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Old 15th Jun 2021, 8:59 pm   #1
cgehansen
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Default Arcam Alpha 8 amplifier crackle

Hi, I’ve pulled an old Arcam 8 amp out of storage but unfortunately it’s developed a fault. There’s a continuous crackling noise on the left channel which can also be heard faintly on the right channel. Pressing the ‘direct’ button makes the crackle faint but still audible. I’ve cleaned all the switches apart from the enclosed selector switch but this hasn’t helped. The noise is the same on all inputs and turning the volume, tone controls or selector switch doesn’t affect it. Does this sound like a cap or transistor in the power supply? Any help would be very much appreciated.
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Old 16th Jun 2021, 5:29 am   #2
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Default Re: Arcam Alpha 8 amplifier crackle

Power supply would likely affect both channels equally. I'd suspect something, maybe a transistor in one channel affecting the other via the power rail.

It's got tone controls, so I presume the 'direct' button bypasses them. So I suspect you have something iffy in the tone control section of the badly affected channel.

With a scope, you'll likely see bias conditions jumping around in time to the crackles. A picture paints a thousand words, that's still a lot more than a DVM

David
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Old 16th Jun 2021, 11:51 am   #3
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Default Re: Arcam Alpha 8 amplifier crackle

Thanks for that. So when I press the direct button the crackle is still audible - just quieter the same as on the right channel. Does that still point to transistors in the tone control section?
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Old 16th Jun 2021, 12:32 pm   #4
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Default Re: Arcam Alpha 8 amplifier crackle

...although having looked at the circuit diagram there don't appear to be any transistors in that section. Plenty on the left channel though. I don't have a scope so I was planning on tackling this one transistor at a time.
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Old 16th Jun 2021, 12:59 pm   #5
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Default Re: Arcam Alpha 8 amplifier crackle

Without a scope, that leaves you changing likely parts and hoping. The CMOS analogue switch is a possibility, but I'd start swapping opamps first.

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Old 16th Jun 2021, 1:44 pm   #6
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Default Re: Arcam Alpha 8 amplifier crackle

https://www.wilkinsons.tv/arcam-alpha-r-9-repair/
This might be worth a veiwing if its of any guidance or help in passing
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Old 16th Jun 2021, 2:16 pm   #7
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Default Re: Arcam Alpha 8 amplifier crackle

Quote:
Originally Posted by HECTOR63 View Post
https://www.wilkinsons.tv/arcam-alpha-r-9-repair/
This might be worth a veiwing if its of any guidance or help in passing
Thanks, yes I noticed that. My amp has no remote control so has a simpler non motorised switch.
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Old 16th Jun 2021, 4:28 pm   #8
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Default Re: Arcam Alpha 8 amplifier crackle

As you have the schematic why not 'split' the amp into sections?
Remove the input to the power amp with the crackle. Hard to see the numbering, but looks like a 1k resistor to Q13?
If the crackle remains fault find the power amp section. Fault find that.
If the crackle goes move back down the pre amp sections one at a time removing the input to each stage.
If you are brave short each input to ground and see where the crackle stops. The fault will be before that section...
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Old 16th Jun 2021, 4:30 pm   #9
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Default Re: Arcam Alpha 8 amplifier crackle

I guess the 'direct' switch is the tone / balance control bypass ?

Did you have the tone controls set to the 'mid' position ?
Have you tried a cleaning procedure on the tone controls / balance control themselves ?

I does rather sound like an issue in the tone control circuitry, the residual noise is being 'induced' into the R channel and the L+R channels when the tone controls are defeated. Given an op-amps ability to reject noise on its power rails I imagine something must be making rather big noise spikes for it to break-through. If that's the case, then one or both op-amps in the L channels tone control would likely be the culprit - these are both dual op-amps and shared with the R channel DUH! If I was sure that this was the case (noise 'nearly' disappears with the defeat) then I would probably cut the supply leads (4,8 on the DIL version) and see if the noise goes away in the 'defeat' position. Z300 first then Z304 if that didn't fix it. As you would expect this will kill the tone controls so stay in the 'defeat' position! Worse case is that you end up soldering the pins back together

dc
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Old 16th Jun 2021, 10:44 pm   #10
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Default Re: Arcam Alpha 8 amplifier crackle

Quote:
Originally Posted by dave cox View Post
I guess the 'direct' switch is the tone / balance control bypass ?

