Hi, I've been given an Alpha 5 Plus integrated amp by a friend who told me that the sound was cutting out as the volume control was increased beyond the 10 o'clock point. If I hadn't said I would take it he was going throw it out.
On testing, I noticed crackling when operating the volume control, which some Deoxit fixed and then I ran the amp until nicely warm and tweaked the IQ adjustment for both channels to spec (4mV), as per the service manual.
So, the amp and all controls, via all inputs, seems to work fine now and at all volume levels EXCEPT for a fairly low level, though clearly audible, noise/crackling, which can be heard through both speakers and headphones. This clearly isn't right. I'm pretty inexperienced at tackling issues like this and all I can report at present is that the board components appear okay from a visual inspection (no bulging caps, no heat discolouration around components on the pcb; etc). Also, I've examined the rear of the board and can't see any bad solder joints. Would be grateful for any advice on what the likely causes might be. I'm pretty handy at de-soldering/soldering and changing components, but am still on a learning curve with the hard part...diagnosing the faults! Many thanks for any help.
On testing, I noticed crackling when operating the volume control, which some Deoxit fixed and then I ran the amp until nicely warm and tweaked the IQ adjustment for both channels to spec (4mV), as per the service manual.
So, the amp and all controls, via all inputs, seems to work fine now and at all volume levels EXCEPT for a fairly low level, though clearly audible, noise/crackling, which can be heard through both speakers and headphones. This clearly isn't right. I'm pretty inexperienced at tackling issues like this and all I can report at present is that the board components appear okay from a visual inspection (no bulging caps, no heat discolouration around components on the pcb; etc). Also, I've examined the rear of the board and can't see any bad solder joints. Would be grateful for any advice on what the likely causes might be. I'm pretty handy at de-soldering/soldering and changing components, but am still on a learning curve with the hard part...diagnosing the faults! Many thanks for any help.
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Many faults result in noise so hunting them down is usually a process of elimination - assuming you understand the functional blocks of the amplifiers, how they are coupled together and what they do. That's the first hurdle.
You do have a clue, if as you say the noise is in both channels, it means there is likely a common source - in this case, possibly the power supply for the preamps, which are based on 7812 &7912 ICs. The A5 and A5 plus models are ageing now and likely need replacement of the electrolytic capacitors in the power supply circuits for both the power amps and preamps. Briefly, the power supplies must deliver clean DC to the circuits they power and that includes their grounding connections (return circuits) which sometime break down due to various causes, including tinkering with them. Even if recapping isn't really a fix, it may well be a good thing to do now as routine maintenance.
The next area of suspicion is the muting circuit at the input to the power amps. This is based on JFETs Q15, Q115 and associated circuits. It holds off the audio output until after the amp is powered up so that the associated thumps and odd noises don't pass to the speakers. If working correctly, you should hear only a faint click followed by a normal, very low level of hiss and hum noise after the amplifier powers up. Check this out and though an oscilloscope is by far the best way to analyse noise sources, with patience you often can check voltages out to see if they are in within their expected ranges and test for the effect of increasing electrolytic cap sizes sensibly, with good parts from the bin.
You do have a clue, if as you say the noise is in both channels, it means there is likely a common source - in this case, possibly the power supply for the preamps, which are based on 7812 &7912 ICs. The A5 and A5 plus models are ageing now and likely need replacement of the electrolytic capacitors in the power supply circuits for both the power amps and preamps. Briefly, the power supplies must deliver clean DC to the circuits they power and that includes their grounding connections (return circuits) which sometime break down due to various causes, including tinkering with them. Even if recapping isn't really a fix, it may well be a good thing to do now as routine maintenance.
The next area of suspicion is the muting circuit at the input to the power amps. This is based on JFETs Q15, Q115 and associated circuits. It holds off the audio output until after the amp is powered up so that the associated thumps and odd noises don't pass to the speakers. If working correctly, you should hear only a faint click followed by a normal, very low level of hiss and hum noise after the amplifier powers up. Check this out and though an oscilloscope is by far the best way to analyse noise sources, with patience you often can check voltages out to see if they are in within their expected ranges and test for the effect of increasing electrolytic cap sizes sensibly, with good parts from the bin.
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Ian, thank you, that's really very helpful. I'm enjoying learning about this area and will definitely persevere. I don't yet have a scope , but given my rising interest may well invest in one (guess 2nd hand would be more affordable). I do, however, have a reasonable hand-held multimeter and also a desktop millivolt meter that I bought to help fix an issue with a cassette deck.
Regarding the muting circuit, I did hear a thump and odd noises when powered on and assumed that was normal, but perhaps I have another clue here if the muting circuit should prevent me hearing this. My other amps have speaker relays, so was expecting to hear the click I'm used to after a few seconds.
