The ultimate DIY? A Stax SRM-T2!

[JoaMat]

Opened up my modified T2 from 2014, same circuit as James’ T2 but smaller board. All 140k resistors look as new. No decoloration at all but they are standing to save space. Maybe they are better cooled but it’s very crowded so I’m not sure about the cooling effect. A year ago I reduced the high voltages by 100V so batteries at 640V but before that it was “full” voltages.

With three 140k resistors you get 1.7mA through lt1021-10 and I guess you can lower that to 1.2 – 1.3mA. 200k resistors gives 1.2mA and 300mW. Going with lt1021-7 as Kerry does you can reduce current further.

[Craig Sawyers]

It was not one of Quad's finest moments that is for sure. And on one channel only, an electrolytic is right above a power resistor, and cooks.

So remedial action is to replace the two 3.3k resistors with something that can actually dissipate 0.35W, and space above the board anyway. And remove the dead electrolytics and mount replacements on the other side of the board away from the heat.

Then remove the crowbars and fit a half way decent DC offset disconnect. Like this https://neurochrome.com/products/guardian-86 or this http://www.velleman.co.uk/contents/en-uk/p193_m4701a.html or this http://www.signaltransfer.freeuk.com/protect.htm just for starters.

[JoaMat]

Several years ago I changed all LEDs (only three per channel compared to 25 in the original DIY T2) in my modified T2 to LM336 2.5V, a voltage reference device in TO-92 case (I also changed resistor values to keep desired current of the CCS).

Pros LED/BJT see Lefferts_LED_Used_as_Voltage_Reference_ED_Feb_15_1975.pdf. Pros LM336/BLT see Sources_101_P1.pdf and Sources_101_P2.pdf.

Overall, at least in theory, I think LM336/BJT wins.

My version of LM336 have operational temperature up to +70 degrees Celsius and I don’t think I exceed that.

Edited May 3, 2021 by JoaMat

[starcat]

Soldered the last cs3675 and thoroughly cleaned the PSU board. And of course a single .1uF 50V is missing and Mouser again has long lead time ot it. 

 

[JoaMat]

Aren’t there other similar .1uF/50V in stock at Mouser? For instance 80-C320C104J5R7301, but there are cheaper.

[starcat]

Finished the PSU and turned on the LV part only, first the trafo, measured, connected the secondary 30VAC winding to the board, measured 12VDC out perfectly. I was about to measure for the relais as I could not hear it clicking as I noticed both C21/C22 getting extremely hot. At this time one of them smoked. No idea how I came up with but those are 25V 4700uF United Chemicon. I think 25V might be on the lower side and 50V should be more appropriate. Any ideas? 

Going to swap them with Nichicon 50VDC 4700uF.

Specs for the LV transformer including 30V-0-30V secondary winding were taken from this page: http://tee8tee4388.blogspot.com/2013/07/t2-diy.html

Edited June 5, 2021 by starcat

[starcat]

Looks like the 30-0-30 spec'ed secondary winding on http://tee8tee4388.blogspot.com/2013/07/t2-diy.html is wrong and has produced overvoltage smoking the 25V cap. 

[Pars]

Haven't looked at the schematic, but 30V secondary would produce 30 * 1.414 = 42.4 V raw DC, less the rectifier drop (1.4V for full wave). So if the caps are right across that, 25V isn't going to live long.

Looked at the schematic, and the caps are right across the secondaries after the bridge. A 30-0-30 seems rather excessive for a +/- 12V supply. 15-0-15 would have been a better choice.

Edited June 5, 2021 by Pars

[starcat]

Exactly! 

And it doesn't end here... Spec'ed is on the same page for the HV transformers 

2pcs of: Pri: 110/220v Sec: 475v/150mA | 285v/60mA | 75v/60mA

where the PCB says 485-CT-485, i.e. CT. 

With other words, I sadly need a full set of new transformers... 

Edited June 5, 2021 by starcat

[G600]

Made the same mistake long ago, in this precise place.

I had to add a small toroid because I ordered the wrong secondaries in my custom toroid.

It's 30 VCT,or 2*15 V

 

Just replace those 2 caps along with the V regs, and you're back on track

Edited June 6, 2021 by G600

[starcat]

But 30-0-30 spec'ed secondary is not the same as 2*15. It should have been spec'ed 15-0-15 unless this is different in the US. Attached what's printed on the PCB as well: CT-AC30-AC30.

Strangely enough the output caps C23/C24 are 50V but the input C21/C22 are 25V only (again according to silk layer on the PCB). 

Edited June 6, 2021 by starcat

[starcat]

Looks like the CT on the HV is optional. 

Looking through the old threads 475V without CT is spec'ed as well. 

So, luckily no new HV transformers are needed, only one for the LV.

Edited June 6, 2021 by starcat

[Beefy]

4 hours ago, starcat said:

 

Might just want to check your soldering on that resistor to the right of the terminal block, the solder hasn't filled in the through plating at what looks like the grounded end. [EDIT] Although looking at it now on desktop instead of mobile, I honestly can't tell if it isn't filled, or if it is just a trick of the light.

