Audio Transformer Get Hot?

[YosemitySam]

Well, I replaced a volume pot in this radio and now it only transmits 0.5 watts,with little if any modulation. Audio transformer is also getting hot. I'm puzzled with this, all I did was fix the volume pot. I now have volume with no transmit. I changed the final cause I had one one handy. Did not work. Could it be the audio IC? Maybe a capacitor? Any help or Idea's would be great. I don't have equipment to test audio tranformer. But, I'm sure you could help me narrow it down without that. It is a Tristar 120 mobel. Thanks !!

[zeppfrog]

did you have good wattage before changing the volume pot? I would check there first. miswired perhaps.

[YosemitySam]

I had good carrier with a little modulation, Before I replaced it. I double checked the wires. All are right there. I drew a schematic before I unhooked the pot, just to be sure I rehooked right. I am getting 13.8 volts to power and out. I don't want to experiment with redirecting wires for fear of smoke. I'm sure I hooked it up right. What could be the possible causes? Thanks

[zeppfrog]

i dont really know the model, but just using some guess work here. if you are sure the wiring is correct on the new pot, and i trust you did a good job on that, then it could be there was a cold solder joint just waiting to be disturbed. it also could be that something attached to the chassis, such as a regulator in the cobra 29 types, got disturbed and lost its insulation against the chassis. testing the solder side with a plastic probe, pressing against the solder connections may reveal a cold solder joint. (while the radio is setup and running) can be tedious, but its a pretty tried and true method. also, look for swollen electrolytic capacitors. if the top of one looks like a baby moon hubcap or worse, its shot. again using the plastic probe (tooth brush sharpened to a dull point works) press lightly on regulators or chassis mounted transistors to see if they are shorted against the chassis. finally the old freeze spray and or a good keen eye looking for burned components.

[nomadradio]

I was unable to locate a diagram for that radio. The schematic is my starting point, whether the radio is in front of me, or across the planet.

I'll pass along my general experience concerning failures that just 'happen'.

I have accidentally bridged solder across adjacent foil pads on more than one occasion. Sooner or later, it happens. Your 'spontaneous' problem sure sounds like this could be the cause.

But the volume control should affect only the receiver, if you did not get it exactly right. I don't see how an error on the volume control would stop the radio from transmitting normal carrier power.

So, you fixed a receiver problem, and now you have a transmitter problem. When a seemingly random failure appears right after a repair, the logical thing to assume is that you accidentally changed something you didn't intend to change.

Best place to look is in the physical vicinity of where you were working.

Could you have left off the insulating shoulder washer that separates the metal tab of the fnal transistor from the grounded heat-sink metal? If that washer doesn't seat properly in the hole in the transistor's metal tab, the mount screw will touch the inside of the hole and bridge the metal tab to ground. The metal mounting tab on that final transistor is "hot". It has your RF output power on it, as well as the modulated DC power coming out of the audio transformer. The one that's overheating.

Second, it sounds as if you do not have the correct size fuse in line with this radio while you are testing it.

Do you have a current meter on your power supply? If the radio's current draw is excessive, this is a big clue. That transformer should NEVER have the chance to get hot with the correct 2 Amp size fuse in line. A fuse that size should blow before that transformer can overheat. And if you allowed the radio to pull much more than 2 Amps, this could damage the audio transformer permanently.

If the audio transformer gets too hot for too long, it will short one or more of the windings. Once this happens, your receiver audio will be very weak and distorted. The audio power chip will also overheat and fail pretty quickly. But before it does that, a 2-Amp fuse should pop and shut it down.

Running it for too long with a too-large fuse in line can add new problems to the one that appeared after replacing the volume control.

But without a schematic, this kind of general advice is all I can offer.

73

[YosemitySam]

Thanks for the reply guy's. After surching high and low I cannot find a schematic any place. If any one happen to have one. It would be a great help if you would post it or mail it to me. As far as the fuse it is a 2 amp and it will not blow I even change it thinking the fuse is marked wronge. Still won't blow. Final is seated correctly with good paste. Checked for solder bridges and exposed wire shorts to no avail. Now, I have a new symptom, after checking the fuse and turning it on. It automaticly goes TX, I think mybe it is, or, was shorted some how. I will change the transformer and Audio IC. But, I won't turn it on until I find a schematic. If someone comes across one please let me know. Until then I guess the project it DOA. Thanks for the help Fella's. 73's

[ramblingman]

If you have good receive audio..The audio i.c. and or audio
transformer is probably o.k..If one of these where open or
shorted.You would have no receive audio,or it would be very
distorted.Sounds like you short is around the r.f. section.
You should double check your work at the final.And also check
the switching transistors this will cause it to key up,if they
short.

[Rick]

There are 3 versions of the Tristar 120.
To help ID your version let us know which PLL is fitted and is it the version with a 3-way power switch, and let us know the IC ref numbers and part numbers.

[timrim]

Check and/or replace the final. 99% of the time when the audio transformer gets hot, its because the final is leaking or shorted. Usually this will blow the spike diode, but I have seen some shops actually jumper this diode to gain an extra watt or two on dead key!

[YosemitySam]

PLL LC7120, IC KIA7217AP not sure about ref numbers yet. Has a HI, CB, and Low swith. Okay, I have checked Final and It is okay. Turns out I was looking at the Power transformer instead of audio. If I had a schematic it would have been more spottable. Sorry, I'm still new and learning. Keying diodes check ok. I found a bad choke near final and the power transformorer was bad, I assume, being melted and getting hot. Haven't replaced them yet. Thanks guys!!

[YosemitySam]

Well, I replaced the Power Transformer, I have also found a few over heated resistors R69, R54, R66, R158, R6, On to replacing those now. Can't power up until I find out If I switched the wires on the on/off switch. Still haven't found a schematic. Oh, I also don't know what L4 choke value is, It was fried also. Boy, I really messed this one up. THanks for the help guy's! Wish I were smarter, But I'm learning!

[ramblingman]

Have you checked c-51 and c-52..These are the two elec. caps
close to the transformer t-1..c-51 is the large cap. should be a
1000 uf @ 16 vdc,,and c-52 is the second largest cap,,close to
c-51 and t-1..c-52 should be a 470 uf @ 16 vdc elec. cap..
These are directly on the b+ voltage going to the audio chip.
I have seen either of these caps short..when you say the choke
l-4 is bad..i guess you know they suppose to read like dead short
full scale on v.o.m. when it is bad,it will read open..And i dont see
a l-4 on the schematic,,i see an l-14..probably you just didnt see
the 1 of 14..Like i say all the chokes should read full scale on v.o.m.

[YosemitySam]

Thanks fella's for all the reply's! I have replaced all resistors that need to be, replaced. Replaced power tranformer. On this board the c-51 andc-52 are c-90 and c-91 I will check those as well. The choke I refured to is L4 this must be another version than the one Ramblingman is refuring to but, I'll bet the schematics are very close. I would like to know how the power wires hook up to the on/off volume pot. So, I can make for sure I didn't hookup wronge, even though, I double checked. Something had to short and cause all this fun. Maybe someone could please explane chokes why they have large and small raps and few or many turns? I know they test like a short unless they are fried in two. Thanks guy's