I pulled a NAD 218 THX amp from the local dump recently. One channel works beautifully, while the other is dead. On the dead (Left) side, one PS fuse was blown. Upon inserting a fuse from the working side (same fuse), the dead channel remained dead.
Schematics can be found in posts 7 and 8 here: AVforum.no - Nad 218 Thx reparasjon. It's pretty complicated to my eye since I don't often work with solid state devices. Plus there's the bridging, soft clipping, protection circuitry, balanced vs unbalanced inputs, etc. etc.
The blown fuse was F502, for V+ for the left channel in the first schematic. These are slow blow 6.3A fuses. V+ measures 72.9, whereas V- measures -73.1. Seems likely the PS is mostly working. I do notice, though, some burn marks around R704 on the PS board. It's a fusible resistor whose solder joints are shot, though it still measures 100R. That resistor is part of the V+ supply, though, so should affect both the L and R channels. I'll resolder or replace it, but hardly seems it could be the problem for the L channel.
Finally, the right side relay clicks on when the amp is turned on, but there's no relay click on the L side with fuse replaced.
Can anyone point to some next steps for me to check this thing out? I'd really like to get it going instead of parting it out - I can't see landfilling it as there are good parts there.
Schematics can be found in posts 7 and 8 here: AVforum.no - Nad 218 Thx reparasjon. It's pretty complicated to my eye since I don't often work with solid state devices. Plus there's the bridging, soft clipping, protection circuitry, balanced vs unbalanced inputs, etc. etc.
The blown fuse was F502, for V+ for the left channel in the first schematic. These are slow blow 6.3A fuses. V+ measures 72.9, whereas V- measures -73.1. Seems likely the PS is mostly working. I do notice, though, some burn marks around R704 on the PS board. It's a fusible resistor whose solder joints are shot, though it still measures 100R. That resistor is part of the V+ supply, though, so should affect both the L and R channels. I'll resolder or replace it, but hardly seems it could be the problem for the L channel.
Finally, the right side relay clicks on when the amp is turned on, but there's no relay click on the L side with fuse replaced.
Can anyone point to some next steps for me to check this thing out? I'd really like to get it going instead of parting it out - I can't see landfilling it as there are good parts there.
Thanks Nigelwright. Can this testing be done accurately in-circuit? That is, will there always be isolation between the pins even in-circuit? I've looked at the schematic and it does appear to be the case, but that often isn't clear to me in such a complex circuit. In some cases it looks like there may be continuity through resistors.
I was also wondering about the relay on the output circuits. The 4th image in post 8 shows a relay (RL301) next to the big-ish coil which apparently switches the speakers on or off based on the output protection/mute circuit in the 1st image (post 7). If there had been a surge that blew the V+ fuse, might that have burned the relay?
Also, the R704 resistor doesn't want to get soldered back into the board. The holes for it are loose and I can't flow solder into the joints. The PCB appears to be coated on the trace side and I can't tell if the solder points are intact or not. I thought I'd next try to solder a direct connection with the next components, bridging the traces, but I don't know if there's any harm in doing so. I THINK I can find the correct solder points for that approach.
I was also wondering about the relay on the output circuits. The 4th image in post 8 shows a relay (RL301) next to the big-ish coil which apparently switches the speakers on or off based on the output protection/mute circuit in the 1st image (post 7). If there had been a surge that blew the V+ fuse, might that have burned the relay?
Also, the R704 resistor doesn't want to get soldered back into the board. The holes for it are loose and I can't flow solder into the joints. The PCB appears to be coated on the trace side and I can't tell if the solder points are intact or not. I thought I'd next try to solder a direct connection with the next components, bridging the traces, but I don't know if there's any harm in doing so. I THINK I can find the correct solder points for that approach.
OK, so I got inspired to check the output transistors before calling it a night. All appear to be free of shorts on the Left side. Will work on that fusible/flame proof resistor tomorrow and report back.
I tested R345 and it measures 82k on the left, and about 13-14M on the right. But the left channel is the non-working one, and I can't see how I could get anything BUT 82k based on the schematic. So it would seem like the Right channel should be the problem.
Or rather the right side WAS the working channel. Now neither are working.
I re-soldered R704 (which measured 106 ohms so I assume is working) which is part of the regulated V+ circuit on the power board and replaced the blown fuse, and that left me with no output at all, and some kind of fault such that the Power LED lights red and remains red. I soldered R704 directly to the solder pad for R706, right next to R704's mounting hole. Measured R706, R706 + R708, and R704 + R706 + R708 and got correct readings, so I'm virtually certain I soldered that end correctly.
Then I did some research to find the correct connection point to Q703 and soldered the other end of R704 to it. Still running through the board to the underside for soldering as that seemed safest for soldering heat. I would have expected to at least have the same level of operation as before doing this, but now the R channel is also dead. I've re-checked resistances of R704 and associated resistors, and all still measure fine, including the total resistance.
Might the PCB for the PS be multi-layered and I somehow inadvertently connected R704 to another trace? There is some slop in R704's holes which could allow contact to other layers within the PCB if that were the case.
In any case, seems like I must have done something wrong with R704. That or there is some protection against running the amp with the top and bottom covers off, as those are the only things different from before.
Thanks for any more advice,
Carl
Or rather the right side WAS the working channel. Now neither are working.
