Stax SRA-7S

[spritzer]

I figured something this rare deserved a thread  of its own.    Let me introduce the Stax SRA-7S, integrated amp for earspeakers.  This was introduced in 1966 for the SR-1 and remained in production into the 70's.  With Stax we never really know when they stopped making something but given it's all tube design, it was dead by the mid 70's. 

 

 

This one was missing just one knob but amazingly they are still in current production.  A full set has already been sourced... 

 

 

Those RCA's really need to go but pretty clean over all.  Pretty much unmolested but somebody decided to remove the fuse holder.  Not a good sign... 

 

 

Amp and phono stage on separate PCB's.  All original parts and all the tubes are original.  Hell the entire amp is a Matsushita advert on the inside...  All of this will be stripped though.  Not a chance components nearly 50 years old are even close to spec.  Interesting tube choice though, ECC83 for the phono stage but the Stax amp uses a pair of 6AU6's, an 12AT7 and 12BH7's for the output. 

 

 

Power supply is as simple as it can be, two diodes feeding stacked caps to give +300/+600V.  The two chassis mounted resistors are 100R so probably there to generate a center tap for the heater but I haven't retraced it yet. 

 

 

Finally the back of the front panel.  Not sure how many of the features I'm going to recreate as pretty much all of this will be scrapped.  I know I'm the collectors worst nightmare but I'm far more interested in making something better than it ever was than keeping broken parts and useless features in there. 

[mypasswordis]

Looks to be in good condition for something so old, looking forward to see how the refurb progresses. Too bad there's no space to fit a regulated PS in there. Is that green circular component a thermistor? Wonder why they decided to coil the leads like that, whatever it is, and for the... diode? too. Speaking of which, is it only half wave rectified? Edit: Nvm about the last part, missed it first time reading your post.

Edited March 28, 2014 by mypasswordis

[spritzer]

No chance on a regulated PSU but I will probably make a small PCB for the PSU as it will be so much neater.  This way I can also upgrade to 220uf/350V or something like that and have a bias supply built in. 

 

The green thing is SD-1, literally Silicon Diode-1.  Half wave rectified as diodes weren't cheap back in the day... 

 

Just tried to clean the front panel and some of the lettering came off so I'll have to clone it.  Find a suitable font, copy all the markings and make something that looks similar. 

[wink]

 

Wonder why they decided to coil the leads like that,

Strain relief.

 

Why does this fill me with nostalgia?

 

I can't number the old radios and B&W tv's, etc I've repaired back in my younger days that had that charming coating of dust and detritus.

It made me very grateful for the invention of the vacuum cleaner.

Edited March 28, 2014 by wink

[spritzer]

It is a lot of fun to repair/refurbish this old stuff, dirt and grime included.  This one was filthy though, here is what the PCB's looked after I had cleaned them twice:

 

 

Also fun to see how things have changed.  All the electrolytics are odd values such as 50uf and the band indicated +, not -.  Also all those 10% resistors must make the magic "vintage tone"...

 

 

Started to repopulate.  Was going to go with some fancy caps but I found a bag of these so I didn't bother to wait for new ones.  The tube sockets are in amazing condition so no need to tamper with them.  One cool thing, the values for the PCB are written by hand on the underside of the PCB with some notes in Japanese. 

 

 

What the chassis looks like now.  Might install an IEC input and I also might replace the transformer as it's just 100V-117V and the PSU is actually a voltage doubler off a single 220V winding.  One of the Hammond units fits in there and would give me a CT winding so a proper stacked PSU.  Would just have to modify one of my old PCB's slightly and it would fit.  Cost is raising quickly though and I still have to figure out what to do with the input connectors, volume/balance controls (TKD or ALPS) and the input selector (Grayhill?).  Already bought new slide switches but the mono/stereo makes no sense so I'll leave it out.  The filter just messes with the earspeaker output so I'll leave it out as well. 

 

 

Here is the current state of the front panel.  As I expected the silk screening didn't stand up to my degrease efforts so it's needs to be redone.  Only problem is that I have no idea what font that it.  Probably something custom but it appears to be as wide as it is tall. 

 

 

A close up... 

 

 

I spent an hour this morning drawing this up.  Biggest problem was the Stax logo which had to be redrawn from a picture.  So if anybody needs to have the old logo as a PSD file it's available...   

[nikongod]

Could you post a schematic of the "filter" function?

[spritzer]

I didn't draw it up but I have the full schematic of the amp sitting in Japan.  I'll post it when it gets here.

[chinsettawong]

Will you redo the silkscreening of all the letters in the front panel?  

 

Looking forward to seeing the finished amp.  

 

Wachara C.

