Fixing hum in Roland Cube 60

[DavidB]

Hi -

I have an older Roland Cube 60 that has hum related to the reverb. I'm looking for suggestions as to how to fix. Here's what I've tried & figured out so far.
- the hum is audible even when nothing is connected to the amp input.
- there's no hum when the reverb knob is turned all the way down, and the hum comes in gradually as I turn the reverb up.
- the hum is there regardless of the position of the volume controls and does not get louder or softer when the volume controls are changed.
- The reverb is otherwise working as it should, and it generates the usual noise when rapping on the case.
- I opened the case up, and found that there was no metal top on the box containing the reverb spring. I rigged up a cover with aluminum foil and cardboard. This reduces the hum, but does not eliminate it.
-the hum is loudest when I put my fingers on the shielded wires (about 18" long) connected the reverb spring to the main amp board.
- there don't seem to be any loose connectors or solder joints involved on the connections between the reverb spring box and the main PC board.
- occasionally, with the reverb knob most of the way up, I hear a radio station coming through. Not surprisingly, turning the reverb box one direction or another changes this.

At this point, need advice what to do? Is it possible I need another filter cap on the power supply? Or is it more likely the reverb coil is picking up the 60 HZ as an antenna even though I've upped the shielding?

Thanks for any/all suggestions!!!

[txstrat]

Can you rule out the tubes?
I mean have you changed them with known good ones? Sometimes tubes develop a hum by themselves.
You might change the tubes one by one to locate the hum to the driver or recovering tube.

[Enzo]

And a certain amount of hum is normal in a reverb.

[Alex R]

I remember this problem in an old Cube 60. It was a smallish box and so the tank couldn't get far enough away from the mains transformer, kind of a built-in problem. So you need to attend to the screening on the reverb cables and the tank making sure all the ground connections are good. (NB only one of the cables is likely to be grounded to the tank surround, this is to avoid a hum loop).

[Robert M. Martinelli]

+1 on what Alex said, it definitely seems to me your reverb recovery coil is behaving like an antenna, picking up some ( AM ) broadcasting station....

I remember the cube 60 to be a small, very clean sounding box, I tried one some.....oh, my, 25 years ago!!!

Is the reverb pan properly grounded? Having improved its shielding is useless if you have grounding/ground loop problems.

Check/change both shielded wires to/from the reverb pan, the amp's pretty old and the cable's stray capacitance forms an LC resonant circuit together with the reverb recovery coil. ( Actually the same happens on the reverb drive coil as well, but its lower inductance and impedance makes this area less sensitive to disturbances ).

Carefully clean the connectors as oxidization could add resistance to the contacts inducing "strange" behaviors ( it also lowers the Q factor of the above mentioned resonant circuit, enabling it to pick up a broader range of radio frequencies ).

Once you' ve done/checked all of the above, only a little hum should be left, but this, as Enzo rightfully says, is to be considered pretty normal when dealing with spring reverb pans.

Hope this helps

Bob

P.S. @ Matt - AFAICR the Cube 60 was a SS amp - BTW, welcome back!

[oc disorder]

In addition to what Alex R said I can recall rotating and repositioning the mains transformer in the bottom of the box to reduce hum induced into the amp.
However if the unit is also picking up radio frequencies grounding and shielding
and termination of the return should be double checked.
Perhaps the 1uF and the 100pF around Q5 could be replaced without too much expense but it seems more likely a grounding issue.

[bigbassman]

I have one of these amps (70's), and there is no audible hum with no instrument plugged in. If your hum situation has not always existed where you operate the amp, the hum may be caused by aging solder joints around the reverb circuit, or by poor connectivity or shielding of the audio cables to/from the reverb pan or reverb control. Both of these conditions can present non-linearities in the signal path; Radio signals are intentionally demodulated/detected by non-linear electronic components, which may explain your unwanted radio reception. Alternatively, the fault may lie in an IC or transistor, or possibly a capacitor in the reverb circuit that is about to fail. Hum caused by component failure is often due to increased current draw from the power supply, which lowers DC supply voltage, thereby making the ac ripple voltage audible.

This amp is a really well engineered and reliable bit of gear, so I doubt the problem has anything to do with any aspect of its design. I'd bet the problem is most likely due to aging components or wiring/connectivity. Check that jacks and any other mounting hardware are tightly fastened. There are no tubes or other user replaceable parts in this amp, so unless you have tech experience, I would suggest you only remove the chassis (unplugged) to check for obvious electrical or mechanical problems. Bridging power supply filter caps with known good ones can identify capacitor problems, but this can also mask problems in other circuit areas, and present a shock hazard.