I'm re-capping a Mac 4100 for a coworker and in exchange he's trading me a 9090db that won't come out of protection. I've got a little thread going on over in the mac sub-forum about that rebuild. I've got the parts lists compiled and I'm waiting to hear back from Mac about suitable replacements for the tape push buttons before I place an order... Now I'm digging into the Sui that was already given to me. As stated the unit is stuck in protection. First things first I checked all fuses and gave a good round of de-oxit to all switches/buttons/pots. All fuses were good, all switches are clean, and we're still in protection. So off to these lovely forums I went and found a treasure trove of information on this topic. This place is seriously awesome. My 90 minutes or so of AK education taught me F2624 would be a good place to start. To that end I pulled that sucker out and found some good news: there are readily apparent issues! Somebody else has been in here previously and buggered it all up! TR01 and TR03 have been replaced transistors that are not a KSC1845 as is recommended by @Trnsfmr in that awesome thread of his. This seems to have caused R11 and R21 to burn themselves up. Also of note are several other replacements done by Mr. Fixit. C05, 06 R44 VR03, 04 TR09, 10, 14 Those have all been replaced. Mr. Fixit made an interesting decision when he replaced VR03 and VR04 by mounting VR04 on the component side and VR03 on the foil side. That doesn't make any sense to me. Considering all that I think it's safe to conclude that this board is the culprit. All parts that have previously been messed with I am going to replace with the correct parts from the best information I can find here. I am also going to replace the other 6 original electrolytic caps and R33 and R34 (even though they test right around 180ohms) while I'm at it. The service manual states VR01 and VR02 are 4.7k and VR03 and VR04 are 1k VR's. I haven't ever sourced variable resistors before. Can someone tell me what I need to do with that information so I can get the correct part? I also hope Littlediode still has the 2sc984's available to use in the TR09, and TR10 spots. After I get this sucker out of protection I plan on completing a full rebuild on it. Other areas the unit needs attention in if I complete the rebuild and the unit is fully functional are: - replace the power switch which is broken off outside the face-plate - find two silver caps for the filter buttons and a knob for the mic volume - LED bulbs - build a nice new hardwood case. I'm really excited for this project. I think it is going to be fun and I managed to get my hands on a "monster" receiver without having to pay an arm and a leg. My coworker feels like he's getting a good deal too because he bought this cheap with the intention of it being a first rebuild project, but now thinks he'll never get to it. So when I suggested this trade he was all about it.