ahess
Member
I'm glad to be adding something to the forum after having followed so many posts!
This is a restoration project for my first-ever audio purchase--a TA-4650 from an enlightened stereo person who brought me into the audiophile world when I was 16. There is a lot of love in this device!
Background
I’ve been using just the amp section, connected to a tube-modified Logitech Transporter as a preamp. The Sony stopped working, then later I heard a “pop” from the speakers. The speaker protection circuit doesn’t kick in anymore--no “click” 5-10 seconds after turn on. I probed voltage on the power board according to the schematic and it seems okay (values are higher than listed (e.g., ~100 rather than ~78, but I understand that there are reasons for that). I’m assuming, then, that the power amp board is where the problem is. The vfets seem good: S-D measures ~1.6 ohms and S-G and D-G pass the diode test. I’ve taken the vfets out.
I’m a self-taught electronics/DIY guy. I can measure continuity, read resistor bands, solder, discharge capacitors, but don’t understand much of the schematics of the Sony. I’ve combed the posts here and gained an enormous amount of information about this amp and what to do, I also have the manual, schematics, board diagrams, and service bulletins for setting bias, replacing 1.5k resistors, and testing the vfets.
I started testing components (sorry, I forgot to take a before pic). I quickly found that some of the resistors that are in a line on both edges of the board were burnt--measuring over 30k instead of 22k and one was completely open. My theory at this point is that these resistors increased in value (I understand that happens over time) and then one finally burned out. That’s all I’ve found and I’m hoping that is all that’s wrong with the amp.
Echowars has recommended that I try to determine the cause of the failure rather than just replacing everything. If you can offer any insights, I’d greatly appreciate it. At the same time, I would want to upgrade the components, so have decided to go ahead with that (2 birds, and all that). If you can offer suggestions for my plan, that would be really helpful:
Plan
Questions
Thanks in advance for any guidance you can offer. I’ll post pics of the finished work!
Andrew
This is a restoration project for my first-ever audio purchase--a TA-4650 from an enlightened stereo person who brought me into the audiophile world when I was 16. There is a lot of love in this device!
Background
I’ve been using just the amp section, connected to a tube-modified Logitech Transporter as a preamp. The Sony stopped working, then later I heard a “pop” from the speakers. The speaker protection circuit doesn’t kick in anymore--no “click” 5-10 seconds after turn on. I probed voltage on the power board according to the schematic and it seems okay (values are higher than listed (e.g., ~100 rather than ~78, but I understand that there are reasons for that). I’m assuming, then, that the power amp board is where the problem is. The vfets seem good: S-D measures ~1.6 ohms and S-G and D-G pass the diode test. I’ve taken the vfets out.
I’m a self-taught electronics/DIY guy. I can measure continuity, read resistor bands, solder, discharge capacitors, but don’t understand much of the schematics of the Sony. I’ve combed the posts here and gained an enormous amount of information about this amp and what to do, I also have the manual, schematics, board diagrams, and service bulletins for setting bias, replacing 1.5k resistors, and testing the vfets.
I started testing components (sorry, I forgot to take a before pic). I quickly found that some of the resistors that are in a line on both edges of the board were burnt--measuring over 30k instead of 22k and one was completely open. My theory at this point is that these resistors increased in value (I understand that happens over time) and then one finally burned out. That’s all I’ve found and I’m hoping that is all that’s wrong with the amp.
Echowars has recommended that I try to determine the cause of the failure rather than just replacing everything. If you can offer any insights, I’d greatly appreciate it. At the same time, I would want to upgrade the components, so have decided to go ahead with that (2 birds, and all that). If you can offer suggestions for my plan, that would be really helpful:
Plan
- Test all the resistors, capacitors, diodes and transistors out of circuit for the power amp board..
- Replace all power supply and power amp caps, resistors, pots, and the death diodes. I’ve started putting together a Mouser parts list. Here it is.
- Replace four 1.5k resistors with four 3.3k resistors, as per service bulletin.
- Set the DC bias to max, install the vfets (new insulators and grease), then set bias to 50v, as per service bulletin #62.
- Replace the 2-prong electrical plug with a grounded 3-prong.
- Have a beer and listen to music.
- Later, maybe replace the power supply/power amp plugs with soldered, wire-wound connections.
Questions
- Any thoughts about what actually happened, and other courses of action I should take?
- When ordering components, can I go up in voltage for caps (e.g., 10v instead of 6.3v) and in watts for resistors (e.g., 1w instead of 1/4w, 3w instead of 2w)?
- Does my plan make sense? Any suggestions?
Thanks in advance for any guidance you can offer. I’ll post pics of the finished work!
Andrew
). I've triple checked the wiring, and gone over all the caps and their orientation.
