I've picked up a used Arcam AVR350 to use in a simple 2.1 configuration. I've owned an AVR300 since new so I'm reasonably familiar with the kit.
Excusing the "quality" diagram below... the setup is a pair of speakers powered by the receiver's own FL and FR amplifier channels, and an external power amp connected to the SUB preout on the receiver; driving a passive subwoofer.
It was immediately obvious that the subwoofer output is very noisy. A loud buzzing sound at any volume level (except for when the receiver is muted). Sound from the FL and FR speakers is fine (no noticeable hiss unless I turn the receiver volume up to levels that I wouldn't want to use for listening).
Having (unsuccessfully) gone through all the usual checks of grounding issues I reduced the setup to just a single speaker, connected to the FR preout on the receiver, and powered by the same external amp:
That still resulted in a lot of noise (interestingly the FR channel via the receiver's own amp is fine, but noisy via the FR preout).
It could point to the external amp being the problem, so I simplified things even further, and just attached a digital recorder to the FR preout phono/RCA plug. Other than (obviously) power to the receiver there's nothing connected (no speakers, no input sources etc):
As far as I understand (with this receiver), when an analog source is selected you'll always get output to the FL and FL preouts. If the DSP is off (Direct button pressed) then it's a simple pass though, and with the DSP on the input gets processed, then sent to the preouts.
Generating a spectrum plot from the preouts with no input source but the amp volume turned up high shows a spike at 50Hz (UK mains frequency) but then significant spikes every 100Hz. This is present regardless of whether the DSP is on (top) or off (bottom). Understandably there's lower overall noise with the DSP turned off (Direct mode).
I've reproduced the above behaviour on different preout channels (though obviously for a stereo input all other channels are off when the DSP is off, as only FL and FR inputs get passed through in Direct mode).
So; it appears that anything that comes out of the preout phonos is noisy (with or without the DSP enabled), and anything that comes out of the receiver's own amp channels is clean (regardless of whether it's first gone via the DSP or not).
The affected board is the MULTI board (sometimes misspelled MUTI in the service manual). Having pulled the board from the chassis, you can see 4 of the 8 preout phonos (labelled in green). The row of capacitors labelled red are (what I assume to be) part of the line level amplifiers for each preout. On the right (labelled in blue) there's a capacitor (C680) and a black cable going to a ground point near the power supply:
The schematic for this board shows the 7 speaker preouts + sub preout on the right, and capacitor C680 is at the top of the page below:
Based on this section of the schematic I'm assuming this area around C680 is providing a ground for this board (the upside down triangle mark). Could it be as simple as a faulty C680 capacitor causing grounding issues and spikes every 100Hz (that only appears to affect the preouts)? I don't have an electronics background so I'm basically at the limit of my knowledge here:
Excusing the "quality" diagram below... the setup is a pair of speakers powered by the receiver's own FL and FR amplifier channels, and an external power amp connected to the SUB preout on the receiver; driving a passive subwoofer.
It was immediately obvious that the subwoofer output is very noisy. A loud buzzing sound at any volume level (except for when the receiver is muted). Sound from the FL and FR speakers is fine (no noticeable hiss unless I turn the receiver volume up to levels that I wouldn't want to use for listening).
Having (unsuccessfully) gone through all the usual checks of grounding issues I reduced the setup to just a single speaker, connected to the FR preout on the receiver, and powered by the same external amp:
That still resulted in a lot of noise (interestingly the FR channel via the receiver's own amp is fine, but noisy via the FR preout).
It could point to the external amp being the problem, so I simplified things even further, and just attached a digital recorder to the FR preout phono/RCA plug. Other than (obviously) power to the receiver there's nothing connected (no speakers, no input sources etc):
As far as I understand (with this receiver), when an analog source is selected you'll always get output to the FL and FL preouts. If the DSP is off (Direct button pressed) then it's a simple pass though, and with the DSP on the input gets processed, then sent to the preouts.
Generating a spectrum plot from the preouts with no input source but the amp volume turned up high shows a spike at 50Hz (UK mains frequency) but then significant spikes every 100Hz. This is present regardless of whether the DSP is on (top) or off (bottom). Understandably there's lower overall noise with the DSP turned off (Direct mode).
I've reproduced the above behaviour on different preout channels (though obviously for a stereo input all other channels are off when the DSP is off, as only FL and FR inputs get passed through in Direct mode).
So; it appears that anything that comes out of the preout phonos is noisy (with or without the DSP enabled), and anything that comes out of the receiver's own amp channels is clean (regardless of whether it's first gone via the DSP or not).