 

Probably not a big deal in the grand scheme of things, but probably worth checking and reflowing if necessary.

Edited June 6, 2021 by Beefy

[starcat]

24 minutes ago, Beefy said:

I honestly can't tell if it isn't filled, or if it is just a trick of the light.

Just a light catch. Appreciate the hint! 

[starcat]

Ordered the new LV transformer. Double insulation between the secondary windings as Craig has suggested earlier and 15-0-15 for the 30VCT winding. 

Edited June 7, 2021 by starcat

[Rinat]

Hi all. I've found that I can't buy 2SA1486 transistors ( 

Could they be replaced with 2SA1413 ? I don't see any differences between them in datasheets except a lower constant power dissipation: 10W instead of 15W. 

Maybe I missed something  important....

[JoaMat]

Isn’t 2SA1413 a smd TO-252 something? Looking at picture below it seems Stax used something like that in the output current sources in SRM-T2.

You might consider KSA1156 for 2SA1486 on heat sink and in batteries. Reduce high voltages to +/-400V and you can use KSA1156 in the output current sources as well. As I see it reducing the voltage is beneficial without drawbacks.

I’ve used STN9360 in batteries and even in output current sources. Bend outer pads slightly outwards to fit holes and solder them in standing. In current sources they will idle at 1/4W.

[Rinat]

2 hours ago, JoaMat said:

Isn’t 2SA1413 a smd TO-252 something? Looking at picture below it seems Stax used something like that in the output current sources in SRM-T2.

I've found that 2SA1413 is the 2SA1486 in a D-PAK TO-252 case. 

2 hours ago, JoaMat said:

I’ve used STN9360 in batteries and even in output current sources. Bend outer pads slightly outwards to fit holes and solder them in standing. In current sources they will idle at 1/4W.

Thanks!! 

Ok, my new plan: 

1) Try to make an adapter for 2sa1413 as the most authentic option. 

2) If the first case would not successful for some reasons, I will try STN9360, thank you, JoaMat. 

Reducing voltage is the last option for me. 🙂

 

I want to make initially the most authentic T2 and only after that I would make another T2 variations. For example I would like to completely change all CCSs in favour  to higher internal options, modern components, etc...

🙂

 

Edited June 14, 2021 by Rinat

[Kerry]

At full voltages you can use K2SA1156 (as JoMat suggested) for Q1, Q2, Q3, Q28 & Q29.  It's what I've been using in my builds and is a good substitution.

I also use STN9360 as a replacement in some areas, but I've redesigned the boards specifically for them.  I don't recall which of the remaining 2SA1486 (batteries & CCS) are mounted on the heat sinks, but if they aren't, it seems like you could use 2SA1413 though I never have.

[Rinat]

Thanks! 

I think I can use 2SA1413 everywhere with a proper heatsink. It could dissipate up to 10W with an "infinite heatsink" by datasheet. 

I can't find what 1486 dissipates more than 10W. 

But I am going to buy STN9360 and 2SA1156 as well just in case. 

Edited June 14, 2021 by Rinat

[starcat]

Hi Kerry, can't the 1156 be used as a 1486 substitute everywhere unless mounted on a heatsink? 

Need to check, might be that Q1, Q2, Q3, Q28 & Q29 is "everywhere" not on a heatsink 🙂

Edited June 14, 2021 by starcat

[Kerry]

Back in the day (for various builds) we used 2SA1156 (-400V) and 2SA1486 (-600V) pretty interchangeably provided the voltages were in spec. K2SA1156 is a direct replacement for 2SA1156.

I would recommend using K2SA1156 where it works.  It is proven and better not to jury-rig something if not needed.

Edited June 14, 2021 by Kerry

[Kerry]

3 minutes ago, starcat said:

Hi Kerry, can't the 1156 be used as a 1486 substitute everywhere unless mounted on a heatsink? 

Need to check, might be that Q1, Q2, Q3, Q28 & Q29 is "everywhere" not on a heatsink 🙂

No.  The CCS has the potential to exceed the -400V rating of the 1156.  Bad things could happen if it does.  The batteries might be possible though I don't recommend it.  It's cutting it close if something goes wrong or while you are doing the initial adjustment on the batteries.

I'm pretty sure Q1, Q2, Q3, Q28 & Q29 are on the heat sinks.  I'm confused by your statement above.  

[GeorgeP]

39 minutes ago, starcat said:

Hi Kerry, can't the 1156 be used as a 1486 substitute everywhere unless mounted on a heatsink? 

Need to check, might be that Q1, Q2, Q3, Q28 & Q29 is "everywhere" not on a heatsink 🙂

use 1156 on the heatsinks, 1486 (if you have them) everywhere else.

[Craig Sawyers]

Good luck sourcing KSA1156 unless you have some already lurking.

Looks like various dates in 2022 for next delivery.