Modulation Transformer - Partial Magic Smoke Release

[LeapFrog]

I have a 29 LTD Classic, I was experimenting with an old tip120 variable power mod I found on the web.
Upon powering up the radio not keying it, I quickly observed smoke, I unplugged it and looked around to see if anything had blown I didn't see anything so I started feeling parts with a finger tip.

The modulation transformer was smoking hot, : ) I powered the radio up once more to confirm that's where the smoke was coming from.. Yup, it got hot again quick.
I unplugged the radio, removed the mod and replaced the driver & final.
Powered it up, no smoke. Radio throws a carrier and has forward swing under modulation.

My question: Any way to test the transformer to see if it's 100% GOOD still?

Is there a tolerance like a specified ohm measurement somewhere?
I'd like to see if any of the varnish melted completely between two windings and created a "semi-short" whereby changing the characteristics of the transformer but not failing completely, is there any way to test this?

Like I said the radio works, the part does not heat up, although I seem to be making a little less avg wattage, perhaps that's due me changing the driver and final. For some reason seeing a little of that magic smoke released has me worried, I'm hoping my fear is unfounded, but it seems the radio is making less power. Maybe it's Placebo effect?

 

[loosecannon]

if you saw smoke, and there isn't a part on the radio that you had to replace because it was burnt, then the modulation transformer is a likely culprit.

transformers have a specific smell when they burn, but you may not recognize this yet.

chances are that you shorted this transformer to ground, and in turn singed some of the enamel coating of the wire.

if this is the case, you will not notice that the resistance of the transformer has changed, but it will be more susceptible to failure now due to the thinner/ burnt enamel coating.

the lesson here is NEVER run your radio without the proper fuse in line.

use a 4 amp fuse, and you will be good for any mod you are doing, while remaining safe.

if you have a really high powered radio (50 watts or so), and the fuse blows on modulation peaks, you can use a 5 or 6 amp fuse in that specific instance, but for general use, a 4 amp fuse is what you need.

you transformer will probably continue to work, but if it gets too hot, it may short and fail on you. however, if you have a 4 amp fuse in line, it will blow before you see the smoke.

here is a good variable power mod for that radio and the 25LTD:
http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

you can use this one to check over the one you used if it's different.
LC

 

[LeapFrog]

Alright thank you for the detailed response and link, the one I used looks extremely similar, in fact without the other schema in front of me I would say it's the same.
I used a 1k pot, and had done the ekl en1230 with a vishay 520N. Any chance the mosfet conversion is flat out incompatible with this variable mod?

Well only time will tell if the transformer will fail prematurely, 4 amp inline fuse it is.
I'm using a 12v "203 watt" 16.5 amp power supply with internal short circuit protection, if a short occurs the psu will go into fault-mode and needs to be unplugged to reset it. I'm suspecting there was no direct short detected by the psu, but the current draw was enough to damage the transformer because there was a short in the radio, and a 3 or 4 amp fuse would have blown before i released the magic smoke.

Sometimes the lesson must be learned through painful experience, well i'm off to get an inline fuse holder and some fuses!

 

[loosecannon]

yeah the inherent resistance of the transformer may have been enough to not register as a direct short to the power supply, and instead it just created a buttload of heat.

you should be able to do that variable power mod with a mosfet conversion.
the variable mod only affects the power going to the driver transistor, which you are not messing with.

i'll bet i can guess what happened to you with that mod!

you probably didn't isolate the metal tab on the transistor from the metal that you mounted it to.

That tab is another collector lead, and if you look at the diagram, the collector goes right to the voltage source that goes through the transformer.
if you didn't isolate that metal tab from the metal heatsink surrounding the driver and final (assuming thats where you mounted your TIP120), then you just provided a direct path for the current to go through the transformer, right to PC board ground through that heatsink.

you need to use a mica insulator along with heat sink compound on both sides of it.
then you need to use a plastic shoulder washer so that screw doesnt short the metal tab to the heatsink.

if this is what you did, just try again using a new TIP120, and proper mounting procedure.

remember that the jumper (JP36 for your 29LTD) needs to be removed, and the coil that the wire gets soldered to it (L15 for your 29LTD) does NOT get one end removed. just leave the coil in place and solder the wire to the trace on the solder side of the board.

good luck,
LC