NAD 218 THX partly dead

[bluescreentt]

Hi

Somehow i f.. up my NAD 218 THX.

little over 1 year ago i changed all caps in the psu, right and left Amp and in the input.
I turned it on and a spart came flying out of the amp.
Little scared i tried turning it on again and no sparks, connected some speakers but i didnt feel that the sound levels / details was the same in buth speakers.
i left the amp under the bed untiul 2 weeks ago.

I have disassembled the amp and visualy checked all components and solder and dont find any problems.

after putting it together again the amp "powers on" (led is on) but the relays dont "click"

so basicly the amp is dead.

any ideas ?

Thomas

 

[BSST]

Hi bluescreentt,

Welcome to the forum!

May I suggest you post a service manual or at least a link? I think you'll get more help if you make it easier for members to study your dilemma.

Good luck!

 

[bluescreentt]

This is all I have and can find

Just to be clear

normaly when i power on the amp RL301 and RL302 engages

 

[BSST]

Hi Thomas,

Thanks for service data.

My typical disclaimer: I'm working from the schematic and service manual and have no first-hand experience with this amp.

Obvious places to start looking are all the power supply outputs--- i.e. +/- 82V supplies and the lower voltage supplies that power the opamps. Looking at power-amp outputs in front of the relays are other initial points to examine. These should help to localize the failure. A less direct attack is to try to determine what fault protection path is preventing the output relays from activating.

Sorry for your trouble. This looks like a very nice, powerful amp and well worth repair. Let me know what you find. I'll try to study circuits and become more familiar with details.

Steve

 

[bluescreentt]

ill give it a try but it will proberly take some weeks 🙂

Thank you

 

[Fsatsil]

I would go back and double/triple check all the parts you swapped before the sparks started flying. If you are lucky you may find blob of solder causing a short or maybe a part was installed backwards.

 

[bluescreentt]

I would go back and double/triple check all the parts you swapped before the sparks started flying. If you are lucky you may find blob of solder causing a short or maybe a part was installed backwards.

checked all with a magnifying glass lamp 3 4 5 times cant find any problems

also it was "working" after the spark but now 1 year later it dont

 

[bullittstang]

I would start from the beginning -- check all fuses, then test voltages --> AC coming into chassis, then to the power supply board, then rectified voltage, then final +/-Vdc. Next check for DC on the speaker outputs -- you must measure before the relays, since they are not engaging, most likely suspect is DC voltage on speakers. Anything over >500mV would be a problem and should be <25mV. If that all checks as good. Try tapping on the relays with something wooden or plastic, tap don't beat on them. Might just be worn out relays and the contacts are stuck/welded closed wouldn't be the first NAD with this issue.
Report back - if you have DC on the speakers, it will lead you to some repair on either the pre-drivers, drivers or outputs, or if your lucky a burnt resistor or two.

 

[bluescreentt]

Thanks for all ideas Guys. Ill give it a go. But as said before. It will take some weeks. Im have to be done building a snow system before middle of this month, before i have time to sit down and TS

1 last question this week

where do i find a replacement for VSB-48SMB
https://www.mouser.se/ProductDetail/Fujitsu/VSB-48SMB?qs=lEfYtVAeAoobn02KKs/%2BqA==

 

[Fsatsil]

Looks like a pretty standard relay you should be able to find a modern part that matches the specs.

 

[bullittstang]

looks like any of these, as well as 8 more in the search (these are sorted by price low to high). I would go with 1 or 3, never had an issue with Omron relays.

 

[tiefbassuebertr]

A friend of me, who operated these NAD power amps in bridge mode on an Infinity Kappa, experienced a total failure of the power amps.
Reason was obviously no responsive protection circuit mode despite built-in overcurrent protection circuit for each output power transistor pair (check out schematics in post #3).

Post #43 under
https://www.diyaudio.com/community/threads/infinity-8-kappa-rescue-project.246951/page-3
show both schematic and impedance plot of the Infinity RS-8 (Kappa series).

Main question in the moment is, whether the bridge mode avoid the protection mode in the right manner.

Next question is, which currently available BjT output power devices are the best replacement for Toshiba's 2SA1302A/2SC3281.
Maybe Onsemi's MJW1302A/MJW3281A (230V 15A 200W hfe: 50-200 30MHz TO-247) or NJW1302A/NJW3281A (250V 15A 200W hfe: 75-150 30MHz TO-3P) ?

Thank you for all advices.
P.S.: in the attachment some internal images of this power amplifier