northpaw
Super Member
Since first picking up a TA-2A off of CL a year ago, I have been very impressed at the sound of these units and have since picked up two SR-2As and most recently one more TA-2A. All of these had minor issues (bad speaker relays, bad Li coin battery, etc.). I purchased the extra units because I feel they are very worthy receivers (and will probably give them to my kids), they were cheap (sold as broken/needing repair), and in part, I was looking for some challenges in learning and extending my repair experience. The last one has fulfilled that wish.
This TA-2A arrived as described: good cosmetically but does not power on. As received, the display does not light, the small LED on the power switch stays solid orange (never goes to flashing green/steady green), and there are no audible clicks of relays. On opening, a little dusty but otherwise not too bad, and no evidence of prior work. But I notice the power supply board has a more ratty appearance than the rest of the chassis. There is some corrosion on the leads of several diodes and resistors, especially near the old glue on the small transformer mounted on the power supply board and around the larger caps; there is also some superficial rust on the large chassis-mounted transformer. Initially I was concerned there was water damage in this corner of the receiver, but closer inspection showed the corrosion was essentially limited to glued areas. Here is a photo as an example (corroded D405 in the middle; some caps already replaced and cover of RL401 removed), and the schematic for the Power Supply board and related parts.
I began measuring voltages on the power supply board and inadvertently noticed the top lid on C403 had 9V on it, so its case had shorted. I stopped and unplugged, and after unbolting the big transformer (as noted elsewhere, the power supply board is a PITA to work on), managed to unseat the power supply board to the point where I could get to the underside. I pulled C403 and it measured 100µF instead of 1000µF, so I replaced it. Then I notice C412, another 1000µF cap, has some bulges at its base. Removed, it tests as failed/unrecognized. I replace it and then, proactively, C405 and C415 through C419. Those tested ok for capacitance, but Vloss was >1% and some had ESR >1Ω. Replacements all had the specified capacitance but a higher voltage rating (50V or higher).
Restarted, but no change in behavior, with the power LED still yellow. I then replaced Q525 on the logic board and the 22kΩ R309L and R309R on the main board, as these are common failure points and I had the parts leftover from earlier work on the other TA-2A. After powering on, I check the voltage across the big power caps and there is nothing. I trace back to relay RL401 on the power supply board. RL401 controls power to the large transformer. and to the second rectifier on the power supply board (D410).
Not being able to access the underside of the power supply board while in operating position, I start to check the voltage on D405 (diode with the corroded leads in the photo above) which is in parallel with the coil on RL401, but it varies from 14V to 0V depending on which way I push it, so I figure a bad solder. I remove it and clean up the corrosion. It tests ok, I think (Vf=686mv, C=15pF, Ir=2nA; the spec for 1N4002 is Ir=5µA, but is that a max limit?). I reinstall it, and now get a steady 14V on either side of it. I also check the coil on RL401 when unpowered, and it gives 187Ω, comparing well to the spec of 200Ω. Looking at more solders on the underside of the power supply board, I reflow the solders on U401 and U402 (these are transistors mounted with heat spreaders, but I am not sure what their function is) as they looked dodgy. At his point I also removed the power switch board and reflowed the contacts on the power switch, although they looked fine.
Powered up, no changes. I pop the lid of RL401 and manually engage it. I get a little spark on contact, and 4-5 sec later, I hear the speaker relay on the main board (RL103) click. The power LED stays steady orange throughout all this, to my surprise. At this point I also check again the voltage across the big power supply caps on the main board and they give a proper 44V.
I take from these observations that the both the large transformer (chassis-mounted) and the small transformer on the power supply board are OK, that the diode rectifiers D401 and D418 (the latter on the main board) are functioning and the relays are getting the voltage they need, but that RL104 is not working and needs to be replaced. I don’t know if Q407 is a potential culprit in any of this.
As for the lack of a green light on the power switch, and the lack of light on the main display, there is one more rectifier on the power supply board (D410) that I think supplies the power for at least some of these; but perhaps other parts of the circuit are involved? Mannegizen as noted that the failure of D410 will cut power to the tuner, etc., and can cause F104 to blow http://www.audiokarma.org/forums/index.php?threads/new-nakamichi-ta-3a-project.732531/#post-9919674. My F401 remains intact (although I note its resistance measures at 0.6Ω and I shall get a replacement for it).
To check D410, I manually activate R104 again and measure 8.0V on the + lead of D410, which is quite a bit less than the 19.5V value on the schematic. So there is a problem there, and I need to replace D410. Anything else? Shortly after making that measurement, F401 blows and the circuit goes dead. Mannegizen knows his stuff.
So while I will make up a parts order, with the W20M rectifiers and suitable relays at the top of the list, what I wanted to ask here if anyone can suggest any further tests I should do, or noticed any errors or deficiencies in my diagnostic efforts so far, and/or suggest further parts to order for likely failouts (I am already covered on the Q525 and the R309L/R on the main board). I am a bit suspicious of U401 and U402 (listed as µPC7805H and µPC7812H, and as ICs), as I don’t know what they do and they might have been stressed, given the heat spreaders and what their solders looked like. Could they be involved in getting to power lLED going from yellow to green, or related to a protection circuit, or are these functions related to something else? I did get the speaker relay R103 to activate once I bypassed faulty RL401, but the yellow LED persisted. My concern is that there is more going in this circuit that I can understand from my limited knowledge and experience. Any advice would be appreciated.