[DefQon]

What a nice piece of history.

 

Not as dirty as that SRA-3S I posted on the other site while ago but too bad your Stax logo and lettering came off as well. I might need the psd files.

[spritzer]

I'll wash the front panel clean and then have the new graphics laser etched which will not wash off... ever!!   

 

I'd just have thrown that SRA-3S in a bucket of water.  Been there though as one of my SRA-12S amps was filthy to say the least. 

[DefQon]

Is there any B+ lines to add in a pro-bias output?

[spritzer]

The output tubes sit at +600V so it's easy to generate Pro bias.  Not that it isn't always easy to generate it... 

[DefQon]

Sounds simple enough, what other rare Stax amp you might be after next time? 

[spritzer]

Ehhh... not sure there is anything I have left.  I need to fix up my SRA-14S and sell it (need a new transformer like they all will) but nothing else I'd want.  If I find a broken T2 I might buy it just for fun. 

[drmlyung]

Hi, Spritzer,

 

Any update for your project?

And any chance to post the schematic of the 7s?

[spritzer]

I did post some updates somewhere but I might as well copy them here:

Not much has happened since I posted that but I'll work on the PSU at some point and then I can do final assembly.  The schematic is here:

[JimL]

Now, that's an interesting design!  If I were rebuilding it and wanted to keep the phono preamp I'd probably hardwire the phono input to the phono board, hardwire the phono EQ in place and convert the tape and mic inputs to more line inputs, but that's just me.  BTW, are those green resistors Kiwame (Koa)?

[spritzer]

The phono stage is of no use to me so it was scrapped and it will be used line level only.  Pretty much had to do it so I could fit the new transformer and PSU board.  I'll run the front at at 300V and the output at 600V as it is simply easier with the regulated PSU.  I replaced everything so the new output filter network will be fun to test out as I converted to balanced. 

Those are KOA SPR2 resistors. 

[gepardcv]

Did you ever find a vector or even raster file of the old Stax logo? Must admit I prefer it to the current one.

[spritzer]

I just made my own.  Photographed the logo in an old manual and then cleaned it up. 

Some progress and now it is ready for testing.  Still missing the normal bias supply but that is trivial at best: 

It is a very, very tight fit and would be an absolute dog to take apart again...

[luvdunhill]

You need to lay off the control key! Or is that required for all those weird Icelandic characters?

[spritzer]

Nahh but these keyboards always go like this after a while.  Still love them though and it is the only keyboard I want to use. 

[zhetanay]

On 3/29/2014 at 4:41 PM, spritzer said:

It is a lot of fun to repair/refurbish this old stuff, dirt and grime included.  This one was filthy though, here is what the PCB's looked after I had cleaned them twice:

 

 

Also fun to see how things have changed.  All the electrolytics are odd values such as 50uf and the band indicated +, not -.  Also all those 10% resistors must make the magic "vintage tone"...

 

 

Started to repopulate.  Was going to go with some fancy caps but I found a bag of these so I didn't bother to wait for new ones.  The tube sockets are in amazing condition so no need to tamper with them.  One cool thing, the values for the PCB are written by hand on the underside of the PCB with some notes in Japanese. 

 

 

What the chassis looks like now.  Might install an IEC input and I also might replace the transformer as it's just 100V-117V and the PSU is actually a voltage doubler off a single 220V winding.  One of the Hammond units fits in there and would give me a CT winding so a proper stacked PSU.  Would just have to modify one of my old PCB's slightly and it would fit.  Cost is raising quickly though and I still have to figure out what to do with the input connectors, volume/balance controls (TKD or ALPS) and the input selector (Grayhill?).  Already bought new slide switches but the mono/stereo makes no sense so I'll leave it out.  The filter just messes with the earspeaker output so I'll leave it out as well. 

 

 

Here is the current state of the front panel.  As I expected the silk screening didn't stand up to my degrease efforts so it's needs to be redone.  Only problem is that I have no idea what font that it.  Probably something custom but it appears to be as wide as it is tall. 

 

 

A close up... 

 

 

I spent an hour this morning drawing this up.  Biggest problem was the Stax logo which had to be redrawn from a picture.  So if anybody needs to have the old logo as a PSD file it's available...   

Hello, please, do you still have the Stax psb file.

[spritzer]

No, I don't. 

[Bingturong]

Sorry for necroing this thread but I got an SRA-8S, pretty much all original except for two switched out knobs. I plan on restoring it but Im stuck on a rather simple hurdle.

 

The fuse, 5mmx20mm nor 6mmx30mm would fit. So what size fuses do these things use? It looks like a 6mmx20mm but as far as I can find, there is none