Repair and restoration of my STAX SRA-12S

[DefQon]

Shouldn't matter then. The old styled case are very old Toshiba.

Sent from my LG-D855 using Tapatalk

[Doug_D]

Tinkerer put his SRA-12S up for sale over the holidays, and I was thrilled to be able to buy it.

I'm sure I my ears are more pyrite than gold, but this is a big step up from the SRD-7 I've been using up to now. Since this SRA-12S has one pro bias output, I now have an amp that opens up a path to upgrades too. And until I saw this unit listed and was reminded of its existence, I really didn't really realize how badly I needed a "control center" for my headphone/2-channel gear. Just a great fortuitous fit for my needs.

One giant step for a man, one profound "ehh" for mankind. 

Now I have to sift through all the voices and decide what Lambda should be the next one to get. What fun !

 

(Now listening to: The Best of the Chesterfields.)

 

[DefQon]

So spritzer I`m finally getting off my ass and getting a replacement toroid made for my 12S is 65VA enough handling for the 12S?

Here is a spec sheet from the manufacture I`m getting it done through based off the secondary windings you mentioned pages back you used to fix your 10S trafo.

 

TI-169015 [960325].pdf

[Tinkerer]

On 1/16/2016 at 11:24 PM, Doug_D said:

Tinkerer put his SRA-12S up for sale over the holidays, and I was thrilled to be able to buy it.

I'm sure I my ears are more pyrite than gold, but this is a big step up from the SRD-7 I've been using up to now. Since this SRA-12S has one pro bias output, I now have an amp that opens up a path to upgrades too. And until I saw this unit listed and was reminded of its existence, I really didn't really realize how badly I needed a "control center" for my headphone/2-channel gear. Just a great fortuitous fit for my needs.

One giant step for a man, one profound "ehh" for mankind. 

Now I have to sift through all the voices and decide what Lambda should be the next one to get. What fun !

 

(Now listening to: The Best of the Chesterfields.)

 

Just saw this post. Glad you're enjoying it. The 10S/12S really are great little amps.

[spritzer]

2 hours ago, DefQon said:

So spritzer I`m finally getting off my ass and getting a replacement toroid made for my 12S is 65VA enough handling for the 12S?

Here is a spec sheet from the manufacture I`m getting it done through based off the secondary windings you mentioned pages back you used to fix your 10S trafo.

 

TI-169015 [960325].pdf

It should be enough but I never measured the current draw of the SRA-10S. 

[DefQon]

Ok so my trafo came, size is just ok as its pretty big for a 65va based off the specs of the above attached pdf. The only problem now is what primary winding wire goes to which pin on the voltage selector?

The wire colours are different on the 12S compared to the T1 and SRM1 MK2 schematics you've posted.

 

The plug reads in following orientation downwards:

240v - 117v

220v - 100v

Edited February 16, 2016 by DefQon

[spritzer]

No idea really as my schematics are all packed up so I can't look at them.  You would need to ohm out the connections inside the plug and mate the socket to that. 

[DefQon]

Fuck sounds like a lot of work. 

[Tinkerer]

The dual primary order should still be structured the same. 117 100 G  117 100  G CG, left or right or shuffle the center tap where-ever.

 

So the 117 plug position should have both the 117's tied together and the grounds tied together

The 240 should have them in series.

The 100 should have the two one hundreds in parallel

And the 220 should have a 100 and 117 in series

 

So easiest way would be to record what pins are connected on the switch in each position, then sudoku it out. Which is just a roundabout way of saying what spritzer said really. At least your new trafo should have all the wires labeled so you don't have to ohm that out too.

 

Just looking at it, I think the top row is all the primaries in proper order. Gray and white are the two 117's and yellow is the CG. But I'd certainly check it.

Edited February 16, 2016 by Tinkerer

[DefQon]

Thanks I`m just going to match the wiring from the plug pin number position based off the floating schematics found online for the SRM1 MK2 since majority of Stax transformers that used the old plug style with the voltage selector plug are pretty much universal to each other in wiring.

[DefQon]

Well it worked. But getting the same (not as much) static distortion going on in upper registers when I turn volume up and up. This problem started happening after I re-did the psu board. I think I fucked up somewhere and will replace all parts again. 

EDIT: Well it turns out the interstage board had a shorted 2SK30 O grade jfet and was messing up the signal path when the volume is turn up with buzzing and with I.S engaged, a huge mono like sound.

Problem persisted after I re-worked the PSU board with replacing all diodes and changing the snubber capacitors for the diodes from 3300pf to 10nf which is the value usually used for capacitors in snubber positions. I hope the MKP10 is fine don't have any ceramic or polyesters on hand. 

Worked a treat after pulling out the boards one by one and the problem disappeared once the I.S board was removed. Now I can stop pulling my hair out.

