Hi Mooly (and all)
OK, I've dusted down the old scope and had a look. Attatched is a poor quality photo of the output.
It seems to clearly show the slight hiss on both channels plus the small hum on the right (top trace). I don't understand the slight sawtooth kick.
The scope is on 10 mV per cm and 2 ms for the photo.
I downloaded a test CD with various sine wave tones. These look fine.
I also tried an old valve (school physics lab) signal generator on square wave. At low volume levels the output looked just as good as the input. I don't have a dummy load so couldn't try much volume but did have it at normal quiet classical listening level.
I also tried playing music using the line output of a decent quality portable CD player running on batteries so no possible ground loop! I couldn't see any trace of instability over the music traces on the scope but I'm no expert.
Where do I go from here?
Thanks again!
OK, I've dusted down the old scope and had a look. Attatched is a poor quality photo of the output.
It seems to clearly show the slight hiss on both channels plus the small hum on the right (top trace). I don't understand the slight sawtooth kick.
The scope is on 10 mV per cm and 2 ms for the photo.
I downloaded a test CD with various sine wave tones. These look fine.
I also tried an old valve (school physics lab) signal generator on square wave. At low volume levels the output looked just as good as the input. I don't have a dummy load so couldn't try much volume but did have it at normal quiet classical listening level.
I also tried playing music using the line output of a decent quality portable CD player running on batteries so no possible ground loop! I couldn't see any trace of instability over the music traces on the scope but I'm no expert.
Where do I go from here?
Thanks again!
Attachments
The spikes could be normal in a set-up like this. If that top trace is the residual hum then the spikes are the rectifiers coming into conduction.
Where do you go from here... it seems as though the thin sound may be a charactersitic of this design. Amps do vary enourmously in subjective quality and if you are sensitive to that then those differences are obvious.
Where do you go from here... it seems as though the thin sound may be a charactersitic of this design. Amps do vary enourmously in subjective quality and if you are sensitive to that then those differences are obvious.
Just for interest (nothing to do with the sound) you could short the scope probe and scope ground lead together. That should give a perfect thin clear trace. Now keeping them shorted touch the two on each channels negative (ground) output in turn.
Does the trace still show the hum and spikes even though the probe tip is shorted to the probe ground ?
Does the trace still show the hum and spikes even though the probe tip is shorted to the probe ground ?
When shorted I get an almost straight line with a very tiny bump where the top of the hum sine wave is when the positive scope lead is held in my hand but disconnected. (if that makes sense)
My biggest concern the the ridiculous sensitivity to ground loop issues. The one and only mains powered device I can connect to it without getting a hum from both speakers is the Alpha 8 CD player. Anything else, including the Alpha 7 tuner causes an unacceptable hum. As I've said before the devices all work fine with any other equipment I've got including a direct substitution of my Alpha 5 amp (same cables etc).
To me, this suggest some kind of earthing fault in this amp??
Thanks
That makes sense.
What happens if you now touch that shorted combination to the negative speaker output lead of each channel.
Not according to my Alpha 5 service manual. That says SMF045 n channel mosfets.
That is where I shorted it, perhaps I'm misunderstanding you?
I have a croc clip on the scope ground lead clipped onto the neg speaker terminal. I then touched the tip of the positive probe onto the clip. When the probe is held in my hand I get an obvious sine wave. Grounding it I get an almost perfect straight line with a tiny suggestion of a bump where the peaks of the sine wave were. It may well be the ancient scope is not up to this!
They are all soldered (no flying leads) as all connectors are mounted on the PCB but obviously there could be a dry joint. I have cleaned the circuit board's one connection to the case and fitted a better screw.
Thanks.
Do you see my point though about the crazy sensitivity to ground loop problems? It seems to me there must be some kind of fault causing this (probably stupidly simple) but it makes it almost unusable.
I accept that probably isn't causing the sound characteristic but if I could eliminate that at least I would have the option of selling it with a clear conscience if I couldn't live with the sound.
All this started wanting a remote volume control
OK, that is interesting.
Neither the CD (which is OK) or the tuner (which is not) have an earth pin in their mains connectors. Removing the aerial from the tuner maybe lessens the hum a little but it doesn't stop it.
You've definitely got an earth problem somewhere - it might take some finding.
The tuner might have a poor 0V connection somewhere.
I was running a modified 8R with an 8P. The 8R had an additional buffer amplifier fitted to give a pre-out so that the 8R and 8P could work as a bi-amp set-up. In that case one of them had to have their EARTH disconnected.
The tuner might have a poor 0V connection somewhere.
I was running a modified 8R with an 8P. The 8R had an additional buffer amplifier fitted to give a pre-out so that the 8R and 8P could work as a bi-amp set-up. In that case one of them had to have their EARTH disconnected.
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Can I just double check when you say 0V connection?
The signal earths (phono socket outers) are not common to the case. I think in the amplifier there is a 200 ohm (from memory) resistor between signal ground and earth. What should I check for?
I can't recall there being any 200R. We're not trying to change a design here cos it was a brilliant amplifier. Just check that everything 0V and Earth is corrosion free, tightly soldered and tightly bolted.
Earth on the 8P is there for safety. If you are happy that the amplifier is SAFE then try disconnecting the earth (only for testing) to see how things pan out. It MUST go back on for safety after the test.
As the Power Amp is the high power component of the Hi-Fi chain it must retain its earth.
Earth on the 8P is there for safety. If you are happy that the amplifier is SAFE then try disconnecting the earth (only for testing) to see how things pan out. It MUST go back on for safety after the test.
As the Power Amp is the high power component of the Hi-Fi chain it must retain its earth.
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Sorry, 100 ohm - R207 between the chassis earth point and 0V.
This was one of the first things that was suggested and checked. The resistor is OK and Arcam suggested a way testing it along with a good bit of the board's earth track. Also OK.
Given that all the sockets etc are board mounted I'm not quite sure what else I can do apart from re-solder their connections?
This was one of the first things that was suggested and checked. The resistor is OK and Arcam suggested a way testing it along with a good bit of the board's earth track. Also OK.
Given that all the sockets etc are board mounted I'm not quite sure what else I can do apart from re-solder their connections?
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