Rega Elicit, need repair help.

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I have a Rega Elicit amplifier and it is making a hissing noise like a radio station on one channel. I have used an oscilloscope to search for the problem and I have located it somewhere in the output of the amplifier stage.
Unfortunately I can´t find where this it is comming from.
I have swapped most parts between channels but found nothing.
The wierd part is that sometimes the hissing sound goes away and then comes back again.
The level remains the same no matter of the volume setting.
THe channel also works perfectly.
If anybody has a tip for me I would gratefull.
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Both channels perform the same. The quality is good.
The hissing sound is at one level and doesn´t change even if you set the volume up. It was even gone after I resoldered a few parts but then it came back again.
I checked the Zobel parts and thex seem ok.
Should I change them?
The resistor is not open.
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The hiss comes from the output stage. I have checked this.
I have used the power supply from the other channel and it still hisses.
When you say "output stage" you infer that you can determine that the source of hiss is in the output/driver transistors without saying how you determine this. The output stage is directly coupled to, and dependent on the power amplifier's low level stages for correct operation and would need to be isolated by disconnecting the VAS stage and substituting an alternative source of bias current to test this, otherwise anywhere in the front end can just as easily be the source, even the input. Unless the magnitude was greater here than following the VAS, I would be doubtful.
As I read it, there are 3 or more possibilities:

a) The amplifier is oscillating at supersonic frequencies and this appears as audible hiss, only because of intermodulation effects in the amplifier. As you find, an oscilloscope is needed to verify this and then to systematically check the function of any small caps and base stopper resistors fitted to the output stage and perhaps feedback path and frequency compensation caps that are often tweaked at the proving stage to control oscillation in marginal designs.

b) RF breakthrough is occurring due to local transmissions or even switchmode power supplies, computers etc. - disconnect speakers and use headphones or shorten leads drastically to see if this alters the effect. Make sure that mains protective earth (where fitted) is securely tied to the chassis.
tip: try the amplifier at another location, in a friend's system etc.

c) A driver transistor or its connections have gone bad (unlikely). Resolder the offending channel output stage transistors and test again. If there are connectors here, remake the connections to ensure they are good.

Remember, few here will have access to the schematic so comments like these are mostly generalised. Also, the fault may manifest itself in the output stage but it could also be controlled further back.
Thanks for all the replies till now.

About the points mentioned from Ian Finch in the previous post.
a) I used an oscillocope to see where the noise is coming from. I separated the whole preamplifier circuit and the noise starts in the last stage. I see that touching the feedback parts there is a change in noise which doesn´t happen in the other channel.
b) The hissing sound is there in other places also. Next room, at a friends house etc.
c) The drivers and output transistors were swapped from left to right channel but the problem remains in the same channel.

I have the schematic so if anybody needs it I can send per Email.
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So, you say "output stage" but it's actually the "power amplifier" you meant. That's OK, so long as we know what you mean.

You are using a scope but not telling us what is visible, any measureable frequency or magnitude etc. How about at least telling us or even posting a screen shot may be helpful. At this point, it would seem to be oscillation and I would turn attention to input stage and voltage amplifier - likely all small signal transistors. In my experience, oscillation that only begins after years of operation is due to component failure or PCB soldering. Sometimes earthing faults are a cause too. I would replace any electrolytic caps in the front end with suitable quality types of same value. Then look particularly at the compensation capacitor(s) across the voltage amplifier and any other small ceramic or film caps as in (a) #7
Edit: I'm referring to parts of the power amplifier here, not the preamp or inputs to the whole integrated amplifier, in case of confusion again.

Of course, these checks need more than a basic DMM and scope to do properly in cases where very high frequency is involved. Experience and a good knowledge of how DC amplifiers work also make this so much easier.
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