Stax SRM1-MK2 Professional

[Equaliser]

I've had over 4 SRM1-MK2 Pro and Non Professionals come and go over the time (and an additional 3 other SRM1 MK2 P.P units) so these are not exactly new to me.

 

Cutting down on the literature and moving onto the important stuff. Ok other than 4 variations that I know of with these amps dating back from early 1980's to 1990's (so they are quite old and the design has undergone a few minor trimmings here and there). This one came to me with 2 missing 900vdc rated HV output devices and resistors drifting out of spec (operating on about 500vdc here so higher than your average tube amp), the SRM1 MK2 is a fully DC coupled, Class A solid state electrostatic amplifier. 

 

This one in particular had 2 missing Toshiba 2SC3148 for some odd reason when the amp arrived to me from Japan. I didn't bother testing the other 2 C3148's because I usually buy a bunch with spares to boot and change all of them unless the original measures equal or better. Seeing that 2 was already missing I assumed this was a previous unrepairable botch job by the owner or tech so I replaced all 4. Anybody who has worked on devices using obsolete Toshiba parts know how difficult it is to get ahold of them, especially the old HV rated parts and there are no real modern equivalents unless you re-design the output stage with a custom circuit fitted with different case styled devices. 

 

Anyway, the balance and dc offset was all over the place, they've been adjusted now from stator to ground and negative stator to positive stator appropriately for each channel. Two 910ohm input resistors measured only 31ohms (yes 31ohms), so I replaced them with Holco 1mohm metal resistors which does the job fine. PSU filter caps have been replaced, same with the coupling film caps from the original ceramics to 630v and 1600v rated Wima MKP10 and 4 category MKP's (usually I leave the ceramics as they measure fine but 2 of them caused major drifts while adjusting the balance). The normal bias 0.1uf coupling cap changed to Wima equivalents, left the pro-bias section alone as they didn't need to be changed, swapped out the old Toshiba 1kv standard rectifiers for ultra fast recovery rated equivalents and new HV devices. The variations of the amp (and it's different normal/pro-bias output is also dependent on the A, B and C serial number). These amps can go anywhere between $300-550 (depending on the condition and it being Pro or non-Pro).

 

One version of the schematic:

 

 

Anyway enough rabbling, some pictures:

 

 

 

 

 

 

 

They sound bloody superb now as I've had the amp run in all day long since 11am. 

 

More Stax pics coming when I can be bothered.

Edited August 3, 2015 by DefQon

[Addicted to music]

@

Here's mine, I still have mine, I need to spend some time on it to work out why the bias is drifting and a I balance issue!

http://www.stereo.net.au/forums/index.php?/topic/48986-stax-srm1-mk2-new-lease-of-life/

[Saxon Hall]

They look remarkably clean considering their age.

Well done

[ThirdDrawerDown]

Beautiful work!  I have a Stax that is only suitable for a "before" photo.  Very sad piece of kit, a world away from you've shown. 

[Addicted to music]

They look remarkably clean considering their age.

Well done

They are an incredibly well laid out, superbly built, look after them and it will service you well.

The other brand that is built like this is the early Naim amps

Today it goes without saying, the Sanders Magtech employ all the current best practice trends in electronic assembly.

[Equaliser]

@

Here's mine, I still have mine, I need to spend some time on it to work out why the bias is drifting and a I balance issue!

http://www.stereo.net.au/forums/index.php?/topic/48986-stax-srm1-mk2-new-lease-of-life/

 

You mean drifting dc offset and balance issue?

 

This amp had it as well (after the amp is powered on for about an hour) and I couldn't adjust the balance properly to 0vdc so closest I got it was around -0.171 which is still fine. Due to the unregulated power supply stage and fluctuating mains we have it is very hard to have it stable, especially with some resistors completely out of tolerance (and you can't measure them in the circuit as the trimpots adjust resistance affecting mostly everything else). As long as you don't have over a few (4-5vdc) volts on the balance and dc offset you're fine.

 

Adjusting balance is negative probe in -L(7'o clock position) or -R(6'o clock position) and red probe in +L(12'o clock position) or +(2'o clock position) and adjust the balance trimpots counter-clockwise (in tiny turns) to get it as close to 0vdc as possible, it will drift so its normal.

 

Adjust dc offset have the negative probe on the back ground post and red probe in the +L or +R and adjust the offset trimpots to as close to 0vdc as possible. It will drift so it's normal.  

 

Going to post some pics on the SRM1 MK1.

[rotary914]

Hello

 

I am new to this forum and I am hoping to find a schematic and bias measurements for a STAX SRM-1 MK2 amplifier

 

I saw a link in this forum that I clicked on, but I got no schematic

 

Thank You for your help