Troubleshooting the Sansui AU-555A

[DatGtrPlaR]

They might benefit from cleaning. I prefer Caig F5 Faderlube for this. D5 is good for switches and other metallic contacts, F5 for pots.

Well, amazingly, F5 on the trimmer pots worked like a charm and I now was able to do The main section output and current adjustments with no problem with both sides at 20 mA. I also went and D5’d and F5’d the rest of the switches and pots on the front panel and everything works as intended. I played a vinyl of Michael Hedges “Ariel Boundaries” through it, and it sounds wonderful. As it is now with stock components (50 years old)it is besting my AU999.(on the list to work on to improve) But, in wanting to learn more about electronics and repair, what should I do now?

 

[Ray Gianelli]

Well, amazingly, F5 on the trimmer pots worked like a charm and I now was able to do The main section output and current adjustments with no problem with both sides at 20 mA. I also went and D5’d and F5’d the rest of the switches and pots on the front panel and everything works as intended. I played a vinyl of Michael Hedges “Ariel Boundaries” through it, and it sounds wonderful. As it is now with stock components (50 years old)it is besting my AU999.(on the list to work on to improve) But, in wanting to learn more about electronics and repair, what should I do now?

Well, considering its age a full rebuild would be in order. New electrolytic capacitors and any known failure prone small signal transistors and/or diodes.

But at the moment, the only other thing you might want to address is the symmetry adjustment.

 

[DatGtrPlaR]

Well, considering its age a full rebuild would be in order. New electrolytic capacitors and any known failure prone small signal transistors and/or diodes.

But at the moment, the only other thing you might want to address is the symmetry adjustment.

Main output section output adjustment (pg18) from the service manual with the oscilloscope? If that’s what you mean, then I did that successfully!

 

[Ray Gianelli]

Main output section output adjustment (pg18) from the service manual with the oscilloscope? If that’s what you mean, then I did that successfully!

Good on you! Just getting a new scope and figuring out how to do that is not a trivial task.

At this point, enjoy the tunes for a while. You've earned it. But even though I'm not a fan of blanket recapping*, this amp is at the age where it is due.


* I say this because some folks do it as a cure all, hoping that it will fix whatever is wrong with their unit. If you had done that it still wouldn't be working since it was a fuse issue! Still kind of tripping that they were both blown!

 

[DatGtrPlaR]

Good on you! Just getting a new scope and figuring out how to do that is not a trivial task.

At this point, enjoy the tunes for a while. You've earned it. But even though I'm not a fan of blanket recapping*, this amp is at the age where it is due.


* I say this because some folks do it as a cure all, hoping that it will fix whatever is wrong with their unit. If you had done that it still wouldn't be working since it was a fuse issue! Still kind of tripping that they were both blown!

Thank you Ray and everyone else for your help! I was lucky to acquire this amp at a great price as it wasn’t working then. I bought it as a foray into electronics and repair. I really don’t know of it’s history prior. I wish I had an answer as to how both output fuses were blown. I learned a great deal so far, and as a novice I have some musings. I wondered what the big deal is about dummy loads. Would I need this? Well, when you turn it up to 11 to test it you do! And no, I didn’t try without it. I also wonder that maybe the volume from 12 O’clock onward, is the “headroom”. Does anyone listen to their amps that loud?
This little Sansui sounds great as it is so far and I’m gonna enjoy it in it’s stock glory nous for a while. I’m tempted to find another 555a to make mods to.
For now I’m gonna look at my AU999, which I was lucky to get cheap as my Qrx 7500A has distortion problems. I think I need some more experience before I tackle that one. The 999 works fine, but a bit lackluster. Low bass, not as responsive EQ, yada, yada…I think I want to try some mods..I took a gander at the service manual and was thinking about testing the balance and bias and noticed it doesn’t call for use of an oscilloscope for the balance as the 555a did. What is the reason for this?

 

[Ray Gianelli]

Thank you Ray and everyone else for your help! I was lucky to acquire this amp at a great price as it wasn’t working then. I bought it as a foray into electronics and repair. I really don’t know of it’s history prior. I wish I had an answer as to how both output fuses were blown. I learned a great deal so far, and as a novice I have some musings. I wondered what the big deal is about dummy loads. Would I need this? Well, when you turn it up to 11 to test it you do! And no, I didn’t try without it. I also wonder that maybe the volume from 12 O’clock onward, is the “headroom”. Does anyone listen to their amps that loud?
This little Sansui sounds great as it is so far and I’m gonna enjoy it in it’s stock glory nous for a while. I’m tempted to find another 555a to make mods to.
For now I’m gonna look at my AU999, which I was lucky to get cheap as my Qrx 7500A has distortion problems. I think I need some more experience before I tackle that one. The 999 works fine, but a bit lackluster. Low bass, not as responsive EQ, yada, yada…I think I want to try some mods..I took a gander at the service manual and was thinking about testing the balance and bias and noticed it doesn’t call for use of an oscilloscope for the balance as the 555a did. What is the reason for this?

Your AU-999 would probably wake up bass wise by replacing the main filter caps. It may be ready for a recap, as some amps age more poorly than others depending on their storage conditions. An amp stored in a hot shed would be more likely to suffer from dried out caps versus one stored in an air conditioned home, as an example.

The 555A has a capacitor coupled output, where the 999 doesn't. You'd set bias on the 999, symmetry on the 555A.