Electronics > Beginners
NAD 218 THX amplifier Repair.
<< < (9/12) > >>
SantaClaw:
I already labeled all the transistors I removed and the spot it came from :)

I'll have the chance to do more work on this tomorrow.

The uni-t 81c is actually a decent meter for normal measuring, by that I mean when it comes to functions, the oscilloscope part sucks though..  I also have an aneng an8009.

My component tester: https://www.banggood.com/DIY-M12864-Graphics-Version-Transistor-Tester-Kit-LCR-ESR-PWM-p-986954.html
I have two, I built a kit for this case as well https://www.banggood.com/Supporting-Shell-For-DIY-M12864-Graphics-Version-Transistor-Tester-Kit-p-989949.html
As with my meter, I can double check any value with the other one.

Though with regards to the component testers, they are calibrated using a capacitor... and since both where calibrated using the same capacitor, I would think the results would be damn near identical/with the same potential faulty readings.

The amp has been without power for 6 months or more, so I assume all caps are dead, though I'll measure the main filter caps just to be sure. I am a fan of mr carlson, though most of it goes way above my head :P
Jwillis:
Arbitrarily replacing transistors is a bit extreme and expensive.

On an earlier post you stated that a "Protection Resistor" keeps blowing .Which one on the schematic is that .What is the reference number?

Bad regulation can cause components to burn up . Bad rectification can cause components to burn up. Shorts can cause components to burn up.
Did you confirm that the output voltages from Main power supply are correct? The schematic shows what voltage should be present at certain points.
I find that printing out the whole schematic helps reduce confusion.

Simply checking the resistance between transistor pins can point you in the right direction .If you get zero resistance between two pins on a transistor, reverse the probes and check again .If it is still  zero ohms that transistor is Shorted . If you get several Kohms between pins and Open or several ohms  when probes are reversed that transistor is probably OK.Move on to next transistor do the same .This can be done with components on board. You do not need to remove the components for this test..

Why do we do it this way? Removing and replacing components over and over Damages the epoxy gluing the traces to the board. So you want to avoid removing components unless you absolutely need to .

We can worry about Pairing transistors or where they come from after we find out which, if any, are at fault.


SantaClaw:

--- Quote from: Jwillis on February 19, 2019, 08:13:44 pm ---Arbitrarily replacing transistors is a bit extreme and expensive.

On an earlier post you stated that a "Protection Resistor" keeps blowing .Which one on the schematic is that .What is the reference number?

Bad regulation can cause components to burn up . Bad rectification can cause components to burn up. Shorts can cause components to burn up.
Did you confirm that the output voltages from Main power supply are correct? The schematic shows what voltage should be present at certain points.
I find that printing out the whole schematic helps reduce confusion.

Simply checking the resistance between transistor pins can point you in the right direction .If you get zero resistance between two pins on a transistor, reverse the probes and check again .If it is still  zero ohms that transistor is Shorted . If you get several Kohms between pins and Open or several ohms  when probes are reversed that transistor is probably OK.Move on to next transistor do the same .This can be done with components on board. You do not need to remove the components for this test..

Why do we do it this way? Removing and replacing components over and over Damages the epoxy gluing the traces to the board. So you want to avoid removing components unless you absolutely need to .

We can worry about Pairing transistors or where they come from after we find out which, if any, are at fault.

--- End quote ---

I was told when this resistor blew, it was proof the transistors where shorted/broken, R345 it's a 82kohm resistor
Jwillis:
Ok . Work your way from the obvious blown resistor outward .
That resistor ties the bases of Q317 and Q319 .That's a lot of current running out of  the Base of a transistor to fry a 1/2 watt resistor . Check Q317 and Q319 for short between emitters and base and collectors and base.

Looking at the transistors 2SA1478 / 2SC3788 label reading left to right Pin 1 is Emitter Pin 2 (center pin) is Collector and right most Pin 3 is Base .
What are your readings.

Check series Diodes D301,D303,D305,D307 and series diodes D308,D311,D313,D315 with Diode Setting of DMM. Forward voltage of 1N4148 should typically between 550mV and 650mV.
SantaClaw:


Right, all set up, I'll print out the schematics and start work tomorrow.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod