Nakamichi CA-7A Troubleshooting: Preamp MC Stage

[avantbored]

Hi all,

Have had this pre-amp with me for a few years and have never had any eletrical or mechanical issues until now!

I rotated this out of daily service about 4-5 months ago; was dormant until I reintroduced it into the main system, driving a Yamaha M-65 on Class A mode.

Chain:

Technics SL1700MK2 // Denon DL-301II (MC)
Nakamichi CA-7A (Gain set to 24dB)
Yamaha M-65 (Class A mode)
Yamaha NS1000M

With PHONO selected on power up, the following was observed after cueing the tonearm with the turntable's "Cue" button:

Loud pop
Amp in/out of protection
Loud hum from turntable

Then normal operation after. Speaker drivers okay.

Normal operation for the next week, until yesterday! Same symptoms.

Have rotated it back out for safety.

Couple thinking-out-loud thoughts:

1) Haven't opened it up yet. Preamp was dormant for a good while; no issues before; have observed similar behavior before with other components, so maybe normal?
2) TT has muting switches controlled by a photo sensor (I think), maybe that's malfunctioning? But have not observed same behavior with 2nd preamp (C-80).

What else might I do before I schlep this to the tech?

 

[GKTAUDIO]

Hmm. What do you mean you got a loud hum from the turntable? Physically, from the turntable?
You've heard these sounds with other preamps?

 

[avantbored]

Hmm. What do you mean you got a loud hum from the turntable? Physically, from the turntable?
You've heard these sounds with other preamps?

Loud and big, as if the grounding wire was disconnected but much louder.

Havent experienced this with other preamps (yet). Was running an external Fosi phono into line on C80; now into built-in stage. Nothing

 

[UncleAng]

It sounds like the problem could be in your TT not the NAK