Don’t buy used McCormack DNA 1990s amps

@soix in response to your initial post. You're being extreme. All DNA amplifiers are not doomed to failure. Do original amps have a much higher failure rate than normal, probably so. However, I'd be willing to bet that 50% or more of them will still be kicking with their original input boards 10 years from now. Being that this is the internet you're going to hear more about failed units than about those that are still working because people go out in search of help when they fail.

I've worked on a couple of DNA-1's recently. They had clearly different manufacturers of the input boards, but the actual layout was identical. One I worked on was a full upgrade on a late model, circa 1997 or 1998, DNA1 Deluxe for a local audiophile. The other was a dead early model that I picked up off of Ebay and have since fixed and fully upgraded including the "C" mod. There are some problems with the input boards and the construction of them. If you have one and take a look at the few IC's on the input board you'll notice that the solder didn't flow through to the top side of the board and up on the legs of those IC's on the board. This is a big issue on the pins that have through-hole connections because those pins can lose contact eventually.

I contacted SMc Audio a while back asking for schematics for the amps and, in addition to saying no to giving me a schematic, they pretty much confirmed my observations. Here is what I was told "The problem is with the through-hole plating between the top and bottom copper layers of these two sided driver boards and attempts to repair them is not recommended or supported." 

All holes on the boards are not considered through-hole. Only about maybe 20% of them are. Through-hole is when a signal comes in on one side of the board and travels through the plated hole and then travels out the opposite side of the board to a different component on the board. Through-hole issues can be corrected and overcome. Its a lot of labor to do so, but the faults are not irreversible. Through hole connections need to have a solid solder connection on both the top and bottom of the hole. When these boards are upgraded care needs to be taken to offset the new capacitors about 1/4" off the board and apply solder to the leads on both sides of the board to make a solid and long lasting connection. 

I think its not a stretch to say that the impetus for the "C" mod initially was to eliminate the AD712 opamps that were driving the DC servo in the DNA amps because of failures. The reason was that if a through hole failure occurred due to a poor solder joint on one of these 2 servo opamps then the servo could drive the output into a heavy DC situation and smoke the speakers. Pulling the servo op amp and installing a manually adjusted DC offset trimmer (aka "C" mod) into the same location basically solves this issue. So IMO the C-mod was a reliability upgrade and not necessarily a sonic improvement. The DC servo works great, when its working properly.

I would ask how many of the amps that SMc upgraded from the late 90's up until about 2014 have failed? In those years SMc was upgrading the stock input pcb's. Has anyone heard of one of those amps failing? Probably not, because the stock amps were already randomly failing back then so Steve implemented fixes for some of the issues he was seeing. However, SMc is now selling a new product to go into your old DNA chassis. Its not easy to sell that new product if the old parts are repairable and if you were happy with how the amp sounded before. How many people would pay $2k+ (I've heard of costs approaching $4k for some) when they might pay $500 - $600 to just get it repaired and not just functional but more reliable than it was and sounding just as good as it did prior?

@nymarty Sorry to undermine the potential sale of your amp and glad your is still working!  And yes, I’ll let you know my thoughts on the amp once I get some hours on her.  
As promised, here’s the reply from SMcAudio on the input board issue…

The through-hole plating issue is a global issue with the driver pcbs for DNA-1, DNA-0.5, HT-1, HT-3, DNA-2 and ALD-1.  It affects the power supply, driver stages and protection circuit on the PCB.

  It is a real problem that caused us and our customers lots of grief as odd failures were repaired only to have a different odd failure crop up weeks or months later. Attempting to repair these boards reliably becomes an expensive game of Wack-A-Mole and often results in solder doing the job of carrying power and signal instead of copper which sounds bad and isn’t reliable.

 The issue does not affect the output PCBs of these listed amps nor do they affect the Virginia Series amplifiers such as the DNA-125, and DNA-225 because these amps only have plating on one side of the PCB (so through-hole plating issues will never be a problem)

 We made the decision in late 2014 to no longer work on these boards and made a replacement PCB that eliminates the potential of plating issues (better PCB material, much heavier plating and redundant vias where necessary)

These PCBs are superior in everyway and have proven to be exceptionally reliable and they incorporate the 20 years of mods and revisions that SMc Audio has developed however the replacement PCB is not a drop-in replacement and requires additional work on the amplifier.

 Hope this helps.  People can always give us a call if they’d like to.

@kchamber 

Thanks for the great post. Please stick around for more on this thread or others. Do you still perform repairs for others? Mind telling us a little more about your background and where you are located?

However, I’d be willing to bet that 50% or more of them will still be kicking with their original input boards 10 years from now.

@kchamber You’d be willing to bet based on what? Faith? Other people’s money? Where does 50% even come from? Outta your butt? You’re willing to “bet” means precisely ZERO to someone buying one of these amps. And by the way, who wants to buy an amp with a 50% failure rate even if you’re right, which you probably are not. The design is prone to failure, especially after 20+ years for the reasons stated by the guys who built the amps, which is not you. Period.

@soix 

Convinced of your belief, you are being harsh and extreme, IME. You also have no basis to claim the converse-that every 1990's McCormack amp has either failed or is on the verge of failing. IMO, kchamber's viewpoint is likely more realistic. I can not think of too many consumer products that have had a 100% failure rate, not even the dreaded Yugo.