I recently reconditoned my Sherwood 7110B receiver from the 1970s with all new aluminum electrolytic capacitors. It now has a low-level 60 cycle (US mains) hum that remains at the same low level at any control setting. The hum is barely noticeable when music plays but is quite annoying when no signal is present. I did not notice the hum when I last tested the receiver with the original capacitors but I can not positively say that it was not present at that time. The receiver is rated for use with 4 ohm speakers and I use it with my 4 ohm monitors.
I replaced the old capacitors with new ones of the same capacitance values and upgraded the temperature ratings when possible. I checked the cap values before installation with a capacitance meter of known accuracy and double-checked the polarity orientation before and after installation. I checked for cold solder joints, bad circuit board traces, loose wires and short circuits, and cleaned the boards with isopropyl alcohol, and still have the hum.
The power supply is a simple design (see attached photo), with rectifiers, filter caps, and Zener diode regulation. It seems to me that the receiver should not hum, but it does. Can anyone help?
I replaced the old capacitors with new ones of the same capacitance values and upgraded the temperature ratings when possible. I checked the cap values before installation with a capacitance meter of known accuracy and double-checked the polarity orientation before and after installation. I checked for cold solder joints, bad circuit board traces, loose wires and short circuits, and cleaned the boards with isopropyl alcohol, and still have the hum.
The power supply is a simple design (see attached photo), with rectifiers, filter caps, and Zener diode regulation. It seems to me that the receiver should not hum, but it does. Can anyone help?
Attachments
The circuit diagram also shows the speaker negatives are being returned to chassis ground. Far better would be to return them to the 0V point between C805/C806.
Thanks to jaycee for your reply.
Perhaps I should add that I have owned this receiver since it was new and I have never before in all the years I used it had this kind of trouble, so I believe the design is sound (no pun intended). Again, the capacitors I replaced were original to the unit when new. I replaced them simply because I thought it was wise to do so considering their age.
Perhaps I should add that I have owned this receiver since it was new and I have never before in all the years I used it had this kind of trouble, so I believe the design is sound (no pun intended). Again, the capacitors I replaced were original to the unit when new. I replaced them simply because I thought it was wise to do so considering their age.
You have said--"it now has a low level hum" ---after replacing the capacitors .
Either one of the new capacitors is faulty OR --your soldering has left a HR join but you also say you checked all that - which are listed in the parts specifications on the schematic .
Many of the electrolytic capacitors were specified in "the old days " as they did not have film equivalents or were more costly to install/manufacture --times change in your position I would have replaced them with film capacitors as many specified values are low .
Try changing the lowest values first for film types.
Either one of the new capacitors is faulty OR --your soldering has left a HR join but you also say you checked all that - which are listed in the parts specifications on the schematic .
Many of the electrolytic capacitors were specified in "the old days " as they did not have film equivalents or were more costly to install/manufacture --times change in your position I would have replaced them with film capacitors as many specified values are low .
Try changing the lowest values first for film types.
Thanks, Duncan. I actually did install film caps in the signal path where possible, but as film caps are larger than electrolytics of the same value they don't always fit in the room available on the circuit board. This includes polyester capacitors, the smallest film caps of which I know.
Seconded, death caps must go - 3-wire mains cable is mandatory, act as if your life depends on it (hint).
That's easily enough to explain more hum pick-up - larger caps have more stray capacitance to the surroundings. Fit electrolytics in the signal path and you problem may go away. But fix the safety issue before you power it up again please....
I logged in on another topic and I wanted to relate my success in eliminating the hum in my receiver.
First, I determined that the hum was not at 60 cycle as I had thought, but at 120 cycles. This led me to suspect a problem with a ground and in my research I found a thread on another site that addressed the same problem with this same receiver. I rechecked all my recap work and again checked all wires and solder joints. I then tightened the mounting screws on the PCB and the power transformer. One PCB screw in particular made continuity between the ground plane and a metal shield on the other side of the board. I cleaned that connection and re-tightened the screw, and et voila! the hum and buzz are gone!
First, I determined that the hum was not at 60 cycle as I had thought, but at 120 cycles. This led me to suspect a problem with a ground and in my research I found a thread on another site that addressed the same problem with this same receiver. I rechecked all my recap work and again checked all wires and solder joints. I then tightened the mounting screws on the PCB and the power transformer. One PCB screw in particular made continuity between the ground plane and a metal shield on the other side of the board. I cleaned that connection and re-tightened the screw, and et voila! the hum and buzz are gone!
A bit if a tangent, sorry. Why would they put the power switch in the mains hot leg and the fuses in the neutral? This can’t be the safest way to do it. Am I missing something?