Electronics > Beginners
NAD 218 THX amplifier Repair.
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SantaClaw:

--- Quote from: Richard Crowley on February 17, 2019, 11:22:09 pm ---1 - Those results from the transistor tests seem OK to me.
2 - What about the transistors that are driving them?  Take those out and test them.
3 - How do the transistors on the good channel test by comparison?
4 - Do any of the components on the bad channel get more hot/warm than on the good channel?

--- End quote ---

1. Well, that's what I thought too.

2. This will be the small transistors type C4793, (Component Q333, Q329, Q321) type A1837 (Q331 and Q335) type A1837 ?
I haven't de-soldered them yet, it's past midnight.

3. I haven't de-soldered them either, though I plan on changing them too if I have to replace any of the others.

4. Yes, the resistor that blew... strangely enough. I never got the chance to see if the transistors got any warmer, as it failed before I could do that.

I am pretty clueless. The closest to an electronics education I have was when I did a year's school to become a car mechanic..

 What I can say that it was powering a set of Dali concept 10 speakers at the time it failed (6 ohm's) this amp is rated 4 ohms min. It produced low/muted/distorted sound on the left channel when it failed, so there was some sound, but not very good sound, no bass/punch and distorted high tones. Could it be the speaker relay somehow ?

There is a diagram posted further up in the thread, I posted a link to the service manual as a pdf
Jwillis:
B(Beta) HFE or Gain are generally the same thing when describing BJTs so the 2SD1718 transistor might have a current gain anywhere between 60 and 200. The gain of the 2SD1718-Q will be in the range from 60 to 120, 2SD1718-S ranges from 80 to 160, 2SD1718-P ranges from 100 to 200.
The 2SB1163 transistor might have a current gain anywhere between 60 and 200. The gain of the 2SB1163-Q will be in the range from 60 to 120, 2SB1163-S ranges from 80 to 160, 2SB1163-P ranges from 100 to 200.
Your test is fine .If your  tester shows the pins 1-2-3 as resistors or open then the transistor is toast.
Download the data sheets for each transistor your putting to test and compare your results.

From experience ,Ebay is not bad for electronic components from China.But here's a tip.If the seller is only selling electronic components or items that are in that general area including modules ,test equipment ,ect.Your probably Ok .
If the vendor is also selling Lingerie and other weird stuff then you probably want to avoid them.
I have purchased probably 90% of my components from Ebay have have found no problems with them.
Not to say that i havn't received fake stuff ,but believe it or not it came from western Europe.I was indeed shocked.
Distributors like  Digikey is good but finding older components or obsolete products is tough. 
Fire Doger:
The replacement resistor was 0.5 watts as schematic says?
SantaClaw:

--- Quote from: Fire Doger on February 18, 2019, 12:04:56 am ---The replacement resistor was 0.5 watts as schematic says?

--- End quote ---

No, it was a 5w one, i didn't have any 0.5w 82K-ohm resistors on hand... Stupid as I am, I asked a radio amateur friend, and he said it would be fine.

I did order some, and I replaced it now when I de-soldered the transistors with a 0.5 watt one.
Fire Doger:

--- Quote from: SantaClaw on February 18, 2019, 12:13:37 am ---
--- Quote from: Fire Doger on February 18, 2019, 12:04:56 am ---The replacement resistor was 0.5 watts as schematic says?

--- End quote ---

No, it was a 5w one, i didn't have any 0.5w 82K-ohm resistors on hand... Stupid as I am, I asked a radio amateur friend, and he said it would be fine.

I did order some, and I replaced it now when I de-soldered the transistors with a 0.5 watt one.

--- End quote ---
5W is more than enough, if everything was ok it would be fine as your friend said.
I thought that you maybe used a 0.25W...

*For feature reference, replacing parts with higher maximum ratings is not always "the bigger the better", for example if you replace a 100mA fuse with a 10A fuse, a working circuit will work fine but in case of fail fuse won't blow up.
Accordingly a 5W resistor on a faulty circuit will put more stress before blowing up than a 0.5W one and may damage additional components or pcb tracks etc...
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