DR #61 - More Sansui - Rebuilding the Amplifier Boards of an AU-717

we're looking at the bottom of a Sansui AU717 in my 60th video I rebuilt the power supply that turned out very nice in this video I'll take a look at both amplifier modules i'll do the same thing to these pull rebuild reinstall and test those as well fortunately these are very easy to remove from the amplifier disconnect these seven connectors remove the four screws on either heat sink on the other side and they slide out so let's get started this is the right channels amplifier board the left and right boards are basically mirror images of each other and so the work on each will be nearly identical on each board there are five electrolytics all of which will be replaced similar to the power supply board the two larger electrolytics were glued down and the glue has started to affect some of the components so after I remove those two larger electrolytics I'll remove the glue remove any affected components clean them if I can or replace them as I need i'm going to replace all three potentiometers we have two for the DC offset one for the bias these are the originals they are single turn and they are very likely oxidized so those will be replaced with new multi-turn potentiometers i will inspect the thermal compound on these two heatsink mounted transistors and if there's some on this one as well and replace as needed i'll also inspect the thermal compound on the two output transistors which are mounted to the heatsink out of sight down here so let's get to work i try to only film when it's quiet but I do have kids and I do have dogs and so sometimes background noise is unavoidable separating the board from the heat sink is as simple as removing these five screws there were five components affected by the glue i was able to salvage the four resistors here but I don't feel comfortable trying to salvage the diode so I'll replace that the right channel is complete now I'll do the exact same thing to the left channel both channels are complete the rest of the rebuild was pretty uneventful it's time to send these back home the amplifier powered on just fine i've had it on for at least 10 minutes now it's time to make the adjustments for the DC offset and bias i'll start with the DC offset you can see I've already made the adjustment for one channel we need to make it for the other channel now we have about 300 molts of offset on that channel there are two potentiometers to make this adjustment you start with VR1 that's the coarse adjustment and then finish with VR2 that's the fine adjustment and now we'll move on to VR2 the spec is plus or minus 5 molts but with these 25 turn potentiometers you can pretty much get it down to 0 mts perfect let's move on to the bias the target for the bias is 20 molts plus or minus one you can see I've already set it up again for this channel let's set it up for this channel going to use VR3 for this adjustment again I have absolutely no idea how people do this reliably with a single turn potentiometer that looks great i'll monitor for about five minutes and make adjustments as needed otherwise let's move on to testing here we go nice clean output from each channel very evenly matched so far how about a square wave those look nice too let's keep going the manual says should get about 85 watts per channel we'll see if we can hit that clipping there that's closer to 100 watts but we're definitely getting our 85 watts this is great i'll button it back up and let's do a final sound test i'll use this NAD712 stereo receiver as a preamp for the final sound test thanks for watching