Hello,
Quite a while ago I asked my dad for his old HK640 that we used to have at home. I remembered it having some problems and was interested in fixing it up and using it. I have made some progress, but am currently somewhat stuck. I've been working on it off and on for quite a while, so I can't remember the whole timeline 100% anymore, but I will try to sum it up as accurately as possible. (service manual is quite easy to find, for instance here)
I remember the amplifier being in use until about 10 years ago. I do not know since when, but for a few years before it being replaced, there was an issue with either one of the left/right channels, or one of the two speaker groups. I think it just did not sound right but it might have not worked at all.
Fast forward to a year ago when I got it from my dad. When I got it I think the left channel did not work at all. I opened it up and noticed C419 looked broken. I also retouched some solder points that looked particularly crappy and cleaned the pcb up a bit. After this, the left channel worked again, although it was slightly more quiet than the right channel. Did not have much time to continue working on it then, but used it for about half a year without much issue. Sometimes the left channel would get very quiet, but this was usually fixed by wiggling the balance pot a bit. A few months ago I left town for about 2 months, and when I got back the amplifier would not work at all. After some investigating, I noticed that one of the output transistors, as well as R445 were broken. I replaced all of the output transistors, as well as R447 and R445. I also cleaned all of the pots. After this, the right channel is completely fine, but the left channel is still broken. I do some testing and find that R449 is broken, and I also accidentally break the idling current adjust trimpot (woops). Replace both of these, still no left channel. I find R407 to be broken and also check most other resistors, who all seem fine. So I replace R407, still no left channel.
Currently, the status is that the right channel is fine, but there is a few volts across the left channel speaker output, while there is no voltage across TP401 and TP403 (the measure points while adjusting the idling current). I am new to audio equipment and have only done a few small repairs of other electrical equipment. However, I can solder (although not the best) and know about components and how to use a DMM. I have checked all transistors with the continuity mode of my DMM for shorts, but have not pulled them out to test them properly. I have also checked most capacitors with the resistance mode while on the pcb but have not found anything obviously faulty so far. I am afraid I am going to have to pull out all of the transistors and possibly capacitors to test them, which with my soldering skills will be a very big task, so I was hoping there might be some other things I could still try beforehand which I might be missing.
Sorry for the long messy story, I wanted to give as much info as possible. Any help would be appreciated.
Quite a while ago I asked my dad for his old HK640 that we used to have at home. I remembered it having some problems and was interested in fixing it up and using it. I have made some progress, but am currently somewhat stuck. I've been working on it off and on for quite a while, so I can't remember the whole timeline 100% anymore, but I will try to sum it up as accurately as possible. (service manual is quite easy to find, for instance here)
I remember the amplifier being in use until about 10 years ago. I do not know since when, but for a few years before it being replaced, there was an issue with either one of the left/right channels, or one of the two speaker groups. I think it just did not sound right but it might have not worked at all.
Fast forward to a year ago when I got it from my dad. When I got it I think the left channel did not work at all. I opened it up and noticed C419 looked broken. I also retouched some solder points that looked particularly crappy and cleaned the pcb up a bit. After this, the left channel worked again, although it was slightly more quiet than the right channel. Did not have much time to continue working on it then, but used it for about half a year without much issue. Sometimes the left channel would get very quiet, but this was usually fixed by wiggling the balance pot a bit. A few months ago I left town for about 2 months, and when I got back the amplifier would not work at all. After some investigating, I noticed that one of the output transistors, as well as R445 were broken. I replaced all of the output transistors, as well as R447 and R445. I also cleaned all of the pots. After this, the right channel is completely fine, but the left channel is still broken. I do some testing and find that R449 is broken, and I also accidentally break the idling current adjust trimpot (woops). Replace both of these, still no left channel. I find R407 to be broken and also check most other resistors, who all seem fine. So I replace R407, still no left channel.
Currently, the status is that the right channel is fine, but there is a few volts across the left channel speaker output, while there is no voltage across TP401 and TP403 (the measure points while adjusting the idling current). I am new to audio equipment and have only done a few small repairs of other electrical equipment. However, I can solder (although not the best) and know about components and how to use a DMM. I have checked all transistors with the continuity mode of my DMM for shorts, but have not pulled them out to test them properly. I have also checked most capacitors with the resistance mode while on the pcb but have not found anything obviously faulty so far. I am afraid I am going to have to pull out all of the transistors and possibly capacitors to test them, which with my soldering skills will be a very big task, so I was hoping there might be some other things I could still try beforehand which I might be missing.
Sorry for the long messy story, I wanted to give as much info as possible. Any help would be appreciated.
Hi WJ44,
Welcome to the forum!
I favor "active" powered trouble shooting rather than out-of-circuit component testing. It's far less tedious and less risky to the equipment. Are you comfortable with live testing and familiar with safety precautions? Sounds like you done at least some live testing. I recommend insulating exposed lengths of your probes with heat shrink to minimize unintended contact with other components while you probe around. Do you have any test equipment other than your DMM?
Would you report some observed voltages on the left channel? Be sure to note negative voltages re ground with a minus sign. Of particular interest would be the amp output re ground, voltages at bases of Q401 and Q403, volts across TP401 and TP403, voltage across C413. Use your instincts and look for voltages that are dramatically different than those noted in the schematic. I'll probably ask for other measurements as we proceed.
Let us know what you find. Good luck!
Welcome to the forum!
I favor "active" powered trouble shooting rather than out-of-circuit component testing. It's far less tedious and less risky to the equipment. Are you comfortable with live testing and familiar with safety precautions? Sounds like you done at least some live testing. I recommend insulating exposed lengths of your probes with heat shrink to minimize unintended contact with other components while you probe around. Do you have any test equipment other than your DMM?
Would you report some observed voltages on the left channel? Be sure to note negative voltages re ground with a minus sign. Of particular interest would be the amp output re ground, voltages at bases of Q401 and Q403, volts across TP401 and TP403, voltage across C413. Use your instincts and look for voltages that are dramatically different than those noted in the schematic. I'll probably ask for other measurements as we proceed.
Let us know what you find. Good luck!