Hey all,
I'm really new to the game, but have worked on pinball machines and some other odds and ends over the years, so working on circuit boards isn't outside of my wheelhouse. I currently have a Rotel RA-840BX3 that's on the fritz (has a broken bias potentiometer). As I'm ordering parts anyhow, I tested the readily available (read: huge 50V 10000uf) caps on the board, and one is dead, so I intend to replace those as well.
I have the schematic and service manual, but have never done a recap. I'm wondering a) if there's any particular way other than poring over the schematic to find the caps required, b) if there are any capacitors that are generally replaced and generally left alone, and c) how a guy goes about learning how that schematic hangs together i.e. what is each part of the circuit doing? I can easily test out a given component, but I don't entirely know how to say "oh, these components are the phono stage; this is the power supply" and so on. I've done some Googling and YouTube-ing and whatnot, but feel like I'm on a bit of a fishing expedition and not catching much. I figured as opposed to taking a shotgun approach, I'd try to learn to do it right. Thanks in advance for the help; really appreciate it.
I'm really new to the game, but have worked on pinball machines and some other odds and ends over the years, so working on circuit boards isn't outside of my wheelhouse. I currently have a Rotel RA-840BX3 that's on the fritz (has a broken bias potentiometer). As I'm ordering parts anyhow, I tested the readily available (read: huge 50V 10000uf) caps on the board, and one is dead, so I intend to replace those as well.
I have the schematic and service manual, but have never done a recap. I'm wondering a) if there's any particular way other than poring over the schematic to find the caps required, b) if there are any capacitors that are generally replaced and generally left alone, and c) how a guy goes about learning how that schematic hangs together i.e. what is each part of the circuit doing? I can easily test out a given component, but I don't entirely know how to say "oh, these components are the phono stage; this is the power supply" and so on. I've done some Googling and YouTube-ing and whatnot, but feel like I'm on a bit of a fishing expedition and not catching much. I figured as opposed to taking a shotgun approach, I'd try to learn to do it right. Thanks in advance for the help; really appreciate it.
I know this is an old post...
I just bought one of these amps via CL and I'm recapping it. This is not my first recap job. I recap anything that is over 20 years old because electrolytic caps have limited life span and they are usually cheap. I don't bother testing the caps I'm going to replace, but I do test the new ones I am installing.
These amps were made to be serviced, so you picked a good one to start with. There is a cover on the bottom of the amp that comes off for access to the bottom of the PCB. They marked component designators on the top and bottom sides of the PCB, making it easy to find solder pads for parts you are replacing. The service manual includes voltages at critical nodes in the circuits. The only thing they didn't do is provide a complete list of parts.
My process is to visually inspect the board and list each and every electrolytic cap I find, noting the component designator, value and voltage rating. Then I use a highlighter to mark out all the electrolytics on the schematic, and compare those parts to my list. Finally I make a spreadsheet with all that info and sort it by capacitance and voltage. Then I go to digikey or mouser to get replacements. On some pieces of gear, I increase the voltage rating on the caps because the voltages in the circuit are too close to the cap's rated voltage. In this particular piece, the caps they used had reasonable margins on the voltage, so the caps can be replaced with same voltage parts.
That said, when buying parts you'll discover that there is a significant price break at 10 pieces. So in equipment that uses say 10 uF at 10, 16, 25, and 35 V, you could buy all those different voltage caps, but you'd be better off buying all of them rated for 35V. As long as there is room on the PCB to fit the parts, you'll be fine. It is often cheaper to buy 10 pieces than to buy 4, or will cost only a little more, so order 10 and you'll have some left for your next recap job.
The RA-840BX3 has 4x 1k 2W 5% carbon film resistors (R641-644) that run hot. In my amp their outer coverings were cracked from the heat. I am replaceing those parts with 1k 3W 2% metal film parts. C652 is marked on the schematic as 22 uF10V. On my amp there was a 220uF cap there, so that's what I am replacing it with. There are two 4.7 uF 10V tantalum caps (C403 and 404) used as coupling caps in the phono preamp stage. I am replacing those with polypropylene film caps as tantalum caps are really not very good for coupling audio signals. You'll notice some of the electrolytic caps are pretty generic looking and others are Nichicon or Rubycon "audio grade" caps. I always replace audio grade caps with audio grade caps.
After recapping, I always set the bias and DC offsets per the manual. You're going to have to be careful when replacing that bias pot- if it is set too high when you power up the amp it may cause the output devices in that channel to overheat. It is probably safe to set it at about mid position when you install it and then tweak it once the rest of the job is done.
I just bought one of these amps via CL and I'm recapping it. This is not my first recap job. I recap anything that is over 20 years old because electrolytic caps have limited life span and they are usually cheap. I don't bother testing the caps I'm going to replace, but I do test the new ones I am installing.
These amps were made to be serviced, so you picked a good one to start with. There is a cover on the bottom of the amp that comes off for access to the bottom of the PCB. They marked component designators on the top and bottom sides of the PCB, making it easy to find solder pads for parts you are replacing. The service manual includes voltages at critical nodes in the circuits. The only thing they didn't do is provide a complete list of parts.
My process is to visually inspect the board and list each and every electrolytic cap I find, noting the component designator, value and voltage rating. Then I use a highlighter to mark out all the electrolytics on the schematic, and compare those parts to my list. Finally I make a spreadsheet with all that info and sort it by capacitance and voltage. Then I go to digikey or mouser to get replacements. On some pieces of gear, I increase the voltage rating on the caps because the voltages in the circuit are too close to the cap's rated voltage. In this particular piece, the caps they used had reasonable margins on the voltage, so the caps can be replaced with same voltage parts.
That said, when buying parts you'll discover that there is a significant price break at 10 pieces. So in equipment that uses say 10 uF at 10, 16, 25, and 35 V, you could buy all those different voltage caps, but you'd be better off buying all of them rated for 35V. As long as there is room on the PCB to fit the parts, you'll be fine. It is often cheaper to buy 10 pieces than to buy 4, or will cost only a little more, so order 10 and you'll have some left for your next recap job.
The RA-840BX3 has 4x 1k 2W 5% carbon film resistors (R641-644) that run hot. In my amp their outer coverings were cracked from the heat. I am replaceing those parts with 1k 3W 2% metal film parts. C652 is marked on the schematic as 22 uF10V. On my amp there was a 220uF cap there, so that's what I am replacing it with. There are two 4.7 uF 10V tantalum caps (C403 and 404) used as coupling caps in the phono preamp stage. I am replacing those with polypropylene film caps as tantalum caps are really not very good for coupling audio signals. You'll notice some of the electrolytic caps are pretty generic looking and others are Nichicon or Rubycon "audio grade" caps. I always replace audio grade caps with audio grade caps.
After recapping, I always set the bias and DC offsets per the manual. You're going to have to be careful when replacing that bias pot- if it is set too high when you power up the amp it may cause the output devices in that channel to overheat. It is probably safe to set it at about mid position when you install it and then tweak it once the rest of the job is done.