Did you have the tone controls set to the 'mid' position ?
Have you tried a cleaning procedure on the tone controls / balance control themselves ?

I does rather sound like an issue in the tone control circuitry, the residual noise is being 'induced' into the R channel and the L+R channels when the tone controls are defeated. Given an op-amps ability to reject noise on its power rails I imagine something must be making rather big noise spikes for it to break-through. If that's the case, then one or both op-amps in the L channels tone control would likely be the culprit - these are both dual op-amps and shared with the R channel DUH! If I was sure that this was the case (noise 'nearly' disappears with the defeat) then I would probably cut the supply leads (4,8 on the DIL version) and see if the noise goes away in the 'defeat' position. Z300 first then Z304 if that didn't fix it. As you would expect this will kill the tone controls so stay in the 'defeat' position! Worse case is that you end up soldering the pins back together

dc
Yes the crackle is loud on the left channel but faint on the right. And also faint when the tone defeat 'direct' button is engaged. I'll take a look at the op amps as you suggest and report back. Thanks for the advice!
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Old 16th Jun 2021, 11:54 pm   #11
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Default Re: Arcam Alpha 8 amplifier crackle

It does seem very common on Arcam amps for the input selectors both motorised and non motorised to go noisy and loose channels on some inputs particularly if they have not had use for a while. I have had to strip down several to get them clean and working again crackle free.
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Old 17th Jun 2021, 1:04 am   #12
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Default Re: Arcam Alpha 8 amplifier crackle

Quote:
Originally Posted by dave cox View Post
these are both dual op-amps and shared with the R channel
Indeed. What's not usually shown in circuit diagrams is that some multiple opamp chips share bias circuitry, things like voltage reference generators used to bias-up current sources and set quiescent currents. If something big happens on one channel, something goes wrong or the amplifier gets driven into its rails, then you can get twitches coupled into the other channel.

There's a fine art in knowing the things they don't show you in spec sheets and diagrams...

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Old 17th Jun 2021, 9:21 pm   #13
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Default Re: Arcam Alpha 8 amplifier crackle

My Arcam crackles on the right channel when the volume control is very low. Moving the volume control eliminates this as the volume goes up. Perhaps it is the volume control?
Also check the fixed resistors, I had one break up (literally, into two pieces) for no known reason (heatstroke?). The whole amp was microphonic until the resistor parted into two pieces.
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Old 20th Jun 2021, 5:15 pm   #14
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Default Re: Arcam Alpha 8 amplifier crackle

I replaced Z304 but that had no effect on the noise. Z300 isn’t on my PCB for some reason. The noise is much worse with the front panel connected to the chassis and quieter when it’s disconnected. When I was testing it R2 blew on the left channel. I’d not noticed before but it looks as though it’s previously been replaced. I swapped it out for a new resistor. The noise is still there but pressing the tone control bypass button now cuts the power supply- the yellow light is still on but no green light. I need to turn the power off and on again with the tone bypass button off to get things working again. I’m not sure what R2 does. It appears to be connected to ground.
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Old 20th Jun 2021, 5:57 pm   #15
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Default Re: Arcam Alpha 8 amplifier crackle

I’ve also noticed some odd looking caps that have been added to R403 and R503. I’ve removed them to see if that makes a difference. The current situation is no left channel on the power amp but the pre out works on both channels. The left channel went out when R2 blew. The tone bypass button sometimes cuts the power but sometimes not. Something is not right somewhere but not sure what.
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