Repairing the Acoustic Research Stereo Remote Control (SRC)
The Acoustic Research Stereo Remote Control (SRC) is an audio control box meant to fit between your preamp output and your amplifier (or crossover, if bi- or tri-amped) input. It allows you to turn your stereo on and off, and control the volume and balance, with an infra-red remote control.
I've had one of these for many years, and while it was a bit "hummy", I loved it for the convenience it gave me.
However, recently, it packed up: I arrived home to hear the stereo on, humming that 60Hz song at near full volume. The remote didn't have any effect, and I had to resort to un-plugging things to achieve quiet again. Before I unplugged it, all the LEDs were lit (flashing, actually) so it looked a bit like this:
Note: Click on the pictures to embiggen.
The assembled unit with all LEDs lit.
Time to crack it open (remove 5 screws from the bottom, lift off bottom and sides):
OK, not much to see here, except to note that the power supply has a small 60Hz transformer. Considering how old this thing is, it's highly unlikely to have a switching power supply in it.
Next, (make sure the thing is unplugged from the mains, huh?) remove the 2 or three screws holding down the circuit board, and the two holding the transformer in. Older units have three circuit board screws, later units have two. Put them and the cardboard bit covering the mains voltage section aside.
Now, carefully remove the board and the transformer, and twist the wiring harness gently so you can see the top of the circuit board.
The yellow circle highlights the main culprit, the plus voltage power supply filter cap. It's 2200uF at 10v in my older unit. The red circle outlines the minus voltage power supply filter cap; it's 330uF at 10v in my older unit.
The above picture shows the board after my first repair, which was only partly successful. Partly, in that it returned the unit to function as it had before, but not fully in that the unit was still quite "hummy". I could measure the hum by putting a voltmeter on the 2V full scale range across the speaker terminals of my bass amplifier. The hum voltage I measured when it was "annoying" was about 8 millivolts; once re-capped, the hum voltage was down to about 3 millivolts.The main culprit. It doesn't look bad, does it?
It eventually occurred to me that there might be another failed (or failing) capacitor in the unit. So I pulled it apart again, put a scope on the minus voltage, and saw huge ripple. Ah-hah -- the minus cap was failing, and that was the source of my hum. (Pity that coin didn't drop years ago.)
The electrolytics in my unit were rated at 10V. Unfortunately, when the unit is "off" (but still plugged in) the power supply is +9.5volts. I don't think there's enough margin on the electrolytics. I replaced them with 16V units. I also increased the amount of capacitance on the positive bus from 2200 to 5500uF by putting two capacitors in parallel, as you can see in the picture. (This taken before I replaced the minus cap.)
When the unit first failed, I purchased another good AR SRC on eBay, and that was quite helpful in realizing that (a) I had a hum problem, and (b) what the correct amount of ripple was in the power supply. The "eBay" unit has 16v electrolytics in it, so I think AR eventually realized the caps were marginal and changed the specification.
Adding more capacitance (and replacing the bad minus cap) has made my original unit quieter than the "new" unit from eBay.
For reference, the positive voltage ripple, measured with a scope at the top of the 2200uF cap, is about .4v peak to peak, and the DC value is 7.5 volts with the unit on, and 9.5 volts with the unit "off". The ripple on the minus voltage is 0.6V, and that voltage is always about 9V, "on" or "off". None of these voltages are critical; don't freak out if you have 7.4 or 7.6 volts on the positive supply. The ripple voltages should be this good, or better (less), however.
There's mains (120VAC) voltage inside this thing. You could hurt or kill yourself if you play with the insides when it's plugged in. Please be careful. I don't want to get angry calls from your widow (or widower).