Troubleshooting

Or… HOW TO SEARCH AND FIND FOR ERRORS, MISTAKES, FAILURES, SHORTS AND PARTS DAMAGED IN A DIY AUDIO AMPLIFIER BOARD:

First job is to check the board, because when we prepare boards, or when we etch at home, we may have failures as copper lines missed, and during soldering you may have shorts.

So, prepare a schematic copy, a printed schematic and a colour pencil or pen and go watching the board through strong light, put a light bulb bellow the board and go inspecting traces, proceed painting each one of them in your schematic, till you have painted all schematic, trace these lines covering or in parallel with the schematic lines, do it very near the black horizontal and vertical lines that constitutes the circuit connection lines... doing this you will find missed lines, shorted lines and interrupted (broken) lines.

Solder once again each solder point, melt the solder and add a little bit of new solder, if you face bubbles in the solder point, there's a bad solder, a problem you may have fixed soldering these points once again.

Then, having all connections removed, as input cable, output cable, positive rail wire, negative rail wire, ground (VBE multiplier too) them measure resistance to search for semiconductors, or transistors shorted, lines shorted that you could not discover with visual inspection...measure resistance, high scale, 2 Megaohms or 20 Megaohms, you may be patient to wait the meter reading, it will charge electrolytic condensers and will having strange numbers floating, depending the polarity, resistance may reduce to zero and them increase again till will be stable reading several kilo ohms, more than 30K, usually you have more than 500K when everything is all right.

Do that from positive to ground, and them invert your multimeter probe tips and measure once again.... register the numbers in a piece of paper, make your notes about the resistances you have found.

Do that now from negative to ground and them invert your multimeter probe tips once again, take notes from the values you have, Now check from positive to negative and invert your multimeter probe points.

All reading should be high resistances... ohms reading, less than 2K, means shorts.

Having shorts...good!, now clean you board with a brush, cut your brush hairs to make the brush tight, reduce the brush hair length to make it strong to remove solder flux, use Alcohol, 92 degrees (to medicine use) or use Kerozene, the aviation fuel, remove all flux, clean your board, let it dry or force it dry using hair dryer or sun light.. Measure once again..... short continues?

Then check each transistor, observe first if collector is connected to the collector resistance, check the base if is is connected to the base resistance or with other component, and check emitter, do that using light bellow the board, watch the component side, and copper tracks will appear because board will be transparent to the light... copper tracks will be dark because light does not cross metal tracks.. them remove the one you have inspected, write with permanent pen the leads position to put it back once again, write (B) for base, (C) for collector and (E) for emitter...

It is needed to remove transistors, to pick them out from the board, sometimes you find transistor shorted, resistance will be zero, but board use to fool us, so, better will be to remove one by one, you will see that this will save time in the place to spend time as you may think, to be thinking and analyzing takes too much time, better is to remove them all and measure them outside, the board has resistances, diodes, and other components that may fool you, you may be reading the next transistor and the one under measurement may be open, not conducting. you may think it is fine, but you were measuring other connected to the first one.. so, remove them from the board, always do that job, seems stupid, you going to see that it is not, save time!

...then go removing transistors, one by one, do not remove all them, go one by one, and you will find the shorted one... them replace that one and continue, you may have other, or others....if you remove all, then you will searching for troubles, as you may return them in a wrong way, you may even install a npn in a pnp place..

Doing that you must check if you are using the correct transistor, and you may check in the internet to be sure about the transistor lead position, so, datasheet is there, in the internet, published to help you....check the base lead, the collector lead and emitter lead, you should download data sheet and watch this careful, also check three times if you are using a NPN in a place you need a NPN and if you are using a PNP in a place you need PNP....so , transistor will be checked...also you will find fake ones, there are NPN fakes that measure alike PNP.... yes...happened this week with a friend.

Check all transistors.... short remains?... then check if output transistor case is insulated related the heatsink, the output transistor's collectors cannot show resistance to the heatsink, heatsink is usually grounded, a wire must go from a heatsink screw (soldered) to the transformer secondary coil center tap pin, the traditional grounding system..... your collector can be shorted with the heatsink, you have already checked all transistors disconnected (out from the board) so, you know they are fine.... so, collector shorted against the heatsink means positive rail or negative rail shorted to ground.

Now electrolytic condensers, remove them and check them, also check if you have installed them in the correct polarity and check the value you have used... if you had electrolytic condensers installed in the opposite polarity, then throw them in the garbage can, they may create future troubles, better to discard these ones submitted to inverted polarity.

Now check resistances, each one of them, go painting into the schematic each one you lift one side to measure.. let the one side soldered and measure, this way you will not have the circuit resistances fooling you... take this chance to check values with your multimeter, check value comparing with the schematic value... do not believe yourself, you’re human, you make several failures each day, the first and main mistake is to think you have not made any mistake.

Doing that, is almost impossible to remain a failure...but it is hard work, do not engage your brain, it is hard, mechanical, automatic work, to desolder, to remove parts, to take notes, to measure, and to return to the board, one by one... use your Hypothalamus, your ancient brain only...work mechanically with low intelligence level needed, give a break and a good rest to your brain and engage the monkey automatic features we all have.

This is what i can do to help you. do not waste your time trying to analyze why negative rail is having so big power consumption, go measuring and you will find your mistake.. measure everything, all capacitors, all condensers, all resistances, all diodes and all transistors.. check trimpot, vbe multipler, supply voltages, supply rectifiers and so on.. if anything wrong was found, give yourself a break, try in another day, you may be tired, more than one hours doing that use to reduce our attention.. try once again latter, or another day.

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