I had previously re-capped the preamp and power amp in my '79 2285B receiver. I figured that one day the power supply caps would go, and I bought a few parts, but it seemed like a daunting job and I left it go for a couple of years.
However last week the left speaker began to buzz loudly, and after a brief period of worry that the output transistors had failed, I figured I might as well re-do the power supply caps, since I had the parts, and I would deal with any further problems when that was done.
Before I describe anything I did, DO NOT DO THIS YOURSELF UNLESS YOU KNOW HOW TO SAFELY DISCHARGE CAPACITORS and have worked with mains powered gear before. I DO NOT RECOMMEND ANYONE TRY THIS, NOR DO I ACCEPT ANY RESPONSIBILITY FOR HARM YOU MAY EXPERIENCE IF YOU IGNORE THIS ADVICE.
That said, I opened up the amp, and I saw brown soot on the bottom of one cap (on the right here):
After discharging the caps (big spark and a snapping sound, glad I did), I unsoldered them:
I cut them open gingerly with a hacksaw near the base (CAUTION, CONTENTS MAY BE TOXIC). There was a small cap encased in plastic, wrapped around with foil and paper soaked in electrolyte, which looked very heat affected in both:
I had bought four of these compact computer-type electrolytics (Nichicon) of the same rating as the originals (I don't have the expertise to modify the design, and I love the 2285B sound, so I wouldn't use higher rated parts) although they are safe to 105C instead of 85C:
Using the diagram on the side of the caps I worked out how to join pairs of caps to match the arrangement in the dual section caps. Basically - one positive terminal to the dual-section A terminal; the negative of that cap to the positive of the other, then to dual-section terminal B, and the remaining negative to terminal "-". I wired them up like this as a test rig and powered up the amp, which worked normally after a few anxious moments:
(in this picture I joined the "B" terminal pairings across both pairs of capacitors [red wires], because they are soldered to the same metal plate in the amp).
Once I knew it worked, I taped the pairs of caps top-to-top (with insulating tape between in case they touched) to fit inside the dual-section cap cans I had emptied out. I then recreated the A-B-negative wiring. In these pics, the red wire goes to A, the brown wire joins the -ve of the top cap to the +ve of the bottom one, and then to the B terminal, and the black wire goes to the -ve terminal:
I taped all around the solder joints with double layers, so that nothing would touch the inside of the can. I then drilled holes through the insulating base beside the large terminals (inside they were steel rivets I couldn't solder to) and passed the wires to the appropriate terminals, then soldered them. They then slipped inside the cans and I taped around the join:
I put it back together, checked the DC offset and bias of the power amp channels (essentially completely unchanged) and cautiously played some music. It sounds exactly as it did before (I think) and seems completely normal - here's hoping I get 30 years out of these caps!
Hope this is helpful to anyone contemplating the same. I'm not sure about other suppliers but this link shows the capacitors I used at the Radio Spares Australia site. They are Nichicon part number LGU1J682MELC, $17 each, so $68 to fix the amp with four of them. Plus quite a few hours of messing around, I hope I can save somebody that time. IF YOU KNOW HOW TO DO IT SAFELY!
However last week the left speaker began to buzz loudly, and after a brief period of worry that the output transistors had failed, I figured I might as well re-do the power supply caps, since I had the parts, and I would deal with any further problems when that was done.
Before I describe anything I did, DO NOT DO THIS YOURSELF UNLESS YOU KNOW HOW TO SAFELY DISCHARGE CAPACITORS and have worked with mains powered gear before. I DO NOT RECOMMEND ANYONE TRY THIS, NOR DO I ACCEPT ANY RESPONSIBILITY FOR HARM YOU MAY EXPERIENCE IF YOU IGNORE THIS ADVICE.
That said, I opened up the amp, and I saw brown soot on the bottom of one cap (on the right here):
After discharging the caps (big spark and a snapping sound, glad I did), I unsoldered them:
I cut them open gingerly with a hacksaw near the base (CAUTION, CONTENTS MAY BE TOXIC). There was a small cap encased in plastic, wrapped around with foil and paper soaked in electrolyte, which looked very heat affected in both:
I had bought four of these compact computer-type electrolytics (Nichicon) of the same rating as the originals (I don't have the expertise to modify the design, and I love the 2285B sound, so I wouldn't use higher rated parts) although they are safe to 105C instead of 85C:
Using the diagram on the side of the caps I worked out how to join pairs of caps to match the arrangement in the dual section caps. Basically - one positive terminal to the dual-section A terminal; the negative of that cap to the positive of the other, then to dual-section terminal B, and the remaining negative to terminal "-". I wired them up like this as a test rig and powered up the amp, which worked normally after a few anxious moments:
(in this picture I joined the "B" terminal pairings across both pairs of capacitors [red wires], because they are soldered to the same metal plate in the amp).
Once I knew it worked, I taped the pairs of caps top-to-top (with insulating tape between in case they touched) to fit inside the dual-section cap cans I had emptied out. I then recreated the A-B-negative wiring. In these pics, the red wire goes to A, the brown wire joins the -ve of the top cap to the +ve of the bottom one, and then to the B terminal, and the black wire goes to the -ve terminal:
I taped all around the solder joints with double layers, so that nothing would touch the inside of the can. I then drilled holes through the insulating base beside the large terminals (inside they were steel rivets I couldn't solder to) and passed the wires to the appropriate terminals, then soldered them. They then slipped inside the cans and I taped around the join:
I put it back together, checked the DC offset and bias of the power amp channels (essentially completely unchanged) and cautiously played some music. It sounds exactly as it did before (I think) and seems completely normal - here's hoping I get 30 years out of these caps!
Hope this is helpful to anyone contemplating the same. I'm not sure about other suppliers but this link shows the capacitors I used at the Radio Spares Australia site. They are Nichicon part number LGU1J682MELC, $17 each, so $68 to fix the amp with four of them. Plus quite a few hours of messing around, I hope I can save somebody that time. IF YOU KNOW HOW TO DO IT SAFELY!
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