I'll focus on the areas you suggest and also make a list of the electrolytic caps needed, check if I have any left over from earlier projects and order the remaining. There doesn't seem to be too many in there and everything is easily accessible, compared to some other equipment I've delved into, so a recap (or partial recap) shouldn't take long.
Cheers, David.
Regarding the muting circuit, I did hear a thump and odd noises when powered on and assumed that was normal, but perhaps I have another clue here if the muting circuit should prevent me hearing this. My other amps have speaker relays, so was expecting to hear the click I'm used to after a few seconds.
I'll focus on the areas you suggest and also make a list of the electrolytic caps needed, check if I have any left over from earlier projects and order the remaining. There doesn't seem to be too many in there and everything is easily accessible, compared to some other equipment I've delved into, so a recap (or partial recap) shouldn't take long.
Cheers, David.
As a postscript, the muting circuits work by slowly charging up their RC timer circuits which should charge up evenly, with the midpoint remaining at about 0V so that the muting works correctly for both power on and off muting.
The associated caps in the RC timers are 4 x 100 uF rated at (I'd suggest) 63V or more because they may be exposed to a near full rail-rail voltage of 55V. According to the schematic, they're numbered C881/C981 and C882/C982 respectively in R & L channels. It would be wise to replace these if you can, in the first recap sweep if you already have problems with muting function. Here's a link to the alpha 5 and 6 service manual which incorporates the tweaks of the "Plus" models on separate schematics. Free download after registering: Arcam Alpha 5 - Manual - Stereo Integrated Amplifier - HiFi Engine
@ Glenn E.
Hi Glenn, These are budget amplifiers and when they were produced, it would have been cheaper to use a few small components than provide a power source and decent relay plus its sensing circuits. Personally, I dislike relays because they are prone to degrade the audio by fouling of the contacts as well as prove difficult to replace but there is the risk of speaker damage with muting, unless the outputs or the power is at least fused. I hasten to add that I haven't seen a case of this happening but I've heard a lot of lousy relay behaviour.
The associated caps in the RC timers are 4 x 100 uF rated at (I'd suggest) 63V or more because they may be exposed to a near full rail-rail voltage of 55V. According to the schematic, they're numbered C881/C981 and C882/C982 respectively in R & L channels. It would be wise to replace these if you can, in the first recap sweep if you already have problems with muting function. Here's a link to the alpha 5 and 6 service manual which incorporates the tweaks of the "Plus" models on separate schematics. Free download after registering: Arcam Alpha 5 - Manual - Stereo Integrated Amplifier - HiFi Engine
@ Glenn E.
Hi Glenn, These are budget amplifiers and when they were produced, it would have been cheaper to use a few small components than provide a power source and decent relay plus its sensing circuits. Personally, I dislike relays because they are prone to degrade the audio by fouling of the contacts as well as prove difficult to replace but there is the risk of speaker damage with muting, unless the outputs or the power is at least fused. I hasten to add that I haven't seen a case of this happening but I've heard a lot of lousy relay behaviour.
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I've misread the power amplifier rail voltages as +/- 27V when they are actually +/- 37V according to a clearer schematic. Apologies folks but the original drafts and most scans are awful quality. Read the voltage rating on caps carefully before ordering as there's no parts list in the linked manual. Err on > side of V ratings if you can't buy the correct one, as they may already be close to a safe margin.
Glenn, as far as I know, rotary encoders have only been used in domestic hi-fi in more recent years. Even now they aren't used in all models and brands. Arcam's Alpha 6 model certainly had a remote controller but it used motor drive pots, as most up-market models did in the mid 1990s when these models were produced. I saw only a few forms of digital volume contollers until I stumbled upon the low cost Chinese DIY kits in more recent times.
They work well enough if the controller is robust (few are) but they usually require a display too and I find they're more than a little tedious and confusing when configured as a single knob, multi-task controller for all the traditional front panel functions. Even so, I have a few preamplifiers that use rotary encoded volume controllers and they're very handy as precision attenuators for test purposes
Glenn, as far as I know, rotary encoders have only been used in domestic hi-fi in more recent years. Even now they aren't used in all models and brands. Arcam's Alpha 6 model certainly had a remote controller but it used motor drive pots, as most up-market models did in the mid 1990s when these models were produced. I saw only a few forms of digital volume contollers until I stumbled upon the low cost Chinese DIY kits in more recent times.
They work well enough if the controller is robust (few are) but they usually require a display too and I find they're more than a little tedious and confusing when configured as a single knob, multi-task controller for all the traditional front panel functions. Even so, I have a few preamplifiers that use rotary encoded volume controllers and they're very handy as precision attenuators for test purposes
I've read this too. It's probably a good idea to replace the J112 FETs anyway, so buy a few now, as most through-hole type JFETs are already obsolete so how long will it be before these are too? RS, Farnell, Mouser and, Digi-key all stock the genuine parts. The cheap stuff from Ebay etc. is dodgy and likely won't function correctly.
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