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Could use a bit of help with a Onkyo A-5

B

brandysparts

New Member
Hello, I am currently trying to fix my fathers old Onkyo A-5 Integrated Amp. It would not start up or give any inclination that power was going through it, so I spent the past few days trying to figure out why when I discovered it was an issue with the thermal fuse in the power transformer. After attempting a bypass of it, I was greeted by it turning on which was a very welcoming sight. I was also then greeted by the smell of a burning resistor, which then promptly blew and caught fire which was promptly extinguished. The resistor that blew was named R638, and I cannot find it on the schematics for whatever reason, so I am a bit lost as to what sort of resistor to use, and could use a bit of help there. I am also looking for advice as to why the resistor blew in the first place, as I am unsure if its just because of the age of the system and being in a moist environment for two days, which I believe is the reason it failed 2 decades ago, or if its because of something I have done wrong. I appreciate any sort of help or advice you can give me, and I hope you have a wonderful day!
 
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brandysparts, Welcome to AK. Looking at the schematic the power amp shows just one channel. The resistor not on the schematic is on the channel not shown. I think it is the other channel's version of R639. But pics would be welcome.
 
NAD80 said:
brandysparts, Welcome to AK. Looking at the schematic the power amp shows just one channel. The resistor not on the schematic is on the channel not shown. I think it is the other channel's version of R639. But pics would be welcome.
Click to expand...
Ohh ok that makes sense. I was able to rectify the issue with a newer resistor and pot, and I can now turn it on without anything catching fire. I am currently trying to figure out why the sound is extremely muted however. I am hoping I just need to clean the electrical contacts as it has been sitting for over two decades now. The sound is extremely muted to the point that you cane hardly hear it, and it only coming out of the right. I accidentally blew a fuse while working on it so I know why it is only coming out the right, but I am unsure about the muteness of it all.
 
Check that the PRE-OUT/MAIN-IN jumpers are in place at the rear
of the unit. There may be a coupling switch, needs to be ON, do
not flick switch with amp powered up.

Since both channels affected could be a power supply issue
to the opamps in tone stage. If you are comfortable taking
voltage measurements on a live unit, suggest check the +B2
and -B2 supply to the power amp (same as tone stage) hopefully
there will be some posts to clip on. expect about +/-17Vdc
 
mbz said:
Check that the PRE-OUT/MAIN-IN jumpers are in place at the rear
of the unit. There may be a coupling switch, needs to be ON, do
not flick switch with amp powered up.

Since both channels affected could be a power supply issue
to the opamps in tone stage. If you are comfortable taking
voltage measurements on a live unit, suggest check the +B2
and -B2 supply to the power amp (same as tone stage) hopefully
there will be some posts to clip on. expect about +/-17Vdc
Click to expand...
Wow, that fixed it right up. You guys are an absolute lifesaver! I'm happy I was able to bring life back to an older piece of equipment like this, it was my fathers first integrated amp he apparently bought with his paper rout money.
 
Sorry guys, evidently I must have messed something up as I have had two transistors burn out because I had a misplaced part, and after replacing the transistors I had a resistor burn along with another transistor. I was able to get some sound out though, so that was nice. I do not believe that I am equipped enough to handle vintage equipment.
 
Welcome to AK. Should have said that before. Well as for fixing the Onyko A-5. Ask questions. Put up some pics as to the PCB looks like. Don't give up that quick. Some great people here. It may take time, it can be fixed if the parts are available. Also check all the soldering on the boards.
 
 
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