NAD C-316bee Power Supply - Page 1
Forum section: electronics / repair

author: cfish
author_rank: Newbie
timestamp: November 09, 2015, 11:31:06 pm
content: I'm just starting to get into trying to repair some simple things. I have a NAD C-316bee amp that was lost to a lightning strike a couple of years ago. I tossed it aside with the intention of harvesting some parts out of it, and finally got back to it. I decided to see if I could find the issue, and the first issue I've found is that the +/- 17V supply isn't working. I found a service manual for a C-315bee online, and that one appears to be very similar so I've been using that schematic. I have attached a PNG snip of the relevant part here.Anyhow, I've been poking around, the transformer appears to be putting about 50VAC into the rectifier, and I'm seeing +/- 35V at R317 and R314 as expected. On the other side of R317 though I'm only seeing 0.08V and that's what I'm seeing on everything after that point in the positive side of the circuit. On the negative side, it settle at about -2.19 V.Anyhow, I'm having a hard time figuring out what to test next. I'd appreciate any suggestions.Thanks,Chris

author: kc8apf
author_rank: Regular Contributor
timestamp: November 10, 2015, 03:53:19 am
content: Q308 won't turn on until there is sufficient current through it's base-emitter junction. With +35Vdc where R317 and R319 are connected, there is a path through R319 to slowly charge C314. As it charges, the voltage where R319 and R316 are connected should slowly increase at initial power-on. When it rises past 0.7V, Q308 should start to turn on and let current through to the +17V rail. When the +17V rail hits +16V, D301 should start to let current flow through R309 causing a voltage to appear where D301 and R309 are connected. That voltage is used as feedback to Q306 which will start to draw current away from Q308's base which will cause less current to flow through Q308's collector-emitter junction.I'd start by hooking an oscilloscope up to where R319 and R316 are connected. You should see a normal capacitor charging curve when you first apply power. If you do and the voltage ramps up and holds at +35V, Q308 is suspect. If it doesn't ramp, it could be C314 or Q306.

author: mij59
author_rank: Frequent Contributor
timestamp: November 10, 2015, 05:24:51 am
content: You already found two open resistors R317 and R314, 35V across a 33 Ohm resistor gives about 35W power dissipation, the magic smoke would have escaped within a fraction of a second.Check for shorts in all active components and the +17V, -17V supply.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 10, 2015, 01:39:38 pm
content: Quote from: mij59 on November 10, 2015, 05:24:51 amI would suggest that if these resistors are bad - and I think they are - there are probably some op-amps etc downstream that are also bad.

author: cfish
author_rank: Newbie
timestamp: November 10, 2015, 08:00:08 pm
content: Thanks everybody for the help. I am pretty certain those two resistors are open, probing them in circuit I very briefly got 33 ohms on one but it was intermittent at best. They just don't look bad at all, no sign of the magic smoke escaping.Anyhow, looking at the service manual, they were nice enough to include two sets of jumpers that seem to tie the 17V supply to the preamp and power amp where they get used. I am going to pull those jumpers and replace those two resistors and then see what else needs attention in the supply. If I can get 17V at those jumpers, then I can see the op amps in the power section that the 17V goes to, so I will start checking to see if those are bad.It may end up being a part it out/throw it away thing anyway, but I figure I might as well give it a go and see if I can get anything out of it first.