Kappas4Life
New Member
OK, now you've jogged my memory on how it looks inside. There are actually 5 sets of emitter resistors - 10 total. They are the larger gray bodied resistors. They will all measure a little differently. In my case, I measured them all and set bias so the average was roughly right. But I've also read where people didn't even measure the emitter resistor voltages and used temperature as an indication of correct bias. I double checked my temperature (using one of those laser point and shoot temperature gauges) to make sure I was in the ballpark.
Actually, before you do anything, I think establishing a baseline is a good idea. It's helpful to know what the amp seems to be doing before you actually touch or change anything. I'd turn on the amp and let it warm up (you don't need to run a signal to it or even have speakers connected). See how the heatsinks feel. If either are getting too hot right away, turn it off. But I'm curious what the bias conditions are like now and the heat of the heat sinks will give you an idea of whether the bias current is low (cold heatsinks) or high (too hot heatsinks). As mentioned, mine were stone cold but the amp still sounded fine. But I've worked on receivers where the bias setting was too low and it did make the channel sound bad and correctly setting bias make the channel sound normal again.
After you get a baseline, unplug the amp, note the position of the bias pots, exercise the pots then set back to the original position, clamp your probes, double-check everthing, then you can plug in, turn the amp on, and measure the voltage across the emitter resistors. Have your screwdriver ready to adjust the pots. Especially if the heat sinks were too hot when you checked your baseline/current state, you're going to want to turn the bias pot(s) down right away. Turn slowly and observe the voltage change. You're really concerned about absolute magnitude - if the voltage reads negative, you probably just clamped the check points in reverse of the way the current is flowing, but that's no big deal.
I drew green boxes around your two bias pots. There's one on each board. I drew a blue box around just one of the 10 emitter resistors. You would clamp to each side of one (or more if you wanted to measure more than one) of those resistors. Luckily, as you can see, they're pretty easy to access and I'd just clip onto the top one of the pair instead of trying to get to the bottom ones. Since there's only one single bias pot that's going to control the bias current to all of those output transistors, you're not going to be able to get them all spot on anyway - you just want to be in the ballpark.
To keep things simple to start with, I'd just focus on 1 or 2 emitter resistors on each side and see how they measure. I'd set to say 50mV to start with (instead of going to 70mV) and leave it for a bit and see how the heat sinks feel. Should be pretty warm, but you should be able to keep your hand on them comfortably for as long as you'd really like. If you do have one of those laser temperature things, shoot it at the heatsinks and you want to be somewhere around 50C ballpark. Keep in mind that it will take some time for the heat sinks to change temp once you make a change. And even once you get it to where you want it, you should monitor it after further warm up to make sure the values have stayed the same after warm up.
View attachment 977619
I measured each. The left channel is cold and measured 9.8 mv and the right is perfect temp and measures in the mid 20s. I turned the left pot and then measured again. The mv kept on climbing. I tried to turn it back a little but couldn't get my tool to work and now the left channel just measures 0mv. Still cold. Did I mess something up? Turned pot maybe 15 degrees clockwise