter intact

 

[Shockwave]

Shockwave,
here are pics of what your talking about this year i remove it and went with a different mod keeping the amc limiter intact

Thanks Willy. That's not the Radio Shack choke I use to install but, it probably works just as well. I agree with keeping the AMC intact since bypassing it can cause every stage from the mic preamp, to the output chip to flat top.

For the customer that was difficult to please without removing the AMC transistor, I found another way to modify the AMC circuit to prevent the AF stages from ever going into distortion while still allowing the full modulators capabilities. It is especially useful if there is real NPC installed.

The idea is to take the input line from the AMC circuit, off of the RF line and connect it to the output of the audio chip. That input to the AMC circuit was off a glass germanium diode but I did forget the number. Now the positive peaks can rise to the full potential of the modulator, regardless of the carrier setting. AMC will only trigger to prevent the modulator from flat topping. If you can prevent the negative peak from hitting cutoff, this will produce louder audio without noticeable distortion.

I'll also mention that when you "volt" the final with only 12 volts and no modulation, you may hit 20 watts of output. However, when you modulate that 12 volts, the peaks can easily hit over 40 watts PEP. When you double the modulated voltage to the final at the same output impedance, output PEP tends to quadruple what it was at 6 volts. The stock 1969 final, will not like this at all.

 

[Handy Andy]

The idea is to take the input line from the AMC circuit, off of the RF line and connect it to the output of the audio chip. That input to the AMC circuit was off a glass germanium diode but I did forget the number. Now the positive peaks can rise to the full potential of the modulator, regardless of the carrier setting. AMC will only trigger to prevent the modulator from flat topping. If you can prevent the negative peak from hitting cutoff, this will produce louder audio without noticeable distortion.

​

 

[Dark Star]

Thanks Willy. That's not the Radio Shack choke I use to install but, it probably works just as well. I agree with keeping the AMC intact since bypassing it can cause every stage from the mic preamp, to the output chip to flat top.

For the customer that was difficult to please without removing the AMC transistor, I found another way to modify the AMC circuit to prevent the AF stages from ever going into distortion while still allowing the full modulators capabilities. It is especially useful if there is real NPC installed.

The idea is to take the input line from the AMC circuit, off of the RF line and connect it to the output of the audio chip. That input to the AMC circuit was off a glass germanium diode but I did forget the number. Now the positive peaks can rise to the full potential of the modulator, regardless of the carrier setting. AMC will only trigger to prevent the modulator from flat topping. If you can prevent the negative peak from hitting cutoff, this will produce louder audio without noticeable distortion.

I'll also mention that when you "volt" the final with only 12 volts and no modulation, you may hit 20 watts of output. However, when you modulate that 12 volts, the peaks can easily hit over 40 watts PEP. When you double the modulated voltage to the final at the same output impedance, output PEP tends to quadruple what it was at 6 volts. The stock 1969 final, will not like this at all.

That does seem like a lot of modding for what it sounds like something that may just fry the 1969 at the end of it all. This is why this forum is so great and why I like to get educated opinions before jumping forward. Thanks for explaining this to me. It sounds like I should just leave it as is and don't mess with it!

 

[Shockwave]

That does seem like a lot of modding for what it sounds like something that may just fry the 1969 at the end of it all. This is why this forum is so great and why I like to get educated opinions before jumping forward. Thanks for explaining this to me. It sounds like I should just leave it as is and don't mess with it!

It is just a matter of adding two parts. But yes, it does benefit from an upgraded final too. People like doing the modification the wrong way because it requires zero parts to be added.

 

[Dark Star]

It is just a matter of adding two parts. But yes, it does benefit from an upgraded final too. People like doing the modification the wrong way because it requires zero parts to be added.

I did try the mosfet 520 upgrade but didn't see much benefit to it so went back to stock 1969. I'm lucky enough to have two older genuine Motorola 1969s that I've been hoarding for years just in case.