 

Anyway for anybody who ever needs to replace the transformer in the 12S or 10S (ones with EI Core type xfmr) and want to retain the multi primary input voltage selector, the wire voltages are as follows in the diagram I made:

In reference to the picture I've posted few posts above that show my voltage selector from my 12S, the wires are interchangeable for the coloured wires below (if your xfmr is like mine).

Grey wire is usually grey.

Blue wire can be brown.

Black wire can be white.

Orange wire can be white.

Red wire can be yellow.

Brown wire can be red.

 

Edited February 21, 2016 by DefQon

[drmlyung]

On 2/15/2016 at 2:27 AM, spritzer said:

No idea really as my schematics are all packed up so I can't look at them.  You would need to ohm out the connections inside the plug and mate the socket to that. 

I wonder when will you unpack and be able to get your hands on the schematics again? I would very much like to have a copy to sort out my problematic 12S.

[Tinkerer]

1 minute ago, drmlyung said:

I wonder when will you unpack and be able to get your hands on the schematics again? I would very much like to have a copy to sort out my problematic 12S.

 

[spritzer]

Yup, that's it. 

[drmlyung]

Thank you both. Will start work on it tomorrow.

[spritzer]

Not sure where to post this but it might just as well be here, an update on the ongoing restoration of my SRA-7S.  This is more a of complete rebuild/redesign but I digress.  I bought the amp some 3 years ago for way too much and well...it was filthy and full of parts which were way past their use by date.  Also the transformer was 100V only so the only solution would be to remove everything and start from scratch.  Now I don't recommend doing this...like at all!!  The amp is about the same size as the SRM-323 but with way more crap crammed in there.  There are also some design issues which needed to fixed plus I needed to decide what to do with this thing.  Since vinyl is dead to me, the phono stage was of no use so I left it out.  That gave me enough space to fit a regulated PSU and a larger transformer.  Just barely though. 

First up was the main amplifier PCB, I stripped of everything but the tube sockets and in went new parts: 

It was littered in carbon comp resistors and paper caps so they all had to go.  Funny though that the values for each component are written in pencil on the back of the PCB. 

Now for the chassis and I decided that new RCA's were the only way to go plus is needed all new pots and switches. 

Stock values except for the filter section.  It was single ended in the stock design but here I used all four channels so the values were halved.  I'm replicating all of the features so the full 6 inputs are wired and even the mono switch.  Way too much work but there is also no rush. 

Now the front panel was a problem.  When it came here it was pretty disgusting.  Some 50+ years of dirt and grime so when I tried to clean it, all the labels washed off.  What you see here is in effect a reproduction of the orignal markings on the stock panel.  This time done by laser so it will last forever.  I went with the shorter switches for the power and mono/stereo for a sleaker look.  The knobs are all brand new but they are identical to the stock ones.  Sato still make them... 

[Kerry]

Sounds like a fun project.

Looking forward to seeing the final results

[DefQon]

Going for a full rebuild plus improvements. 

I'm interested in your faceplate cad/photoshop file as I need one done to my SRA-3S and the logo is a issue at the moment.

[spritzer]

Shoot me an email and I can give you the PDF.   

[Wes]

I have a SRA-12S.  I used it for years and one day, the dreaded 60hz hum.  I have an electronics background and as a hobby I work on tube equipment, but I am reluctant to work on this myself. It's a pretty special piece to me. Is this something that any competent professional tech can fix, or should I send it to a particular repair place?

Not sure on the condition of the headphones.  They look great but I haven't listened to them in 20 years.

 

[spritzer]

For hum I'd look at the rectifiers and caps. they are all probably well past their prime.  I would swap them all out regardless just to be on the safe side. 

[Wes]

Thank you.  I had suspected as much but my confidence repairing this piece is almost nil. It has tremendous sentimental value.  A question on the earspeakers.  As I rarely used them, and they weren't energized much, how would effect the condition of the diaphragm? They've spent the majority of their life in a bag in a drawer.  I'd never had cause to use headphones, until recently.

[spritzer]

It's hard to say really, the old Stax is hit and miss but if they were never exposed to direct sun and never arced then they should be just fine. 

[mingus]

Hello Gentlemen
I just bought a Stax SRA 12s which works extremely well except that it has a headphone channel which no longer works. On the other hand it needs a serious overhaul, and I am desperately looking for the service manual for this device so that I can do a serious overhaul.
I live in France and I looked everywhere for this document without result. While going through your discussion about this amp, I realized that some of you have it, but the download links are dead.
Can you please put them back online that would do me a great service and allow me to fix my Stax.
Thank you beforehand.

[mingus]

Here we are, I found it!!!!
A big thank you to bobkatz who was kind enough to send it to me.
If I can be of service to you in turn, it will be my pleasure.
See you soon....