Hi there,
I've been trying to fix this Akai AM2600 for a while, but I just can't seem to figure out the issue.
The thing is, the amp was run with blown fuses for a while (by inexperienced people), so all the power going to the speakers came from the preamplfification/volume control circuitry.
Ever since then the amp has been humming at 50hz
I can turn down the volume using the volume knob. With the knob all the way to the left (off) the humming is gone.
I've drawn the path the audio takes from the aux input, to the volume knob PCB in red.
The lines marked with a green 'X' are lines that I have disconnected. Whenever these lines are disconnected, the humming is completely gone. Meaning it has to come from either the Equalizer PC Board (weird name, it doesn't equalize?) or the Tape PC Board.
The audio signal doesn't pass through any active circuitry on these PCB's however, so I've been left wondering where the hum could come from
I've also disconnected the lines with the blue 'X', this didn't change anything however.
Does anyone have ANY clue as to wat might be going on here?
Thanks in advance!
PS. I've also attached the service manual
I've been trying to fix this Akai AM2600 for a while, but I just can't seem to figure out the issue.
The thing is, the amp was run with blown fuses for a while (by inexperienced people), so all the power going to the speakers came from the preamplfification/volume control circuitry.
Ever since then the amp has been humming at 50hz
I can turn down the volume using the volume knob. With the knob all the way to the left (off) the humming is gone.
I've drawn the path the audio takes from the aux input, to the volume knob PCB in red.
The lines marked with a green 'X' are lines that I have disconnected. Whenever these lines are disconnected, the humming is completely gone. Meaning it has to come from either the Equalizer PC Board (weird name, it doesn't equalize?) or the Tape PC Board.
The audio signal doesn't pass through any active circuitry on these PCB's however, so I've been left wondering where the hum could come from
I've also disconnected the lines with the blue 'X', this didn't change anything however.
Does anyone have ANY clue as to wat might be going on here?
Thanks in advance!
PS. I've also attached the service manual
The thing is, the amp was run with blown fuses for a while (by inexperienced people), so all the power going to the speakers came from the preamplfification/volume control circuitry.
Please, explain with more detail. IMO it is a complete nonsense. If is fuse is blown at speaker output or mains input, no sound in absolute.
Also, no one preamp is capable of drive a loudspeaker, this is why a power amplifier is added inside or outside the unit.
Please give us more details.
Please, explain with more detail. IMO it is a complete nonsense. If is fuse is blown at speaker output or mains input, no sound in absolute.
Also, no one preamp is capable of drive a loudspeaker, this is why a power amplifier is added inside or outside the unit.
Please give us more details.
The 2 main 5A fuses were blown (bottom of the schematic, CEE part). These supply the + and - 50V rails for the main output. Meaning the main transistors could't output anything.
The 2 630mA fuses weren't blown however, meaning the preamp circuit still had power
The very little power that did go to the speakers, must've come from the preamp circuit. To be fair, it wasn't much power, and the sound quality was VERY bad. But there was some sound, which was supplied by the preamp circuit, because the main transistors didn't have any power supply.
OK, now we are underdtanding better.
Let's see: the first thing you surely will want to do to work with electricity, is a current limited power supply. This is done getting a light bulb (those old with wire filament, no CCFL nor fluoro nor led), with its socket, a pair of cables 50cm long and a pair of cuccodrile/alligator clips, or a blown fuse. Thus, you need to remove the original fuse and wire the lamp temporarily in the set. A 100/150V lamp suffices. Build this elementary series lamp to protect you and the amplifier.
Also check with a DMM the power transistors, their drivers and all low value resistors (say, 1K or less) and signal diodes. Surely one or more are open or shortcircuited and will need to be replaced.
At same time you can remove the wires that carry the power stage from the PSU and leave the last alone. So, with the bulb in place of the fuse, give energy to the PSU alone. If all is OK you will see a flash on the lamp while storage capacitor(s) is/are charged, after it the bulb should remain dark. If still brights, one or more diodes are fried in the PSU or caps are very leaky and will need be replaced. If all is OK, disconnect supply from mains and discharge capacitors using a 1K power resistor or the same lamp. Never let the caps remain charged while working in the apparatus.
Let's see: the first thing you surely will want to do to work with electricity, is a current limited power supply. This is done getting a light bulb (those old with wire filament, no CCFL nor fluoro nor led), with its socket, a pair of cables 50cm long and a pair of cuccodrile/alligator clips, or a blown fuse. Thus, you need to remove the original fuse and wire the lamp temporarily in the set. A 100/150V lamp suffices. Build this elementary series lamp to protect you and the amplifier.
Also check with a DMM the power transistors, their drivers and all low value resistors (say, 1K or less) and signal diodes. Surely one or more are open or shortcircuited and will need to be replaced.
At same time you can remove the wires that carry the power stage from the PSU and leave the last alone. So, with the bulb in place of the fuse, give energy to the PSU alone. If all is OK you will see a flash on the lamp while storage capacitor(s) is/are charged, after it the bulb should remain dark. If still brights, one or more diodes are fried in the PSU or caps are very leaky and will need be replaced. If all is OK, disconnect supply from mains and discharge capacitors using a 1K power resistor or the same lamp. Never let the caps remain charged while working in the apparatus.
Do I surmise correctly that the power amps now operate correctly?
Do you get working audio from AUX input but with hum in background? If audio path passes signal, how severe is hum?
Same hum problem from any input? Same hum in both channels?
Does the ground path at pin 5 "X" have to severed to stop the hum?
Do you get working audio from AUX input but with hum in background? If audio path passes signal, how severe is hum?
Same hum problem from any input? Same hum in both channels?
Does the ground path at pin 5 "X" have to severed to stop the hum?
I don't really know how or why, but I ended up desoldering and testing some stuff, and after a while the hum went away.
Thanks for your suggestions anyway! 😀
Thanks for your suggestions anyway! 😀
Let me guess that next time you power on (wait until tomorrow) that hum is back again. So it could well be time to replace all the 40+ year old electrolytic type caps because electrical faults in components don't fix themselves. Plastic film caps may "self-heal" to some degree and sick electrolytic caps may work better when warmed up but that isn't the same as restoring or making them like new. Replace them with equivalent grades and types. Note that there are a number of only 10V rated solid aluminium caps in the signal path - these are easily damaged by tinkering about with testing them. Test out of circuit.
Therein lies a problem for dissatisfied users and wannabe DIY repairers:
When is something broken or deteriorated beyond serviceability? Should I also pay someone just to confirm or dismiss my suspicions but no more?
We probably need to be active repairers to make those calls anyway, unless we have the relevant experience and tools to check components ourselves. Either way, it will cost $$ just to find out so we may as well have any suspect elcaps replaced when they have reached > 40 years in situ, like in this ol' fella.