Experienced service techs - thoughts on this voltage rail & acceptable noise - Page 1
Forum section: electronics / general

author: Jonny
author_rank: Regular Contributor
timestamp: November 02, 2013, 04:44:59 am
content: Hey all. Been trying to repair an Onkyo TX-SR803 amplifier which claims to have a DSP error most of the time but sometimes works so the DSP itself is ok. I'm thinking caps, but I'm having a hard job proving it after testing a few showing normal capacitance (unable to test ESR) and scoping the rails shows relatively clean DC. There's a +9V rail that feeds some of the 5V regs that has horrendous sawtooth wave of 800mV peak-peak 100Hz (full wave bridge 50Hz in).There's a heafty 10,000uF cap at the PSU end. Granted it's not regulated, it's just a bridge into that cap. I tried a higher value in place with no difference.So I'm wondering if this could possibly be normal.....maybe it is since the voltage out of the regs it feeds seems to be noise free, even if a little low at 4.8. Also from your experience, acceptable max ripple noise before processors go AWOL?Cheers

author: Whuffo
author_rank: Regular Contributor
timestamp: November 02, 2013, 06:00:12 pm
content: That big cap is bad. You've probably got many other dead / dying electrolytic caps in that amp.

author: David_AVD
author_rank: Super Contributor
timestamp: November 02, 2013, 09:05:05 pm
content: Replace the regulator as it's cheap and will at least remove it from the suspect list. Make sure the one you replace it with has a nice low dropout voltage spec. It can vary a lot between seemingly similar types.

author: tszaboo
author_rank: Super Contributor
timestamp: November 03, 2013, 04:27:08 pm
content: The capacitor can be gone, but also, you might just have a short or dead IC or transistor somewhere loading that 9V rail. 10mF and 800 mV means about 20 Ohm load. Can you measure the 9V with Ohmmeter when the amp is off? Or measure currents if it is possible.

author: Jonny
author_rank: Regular Contributor
timestamp: November 07, 2013, 06:27:39 am
content: Here is a schematic. Right after the bridge are the filter caps, there is no reg for +/- 9V. Resistance on the 9V lines is in the k ohms. Unable to measure the rail while unit switched off. I guess there could be something overloading it when on but the outputs of the regs it feeds aren't bad. Maybe I should pull boards until it comes right, that will narrow it down.

author: David_AVD
author_rank: Super Contributor
timestamp: November 07, 2013, 10:46:08 am
content: Q951 is an LDO regulator, but I'd still replace it as a precaution. Also check that all boards are securely mounted as many hi-fi amps use the chassis as an earth return.

author: envisionelec
author_rank: Frequent Contributor
timestamp: November 07, 2013, 03:39:25 pm
content: Quote from: Jonny on November 02, 2013, 04:44:59 amI owned this exact amp. The DSP has a hardware flaw - it is not, I repeat - NOT a power supply issue. I confirmed this with Onkyo indirectly through their authorized service centers. It is likely a die bond failure. I went so far as to power it externally. It STILL reset, sent gawd awful bursts of white noise through my speakers and gave errors. There is nothing you can do except replace the whole DSP board with another that WILL fail. Mine did that and then I...WIDLARIZED IT

author: envisionelec
author_rank: Frequent Contributor
timestamp: November 07, 2013, 03:41:26 pm
content: Quote from: David_AVD on November 07, 2013, 10:46:08 amI did that. Changed nothing. See above.Any hi fi amp worth its salt does not use the chassis as earth return. Sony was (is?) the exception, but only for signal GND reference.

author: Jonny
author_rank: Regular Contributor
timestamp: November 07, 2013, 06:12:04 pm
content: Thanks envisionelec, that's about all that makes sense given when it did decide to work there were no apparent issues even though that 9V was still shocking. DSP is almost too hot to touch. I don't like systems designed that way. Chip design flaw probably exacerbated by long periods of high temp. Wouldn't kill em to put a 50cent heat sink on it. Ah well