Pioneer QX-8000 Hum issue

[michael_w]

Hi,
Here is a bit of a mystery which I have been unable to solve. I have just finished a rebuild of a very nice quad receiver (with the exception of the tuner board) changing out all of the electrolytic caps and all of the transistors on the tone and Q boards (plus a pair on the phono board). When bench tested everything I can measure seems to be within spec. However, with high sensitivity headphones, a small bit of 60 Hz residual hum can be heard on the front speaker headphone jack. It is still present but very much softer on the rear speak headphone jack (not really noticeable). With regular speakers it is difficult to hear, even when the volume pot was set to a minimum. The volume of this hum is independent of volume level. Oddly it appears only when the pre-amplifier signal is directly coupled to the amplifier input. It is not detectable when using the pre-out (sending the signal to another amplifier) or when using the amplifier input and another audio source. The amplifier section is dead quiet. If I use a shielded RCA cable to couple the pre-amp output to the amplifier input ("bypassing" the switch) I can still hear it. It is quite hard to see on my scope's at the maximum sensitivity at 2 mV/div.

I have tried all of my low tech tricks with strategic placement of coupling caps without success.

Does anyone have any ideas of what the cause might be and how to suppress the slight hum?

Thanks in advance, Michael

 

[michael_w]

Hi,
Well I've been obsessed with this and so I've probed much more thoroughly. At fault is actually the lack of sensitivity in the oscilloscope I'm using. I now see that hum actually originates in the tone control board but it needs the amplifier gain to actually be seen. Even the resistive load from the headphone makes the signal almost unobservable on the scope. The tone board has four identical circuits but in the rebuild I used slightly higher rated 100uF caps in the two channels that have the hum. Instead of 25V they have a 50V rating (Nichicon, one FG and one KZ) but I have a hard time believing that is the origin. Everything else is partwise identical. I don't have any spare 100uF, 25V caps on hand to test things. I also need to put together an external low noise amplifier circuit so I can probe the amplifier board and use the scope. M.

 

[mbz]

Barely noticeable hum is part of vintage, you could live with it or...
If it is 60Hz (not 120Hz rectified) then either a problem with rectification, thought doubtful since other channels would be
affected of 60Hz AC power is being induced(?) into the DC or audio. Look at AC supply wires from cord to switch to transformer
then from secondary to rectifiers, check proximity of other cables and try to separate. Possible you removed some boards
and disturbed the wiring...

 

[michael_w]

Thanks for your thoughts. I was going to write back that I had tried moving the wiring without success but then, through a bit of serendipity, I moved the wire carrying the audio signal from the volume board to the tone board out something like three or four inches from its original track (and well outside the physical range allowed with the shielding in place). Lo and behold the hum diminished to almost nothing. Unfortunately unless I take a hack saw to the shielding that position is just not accessible. All I could do was move the wire to it farthest distance from the other wires in the original track within the confides of the rigid shielding. It is much better but not perfect. I suspect with a proper shielded cable it could be further improved but I just don't have any on hand to try. It is good enough in its current state. Thanks again, M.