well it's peculiar to me anyway. I am recapping one of these and there is a hum that ONLY occurs when the balance control is half way left or right, NO hum when it is in the middle or fully right or left. The hum is on the opposite speaker to the balance setting. It is there when the volume is fully off and does not increase with volume, the music will drown out the hum! any ideas as to where to look? I've tried resoldering, it also gets slightly louder when I touch the chassis, but this does not have an earth wire to the mains
Have you increased the value of the main reservoir caps over what was fitted originally ? If so then that might just be contributing to the issue.
These amps do have a slight residual hum that seems to be caused by a compromised board layout and the sharing of the supply and grounds between channels. That said, its not particularly noticeable.
If its an amp you have owned from new then you know what is normal and what is not. If its an aquisition then you could be looking at other causes... man made issues or damage/cracked print.
These amps do have a slight residual hum that seems to be caused by a compromised board layout and the sharing of the supply and grounds between channels. That said, its not particularly noticeable.
If its an amp you have owned from new then you know what is normal and what is not. If its an aquisition then you could be looking at other causes... man made issues or damage/cracked print.
OK, that sounds a slightly odd issue then. I've no quick fixes either I'm afraid, its going to be a case of looking with a scope to see what is going on and making sure the rails are OK. A shorted diode in a bridge would draw heavy current, an open diode would alter the ripple make up...... this is where you need to scope it all now and see what is what.
The problem has to relate, in some way, to the source impedance presented by the balance pot at its wiper - it is low in the middle or at either extreme and highest in the middle between these spots. Could be electrostatic coupling to that node, but a grounding issue in that area is also a possibility.
Have you already replaced the caps in the regulated power supply? Just to rule that out. Otherwise the issue is probably designed-in and may require staring at the board layout extensively.
Have you already replaced the caps in the regulated power supply? Just to rule that out. Otherwise the issue is probably designed-in and may require staring at the board layout extensively.
Hi dynoPort,
Also check for oscillation. Often you will only hear a hum. You need an oscilloscope for this.
-Chris
Also check for oscillation. Often you will only hear a hum. You need an oscilloscope for this.
-Chris
Hi Dyno,
I think that you have got a faulty or very dusty balance pot. The one half of the internal pot track is a short, the other half a 100k carbon track (Note that the 820AX schematic is wrong, one of the channels should show an inverted drawing of the balance pot). Anyway, in operation, you will have an effective bypass or grounding of the balance at mid position and at either extreme. in between these three, you could get a very high impedance with a possible hum pickup in one of the channels.
I would try cleaning the pot - or replacing it.
Hope this helps,
cheers,
Per
I think that you have got a faulty or very dusty balance pot. The one half of the internal pot track is a short, the other half a 100k carbon track (Note that the 820AX schematic is wrong, one of the channels should show an inverted drawing of the balance pot). Anyway, in operation, you will have an effective bypass or grounding of the balance at mid position and at either extreme. in between these three, you could get a very high impedance with a possible hum pickup in one of the channels.
I would try cleaning the pot - or replacing it.
Hope this helps,
cheers,
Per
Hi dynoPort,
Per has a good suggestion. Just remember to use as little cleaner as you possibly can. It will also attack the protective lubricant on the carbon - wiper track. This type of damage will show up after some time and is something that can only be fixed by replacing the control. If you do elect to replace that control, keep in mind that the new one is as old as the one in your unit. You need to give it a tiny shot to clean the slip ring in the center (the source of the noise, not the carbon track!).
-Chris
Per has a good suggestion. Just remember to use as little cleaner as you possibly can. It will also attack the protective lubricant on the carbon - wiper track. This type of damage will show up after some time and is something that can only be fixed by replacing the control. If you do elect to replace that control, keep in mind that the new one is as old as the one in your unit. You need to give it a tiny shot to clean the slip ring in the center (the source of the noise, not the carbon track!).
-Chris
I've already used switch cleaner, I'd expect crackle rather than hum with a dirty pot?
The hum does go up with volume, not as I stated before. Am about to remove the bridge rectifier to test.
The hum does go up with volume, not as I stated before. Am about to remove the bridge rectifier to test.
From your description I still think that the problem lies with the balance pot. You are quite right to try to sort out the source(s) of the hum, but the pot seems to me to be the pickup culprit. Did you check/re-solder the PCB joints? Does the hum go away if you actively wire bypass the balance pot?
Sorry, I just saw that you had already tried re-soldering the pot.
Yes, dust will make the pot crackle, but other contaminants (tobacco smoke particles or grease gunk - whatever) can give strange effects indeed on a mechanical wiper on a carbon track.
You may have read the lengths I went through to get the 50/100Hz noise down in my 820AX project (hum is mainly 100Hz). The main things that really worked were putting in 100ohm/100uF filters to the input stages - and replacing the E-type mains transformer to a toroidal, I would definitely recommend the Maplin £25 ebay one. But that requires you to be confident in going in to the mains voltage part of the amp.
Yes, dust will make the pot crackle, but other contaminants (tobacco smoke particles or grease gunk - whatever) can give strange effects indeed on a mechanical wiper on a carbon track.
You may have read the lengths I went through to get the 50/100Hz noise down in my 820AX project (hum is mainly 100Hz). The main things that really worked were putting in 100ohm/100uF filters to the input stages - and replacing the E-type mains transformer to a toroidal, I would definitely recommend the Maplin £25 ebay one. But that requires you to be confident in going in to the mains voltage part of the amp.
The volume and balance controls are passive and work in parallel and the hum level increases when the volume is raise or one channel level is increased by counterbalance against its' partner.
The hum also increases when you touch the chassis.
It has been suggested you have an earthing problem which I think is the more likely clue to the source of your problem. The the inverting input of an operational amplifier needs a reference to zero volts and if the connection is compromised it allows space for signals to be picked up and amplified - from radiated mains and simply touching the chassis.
C403,C404, c605, and C606 should connect to the zero volts reference. These would be sensitive to extraneous inputs if the connection is compromised. If the recap involved replacing these parts I would go back there first to check the soldering and continuity. If not due to aging there could still be a cold solder joint involving a capacitor and the series resistors to an inverting input in need of attention.
The hum also increases when you touch the chassis.
It has been suggested you have an earthing problem which I think is the more likely clue to the source of your problem. The the inverting input of an operational amplifier needs a reference to zero volts and if the connection is compromised it allows space for signals to be picked up and amplified - from radiated mains and simply touching the chassis.
C403,C404, c605, and C606 should connect to the zero volts reference. These would be sensitive to extraneous inputs if the connection is compromised. If the recap involved replacing these parts I would go back there first to check the soldering and continuity. If not due to aging there could still be a cold solder joint involving a capacitor and the series resistors to an inverting input in need of attention.
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Ok folks how do you cure the hum? .... put the bottom cover back on !!
seems to make an important connection!
seems to make an important connection!
Really?
Tried that a hundred times with no discernible effect on the 50/100Hz.
But I am all for employing any friendly anti-hum gremlins and if it worked for you - good on you mate! Well done!
Tried that a hundred times with no discernible effect on the 50/100Hz.
But I am all for employing any friendly anti-hum gremlins and if it worked for you - good on you mate! Well done!
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