Need help with power supply short - Page 1
Forum section: electronics / repair

author: synth-dude
author_rank: Contributor
timestamp: November 18, 2016, 07:55:19 am
content: Howdy, I've been trying to fix a power supply in a NAD AV-716 surround sound amplifier. It had other issues with stand by etc, but those seem fixed, I've zero'ed in on the main issue and seem to be a short to ground somewhere.I've got the schematics for the amp, and measured all the test points, the only part not working correctly is the part that produces the 5v used for all the digital logic. See attached schematic below.The problem is R927, it gets hot and nearly blows if I dont cut power. The voltages across it are 19v on one side, and 4.9v on the other, the test point should be 18.7 v, so something is shorting to ground to sink that much current. I thought it might be the regulator further along at Q923, but it tests fine out of the circuit. Caps C930 and C929 also test OK out of the circuit. Funny thing is test point where it should be 15.6 v is actually 0v, so I thought R935 was shorting, but it tests OK. I'm completely stumped now what might be going on, so I've posted this as a last resort before I junk this amp.Thanks for any help anyone might be able to offer.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 18, 2016, 08:15:28 am
content: How about D931 and R934? what do you measure on both sides of R934?

author: synth-dude
author_rank: Contributor
timestamp: November 18, 2016, 08:56:27 am
content: Hi, across D931 I get 0.9v or similar, but across R934 I get 19v, and it too gets really hot after a few seconds. Ohms wise its reading 330 Ohms, so appears ok. I pulled C936 out and it tested fine out of the circuit, but appears to short to ground in circuit.

author: John.S
author_rank: Contributor
timestamp: November 18, 2016, 09:20:00 am
content: Hi ,It almost seems like you have 2 problems in the power supply.My guess for the 5V circuit the only problems can be D926, C929, C930 or Q923 ( that can be defective or pulling too much current, is it also getting hot?).So leave Q923 out and power up, what are your voltages then. If the voltage goes to 19 volts, the problem is Q923 or the circuit it is feeding. If the voltage stays lower than most likely C929 is the couse ( cince you measured D926, and found OK.)For the 0V point, also the capacitor C936 is suspect to be faulted. even with shorted diodes and fine working capacitor C936, you would suspect some voltage there, so there must be a dead short there. As far as I can see now the only short can be that capacitor.greetings John.

author: synth-dude
author_rank: Contributor
timestamp: November 18, 2016, 09:35:05 am
content: Hi John, thanks for that idea. I pulled out Q923 and the device itself tested ok, but I didnt measure the voltages in the circuit without it, so I'll do that now.As for C936, I pulled that out and tested it, and the cap retained voltage perfectly fine, at 14v the cap would only drop by 0.1v per second on my multimeter. Does that cap sound good or could it still be bad though it holds charge?Thanks,Chris

author: John.S
author_rank: Contributor
timestamp: November 18, 2016, 10:48:25 am
content: Hi chris,Well, as far as i can remember i never had a cap with some capacity left to have a complete short, only low resistance, coursing problems to overload the supply but, never think you have seen everything.Can you ( with no power on of course ) measure a short to ground with your ohmmeter then.With the things you measure and feel (hot resistor) I suspect the short to be detectable with measuring resistance.success.greetings John.

author: Anks
author_rank: Frequent Contributor
timestamp: November 18, 2016, 11:49:50 am
content: D929 could also be your issue. The short could also be after the regulator.

author: oldway
author_rank: Super Contributor
timestamp: November 18, 2016, 12:13:04 pm
content: Feed C929 with 15V from an external power supply with 1A current limitation and see what is getting hot.

author: synth-dude
author_rank: Contributor
timestamp: November 18, 2016, 01:47:14 pm
content: I removed the regulator Q923 and voltage returned to normal on the 20v rail, but R930 now gets hot. I measured R932 and it was shorting to ground, so I replaced that resistor, which brought the -34v rail up to -23v, which is a win. R930 being hot, I thought Q924 might be shorting collector to base, so I replaced it but nothing is different, it didn't fix R930. I havent tested the zener at D929, are zener diodes ok to test in circuit, or must be pulled out first?Thanks for your help so far, making progress I think.See below updated schematic showing voltages with the regulator pulled out.

author: strawberry
author_rank: Super Contributor
timestamp: November 18, 2016, 03:12:53 pm
content: CPU is leaching magic smoke ...

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 18, 2016, 04:28:07 pm
content: I worked on a 1700 which was very similar. There was one or two audio switching IC's involved with the input and / or tape monitor functions that was loading down the supply. I removed the resistors that were burning up and jumpered in an external supply - the shorted IC's got hot almost immediately. Reduced that portion of troubleshooting time down to minutes.

author: synth-dude
author_rank: Contributor
timestamp: November 19, 2016, 12:28:04 am
content: It seems the 15v circuit which is producing 0v might be powered by another part of the board, now disconnected, so I'll disregard that for the moment.I've redrawn the negative supply section and measured all the voltages, can anyone tell me if the zener diode (rated at 6.3v) should have 1v across it? I'm suspecting that may be the fault. Also seems very weird that 10v is "disappearing" somewhere, the rails are -20v, and across the zener and 3.3k R I get approx -20v, but the 22 R resistor is 10v, and the emitter is near 0 volts, is the zener sinking the rest? If so why isn't it showing on my meter? Very strange.See attached schematic + voltages measured in circuit.Thanks,chris

author: John.S
author_rank: Contributor
timestamp: November 19, 2016, 10:34:19 am
content: Hi Chris,The voltage over the zenerdiode should be 6,3 Volts, ater all, that's what it's made forThe voltage over R932 must be around 5,6V ( 6,3 volts of the zener - PN voltage of around 0,7 volts of the transistor).that resistor 930 is getting hot, I believe that, it's dissipating more than 4 Watts now.So to me it is likely that the zener is not OK ( is the zener also getting hot ?).Check the value of R931, if it has gone low on resistance it can blow a new zener.Also check the transistor.Greetings John.

author: synth-dude
author_rank: Contributor
timestamp: November 20, 2016, 07:19:08 am
content: I replaced the zener diode with same spec device, all resistors and the transistor, yielded nothing. Same voltages. I've archived the project. Thanks for your help.