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Stax SRM-1/Mk2 Re-cap - Page 2

post #16 of 46
Thread Starter 

Alright the operation is complete and the patient lives. I used the capacitors mentioned in the previous two posts.

I ordered them from mouser and it cost about $25 shipped. 

 

The four power supply capacitors removed. as seen from the top back. 

 

 

HV Capacitors.

The new capacitor is on the right. It is significantly smaller in both height and diameter.

I had to bend the pins outward to get them to fit on the board but that was easy and caused no issues that I can see.

 

 

The four new power supply capacitors installed in the SRM-1/MK-2

If you look closely you can see the much larger white outlines where the old capacitors used to rest 

 

 

The next step is setting the balance and offset. I'm letting the unit warm up for a couple of hours so it can be done right.

Here is what spritzer has said on the subject in the past: 

 

Post A:

There should be two pots per channel, one marked balance and the other one marked offset.  To adjust them you need a multimeter set to DC volts and you insert the probes into the Stax socket.  To adjust the balance you put probes between the + and - outputs for that channel and adjust for 0VDC.  Now to adjust the offset remove the probe from the - output and connect it to ground (i.e. anywhere on the chassis, including the grounding post in the back) and adjust for 0VDC.  Now repeat for the other channel.  While you can do this with the amp cold it is best to do it when it is running at its regular operating temperature as heat is a factor here.  The Stax pinout can be found all over the place. 
 
Post B:
To set the DC offset you first set the meter to 100V or more DC.  Then you put the red and black probes in the+ and - sockets for each channel and adjust the balance pot for that channel (which is marked on the PCB) for 0VDC.  Once that is done then you remove the black probe from the - output and connect it to ground.  Now you adjust the offset pot for 0VDC as well. 

Stax Pinout: (Found somewhere on Head-Fi)

 


Edited by spaceace1014 - 2/14/13 at 6:15pm
post #17 of 46

Great work! Can't wait to see how the rest goes.

post #18 of 46
Thread Starter 

The Balance and offset was a cakewalk. I used a small electronics screw driver to turn the little white pots. They were held in place at first by what seemed to be a small transparent sticker used to keep them from turning. Using the screw driver I just turned a little bit harder and they became free. I did this to each one before starting and after their initial resistance they turned easily. 

 

Strangely illogical.

 

 

Adjusting the Right channel balance

 

 

 

One thing I noticed is that the voltage kept moving +/- .5VDC for both balance and offset in both channels. I assume unless someone says otherwise that this is a result of the power supply design and less than perfect power and is normal. The unit is now within less than a volt of 0 plus or minus which should be pretty decent in a 580v system. 

Everything sounds good.

post #19 of 46
Thread Starter 

So, with that done what should be next from a sound quality perspective? 

post #20 of 46

.5VDC is really nothing to worry about. Are you planning on replacing the smaller caps/diodes as well?

 

Also, it seems like your amp is the same design as mine and I wouldn't mind doing a recapping of my own. Although the pro bias circuit on mine is modified compared to yours (I'll have to ask the original owner of the amp exactly what was done). Anyway I took a few pics of it here: http://imgur.com/a/zeHsp

post #21 of 46
Thread Starter 
Quote:
Originally Posted by loligagger View Post

.5VDC is really nothing to worry about. Are you planning on replacing the smaller caps/diodes as well?

 

Also, it seems like your amp is the same design as mine and I wouldn't mind doing a recapping of my own. Although the pro bias circuit on mine is modified compared to yours (I'll have to ask the original owner of the amp exactly what was done). Anyway I took a few pics of it here: http://imgur.com/a/zeHsp

 

That's what I'm wondering. I don't know what I should do next or where to where to start on whatever that might be. Diodes, film caps, transistors, resistors, stepped attenuator...? 

Those look like the original caps just looking at their size. Just get the smaller 35v cap from post #13 and the three 400v caps from post #14 and its an easy swap. I tried to be as explicit as possible with the posts here so they could be followed later. Let me know if something isn't clear so that I can change it. 


Edited by spaceace1014 - 2/15/13 at 12:15am
post #22 of 46

The small drifts in voltage are normal for any amp but the unregulated power supply in the Stax amps makes it worse. 

 

As for other mods, there is nothing meaningful you can do without replacing the chassis, transformer etc. 

Quote:
Originally Posted by loligagger View Post

.5VDC is really nothing to worry about. Are you planning on replacing the smaller caps/diodes as well?

 

Also, it seems like your amp is the same design as mine and I wouldn't mind doing a recapping of my own. Although the pro bias circuit on mine is modified compared to yours (I'll have to ask the original owner of the amp exactly what was done). Anyway I took a few pics of it here: http://imgur.com/a/zeHsp

 

I truly don't get what was the plan with that "Pro bias" supply....  confused.gif  Google "SRM-1 Mk2 schematic" and there you can find the actual values to put in those spots to make it Pro bias. 

post #23 of 46
Thread Starter 
Quote:
Originally Posted by spritzer View Post 

 

As for other mods, there is nothing meaningful you can do without replacing the chassis, transformer etc.  

 

I'll leave it alone then. Thank you again for the all help.

 

That was fun. Now I want another project. I want to try building an eXStatA...

post #24 of 46

The Exstata is just a messed up copy of the SRM-1 Mk2 with the original being far superior in every way.  Get some KGSSHV boards and build that instead. 

post #25 of 46
Thread Starter 

There are two reasons that I want to build an amplifier. The first is because working with electronics on this scale is fun and the second one is to learn more about electronics and circuits in general. I want to build an electrostatic amp because its something I would use which makes it easy for me to set aside significant time and money to build it. Having said all of that while the most "dangerous" thing I've ever done electrically was replace the HV Caps on the amp in this thread the most advanced thing I ever built was a $30 FM radio kit from radio shack. I know how to solder to some degree, I know which direction to put a capacitor in, and I know theoretically how to read a resistor code but that's about as deep as I've been. If after saying that you don't think its a complete lunatics errand to build the KGSSHV I'll begin the research... and get something better than a pencil soldering iron. 

I would love to have a KGSSHV by the way. I've heard great things about it and I'd rather build something better. I had already heard mixed things about the eXStatA but it seemed significantly more straightforward especially given my general inexperience.


Edited by spaceace1014 - 2/16/13 at 3:50pm
post #26 of 46

I'm a certified DIY addict so no need to rationalize it to me.  redface.gif  We made the KGSSHV as easy to build as is possible with something so complicated but it does take some skill to build it. 

 

I've had a new poor mans electrostatic amp on the books for a while now and I may have finally settled on a circuit that anybody can build.  It will be a tube amp though as they are just easier to deal with and high voltage transistors are getting thinner on the ground. 

post #27 of 46
Thread Starter 

Research it is :)

post #28 of 46

Just did the balance and offset on the SRM-Monitor as above - one was out by about 10V - all within +/- 0.5V now. Sounding great. Didn't do a before and after listen however. It's a bit tricky with the balance and offset labels interchanged LOL.

post #29 of 46

Have you replaced the capacitors in the SRM-Monitor yet, John?  It would be well worth doing and you can even upgrade the size a bit to boost performance. 

post #30 of 46
Quote:

Originally Posted by spritzer View Post

 

I truly don't get what was the plan with that "Pro bias" supply....  confused.gif  Google "SRM-1 Mk2 schematic" and there you can find the actual values to put in those spots to make it Pro bias. 

 

Apparently it was to take the pro bias voltage off the trafo (and thus to compensate for that). I could ask for more details / take more pics if you want.

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