Leestereo's Restoration/Upgrade of a Sansui AU-555A

[JimEGR]

OK I think I should probably feel foolish! The L fuse was blown (and I took that last voltage measurement on the wrong side of the fuse). I should've checked this first.

I would expect no sound on that channel if the fuse was blown though - is that a false assumption? And shouldn't there be zero volts on the load side of the (blown) fuse?

 

[Leestereo]

There is still some voltage/power available to the left channel from regulated supply for the front end, i.e., 55V at the "K" point in the schematic.

 

[JimEGR]

OK. And that would explain why TR801 was fully powered still. Thanks Ben! Appreciate your time on this. Happy to have my AU-555a back in rotation.

 

[ohmless]

Years into this thread and I'll add another big thanks to Leestereo for his great work, and patience in answering all the questions. I may have pulled off a rebuild on my own but it wouldn't sound like this.

One thing about the 6800uf 42 volt coupling capacitors...I couldn't bring them up through any of the major suppliers. Then I clicked on Leestereo's Mouser link in post #426 and there they were. For anyone else taking on this project as of this date 237 are still available by clicking the link.

 

[Joe_music]

I all, so I've been reading this forum and I've collect and bought all component needed to make my Sansui a little younger again!

I got the power supply section all done with following voltage reading on + end of each capacitors :
C001 : 57.7Vdc
C002 : 54.8Vdc
C003 : 46.8Vdc
C004 : 41.7Vdc

Now my tone control board (F-1272-1) is all done and will be going to work on the phono. I have few question as far as testing goes... I don't have a dim bulb tester right now, not sure if I need one... I was thinking of simply going into a short listening session after the phono preamp. I think I need to adjust AC balance and bias since the power amp cap has been upgraded ?

If I adjust those values with the stock main board (F-1266-1) I could go and take a listen to validate everything is good and working as it should ?
( I will check that I have 0Vdc at the speaker output at idle before plugging anything).

Thank for the help!

 

[Joe_music]

Another question, about transistor gain... as @Leestereo mentioned some approximative number of transistor gain for tone & phono. I wonder if it is ideal to have gain closest to each other than higher or lower of those generic reference published.
ex., Reference gain of 450, would a pair of 452 & 446 or 458 & 457 would perform best ?

For TR701-702 the closest matching gain pair I have from 380 will be hFE : 426, 428 or 429.
Those are actually the lowest gain of my 40 transistor... a gain of around 400 was recommended for the phono preamp...
Any other suggestion or I should order more to have close to hFE : 400? I think I understand the purpose of those been gain matched but didn't know it was needed and not sure if this depend of circuit design and values, etc.

Is there a gain suggestion for TR801-802 as well ?

 

[Brian1970]

You probably want a dim bulb tester. They're cheap and easy to make and can save expensive parts if you make a mistake. Feel free to check voltage on the speaker outputs but it's a capacitor coupled amp and there's no way to adjust it. The check would only tell you if you've done something very wrong.

 

[Leestereo]

Another question, about transistor gain... as @Leestereo mentioned some approximative number of transistor gain for tone & phono. I wonder if it is ideal to have gain closest to each other than higher or lower of those generic reference published.
ex., Reference gain of 450, would a pair of 452 & 446 or 458 & 457 would perform best ?

For TR701-702 the closest matching gain pair I have from 380 will be hFE : 426, 428 or 429.
Those are actually the lowest gain of my 40 transistor... a gain of around 400 was recommended for the phono preamp...
Any other suggestion or I should order more to have close to hFE : 400? I think I understand the purpose of those been gain matched but didn't know it was needed and not sure if this depend of circuit design and values, etc.

Is there a gain suggestion for TR801-802 as well ?

The hFE of the transistors installed in the 555A restoration are typical of what was available at the time from Mouser/Digi-Key. IMO, transistors that you have on hand are fine to use; note that Sansui often specified higher gains for the 2nd set of transistors in a circuit, e.g., TR603/604>TR601/602 or TR703/704>TR701/702. For TR801/TR802, the use of hFE>400 is equally good.

 

[Joe_music]

Thanks for the info mister Lee I really appreciate! I missed that and wasn't aware of that. I made a little change of my transistor paired for Phono, Tone & Main amp :
I used KSA992 for the main amp and KSC1845 for tone & phono board.
TR601/602 : hFE 484
TR603/604 : hFE 499
TR701/702 : hFE 469
TR703/704 : hFE 477
TR801/802 : hFE 415

I'm currently finishing up assembly and will be going to set the adjustment soon. Can't wait for the first listening session!

 

[Joe_music]

Well the work was completed succesfully. Setting un AC balance and bias is pretty easy IMO. I did the adjustment over time... not just once as the value stabilized over time with temperature. Did a second adjustment after fully cooled down and on third power up, settings we spot on.

I recommend measuring the old VR to give you a approximative idea of the new one. Personnally, it did help but i still had some adjustment to you. It simply help you be closer to the desire value at first startup. * First start up with a DBT *

I'm happy to share a updated list of component & tips and tricks for people who are interrested.

Some picture of the final work ! Can't wait for the first listen!
Thanks M. Leestereo!

 

[petesui]

I'm happy to share a updated list of component & tips and tricks for people who are interrested.

Some picture of the final work ! Can't wait for the first listen!
Thanks M. Leestereo!

That would be great, thanks. I have two 555As, both in good working original condition. I hope to restore one of them soon.