Update on AU-517: I changed out the ten (10D7C) silicon diodes to MUR1520 and two transistors (TR602 and TR603) to KSC1845. I also changed R607 from a 100k 1/4W resistor, to a Dale CMF55 330k 1/4W resistor and C601 from a 47uF/50V capacitor to a 100uF/100V Panasonic. The last thing was I replaced the 4.7k 1/2W (R616) resistor with a IΩC RN60C 4.7k 1/2W . After these changes, the AU-517 is back up and running again! *Cue the music*

Note: changing the values of R607 and C601 comes from the AU-517/717 repair guide that I have. Changing C601 to a 100uF helps to further reduce AC ripple. Changing R607 to a 330k ohm helps AC ripple as well as decreasing the bias on TR602. C601 was changed to a 100V capacitor, since the voltages seen on the stock 50V capacitor are reaching upwards of 40-45V!

After the changes above, all measurements are spot on!
(All AC Voltage measurements are RMS values, from a 40k count TrueRMS Tenma 72-1020)

Transformer Voltage: 35.6 V(AC) (Quoted at 35.5 V(AC) on schematic)

TR06 Bias Voltage: 3.000 Volt for both drivers (3V is the nominal voltage, user sets it by trimpot) Thanks to a user on AudioKarma.org, I realized I could set the bias based on TR06’s Collector to Emitter voltage. My driver “test terminals” aren’t connecting for some reason, ground is open. When you check TR06, note it’s not the two outside pins. Check my upload of the schematic!

Rail Voltage to Drivers: +48.7V and -48.7V (+- 48.5V on schematic)

AC Ripple on Rails: 0.19V(AC)-0.24V(AC) (0.2V(AC) Is the quoted number)
You want no more than 0.3V(AC)-0.4V(AC). 0.4V(AC) is a sign of weak capacitors and 0.5V+(AC) is a sign of bad ones. The closer to 0V you can get, the better. You’ll never get to 0V, but you can try! This is an inherent property of rectification, even with great filtering on the mains. Especially when you consider a 52 year old amp is pushing near 50V(DC), at some points in the circuits. If I replace the remaining eight, 30D series diodes, I can probably improve on this.

Latching time: 1-2 seconds (couldn’t ask for better), not even my Yamaha or Pioneer home theater amplifiers are able to do it that fast, most of the time.

When I finished my restoration article, I thought the amplifier was totally fixed. However, after a year or so went by, I started to notice the amplifier relay didn’t seem to want to latch. It wasn’t always happening, but it was enough to cause me to put the amp aside for a while. If you have this happening, you should do the same, because when the relay won’t latch, one of the three fault conditions below is occurring.

Do not try and bypass the relay, or you could fry your power transistors. You could also have a bad/dirty relay, it’s impossible to know which one it is, without doing diagnostic work. The easiest things to check are: AC voltage coming out of the two transformers, your DC voltage going into the driver boards (multi-meter in DC voltage mode), AC ripple across the same voltage rails (multi-meter in AC voltage mode). Start there and if you need help, you can comment here or in the restoration article above.

Stock Transformer voltage: 35.5 V(AC)
Stock DC Rail Volage to Driver: +50V and -50V
Acceptable AC Ripple: < 0.2-0.3V(AC) (Anything over 0.5V is unacceptable)

The power and protection circuit has multiple protections built into it, such as: over-voltage protection (on the speaker terminals), too much AC ripple across the rails is protected, allows a startup delay, to prevent speaker thump (by way of an RC circuit at the relay) and it protects from over-shoot voltages during the startup process.

Sansui AU-517 Power and Protect board with stock silicone diodes

These protections are accomplished by the transistors, which is why replacing TR602 and TR603 is recommended. It wouldn’t hurt to replace them all (with the appropriate substitution part: KSC1845 (works for TR601-603, 605-607). TR601 monitors the output of the speaker terminals for over-voltage conditions (Sansui calls it abnormal voltages). TR604 is the only PNP transistor in the lot, the 2SA733 can still be purchased new.

If you look below, in the set of four silicon (black) diodes (right hand side of the board), you’ll see two much larger diodes in the center. Those two diodes are the reason I bothered to tear the AU-517 back down, again!

Sansui AU-517 with stock silicon diodes, re-capped with most resistors upgraded.
AU-517 Power and Protection Board (re-capped and mostly restored)

I continued to do research and read about how others fixed their Sansui AU-517/717 Power Protection Boards. I ran across a few others who had replaced silicon diodes with diodes that didn’t match the stock characteristics. I had made the same mistake myself, this was one of the earliest things I did to the amplifier!! I just threw some large silicon diodes in that I knew were definitely able to handle the same currents. I did this because the stock ones were smoking, so I assumed they were bad. I put in the closest thing to stock I had on hand. That was a rookie mistake, even though it makes logical sense (back 2+ years ago).

Sansui AU-517 Power and Protect board with MUR1520 diodes installed.

You have to dive deeper to understand why this is; the main things are the forward voltage drops can vary: 0.5V to 1.5V (and higher). Also, the diode’s recovery time matters in a circuit like this. The stock IR 10D7C diodes are the fast recovery type.

Sansui built this circuit to protect the rest of the amplifier from the three conditions I outlined above. In doing so, they built something that is highly sensitive to any component changes. I knew this on capacitors, transistors and resistors, but for some reason, it slipped my mind on the diodes!

Sansui AU-517 silicon diodes upgraded with MUR1520. Re-capped with most resistors upgraded.
Close-up of Sansui AU-517 Power and Protect Board with silicon diodes upgraded to MUR1520
Silicon Diode included for size comparison

What I found in my research was to use the MUR1520 Ultra-Fast Recovery diodes in a TO220 package, these have a forward voltage drop of 0.85V.

MUR1520 diode datasheet

I’ll report back later, on if this was enough to fix my issues with the relay not latching. I am hopeful, knowing what I know now, since I last worked on this piece of equipment. I recently had Semiconductors-I and I enjoyed that course.

I am now in my Senior year of Computer Engineering and doing really well! Looking forward to getting my degree in a year.

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