Nakamichi 481

calman46

Journeyman
Joined
May 9, 2012
Messages
115
Location
MA
#1
I'm working on nak 481 . New belts and tire and recapped (e-caps and orange film). The deck plays fine but is not recording correctly . I did record with it before the recap but I didn't playback the recorded tape , I just went by the meters and didn't use all 3 tape types . So I'm not sure if the problem was there before the recap . The problem is the deck will record in ZX but not in SX or EX . The recorded ZX is attenuated I think as the deck's meters on playback are no where near their level when recording . SX is severely muted I barely hear playback and EX is dead quite . I did notice that the green side polarity markings for C401 ,C402 are reversed, the component side (matches schematic) is right (the way I have it). The voltages are a little low so I was looking for a problem . PIN # 5 on IC 101 & IC 201 is 17.4 Vdc and 18Vdc and should be 21.5Vdc .The voltages are a little low all back to the PS. I have clean the switches a couple of times . I did a test record while switching between ZX, SX, EX . Listening with headphones to the recorded tape ,ZX seems to play louder than what meters are displaying ,SX mutes way down and EX is silence . Any ideas ? the low voltage or switches ? thanks
 

nakdoc

Journeyman
Joined
May 11, 2011
Messages
317
Location
Nashville, TN Music City
Tagline
highly biased
#3
Check for a burned 68 ohm 2w resistor in the bias oscillator. It is the DC supply, and often burns and changes value, causing the bias to be much less than it should be.
 

calman46

Journeyman
Joined
May 9, 2012
Messages
115
Location
MA
#4
The 68 ohm resistor looks good . I'll lift a leg to be sure . I did replace the bias and record level trimmers on EX only . Replacing the bias trimmer didn't fix the problem , replacing the record level trimmers return the volume level back but the sound is still distorted . The recorded music sounds kind like a radio out of tune . The new trimmers are at their mid point (25k) . I 'm going to turn the bias down and hear what happens.
 
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