I did a stupid thing... (Nad C320 red light)

[skouliki]

I was reading the Nad c320 service manual and decided to do the alighment procedure. Everything was going well until i shorted TP2 and R242 with the pin of my dmm....a spark came out the resistor, and now the led of the amp stays red instead going to green, plus the heatsinks of the drivers are getting really hot. I havent got any background in electronics, i m just doing really basic stuff: changing capacitors and resistors etc. I included a link to the service manual. If somebody has any insight on how to repair this little amp i would be grateful


nad_c-320_sm_[ET].pdf - 4.0 MB

 

[Tekko]

Take it to a repair place, as you likely caused most of the transistors on the board for that channel to blow.

Outputs are guranteed to be gone, same with the drivers.

 

[FenderBender11]

Replacing all semiconductor which you think it might have affected is a good start.

 

[skouliki]

Thanks for your input guys. For the past 6 months i was upgrading the parts of this amp and was very pleased so far...changed electrolytics to Black gate and Elna cerafine,ceramics to silver mica, carbon resistors to metal film, polyester caps to mkp1837. The amp was totally transformed. Anyway, next time i'll be more careful. Good thing i have spares of the output transistors, but i think it would be hard to find replacements for sd669/sb649. Are there any suitable replacements for these?

 

[WuYit]

BD139/140

 

[nigelwright7557]

Confuscious he say if it its working leave well alone !

Done it myself, dropped my jewellers screwdriver while tweaking a pot.
Should have bought a plastic tweeker !

 

[skouliki]

Confuscious he say it its working leave well alone !

Done it myself, dropped my jewellers screwdriver while tweaking a pot.
Should have bought a plastic tweeker !

So true! And i have to add that only 3 hours of sleep after the night shift seriously impairs decision making and stability of hands.

 

[destroyer X]

Protection circuit was damaged....Q21 gone... and others may be dead too

You must send it to a skilled repairman if you do not feel yourself good to to that.

After night shift we are good for the bed only.

regards,

Carlos

 

[phonomac]

Hi there,
Second source SB649/SD669, manufactured by Hi-Sincerity Microelectronics Corp of Taiwan, are available from eBay seller audiowind-2005.

They are marked as HSB649A and HSD669A, hFE grade C, and as far as I can tell from basic measuring apparatus meet the spec. The datasheet is available from the Hi-Sincerity website.

No connection, just a customer.

Hope this helps.

Regards

Angus

 

[skouliki]

Today i finally managed to replace the output transistors and the drivers but the amp still refuses to go out of protection mode.... any thoughts?

 

[nigelwright7557]

Today i finally managed to replace the output transistors and the drivers but the amp still refuses to go out of protection mode.... any thoughts?

Any DC on output ?

Did you set the bias ?

 

[skouliki]

i measured the output on the speaker terminals(for about 2 minutes) and i got a reading of 13mV on left and 15mV on the right channel.... but i didnt set the bias (which i know think i should have done...)

 

[odysseybmx414]

Did The resistor you shorted the test point to open up or change value? I couldn't download the service manual so I can't look at it.

 

[skouliki]

The resistor was visually ok and i tested it - the value was normal. Cant remember the exact value right now but i will look into it when i get back from work

 

[skouliki]

Just checked the resistor again and it reads 47.4 ohms, which is normal.

 

[digits]

Systematicaly pull all transistors and test them with a multimeter....

 

[Deano1970]

Hello i would try testing the voltages at your protection ic, it looks like pin 2 detects dc voltage from the amplifiers left and right channel, through the limiting resistors R445 for the right channel and R446 for the left. that way you will get an ideah of what is puting it in protection mode and can track it back from there. I had a Marantz in protection mode and i found dc voltage at the protection ic dc detect pin. when i mesured the voltage from the other side of the limiting resistor on my left channel i found there was 56v dc in the audio path, i then tracked that voltage back to the pre amp ic that had become faulty. i hope this helps

 

[skouliki]

I adjusted the bias. It reads 8mV on the left and 9mV on the right channel. The dc offset reads 30.8mV and 30.7mV respectively. But still no go. The heatsinks of Q41 and Q42 still get really hot and i have already replaced these chips (the schematic indicates sb669/sd649 but i found out that my amp had BD139/140).Thanks deano and everyone for your suggestions. I will definately look at the protection ic if its the cause of the problem.If it doesnt work i will replace all remaining transistors altogether.

 

[Coconuts 500]

I recommend that you test all the transistors for shorts, before replacing anything.

 

[ingenieus]

I included a link to the service manual. If somebody has any insight on how to repair this little amp i would be grateful


nad_c-320_sm_[ET].pdf - 4.0 MB

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