[HELP] Nakamichi TA-2A issues

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konan2002

Member
2015-03-31 5:42 pm
Hi,

A few weeks ago, my TA-2A started to have no sound at all. I removed pre-out & main-in jumper and directly connect another preamp to the TA-2A to check if the main amplifier works. Sounds came out with that setup, that means the main amplifier section is OK.

After tracking down, I found that the voltage inputs to the digital switch IC U204 (LC7818) have issues. The voltages to the pins 11-15 are all high (18V) and do not drop down to low (0.6V) to activate switches. So, I checked the logic board and check the voltage outs from U503 MPU. It looks like working fine because output voltages in pin 55-51 change from 0V to 5V when I pushed the front panel source switches. Now, I got stuck at the resistors and TR parts between U503 and U204. The voltage at the base of Q521 reads 5V, not 1.25V, when pin 54 shoots out 5V! No voltage drop at R595. Even when I changed the source switch, there are no voltage drops at the bases of all TR's. I mean 5V at the base of Q524 when I selected "tape source", 5V at Q523 for "AUX source", etc.

Does that mean that all the five resistors R594-598 are shorted? Or there's problem in other places? If so, which parts should I look at?

Thank you.

An externally hosted image should be here but it was not working when we last tested it.


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M

Mannegizen

Member
2013-12-10 7:45 pm
Hello!

I would suspect that the +/-18V regulation has a bad solder.

It feeds the IC's U204/U203, the OP-amps U201 for Phono, U202 for Tuner buffer and the tone board.

If either Q207 or Q208 has a bad solder the regulation will lose either +18V or -18V Which will cause silence and maybe stop you from changing inputs.

These are easy to find on the main board since they scorches the board to a darker brownish tint. Not far from the speaker relay.

Also check the solders on Q202, Q203, Q209 and Q210.
 
K

konan2002

Member
2015-03-31 5:42 pm
Hi Mannegizen,

I just checked the soldering of those TR's as you suggested. They all look good. I resoldered them anyway but I am still having the same issues. No voltage drops across R594-598. I checked the resistance to see if they are gone bad but the readings are all correct 10 Kohms, which means the resistors are good. I am almost giving it up. No clues what's wrong with it......
 
M

Mannegizen

Member
2013-12-10 7:45 pm
You checked that you had both +/-18 volt?

And power led stops flashing after 5 seconds?

Another possible fault is that the mute is stuck.

What is the voltage on pin 1 on CN-11, "out mute" pin 14 on U503.

Should be ~5V at power on and should drop to a few mV after speaker-relay engages.
 
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