As it is, I am planning to order replacements for several of the other components on the power supply board, esp. the corroded 1N4002 and other diodes, and transistor spares for the Q40x on the board. Fortunately, eedork has provided a detailed list for the TA-3A with Mouser order numbers (http://www.audiokarma.org/forums/index.php?threads/new-nakamichi-ta-3a-project.732531/), most or all of which applies to TA-2A as well.
This TA-2A arrived as described: good cosmetically but does not power on. As received, the display does not light, the small LED on the power switch stays solid orange (never goes to flashing green/steady green), and there are no audible clicks of relays. On opening, a little dusty but otherwise not too bad, and no evidence of prior work. But I notice the power supply board has a more ratty appearance than the rest of the chassis. There is some corrosion on the leads of several diodes and resistors, especially near the old glue on the small transformer mounted on the power supply board and around the larger caps; there is also some superficial rust on the large chassis-mounted transformer. Initially I was concerned there was water damage in this corner of the receiver, but closer inspection showed the corrosion was essentially limited to glued areas. Here is a photo as an example (corroded D405 in the middle; some caps already replaced and cover of RL401 removed), and the schematic for the Power Supply board and related parts.
I began measuring voltages on the power supply board and inadvertently noticed the top lid on C403 had 9V on it, so its case had shorted. I stopped and unplugged, and after unbolting the big transformer (as noted elsewhere, the power supply board is a PITA to work on), managed to unseat the power supply board to the point where I could get to the underside. I pulled C403 and it measured 100µF instead of 1000µF, so I replaced it. Then I notice C412, another 1000µF cap, has some bulges at its base. Removed, it tests as failed/unrecognized. I replace it and then, proactively, C405 and C415 through C419. Those tested ok for capacitance, but Vloss was >1% and some had ESR >1Ω. Replacements all had the specified capacitance but a higher voltage rating (50V or higher).
Restarted, but no change in behavior, with the power LED still yellow. I then replaced Q525 on the logic board and the 22kΩ R309L and R309R on the main board, as these are common failure points and I had the parts leftover from earlier work on the other TA-2A. After powering on, I check the voltage across the big power caps and there is nothing. I trace back to relay RL401 on the power supply board. RL401 controls power to the large transformer. and to the second rectifier on the power supply board (D410).
Not being able to access the underside of the power supply board while in operating position, I start to check the voltage on D405 (diode with the corroded leads in the photo above) which is in parallel with the coil on RL401, but it varies from 14V to 0V depending on which way I push it, so I figure a bad solder. I remove it and clean up the corrosion. It tests ok, I think (Vf=686mv, C=15pF, Ir=2nA; the spec for 1N4002 is Ir=5µA, but is that a max limit?). I reinstall it, and now get a steady 14V on either side of it. I also check the coil on RL401 when unpowered, and it gives 187Ω, comparing well to the spec of 200Ω. Looking at more solders on the underside of the power supply board, I reflow the solders on U401 and U402 (these are transistors mounted with heat spreaders, but I am not sure what their function is) as they looked dodgy. At his point I also removed the power switch board and reflowed the contacts on the power switch, although they looked fine.
Powered up, no changes. I pop the lid of RL401 and manually engage it. I get a little spark on contact, and 4-5 sec later, I hear the speaker relay on the main board (RL103) click. The power LED stays steady orange throughout all this, to my surprise. At this point I also check again the voltage across the big power supply caps on the main board and they give a proper 44V.
I take from these observations that the both the large transformer (chassis-mounted) and the small transformer on the power supply board are OK, that the diode rectifiers D401 and D418 (the latter on the main board) are functioning and the relays are getting the voltage they need, but that RL104 is not working and needs to be replaced. I don’t know if Q407 is a potential culprit in any of this.
As for the lack of a green light on the power switch, and the lack of light on the main display, there is one more rectifier on the power supply board (D410) that I think supplies the power for at least some of these; but perhaps other parts of the circuit are involved? Mannegizen as noted that the failure of D410 will cut power to the tuner, etc., and can cause F104 to blow http://www.audiokarma.org/forums/index.php?threads/new-nakamichi-ta-3a-project.732531/#post-9919674. My F401 remains intact (although I note its resistance measures at 0.6Ω and I shall get a replacement for it).
To check D410, I manually activate R104 again and measure 8.0V on the + lead of D410, which is quite a bit less than the 19.5V value on the schematic. So there is a problem there, and I need to replace D410. Anything else? Shortly after making that measurement, F401 blows and the circuit goes dead. Mannegizen knows his stuff.
So while I will make up a parts order, with the W20M rectifiers and suitable relays at the top of the list, what I wanted to ask here if anyone can suggest any further tests I should do, or noticed any errors or deficiencies in my diagnostic efforts so far, and/or suggest further parts to order for likely failouts (I am already covered on the Q525 and the R309L/R on the main board). I am a bit suspicious of U401 and U402 (listed as µPC7805H and µPC7812H, and as ICs), as I don’t know what they do and they might have been stressed, given the heat spreaders and what their solders looked like. Could they be involved in getting to power lLED going from yellow to green, or related to a protection circuit, or are these functions related to something else? I did get the speaker relay R103 to activate once I bypassed faulty RL401, but the yellow LED persisted. My concern is that there is more going in this circuit that I can understand from my limited knowledge and experience. Any advice would be appreciated.
As it is, I am planning to order replacements for several of the other components on the power supply board, esp. the corroded 1N4002 and other diodes, and transistor spares for the Q40x on the board. Fortunately, eedork has provided a detailed list for the TA-3A with Mouser order numbers (http://www.audiokarma.org/forums/index.php?threads/new-nakamichi-ta-3a-project.732531/), most or all of which applies to TA-2A as well.