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Issue with SA-7800

Ryan Walton said:
FINALLY! A possible issue? Does that mean point 21 itself is bad or a component in between?
Click to expand...
it means there is a problem in that regulated negative supply ..just need to home in on it now .
or depending on your reply about it being negative or positive voltage . i can only guess right now .
 
petehall347 said:
it means there is a problem in that regulated negative supply ..just need to home in on it now .
or depending on your reply about it being negative or positive voltage . i can only guess right now .
Click to expand...
It was definitely positive 13.9v so does that mean too much voltage is getting there?
 
no it is most likely saying the negative is missing and the positive is simply finding its way back there because of it .. .
 
I'm having trouble finding diode 33, so i took a pic and measured all the voltages around that area. In in this pic there are 4 diodes on the left and 5 diodes on the right, bottom right is the zener diode. You can also see q38 in upper right. Hope you can read my chicken scratch. I'm thinking one of these (possibly top left?) is d33, but not sure.
diodemeasure.jpg
 
Ryan Walton said:
I'm having trouble finding diode 33, so i took a pic and measured all the voltages around that area. In in this pic there are 4 diodes on the left and 5 diodes on the right, bottom right is the zener diode. You can also see q38 in upper right. Hope you can read my chicken scratch. I'm thinking one of these (possibly top left?) is d33, but not sure.
diodemeasure.jpg
Click to expand...

You need to take a break my friend. Those are ALL resistors you have highlighted / measured. NONE of those are diodes except the zener bottom right.
 
now locate q35 .. tape up your probes so only the tip is showing to minimise short circuits ..get voltages at the 3 legs of q35 ,
 
There is a lot less risk of shorting stuff out if measure on the solder (under) side of the board where possible.

This is very true of transistors. Try to poke the legs on the component side can lead to BANG.
 
Ryan Walton said:
Not tired...just new at this. They all looked like diodes. Woops
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Just double check your component identification knowledge. Anything with colour bands is a resistor. Diodes are genrally black with a silver band or Zener usually orangy with black band.
 
Markoneswift said:
There is a lot less risk of shorting stuff out if measure on the solder (under) side of the board where possible.

This is very true of transistors. Try to poke the legs on the component side can lead to BANG.
Click to expand...
What exactly leads to the short? I know you should have the black on the chasis or ground area....is it when the positive touches more than one of the legs at the same time?
 
Ryan Walton said:
What exactly leads to the short? I know you should have the black on the chasis or ground area....is it when the positive touches more than one of the legs at the same time?
Click to expand...

Yes exactly. Touching more than one leg with same probe is never good. Occasionally I have touched two components with the exposed shaft of one probe and caused myself more work ! That's why I favour working on the solder side. Often easier to follow traces between components too.
 
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