Yamaha V3900 - Bad Power 3 - Where to start? - Page 1
Forum section: electronics / repair

author: NFG
author_rank: Newbie
timestamp: July 11, 2019, 12:25:00 am
content: My V3900 recently shut off and won't power back on. Service manual and diagnostic mode reveals that PS3 is reading 180, and its normal range is 140-179.Power 3 seems to be the big one in the middle so I really look forward to removing every. other. thing. to get at it, but my question is...Where to begin? PS3 handles a LOT of stuff, with a lot of voltages. I can't see any way to measure themin situso... Any pointers would be appreciated.

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 11, 2019, 07:44:36 pm
content: Page 21 of the service manual shows a "service position". They say extension cables are necessary but I have found in the past (on other models) one can often work around this. If not, then it may be necessary to solder in small wires to appropriate test points to bring them out to where you can make your measurements.The RX-V3900 is among the WORST Yamaha models for serviceability.

author: NFG
author_rank: Newbie
timestamp: July 11, 2019, 10:07:14 pm
content: Thanks for that. I've started poking at the guts of it now. At the risk of revealing myself to be hopeless, what am I looking for, to resolve the PS3 issue? I checked the voltages running to the AVideo board, (+5, -5 and +9) and got 4.95, -4.96 and 8.95 respectively. That seems good to me.Is that out of tolerance? Is there something else to suspect?

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 12, 2019, 03:18:38 pm
content: PS3 pertains to the +5VA、-5VA、+9VA linesOne of them is wrong (probably). Since the error indication in mildly HIGH, I would suspect the -5V line is low.All come from the Power(3) board. I don't know if these are the ones you've already checked, there are many power supply lines in these things.If so there could be a floating ground or a summing network resistor bad. It's rare, but it happens. In the past I have not been above fiddling around with summing resistor values, at least experimentally.

author: NFG
author_rank: Newbie
timestamp: July 16, 2019, 09:34:02 am
content: I don't often mess with negative voltages (this may be the first time!) but if I were to simply adjust the summing resistor in an attempt to solve the problem, I'd use a lower value resistor, which would increase the amount of -5V, bringing the total for PS3down, is that right?So, replace the 27k with a 24k or 22k, fire it up and hope the problem's solved for the next 8 years? =)

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 17, 2019, 11:02:06 am
content: Might take some experimentation on the resistors, but I'm not above cheating if it gets me out of a troubleshooting jam.If it's a deteriorating issue, the cause may become more apparent later.

author: NFG
author_rank: Newbie
timestamp: July 23, 2019, 12:29:16 am
content: Well I threw a 24k resistor on R77 and it brought PS3 down to 146 from 180. The desired range is 140-179, so that's well within spec... For now!I guess we'll see what happens down the road. =)Thanks for your help, @mzacharias.

author: mzacharias
author_rank: Frequent Contributor
timestamp: July 23, 2019, 01:20:21 pm
content: Quote from: NFG on July 23, 2019, 12:29:16 amIn my view anything that would get me out of a 3900 would be a WIN. If it does quit later on perhaps the actual fault will be more obvious.Best of luck going forward.