Sparks on nad 304

[Max Merc]

Hello,
after finishing a long recap and resistor upgrade on my NAD 304, I was following the alignement procedure for offset and idling current.
I did it one time and after listening some music for 10 minuts, I disconnected the speakers for a 2nd and final adjustment. While I was doing it the wire connected to the TP301 touched the metal case ( ground) of the amp, before I had the time to move it I saw some sparks.

Now the pot turns without any change in the idling current, it always shows something around 0.01 mV, on both channels same stuff.

I thought the amp was ready for the bin , but actually I connected to a pair of "suicide speaker" and it works.

I'm still worried about the idling current, do you have any idea of what happened? What component could be faulty ?
I tested the 10k pot with a multimeter and they seems ok.

Thaks for your help

Max

here the service manual with the schematic https://drive.google.com/open?id=1tshjjFy3IgLueLhKdxYv2tIVU6LSiPwt

 

[slimecity]

Not sure about where you need to look specifically but you can still get sound from an amp with low bias. Usually the bias pot connects to a transistor that sets & regulates bias - you will see it on the SM.

It woud be pretty unusual I would have thought to cause the same problem on both channels though?

 

[Max Merc]

Not sure about where you need to look specifically but you can still get sound from an amp with low bias. Usually the bias pot connects to a transistor that sets & regulates bias - you will see it on the SM.

It woud be pretty unusual I would have thought to cause the same problem on both channels though?

Any tips for finding ithat transistor?

 

[Max Merc]

I'm stock, don't know how to proceed :-(

 

[slimecity]

Find and download the service manual if you are serious about trying to fix this yourself. go to hifiengine.com and register there.

Otherwise, I'd recommend that you take this to a tech to be repaired.

 

[Max Merc]

Find and download the service manual if you are serious about trying to fix this yourself. go to hifiengine.com and register there.

Otherwise, I'd recommend that you take this to a tech to be repaired.

I have the SM. I checked the Q317 and Q318 which seems to be the bias transistor and they are ok.

I also checked 38v on C804-C805,
56V on C914 - C 915
26V on C912 - C913, all seems ok.

if someone have time and would like to help, here a detail of the schematic withh the TP301 and the bias trim highlighted detail schema nad 304.pdf - Google Drive

here the service manual with the full schematic https://drive.google.com/open?id=1tshjjFy3IgLueLhKdxYv2tIVU6LSiPwt


thanks
Max

 

[Max Merc]

I rechecked the 56v rail with the probes in the right place, it gives me about +/-61V on the multimeter (black probe on ground and red probe on C803 61.4V then C806 -61.6V).