Volting a Cobra 148 GTL final?

[Handy Andy]

Oh I can give you a L-o-o-ng story over the issue of Stainless ...

Happened at an earlier job not too long ago, before - and because - it was how I got the last job I had to give up for Mom to take care of family issues.

Yeah, Stainless being high in Carbon, Nickel and other trace materials - and it's sheer brittleness in some "mixes" the (in)ability to machine and condition it - not everyone like the results nor would they...LOL!

There was a discussion or two about the LAMINATE cores and the different alloy mixes used to make some transformers more efficient at permeability without too much "memory" or latency effects back in the CB Tricks days...

 

[1iwilly]

Dark Star if you change your mind and not doing the mod if you want to sell the transformer i'll buy off you ??

Well I picked one of these up from fleabay just in case I feel motivated to do this mod down the road....maybe with a Mosfet final to handle the extra.

 

[kc8mob]

This fella talking about coupling the audio AC signal to the 12vdc feeding the final is absolutely correct.

The cap mentioned blocks DC voltage from the output stage (pin) from the audio amplifier IC and the low value filter choke presents a higher impedance to the audio signal then the final output transistor, thus the audio sinewave (signal) imparts its effects on the final and modulates it just like the driver stages. The choke also keeps the 12v power rail from eating the AC audio signal due to its impedance or equivalent load it would present.

Why the hell this never occurred to me I dunno but by reading the old Motorola engineers comments I realized right away this same principle applies to export style rigs with shorting bars in the back.

I'm an original 2950/2970 hound. Had them in high school and you the deal.

Anyhow I always run my 2312 finals in either model off of 12vdc thinking I'm doing the 2sb817 AM regulator (Stock is 2sb754) a big favor!

Well Damnit. They need Audio signal applied or "modulated" to the 12vdc supplying them too.

I would add that the final bias in a 148 is set under no signal ssb operation and in ssb mode the 148 applies 12vdc to the final. Granted the duty cycle is clearly different but people should run additional and properly mounted heatsink surface area to the back of a 148 if you're going to smack it in the ass and call it names.

I love my Philippines 148gtl. My 11 year old uses it to work me mobile. I'm going to make these changes to it and see what the differences are by recording my current improper NPC mod and then this more complicated and seemingly correct way on my other HF setup.

Glad I came across this

 

[kc8mob]

Going to a stud mount final is also and interesting idea. There was a MRF450a as well as MRF455a

I even have both. The 450a has allot of gain if I remember correctly.

There will be pictures added to this soonish

 

[mailman]

I think doing mods to learn more about radios or to see the results first hand is always fun but before attempting any mod I think people always need to ask themselves some questions regarding what they hope to accomplish.

In most cases when people analyze what it is they want (louder audio, more watts, etc) they'll find that they can accomplish most of the results without having to replace 10 different parts and retuning an entire radio. Especially if they spend a week messing with the radio only to match it up with a crappy antenna.

True story: When I lived in Canada there were two friends who both bought the same radios brand new from a local. The one guy spent a bunch of time working on his radio doing all the mods he could find listed online, swapping out all the parts for the different values and then got on the air and he was bragging about the swing and output (granted he was pretty loud and nasty sounding). His buddy got on the air and sounded almost as loud but with cleaner audio and a local says what did you do to your radio and the friend replied, "I turned up the modulation pot and switched out the stock mike for a power mike"

Of course there are some mods out there that are worth doing so never say never.

 

[mailman]

could someone point me to the r131 cant seem to find it thanks

 

[mailman]

I don't understand why people don't like volting finals on a 148/Grants? Side band the finals get 12 volts, on AM it only gets 6 volts. All you do is move TP7 to a 12 volt source. Most people recommend adjusting the final bias for 100ma instead of 50ma. Easy mod for a few more watts without doing the NPC-RC.

It does NOT affect sideband! Volting the final is not in the same category as the NPC mod.

Volt Final and cut R131, Thats all that ever needs to be done to a 148/Grant!

I usually get 20+ watts out on AM without the NPC crap! The 148/Grant swings way better than a cobra 25 or 29!

could someone point me to the r131 cant seem to find it thanks

 

[mailman]

I don't understand why people don't like volting finals on a 148/Grants? Side band the finals get 12 volts, on AM it only gets 6 volts. All you do is move TP7 to a 12 volt source. Most people recommend adjusting the final bias for 100ma instead of 50ma. Easy mod for a few more watts without doing the NPC-RC.

It does NOT affect sideband! Volting the final is not in the same category as the NPC mod.

Volt Final and cut R131, Thats all that ever needs to be done to a 148/Grant!

I usually get 20+ watts out on AM without the NPC crap! The 148/Grant swings way better than a cobra 25 or 29!

could someone point out where r131 is please thanks

 

[guitar_199]

could someone point out where r131 is please thanks

Right below TR26...under the audio chip in this drawing.