I originally posted this project on vintage audio but I think this might be the right place for this. 

this post is long and detailed, but I figured I’d share the project and get some feedback. Apologies for the rough picture angles and quality — I took most of them while I was actively working on the amp, long before I planned to post anything.  
A few months ago, I finished work on what is easily the nicest — and heaviest — amplifier I’ve ever had on my bench. The sound is unbelievably warm and beautiful. I’ll never forget this one.

Backstory  
A local guy I do a lot of repair work for imported this amp directly from Japan. He held onto it for a few years before listing it for sale. A buyer from a few states away planned to come see it, but when the owner pulled it out to demo, the protection light was on. He brought it to me and asked if I could fix it within three weeks so the sale could still happen.

Lessons Learned  
Working on this amp taught me a few things:

1) Check input voltage — Always verify the allowable input voltage on imported gear. Thankfully this amp is a tank and survived the extra 15–20 VAC from running on US 120V mains. In hindsight, that mismatch explained why none of the rail or internal voltages matched the service manual/rating on back of amp. I eventually ran this on a variac at 100VAC but I got lucky.

2) Use probe covers — Always use insulated probe tips so only the smallest amount of metal is exposed. I learned this the hard way: while checking one of the eight output transistors, I accidentally bridged two points and blew the transistor apart. Sparks literally shot out, and one of the legs vaporized causing a continuous fountain of sparks. Ironically, those transistors turned out to be the root cause anyway.

3) Be cautious when setting bias — On older amps, keep your finger on the power switch and watch the bias for a minute or two before relaxing. If it jumps, you want to be ready.

The Actual Repair  
The amp was in protection because the right power amp module was stuck on the positive rail at the output. This design uses a bridged architecture, so each channel has a hot and cold section.  
Initially, none of the output transistors tested shorted with a multimeter, but after I accidentally blew them up during probing, I needed replacements anyway. I pulled and tested every transistor on the board — all were in spec.I also in circuit tested every electrolytic capacitor - all were in spec.  
Finding suitable replacements for the Sansui LAPT‑style outputs was a challenge because they require very high transition frequency. I eventually sourced 16 transistors (8 NPN, 8 PNP), even though I only needed four of each.  
After installing the new outputs, I began setting the bias. Suddenly I saw smoke and smelled burning. The bias pot was dirty, causing the bias to jump erratically. It locked into a high‑current condition and cooked the brand‑new output transistors along with all the emitter resistors.  
I replaced the outputs again, cleaned the bias pot thoroughly, and that finally resolved the issue.

Picture Guide

Picture 1 — The amp as I received it. The owner removed the wood panels and brass feet to make it lighter. The unit weighs around 80 lbs.

Picture 2 — Inside view with the top cover removed.

Pictures 3 & 4 — Parts I replaced. You can see how two of the transistors literally exploded.

Pictures 5–7 — Replacing the outputs and remounting them to the heatsink.

Picture 8 — some of the test results for transistors I pulled.

Picture 9 — The repaired module back in place.

Remaining pictures are just random shots I took. 

Repaired Sansui AU-Alpha 907 DR and learned a few lessons along the way