100Hz hum on preamp output

[QSilver]

As the title suggests, I've got a hum out the output of my pre-amp. It's a Pioneer C-90 paired with an M-90. I've rested the M-90 separately and its totally silent. When reconnecting to the C-90 I get a hum through the speakers.

When hooking a scope up to the output, I've got a small waveform, partly a square wave at 100Hz. It isn't effected by the volume control to I'm guessing the fault is after the volume control in the pre-amp?

Does the 100Hz point to a power supply issue? The unit has been recapped about a year ago and has done this on and off. Occasionally a light tap on the lid would stop it but I'd rather try to stop it happening in the first place. I've never dealt with 100Hz hum fault tracing so would appreciate some pointers or tips on what to try probing next to find it.

I'm in the UK so if your reading this in the US it would be 120Hz hum, double the line frequency. Oh, one other thing, the 100Hz waveform appears to be inverted between left and right.

Thanks for reading!

 

[QSilver]

For future reference, I think I've found the culprit. There are two boards in close proximity to each other in these units. The Power supply board and the main board.

I made some errors in my original post. The noise was affected by the position of the volume control which lead me to this...

On the power supply board, Q208 has a very large and tall heatsink on it that is not isolated. This means the heatsink is live with the unregulated output of the rectifier. This is where the 100Hz comes from.

The heatsink is less than 5mm from coming into contact with the trace side of the board above it, the main board. Directly above this heatsink is the input buffer amp circuit for the pre-amp. On my unit, the inulating stick on material was missing and only some epoxy on the board was left. On this, a slightly long component lead from a resistor in the input buffer was blackened and able to touch and scrape the heatsink below with the unregulated voltage on it.

I'm making an educated guess here, that the votlage was being passed to the input buffer, which as then blocking the additional DC voltage, but passing the 100Hz residual ripple from the diodes into the amp and amplifying it.

Basically, I cut the overly long component lead short, and added some new pieces of self adhesive foam to the underside of the main board to insulate the heatsink from the traces of the above circuit board.

All back together and the pre-amp is silent - quieter than I've ever known it so I think it's problem solved!

I hope this helps someone else in the future! It's worthy of an advisory check when coming across any of these units. As the boards age, they tend to sag and this might make this problem worse.

I'll report back if it comes back but so far, so good!