The affected board is the MULTI board (sometimes misspelled MUTI in the service manual). Having pulled the board from the chassis, you can see 4 of the 8 preout phonos (labelled in green). The row of capacitors labelled red are (what I assume to be) part of the line level amplifiers for each preout. On the right (labelled in blue) there's a capacitor (C680) and a black cable going to a ground point near the power supply:
The schematic for this board shows the 7 speaker preouts + sub preout on the right, and capacitor C680 is at the top of the page below:
Based on this section of the schematic I'm assuming this area around C680 is providing a ground for this board (the upside down triangle mark). Could it be as simple as a faulty C680 capacitor causing grounding issues and spikes every 100Hz (that only appears to affect the preouts)? I don't have an electronics background so I'm basically at the limit of my knowledge here:
Its far more likely to be a power supply issue on the main motherboard. Unfortunately, you just have to get in there and take everything out to be able to work on it. You can check all the regulator voltages beforehand, but you're still going to need to pull it all apart because there can be lots of bad solder joints on the motherboard.
They are bear to work on. I have 250, 300 and 350. Re-capped and changed rectifiers on motherboards of 250&300. AVR250's DSP is shot and correcting PSU regulator for that board didn't help but it did solve temporarily AVR300 . I've given AVR 300 to a friend but shortly after I thought I fixed it's issues it started to have problems waking up from stand by mode and then it died.
AVR350 is the most reliable. It has re-designed mother board with properly sized and spaced rectifier diodes and better caps.
Still I would encourage you to fix DSP PSU regulator voltage to proper 3.3V instead 3.7V it is fed. I haven't played with line level outputs so I don't know if this is a common issue. I sourced all this junk locally for close to nothing imaging I will be able to fix it relatively easy with the help of Youtube 🙂 given the reputation of legendary British brand. They best place for this line of receivers is the bottom of the lake...
AVR350 is the most reliable. It has re-designed mother board with properly sized and spaced rectifier diodes and better caps.
Still I would encourage you to fix DSP PSU regulator voltage to proper 3.3V instead 3.7V it is fed. I haven't played with line level outputs so I don't know if this is a common issue. I sourced all this junk locally for close to nothing imaging I will be able to fix it relatively easy with the help of Youtube 🙂 given the reputation of legendary British brand. They best place for this line of receivers is the bottom of the lake...
It's just really strange that (if it is a PSU issue) that it appears to affect the preouts but not the receiver's own amplifier outputs. I don't really know where I'd even begin to look to try to diagnose that.
I do have the complete service manual (too large to add as an attachment here unfortunately) but if anyone felt they might be able to offer some suggestions I'd be happy to email a copy.
I do have the complete service manual (too large to add as an attachment here unfortunately) but if anyone felt they might be able to offer some suggestions I'd be happy to email a copy.
I've started to take the various boards out, and I see what you mean: not particular "user friendly " to take apart.
I'll inspect what I can and see if I can make sense of the schematics.
I'll inspect what I can and see if I can make sense of the schematics.
I've been casting my (untrained) eye over the schematics, and it does appear that there's a pair of regulators on the main board that supply -12V and +12V to the MULTI and AUDIO board (U104 and U105 in the schematic below). Most of the other boards take different voltages from other regulators.
On the MULTI board it looks as though there are several analog multiplexers; I assume for switching the various source options (from DSP, direct from analog stereo, direct from analog multichannel) to the line level amplifiers for each preout connector. They appear to be powered by a -4V7 / +4V7 signal; but they're generated using some discrete components on the MULTI board from the incoming +/-12V signal. Whether that's the problem (or the incoming 12V supply) I don't know.
There are regulators on the MULTI board producing 5V and 3V3 signals, but they're used by a codec IC that's not in the signal path when the receiver is running in Direct analog mode (and as the problem occurs even in Direct I'm going to ignore those as an issue for the moment).
The regulators on the main board aren't expensive to replace; but is the noise problem a realistic symptom of a bad regulator, or more likely a bad solder joint (or dodgy ground connection)?
On the MULTI board it looks as though there are several analog multiplexers; I assume for switching the various source options (from DSP, direct from analog stereo, direct from analog multichannel) to the line level amplifiers for each preout connector. They appear to be powered by a -4V7 / +4V7 signal; but they're generated using some discrete components on the MULTI board from the incoming +/-12V signal. Whether that's the problem (or the incoming 12V supply) I don't know.
There are regulators on the MULTI board producing 5V and 3V3 signals, but they're used by a codec IC that's not in the signal path when the receiver is running in Direct analog mode (and as the problem occurs even in Direct I'm going to ignore those as an issue for the moment).
The regulators on the main board aren't expensive to replace; but is the noise problem a realistic symptom of a bad regulator, or more likely a bad solder joint (or dodgy ground connection)?