curiousmarc
patreon
My NAD amp is fine, thank you very much.
🕑 Added 2020-07-13 06:40:41 +0000 UTCI took out the old HP 428B current probe to check that the current going in my power supply components is normal, and that I just didn't cover up another fault, as the conspiracy mill says on YouTube. It is a very old instrument (using tubes!), but still in perfect calibration. I checked it of course, it is within 1% of my Tek DMM. Conveniently there are jumpers on the board for the 55V rails, I just made them longer so I could use the current probe.
It measured 237 mA on the top rail, which is right around what I expected (bottom rail is lower at 170 mA, as it does not have the extra 12V supply to support). Using that figure, one can calculate that the original components were indeed running within spec: the 3.3 Ohms 0.5W resistor was running at 0.19W, and the 3.3V 1W zeners were running at 0.78W. The replaced ones have of course more margin, but what I was really after was reducing the local temp to protect the thing that actually failed: the poor quality PCB.
So the amp is fine, thank you very much, there is no short or over-driven components or any other kind of imminent doom. Some cooked caps will sure need replacement, but since they are just decoupling and not filtering caps, even that should not have a measurable effect on power consumption [edit: and it didn’t, did not even change by 1 mA].
As expected, NAD cut it a little close with the Zeners, which they apparently corrected in the later models by putting a single bigger one, tells me the one commenter that actually looked instead of speculating. It is of course hopeless to get that point across on the YouTube comment section. But Patrons know better.
Comments
CuriousMarc
Fantastic! Congratulations on your repair!
Thank you Marc, this video helped me build the confidence to repair my old Denon receiver that I love so much (bought it in 2000 with my first money :) ), troubleshooting was easier actually: just a voltage regulator to replace, but dismantling the beast to gain access to the faulty component was a feat in itself I could never have done if I wasn't watching your channel ! Big Thanks !!
John McCormick
I have a 1978 Pioneer SX1080. When I tell people online the first thing they ask if I have recapped it. No.
