Denon AVR-789 Receiver Repair Advice? - Page 1
Forum section: electronics / repair

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 29, 2019, 07:08:17 am
content: Got this amp for free.It powers up, starts its self tests, then powers down, and flashes the power light red.If I disconnect the main AC power to the amplifier board, so the rest of the amplifier still powers up, it passes its self test, and doesn't shut down.I expected to find a shorted output stage, but now I've completely torn it down (as half the parts aren't accessible) they all appear to be fine.I prefer devices that set the faulty part on fire when they fail...Thanks!

author: kripton2035
author_rank: Super Contributor
timestamp: December 29, 2019, 09:20:14 am
content: when it's not the output stage that fails on the denons, it's some power supply that is not in the tolerance.check every power supply output, one must be missing or out of specs.the schematic of the amplifier is mandatory to fix them.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 29, 2019, 09:25:45 am
content: Quote from: kripton2035 on December 29, 2019, 09:20:14 amCan't test voltages on something that doesn't turn onI found the issue though.The 78/7915 regulators that are known to fail, had been replaced before.However, the rectifier and current limiting resistors that feed the regulators had failed.Two diodes were short circuit, and half the resistors had gone open circuit.I had tested the regulators themselves out of circuit.I've replaced them again now anyway

author: kripton2035
author_rank: Super Contributor
timestamp: December 29, 2019, 10:47:50 am
content: Quoteyes you can. only the amplifier doesn't turn on, the power supplies output their voltages anyway.and yes often some 78/79xx fail. 7x15 are good candidates.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 29, 2019, 10:59:54 am
content: Quote from: kripton2035 on December 29, 2019, 10:47:50 amNo, the amplifier was going into "protect" mode, and shutting everything down, apart from its standby power supply.

author: jogri
author_rank: Frequent Contributor
timestamp: December 29, 2019, 11:02:17 am
content: Do you know if your 789 has some hand-soldered resistors on the underside of the bottom PCB? I have a 1909 (same board) that won't start up after i replaced a few opamps and caps on the input board, and it looks like my AVR blew a few caps before i got it (it had some new Nichicons on the power supply when i received it). Now i don't know if those resistors (a few 1/2 W resistors with foam between them and the pcb) are supposed to be there (design fail, forgot to add them) or if someone added them for whatever reason.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 29, 2019, 11:03:34 am
content: Quote from: jogri on December 29, 2019, 11:02:17 amYes, my board has them. Sorry I didn't take any pics, and its all back together, but I can confirm, they're a factory fitted bodge.

author: tooki
author_rank: Super Contributor
timestamp: December 29, 2019, 05:27:48 pm
content: Quote from: AmmoJammo on December 29, 2019, 10:59:54 amNo, kripton2035 is absolutely right.FYI, here's how you would have measured that, since you certainly can do it in this kind of situation: hook up a meter to a rail, and then turn it on, and see what voltage it reaches before protection shuts it down again.