When output transistors short through, high voltage comes out the base line to the collector or emitter of the driver transistor. Usually collector. If the driver transistor is not rated to withstand that high voltage (usually not), it blows up too.
Hum trimming comes waaay after replacing blown up parts. Once high voltage gets into the driver area, resistors, capacitors, diodes, even IC's have to be checked.
when I replace drivers I usually use mje15032/33 which cost a little more but can withstand any audio voltage no matter how stupid the output section still is. If you consider the $12 freight of ordering a second box of parts, the extra $2 for the mje15032-33 is not so expensive.
And put a little heat sink on the drivers if the cheapskates that designed the amp didn't. Predrivers too, if it has those.
Hum trimming comes waaay after replacing blown up parts. Once high voltage gets into the driver area, resistors, capacitors, diodes, even IC's have to be checked.
when I replace drivers I usually use mje15032/33 which cost a little more but can withstand any audio voltage no matter how stupid the output section still is. If you consider the $12 freight of ordering a second box of parts, the extra $2 for the mje15032-33 is not so expensive.
And put a little heat sink on the drivers if the cheapskates that designed the amp didn't. Predrivers too, if it has those.
Okay thanks for that solution! I check the drivers and they don't appear to be readily available anywhere.
I'm still curious as to what caused the fire cracker effect some 1 meter above the amp (lid was off when it happend).
I'm still curious as to what caused the fire cracker effect some 1 meter above the amp (lid was off when it happend).
I blew the top right off some metal TO3 transistors that were reading .44 ohms CB as I told you. Resistors, like output emitter resistors, can explode too. Wear safety glasses during tests- except-
The exploding stopped on my amp when I put a resistor in the AC line. It is usually called a "light bulb" box, for the 100W bulb people usually put in series with the hot AC line. If the amp has a serious problem, the light bulb lights up.
Except on my 1300 W PA amp, the +-15 supplies wouldn't work for the op amp with a 100 W bulb. I ended up using a dead 1500 W room heater element for the resistor on my PA amp. Still, it kept parts from exploding so I could measure the voltages and see what was wrong.
Diodes should read .6 v forwards or -xx volts backwards, unless zener, zener voltage backwards. Resistors should drop some voltage, but V^2/R should be less than the power rating. Capacitors should have different DC voltages on either side. Transistors in the no-music mode should have about .6 v B-E (or reverse) and about 1/4 to 3/4 supply voltage on the collector. Op amps should have both inputs and outputs same Dc voltage, unless used to drive a LED or some non-amplifier purpose. Traces should have the same voltage on the leads of the components on both ends (I had some burned traces, and later bad solder joints).
That is how you find a lot of bad parts, by poking around and reading voltages with the amp on the bulb and stable.
Capacitors, I don't have a meter, I finally just had to change a few, about half I replaced were zotted in my amp, especially the 50 v rated ones.
More fun than crossword puzzles, repairing class AB amps.
Best of luck.
The exploding stopped on my amp when I put a resistor in the AC line. It is usually called a "light bulb" box, for the 100W bulb people usually put in series with the hot AC line. If the amp has a serious problem, the light bulb lights up.
Except on my 1300 W PA amp, the +-15 supplies wouldn't work for the op amp with a 100 W bulb. I ended up using a dead 1500 W room heater element for the resistor on my PA amp. Still, it kept parts from exploding so I could measure the voltages and see what was wrong.
Diodes should read .6 v forwards or -xx volts backwards, unless zener, zener voltage backwards. Resistors should drop some voltage, but V^2/R should be less than the power rating. Capacitors should have different DC voltages on either side. Transistors in the no-music mode should have about .6 v B-E (or reverse) and about 1/4 to 3/4 supply voltage on the collector. Op amps should have both inputs and outputs same Dc voltage, unless used to drive a LED or some non-amplifier purpose. Traces should have the same voltage on the leads of the components on both ends (I had some burned traces, and later bad solder joints).
That is how you find a lot of bad parts, by poking around and reading voltages with the amp on the bulb and stable.
Capacitors, I don't have a meter, I finally just had to change a few, about half I replaced were zotted in my amp, especially the 50 v rated ones.
More fun than crossword puzzles, repairing class AB amps.
Best of luck.
You can test most of the amp without turning it on.
Visual inspection first.
Resistors, diodes and transistors with DMM.
Capacitors with LCR meter.
If all ok use lamp in series with mains input to test live.
Another method I've used is to disconnect the + and - supplies from the amp in question and to insert 47ohm 1/2 W resistors in series with each supply. On power up if all is well the sacrificial resistors remain intact. you can then make fairly meaningful measurements ie midpoint voltage etc. On the other hand if the resistors smoke you know you still have a problem.
The prices you mention for semicons are a bit to expensive it seems to me.
And do not buy them via ebay! Buy them via eg. RS or Farnell. They deliver them right to your doorstep, even the next day. Plus the price is OK.
Then listen to the others with plenty of experience. It will save you money and time after all.
Test all semicons out of circuit, Period!
Ask for advice BEFORE applying power!!!
And do not buy them via ebay! Buy them via eg. RS or Farnell. They deliver them right to your doorstep, even the next day. Plus the price is OK.
Then listen to the others with plenty of experience. It will save you money and time after all.
Test all semicons out of circuit, Period!
Ask for advice BEFORE applying power!!!
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