Quad 33 more hum

[PaulR]

I bought a Quad 33/303 last summer and did the Dada modifications. There were hum problems with it, the worst of these I cured with help from forum members. That left a slight residual hum independent of volume setting which I just put up with.

By coincidence a friend gave me the same set up complete with FM3 which he was replacing. I put it away pending deciding what to do with it. I recently decided to try the unmodified 33 in place of mine. The result was no hum whatsoever. I tried swapping the removable pre-amp boards but the hum remained with my unit. I have just taken it apart again to go over my soldering and make sure that the various parts of the chassis are tightly screwed together but the hum remains. I assume the problem is with the power supply but I hesitate to unsolder it to try swapping it over.

I cannot say whether it was humming when I bought it as I just tried it for long enough to make sure it worked.

Has anyone had similar problems following the Dada modifications or have any suggestions other than just use the unmodified unit? The only thing that occurs to me is that, although counter intuitive, the increased value of the electrolytics could somehow be somehow contributing to the problem.

It is a 100Hz hum by the way

[PaulR]

An update to this - I have discovered that if I bypass the tone controls by pressing the cancel button the hum is much reduced, but not completely. The button has no such effect on the unmodified unit.

[peterpiper]

Hi Paul, the inductors in the filter cct pick up hum. This is mentioned in the 33 user guide about how not to place the 33 beside transformers

[PaulR]

Ah, thank you Peter. That explains why the hum goes when the cancel button is pressed.

It isn't actually near any other transformers, and the other unit isn't affected in the same way. I wonder whether it is just particular unit picking up hum from its own wiring or transformer.

I compared the dressing of the various leads in the two units and they are the same. The only thing I will have disturbed are the leads to and from the power supply when I freed it slightly to replace the caps. That was minimal though and they look identical in both units.

I have removed the screws holding the various parts of the chassis together and put locking washers under the heads before screwing them back tightly. I hoped that there may have been a poor electrical connection between them affecting the screening but it hasn't helped.

It is probably one of those mysterious things that will never be solved but I will keep fiddling with it from time to time to try to sort it out.

[alanworland]

I had a problem with my 33 and put it down to the switches. It went back to Quad twice and seemed ok for a while but the intermittent problem returned, I carried out the Dada mods and it was still there!
In a desperate measure I resoldered all joints on the main board although they all looked good and the problem hasn't occurred since. Mine is silent.
Might be worth a try?

Alan

[PaulR]

Thank you. Yes absolutely worth a try. I might not get round to it for a little while though.

[mikes08]

I have added Dada mods to both my 33's but have no issues with hum. As above might be worth re-flowing the solder joints. I assume hum in both channels. Could it be an earth loop?

[PaulR]

Thanks Mike. I know they are very sensitive to hum loops and have taken care to avoid them including isolating the screen of the radio aerial which was causing hum. I would have expected a hum loop to affect both units but the hum is only on one of them.

I think that re-flowing the solder is the best thing to try.

[PaulR]

Apologies for the long delay in updating this thread.

I have just re-flowed all the joints on the tone board and the hum is still there. I have just noticed however that it is affected by the bass control. Turning it right down removes the hum as does bypassing the tone controls. Does this help with a diagnosis?

[Gabe001]

Not familiar with the 33 circuit, but in general,

1. check whether the body of the tone pot is properly grounded, and

2.that there is a ground path for the audio signal at the tone control

[dave cox]

Both channels the same ?
Quad revised the circuit, are both examples the same version ?

The cancel button activates in 4 separate sub-circuits - 2 for each channel.
The 1st bypasses the 'variable' bass / treble and the 2nd the HF roll-off.
Separating these 2 functions (for diagnostics) would be helpful but might be tricky to do in practice. The HF roll-off looks like it could be bypassed so I would start by doing that and see if the problem goes away

dc

[PaulR]

Thank you for the replies. I will investigate and report back

[PaulR]

Apologies for the delay in replying.

All the pots seem to be properly grounded - 1 ohm or less between them and the chassis.

The hum is equal from both channels.

Regarding the tone controls, the hum actually reduces to nothing as the bass control is turned up - I hadn't noticed that before, I assumed it was the other way round. The slope control makes no difference.

Both examples appear to be the same circuit wise.