I re-soldered R704 (which measured 106 ohms so I assume is working) which is part of the regulated V+ circuit on the power board and replaced the blown fuse, and that left me with no output at all, and some kind of fault such that the Power LED lights red and remains red. I soldered R704 directly to the solder pad for R706, right next to R704's mounting hole. Measured R706, R706 + R708, and R704 + R706 + R708 and got correct readings, so I'm virtually certain I soldered that end correctly.
Then I did some research to find the correct connection point to Q703 and soldered the other end of R704 to it. Still running through the board to the underside for soldering as that seemed safest for soldering heat. I would have expected to at least have the same level of operation as before doing this, but now the R channel is also dead. I've re-checked resistances of R704 and associated resistors, and all still measure fine, including the total resistance.
Might the PCB for the PS be multi-layered and I somehow inadvertently connected R704 to another trace? There is some slop in R704's holes which could allow contact to other layers within the PCB if that were the case.
In any case, seems like I must have done something wrong with R704. That or there is some protection against running the amp with the top and bottom covers off, as those are the only things different from before.
Thanks for any more advice,
Carl
Oldie thread, but relevant. I am in trouble with my 218 THX and I have the exact same issue..
dead left channel..
The right side R346 measures 84.7 K-ohm (Working, though maybe down on power?)
The left side R345 measures 20.34 M-ohm - Removed it from the board, it's blown... no connectivity out of circuit 😱
Hopefully replacing it with another 82k 0.5W resistor will fix it 🙂
So should I up the watt rating for the resistor?
Also, none of the transistors are shorted out.
dead left channel..
The right side R346 measures 84.7 K-ohm (Working, though maybe down on power?)
The left side R345 measures 20.34 M-ohm - Removed it from the board, it's blown... no connectivity out of circuit 😱
Hopefully replacing it with another 82k 0.5W resistor will fix it 🙂
So should I up the watt rating for the resistor?
Also, none of the transistors are shorted out.
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I'd guess some transient issue caused the fault, in which case upping the wattage rating won't likely help. I gave up on mine though it's still on my shelves next to my bench. Please let me know if replacing R345 helps. I may get back to this at some point, particularly if you have success. I've also considered re-purposing my case and PS trafo and a few other parts for a DIY Pass build. Still thinking it through.
I'll keep you posted, ordered a pack of 100 1% tolerance 82k Ohm 0.5W resistors.. may take a while unless I can find some locally.
I managed to source a new resistor locally, amp is working 100% so far Woowee !!!
been sitting for two to three years over a 0.2$ component... lol
been sitting for two to three years over a 0.2$ component... lol
LOL! Thanks for posting that. It would be interesting if it's the same with mine. If I ever get back to it I'll post my results.
No, haven't fixed it yet (it's still on the shelves) but at this point I'm likely to repurpose the iron and chassis. After 8-9 years who knows what else I'll have to deal with? Dried caps? Dried thermal grease under the output transistors? I can pull it up on a variac but not sure that will help on a sand amp. I just have too many projects on the bench. I'm working to clear them out now, but I keep going to the dump and finding cool items to fix and either incorporate into my system or pass on to others. So far they have all been much easier to repair.
The blown fuse is 6.3A, so that your left channel driver transistors or output power transistors were shorted, simple check them and replacement can fix it.
Have worked a lot with a Nad 218Thx now and if R345/R346 is within spec (replace those to 2 or 3W)
CHECK R731/R732 (2.7K 0.5W) this are the dropping resistors for the relays. I'll bet one of this is out/ to high value) and the relay won't click on. They're also going insanely hot so replace with 2 or 3W.
Located near protectionchip/ near back on middle psu board.
Also check the 4x100uf/100v and the 4x100uf/25v caps in the regulator. Seen few of those shorted out.
The 218 is very sensitive to blow fuses on the V+ and V- iff you accidently short the sp. cables together. Fuse (502-505)
I think thats what happened here.
Got myself a 218Thx.
Not tested yet, but think it needs some new caps....
Any suggestions for mods are welcome.
Will surely check above mentioned components for weakness and wear...
Cheers
Not tested yet, but think it needs some new caps....
Any suggestions for mods are welcome.
Will surely check above mentioned components for weakness and wear...
Cheers
The whole middle psu board and regulator section need overhaul, replace both caps and resistors. Many are underdimensioned, like the 1W replace with 2w (vishay pr02 for example)
Ceramic caps with silver mica and try to replace the a970/c2240 also with orginal toshiba. They run very very hot. New grease etc etc.. repair foiltances with 1mm copperwire etc.
the a1837/4793 can be replaced with mje15030/31 or similar and the others with ksc2690/??
After that recap both channels (replace also those 82k resistors to some 2-3w flameproof ones) and new relays from Omron. Start there. On left and right channels you can now use Polymer caps from Nichicon instead.
Ceramic caps with silver mica and try to replace the a970/c2240 also with orginal toshiba. They run very very hot. New grease etc etc.. repair foiltances with 1mm copperwire etc.
the a1837/4793 can be replaced with mje15030/31 or similar and the others with ksc2690/??
After that recap both channels (replace also those 82k resistors to some 2-3w flameproof ones) and new relays from Omron. Start there. On left and right channels you can now use Polymer caps from Nichicon instead.
Tnx for answer.
Haven't looked at the schematics yet. But u mean I can use polymer caps for all on both channels.
I'm not expert in this.
Cheers
Yes, replace the electrolytics with them. The filmcaps from Thomson is OK for now.
Regarding polymer, there is some 10uf/100v/160v and they also can bepyssed same time with 0.1uf filmcap.
Regarding polymer, there is some 10uf/100v/160v and they also can bepyssed same time with 0.1uf filmcap.