NAD 218 THX with dead channel

[Carlp]

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.

[nigelwright7557]

I would check the output transistors next.
Look for shorted devices.

[Carlp]

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.

[Carlp]

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.

[Assies]

Hi.
Check the R345 in left channel. This resistor is commen to burn of, then no sound.

[Carlp]

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