Nakamichi TA-2A with an interesting problem

[nickhead]

Hey guys! Yes, I have returned from the grave We got custody of our granddaughter and I haven't had the time or energy to do much electronics work. But I did pick up another of my favorites a few months ago. An almost mint TA-2A. Just now getting around to open her up.

She's been running fine since I got her. Some noisy controls, but nothing major. Finally got her topless on the bench to do some maintenance. First thing I see is a leaky battery. So I removed the old battery. Don't have a suitable battery holder so left it empty for now. I dusted her out, cleaned all the controls and fired her back up. Works like a charm. Interestingly the station memory was not reset. So I unplugged her and plugged her back in a few times, same thing. Figure there's a capacitor in circuit somewhere, so I leave her for a few hours unplugged.

Come back in a few hours, plug her, turn her on, protect relay clicks, speaker relay clicks, then nothing but a load hum from both speakers. Volume control does nothing. Also noticed that there is no light above any of the mode selector buttons. Pressing one of the mode buttons momentarily blips the humming, but the light doesn't light and the hum continues.

I've checked the power supply, all is good. All PS caps test good, as do all output voltages. Random voltage checks on main and logic board all look good. Q525 is an a953. Looks like a pro job, very clean replacement.

Any ideas what to look for to diagnose this?

Nick

 

[avionic]

Interestingly the station memory was not reset.

So I removed the old battery.

The battery was for station memory...

 

[avionic]

Come back in a few hours, plug her, turn her on, protect relay clicks, speaker relay clicks, then nothing but a load hum from both speakers. Volume control does nothing. Also noticed that there is no light above any of the mode selector buttons. Pressing one of the mode buttons momentarily blips the humming, but the light doesn't light and the hum continues.

I've checked the power supply, all is good. All PS caps test good, as do all output voltages. Random voltage checks on main and logic board all look good. Q525 is an a953. Looks like a pro job, very clean replacement.

Start checking power supply grounds...

 

[mondialfan]

If was working and suddenly stopped working while you're performing work inside then suspect something that you disturbed. I would start with turning the unit upside down and blowing out to make sure something conductive didn't fall inside and make its way into the circuit then reseat all connectors and while doing this look for any wires that might have been stressed and broken during the maintenance.

 

[northpaw]

Also noticed that there is no light above any of the mode selector buttons.

Check the 18V power supply (not on the PS PCB), as this powers those LEDs.

You don't say how quickly you turned the unit on and off several times, but doing that quickly should be avoided.

Also a good idea to check F401 on the PS PCB; if it is out, there will be no 30V or 12V on the CN-21 connector (don't know if you checked that connector).

 

[nickhead]

I finally found a wrong voltage, which led me to the problem. Apparently I swapped 2 cables on the logic pcb at some point. Putting them back where they belong fixed the problem. Thanks, guys, for all the suggestions!