• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

chasing dim bulb short

S

sdinfoserv

New Member
I'm chasing a dim bulb short on a Nikko sta-4030.
I think the short is on the main amp PCB.

dim bulb is 100w, the Nikko is only 18wpc.

Reg PCB Main Amp
pins pins
8A,B Left, right input from tone board
8C,D Left, right output (to phone jack/speaker selector)
9C ---> 8E,F power inputs from reg block

9A ---> power 2 tuner PCB
9B ---> power 2 EQ PCB + tone PCB

all connected - dim bulb lit, case lights off.

disconnect 8E,F dim bulb dark, case lights on.

if power to main amp connected, dim bulb lit, no case lights on.
does not matter if power to tuner or EQ/Tone connected or disconnected
does not matter if inputs from tone to main amp connected
does not matter if speaker outputs connected to main amp.
dim bulb lit with any connection combinations to main amp.

Can I safely assume the short is on the main amp PCB?

on the main amp PCB:
I've replaced the caps and tested the transistors with a peak atlas DCA75 pro. It finds no issues. I've cleaned and put new thermal paste on the output transistors and replaced the mica insulators. None of the resistors look burned, 80% of them I've tested out of circuit and they're within spec.
I've double (and triple) checked the caps polarity, transistor pinouts, and the traces for solder bridges - none found.

Suggestions?
 

Attachments

  • my_drawing.png
    my_drawing.png
    483.3 KB · Views: 29
Register to hide this ad
excellent suggestion, I'll try that tonight. It's the basics you overlook when you stare at a problem too long.... thanks!
 
short appears if either side or both get power. Dim bulb stays lit, no case lights. Is the problem with another board perhaps, or should I pull / review all components on that board?
 
Might have and issue with D901 or D902. One or more diodes in the bridged rectifier could be shorted. They are dual diodes. One is a common cathode and the other is a common anode. So don't get them confused. :thumbsup:
 
Last edited:
It's entirely possible both channels of the main amp board have issues. Could the outputs be blown (shorted) on both channels? Could be.
 
just to follow up on this... I switched projects for a few days to reset the minds perspective. After pulling and testing the main output transistors, 3 of the 4 were blown. Not sure if I did something after the initial test to blow them, but either way, after replacing the outputs, it passes dim bulb. Thanks for the inputs.
 
You must log in or register to reply here.
Back
Top Bottom