Nakamichi TA-2A - power supply board

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.

power_board.JPG TA-2A_power_board_schem.jpg

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.
 
7805 and 7812 are positive voltage regulators, 5 and 12 volts respectively. They should be ok as long as they are outputting said voltages.
 
Those images are being resized. If anyone needs a better schematic, I can post elsewhere and link, but what I show was taken from Page 28 of the TA-2 Service Manual (from HiFiEngine).

I just opened up my other (functioning) TA-2A and measured across D405, and got 10.95V on one side, and 0.1V on the other. So the 14V I am getting on both sides of D405 on the problem unit is not right.

Ivan, thanks for the info on the U401-U402. I will check them.
 
U401 gives 13.9V -- 0.61V -- 5.63V on the three leads. Seems the 13.9V is somewhat high.
U402 will have to wait until I have a functioning rectifier (D410) for it.
 
Having the same voltage on both ends of D405 means the relay is NOT energized. Probably due to Q407 not being turned on, it should have at least .6VDC on the base, check both ends of R404.

The input voltages to the regulators are raw/filtered DC and depend on the AC voltage coming out of the wall.

The base of Q406 should have 13VDC on it because of the Zener diode and the emitter of Q406 should have approx. .6VDC less than the base giving approx. 12VDC on it's emitter.

Craig
 
I would start by checking the voltages on CN-9 on the control board.
How I've have it.
(pin 1 is the position nearest the display)

CN-9 _______________ Standby (orange) ____ ON (green)
1 reset ________________ 5V _______________ 4.9V
2 power off ____________ 5V ________________ 10V
3 relay ________________ 0V ________________ 5.6V
4 +4,5V (display)________ 0V ________________ 5.4V
5 +5V ________________ 5.7V _______________ 5.6V
6 ground ______________ 0V ________________ 0V
7 +10V _______________ 14V _______________ 10V

That ended up hard to read (better now...)
 
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Thanks, Craig. I first measured R404 and it had nothing on either end. Then I pushed Q407 back so I could see the leads. They were quite corroded. As I pushed my probe against one of them firmly, to get through to metal, RL401 activiated, and then deactivated with the pressure off. Obviously I need to pull this board out again and reflow more solder joints. Corrosion in Q407 (and probably others) might also be a problem. I should just order all of these and replace.

Edit -- I think the relay activation may have resulted from me accidentally straddling the collector and base on Q407 (middle and the lead furthest from the relay on the layout diagram; I often have trouble with the Japanese notations for transistors on the layouts). Q407 is probably dead.
 
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Thanks, Mannegizen. Is it OK to pull CN-9 free and then power up (easier to get at the pins on the cable), or do I lift up the control board and use the underside to get at the leads?
 
Thanks, Mannegizen. Is it OK to pull CN-9 free and then power up (easier to get at the pins on the cable), or do I lift up the control board and use the underside to get at the leads?

If your probes are pointy enough you can just stick the probe in between the cables an the plastic off the connector.
 
OK, I have some needle probes I have not used yet, good time to start.

Edit -- here are the measurements:

1___5.2V
2___5.3V
3___0V
4___0V
5___5.64V
6___0V
7___13.9V

Matches up pretty well with the Standby condition
 
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Edit -- I think the relay activation may have resulted from me accidentally straddling the collector and base on Q407 (middle and the lead furthest from the relay on the layout diagram; I often have trouble with the Japanese notations for transistors on the layouts). Q407 is probably dead.

Pinout on these are ECB, if you you shorted between E and C the transistor should be OK.
 
Pinout on these are ECB, if you you shorted between E and C the transistor should be OK.
Well, I said that wrong. I straddled the two leads closest to the relay, so I think those are the emitter and collector. (was looking at the layout and forget to invert my view). I just measured the voltages on the 3 leads on Q407. The middle lead is 13.9V, the two ends are 0V. I will avoid embarrassing myself further by not trying to say which pin is which.

Edit: By closely comparing the traces on the layout with the schematic, I can confirm that I shorted E and C. I verified the other pin is the base, as it has continuity with the near end of R404. And both ends of R404 are at 0V, as before.
 
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The base of Q406 should have 13VDC on it because of the Zener diode and the emitter of Q406 should have approx. .6VDC less than the base giving approx. 12VDC on it's emitter.

Thanks for this. My unit (TA-2A) doesn't have a Q406 (that transistor and the additional zener and resistor are marked on the silkscreen but not populated). I believe it is only present for "TA-2(other)" units.
 
What is the voltage on pin 3 on U503.
(the MPU, the big integrated circuit to the left of the battery seen with the display facing you.)
 
Pin 3 has 5.57V on it. Glad I got the needle probes out...

Edit: Perhaps I should restate that rectifier D410 is out? I get only 0.4V on its + lead. In case it is important to what is happening on the control board.
 
Edit: Perhaps I should restate that rectifier D410 is out? I get only 0.4V on its + lead. In case it is important to what is happening on the control board.

It wouldn't stop the unit from powering up.

All stuff on the display wont lit up and tuner will not work tho.
 
Pin 28 is ___5.28V
Pin 29 is ___0V

I usually think of myself as careful, but I bridged 28 and 29 with the needle, and when I did, the voltage went down to about 1.x something, and a relay clicked (pretty sure it was the speaker relay).
 
Well, I'm kinda stumped at the moment, no ideas comes to my mind.

I can make my unit lock up an go in to standby if I either short C536, pin 3 with ground or with pin 2 or 4.
In this state it will not respond to my remote or if I press any of the input buttons, Phono, CD, etc ...

But turning the unit off and on again will turn it back in working order.

It could be that there is a problem with C536, causing a momentary short to ground during power up.

Or that the U503, a small micro computer is locked in standby mode or simply busted.

One way to try to reset it could be by first turning the unit off, unplug power cord, CN-9 and desolder the battery.
Then you short all legs on U503 with tin foil, could take many tries, but the basic idea is to try drain all residue voltage/current that might be left in the MPU.
 
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