Hi, I've taken an old Arcam Alpha 8 Integrated amp out of storage and unfortunately it has developed a fault. There's a constant crackling mostly on the left channel but also faintly audible on the right. It's also only faintly audible with the 'direct' button pressed to bypass the balance and tone controls. Could this be a faulty transistor in the left channel? Cleaning the pots hasn't made a difference and the noise doesn't change with the volume or with different inputs. Any help would be appreciated.
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If the noise is unaffected by volume its in the power amp itself - perhaps loose ground connection, failing capacitor, cracked trace, dodgy solder joint - narrowing this down to the component level is sometimes doable with an (insulated) poking stick - tap the various components, press down on the pcb in various places, the connectors, etc.
Storage suggests corrosion as a possible factor - check the grounding screws/bolts are fully tight. Wiggle all the connectors. Also electrolytic caps may be old and desicated - this could account for the break-through to the right amp channel (power rails no longer strongly decoupled to ground).
Storage suggests corrosion as a possible factor - check the grounding screws/bolts are fully tight. Wiggle all the connectors. Also electrolytic caps may be old and desicated - this could account for the break-through to the right amp channel (power rails no longer strongly decoupled to ground).
Thanks. All the ground connectors look good and there's no corrosion. It sounds like there may be something wrong in the tone control circuit. The 'direct' button greatly reduces the noise when its pressed in and there is some switch noise when engaging and disengaging that switch which doesn't sound right.
It does bleed though to the pre outs which would suggest the problem is with the power amp. However the behaviour of the noise is the same though - if I bypass the tone controls then the noise becomes very faint and at times inaudible. There are a couple of op amps in the tone control circuit - maybe that's where it's coming from.
Eh? jaycee just advised that if the noise was audible at the pre. outputs, the source of the noise would be the preamp. Confused or is it just me?
FWIW, many 1980s Japanese amplifiers had tone control bypass switches which were fragile little alternate push-push switches with silver plated contacts. These were great for a number of years but then started to crackle as the silver tarnished and occasionally, the sound would even disappear completely on one or both channels. I'm not suggesting this is your problem in your ARCAM 8 but do check all the likely mechanical switches, sockets and plugs, particularly the ground connections in the signal path, for consistent, reliable connections. The best quick solution was often to ditch the tone controls and bypass them because replacement parts were unlikely.
As suggested earlier, time in storage almost always has some bad effect on audio gear, depending on just where its stored, humidity levels, temperature cycling etc. Lofts and garages are notorious for ruining your prized gear, even when there is no apparent dampness or dust lying about. Mostly, its about metals, oxidation and corrosion - thin, hard film effects that may be invisible on what appears to be clean, conductive surfaces.
FWIW, many 1980s Japanese amplifiers had tone control bypass switches which were fragile little alternate push-push switches with silver plated contacts. These were great for a number of years but then started to crackle as the silver tarnished and occasionally, the sound would even disappear completely on one or both channels. I'm not suggesting this is your problem in your ARCAM 8 but do check all the likely mechanical switches, sockets and plugs, particularly the ground connections in the signal path, for consistent, reliable connections. The best quick solution was often to ditch the tone controls and bypass them because replacement parts were unlikely.
As suggested earlier, time in storage almost always has some bad effect on audio gear, depending on just where its stored, humidity levels, temperature cycling etc. Lofts and garages are notorious for ruining your prized gear, even when there is no apparent dampness or dust lying about. Mostly, its about metals, oxidation and corrosion - thin, hard film effects that may be invisible on what appears to be clean, conductive surfaces.
Yes it’s me confused. I meant preamp and not the power amp. I’ve checked and cleaned all the switches and grounds apart from the selector switch which I would need to dismantle to clean.
While testing the amp R2 blew. It looks as though it has been replaced before so I wonder if some dodgy repairs have taken place in the past. Some caps have also been added across resistors R403 & R503. I've replaced R2 but now the left channel doesn't work at all. The noise has gone though. The pre-outs work normally but the tone bypass button intermittently cuts of the power for some reason. Also the circuit diagram I have doesn't exactly match my PCB which doesn't help. I wonder if there were some revisions to the design as the diagram I have is from 1998 but the PCD is dated 1996.
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the link may be of some help, it tells why the caps were added.
Taming the Oscillating Op Amp - The Signal - Archives - TI E2E support forums
OSCILLATION IN AMPLIFIERS - Analog Modules
Taming the Oscillating Op Amp - The Signal - Archives - TI E2E support forums
OSCILLATION IN AMPLIFIERS - Analog Modules
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So it looks like someone has tried to fix a problem in the past by fitting the caps. R2 blew before I removed the caps but after I'd changed the op amp in the tone control circuit. Now the left channel is dead (with just some faint hiss) but the pre out is fine. Not sure why the tone bypass should cut the power off. I should probably use a bulb current limiter before powering it up again.
It is better not to introduce a modification when troubleshooting. Chances are that the new opamp was oscillating/made the device oscillate. The caps were added for a reason. So putting back the 100 pF capacitors and the old opamp seems wise. If R2 blew your amplifier certainly was oscillating. Fixing the oscillation should be done first.
It would help tremendously to speak of exact names or type numbers if you want help from members here. So please not "the caps", "the opamp" but C403, Z301 etc.
It would help tremendously to speak of exact names or type numbers if you want help from members here. So please not "the caps", "the opamp" but C403, Z301 etc.
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The op amp was a straight swap - a new RC4560D as I thought the old one may have been causing the noise on the tone controls. It's shared accross both channels. The 100pf caps were a modification soldered onto the component side of the board presumably when R2 blew previously. I'll put them back.