Jump to content

Repair and restoration of my STAX SRA-12S


Quad
 Share

Recommended Posts

Thanks for that. 

 

Here are some pics I took before disassembly of the amp of what I was referring to (I've changed the 2.2kohm resistor to 22kohm Kiwame's as I want pro-bias through this):

20150325_131942_HDR_zpsiysqv1er.jpg

 

What is the purpose of the 100uf 25v cap? Also looking at your old pics few pages back, is one end of the 220kohm resistor connected to one of the empty tabs (essentially GND) and the other end connected to where the 22kohm resistor is, which then another wire is feeding from the 22kohm point to a 5Mohm ballast which is connected to one of the bias pins on the output jack?

 

Where about's should the 1uf 1kv filter cap connect to? Can 10/100nf caps be used as well?

 

20150325_131937_zpsyd5xehan.jpg

 

 

20150325_131920_zpsjnwt0uts.jpg

 

So the round and RA-1 diodes sandwiched in between the ceramic film capacitors be replaced to UF4007?

Link to comment
Share on other sites

You can modify the stock bias supply too as it its run off the same line.  Replace all the resistors though, they are all passed it. 

 

I put the pro bias supply in the middle of the tags as they aren't connected to anything.  The filter cap goes between B+ and ground, from it goes a 22K resistor to a junction.  One 220K resistor goes from that to ground and also the 4M7 resistor to the output. 

 

The diodes should all be changed and the caps too. 

Link to comment
Share on other sites

You can modify the stock bias supply too as it its run off the same line. Replace all the resistors though, they are all passed it.

I put the pro bias supply in the middle of the tags as they aren't connected to anything. The filter cap goes between B+ and ground, from it goes a 22K resistor to a junction. One 220K resistor goes from that to ground and also the 4M7 resistor to the output.

The diodes should all be changed and the caps too.

So UF4007 can be used to replace all diodes including the funky looking ra-1 types on the ps board?

Link to comment
Share on other sites

  • 1 month later...

What a shit day has it been for me.

 

So all the parts and everything came and I rebuilt everything on this 12S except the phono boards and the 1/2 watt resistors on the output stage boards, wired everything properly. Trafo is wired up good and proper with the jumper set to 240v. Powered her on and noticed something unusual. Trafo which gets warm to hot-region on IDLE. Output stage boards are hardly putting off heat. Plug in headphones and a source, no sound unless I put it on full blast there is distortion and a miniscule amount of sound coming out.

 

Not good as I knew there was one thing wrong:

 

Trafo fucked.

 

Well so I powered it up, put DMM on VAC and measured the secondary windings to the psu board. Measurements should be 260 - 0 - 260 and 26- 0 - 26vac, so I measured mine. 

 

Nope not even fucking close. Measurements read something like 17-0-70 and so forth. What I also noticed is the trafo gives off this weird whooshing noise when the amplifier is on.

 

But the amplifier worked before I pulled it apart and the chassis (after it was cleaned) and the trafo (untouched and foam wrapped in a box) and now everything is soldered together and its not working.

 

I did replace the mains cable. The old cable had a red wire from AC cable to switch, green wire to ground and a black wire to fuse holder (fuse holder then goes to universal jumper for AC primary). Red, Black and green was the color code AC wiring for 80's Australia, after the 90's they changed it to Brown, Blue and Green/Yellow. Red is hot/live wire (brown) and black was neutral to fuse holder (blue) and green/yellow is ground. 

 

Any suggestions. And any recommendations on the trafo specs and exactly where can I buy a trafo that is identically rated to the current one in the Stax amp also with multi-tapped primary windings?

Edited by DefQon
Link to comment
Share on other sites

Disconnect everything and then wire up the transformer again.  I don't see how you could damage the transformer like that... 

 

Also, never power anything up without the first job being to test the voltages. 

 

Plenty of custom transformer options.  None will look like the stock one though. 

Link to comment
Share on other sites

Thats exactly what Iwas thinking its hard to kill a power trafo.

I think the trafo is out of phase or something which is burning up the primary windings.

Is live(hot) wire straight to switch or fuse holder and then neutral to switch? I think the mains wiring is messed up.

Link to comment
Share on other sites

Ok tested by isolating the secondary leads from the ps board. Trafo gave me completely off measurements. Should be 260 0 260, it gave me 80 50 190. 26 0 26 winding gave me 140 110 140. All on vac. Tried two dmms an itt mx52 and a old fluke gave same results.

What are my replacement transformer options with what specs like current rating for the secondary windings etc.

Link to comment
Share on other sites

Yep, black probe in the chassis ground holder on the amp, transformer is still secured to chassis just unsoldered all secondary taps to test each winding to see the voltage. 

 

The relay doesn't even engage possibly due to the completely off power supply. 

Edited by DefQon
Link to comment
Share on other sites

Best way to measure AC is reative to each other so for single tap, one end to the other.  CT to each end etc.  That's why you are getting results which make no sense. 

 

Also with everything off and not connected, measure the DCR of each of the windings.  That will tell you if you have internal shorts. 

Link to comment
Share on other sites

Ok

 

Ok bad news, tried to do the winding measurements, powered it up and tripped the fuse. So I put in another fuse this time magic smoke start sipping through the El core case, powered off immediately.

 

So the transformer is absolutely dead....what are my options.

Edited by DefQon
Link to comment
Share on other sites

But its bid only. Price will go up and up. Anyway placed a bid if I'm lucky. The AUD is so low I've stopped buying things over $100 from the US due to the currency depreciation. It's coming up as $340 at the moment including the $89AUD shipping. This auction will end at about $400US. About $600AUD after shipping.

 

Anyway prefer to restore my own 12S then buy another one and waste more money fixing that one. 

Link to comment
Share on other sites

Spritzer any idea on the current rating for the secondary windings? Shouldn't be too much since the amp runs with 1A.

 

There is no spec available for these transformers, not even in the service manual.  I did replace one in a 100V (SRA-10S) unit and used these specs, 520VCT-100ma, 52VCT-250mA

Link to comment
Share on other sites

  • 1 month later...

I have a problem with my 12S.

It's a restored unit with a Pro bias made by spritzer.

But one channel is dead, the problem comes from one of the board with 2SC1167 transistor.

The amp took time to die. First, there was like an interference noise on the left side, and sometimes it disappeared. After a week of use, the channel stop working and trade the intereference noise for a buzzing noise with a very low sound.

 

I want to know which component is likely to die on that board?

I don't have the knowledge to detect a defective component. What step I should follow?.

 

Thank you  

Edited by charlo89
Link to comment
Share on other sites

Are you sure that the channel is dead and it is not a switch issue?  Measure the top of the large resistors for DC voltage.  You should have roughly 0VDC between them and each one relative to ground is +290VDC. 

 

Replacement parts for the output devices are all but impossible to source.  You need low Cob which is the truly hard part. 

Link to comment
Share on other sites

I was going to suggest switching the boards to see if the problem goes to the other side. Have checked the solder joints? The 5 watt Riken 50kohm resistors get stinking hot and on my unit the solder was completely fractured and broken which gave the amp (before restoration) a wishy washy static noise. A solder reflow fixed the problem.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...