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The ultimate DIY? A Stax SRM-T2!


spritzer
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I got confused by the transformer center tap and the instruction from SummR and the marking on the board (Tran’s board).  It marks 30-30-CT, and so when I measured the transformer, the green and violet output was 37.82V, the violet+white is 18.8V and green+white is 17.6V.  So I figured the 30-30-CT would go Violet-Green-White ?  However Richard told me Violet is the Center Tap ?

 

So I fired my PSU on with Violet-green-white, and checking LV before the HV click on, my +12 is perfect....however my -12 is shooting -16.5V ? I replaced 7912cv and still the problem persist. Anyone have any insight ? Thanks

 

 

Edited by Whitigir
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On 10/18/2017 at 5:32 PM, mwl168 said:

I recall these 78xx regulators some times overshoot without load. Maybe try putting a load on the output and re-measure?

 

On 10/18/2017 at 5:33 PM, spritzer said:

Yeah, the ST ones certainly overshot with no load. 

I see, and thank you so very much for the quick respond and the confirmations.  This explains the 7812 which supplies the relay load is regulating 12.1V and then drifting to 12.65V and down to 12.16V again

 

I fired her up again and measured the HV together with the Bias, and ignore the -16.5V as I have no load to put it on.

 

+12: 12.01-12.1V

-12: -16.75V

Low Bias: 276

High Bias:: 565

+500: +496V

-560: -555V

-260: -242V

-500: -494V

I also tapped into the Relay supply for the Led indicator to avoid tapping into the regulated from the output....but I guess it doesn’t matter since the LT7912 tends to overshoot without loads ?

the Led is only an indicator and not a functioning switch :D

 

 

 

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Edited by Whitigir
Re-done wires to minimize capacitances
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Does the LED supposed to be flashing together with the filaments on e88 ? When I tried turning it on and the low voltage kicked in, the led are flashing and the filaments too, but no smoke or anything.  So I turned it off before HV kick in as I want to make sure.  Also, under load, now the negative 12 is reading -11.96V

Edited by Whitigir
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1 hour ago, GeorgeP said:

Do you mean the LEDs on the amp board? In any case that should not be happening. Only the filaments should be on (constantly), and not flashing. 

Yes, the filaments on J&J e88 is flashing and the led does too, the only thing burn constantly is el34 filaments.  This is first turned on without HV kicked in.  Is this because the tubes ? Or did SummR messed up my Fillament amperage Details again

Edited by Whitigir
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so after thinking about it a while, something is wrong with the hv delay circuit which turned on with the tubes cold. this has to be fixed first.

when the tubes are cold, the output voltage is going to go to +500v, and thru the 100k feedback resistor is going to put 500v on the cathode of the input tube. With the filament of the input tube grounded, there is 500v cathode to filament.  Guaranteed to cause a light show and permanently trash the input tubes.

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Thanks you Kevin, it was sparkling and Led was blinking.  So now, back to my little delay Circuit. After pulling many active parts, and ended up checking out “working”, but replaced anyways. I remembered that I hooked up the front Led power switch into the coil supply with a resistor before (45 seconds delay), and then I became a smartass and took the resistor out, just leaving the Led directly Tap into the coil +12V and Ground.......didn’t even check it as I overlooked it as it wouldn’t affect anything and forgot about it.  Then I hooked up the amp board, and HV came in immediately to result in this Sparkling crazy light show.

My apologies to Kevin, Soren, George that my dumb mistake happened and took up your time.  Lesson learned.

1/ Defintely don’t hook up the LED (any kind) directly into the Coil Supply.  The regulator won’t work without load anyways, so better hook LED into the 12V output

2/ Defintely Have to have the Delay Circuit working before hooking up your amp board.......otherwise, the freaky light show happen, cooking tubes, and possibly giving you a heart attacks and possibly collateral damages.  I cross my fingers that nothing is wrong as I measured and read around on board, and they turned out “ok”.....until next power up.

 

* for all that said * my PSU is working correctly once again, relay clicked on at 60 seconds timed.  Though strangely my High Bias is at 563 and low Bias is at 273 which is lower than last time measured.

the 7912 definitely overshoot the regulated negative rail without anything hooked up, it read -16.40.  Now, when hooking LED into it, it read -11.96V

Edited by Whitigir
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4 hours ago, Whitigir said:

Thanks you Kevin, it was sparkling and Led was blinking.  So now, back to my little delay Circuit. After pulling many active parts, and ended up checking out “working”, but replaced anyways. I remembered that I hooked up the front Led power switch into the coil supply with a resistor before (45 seconds delay), and then I became a smartass and took the resistor out, just leaving the Led directly Tap into the coil +12V and Ground.......didn’t even check it as I overlooked it as it wouldn’t affect anything and forgot about it.  Then I hooked up the amp board, and HV came in immediately to result in this Sparkling crazy light show.

My apologies to Kevin, Soren, George that my dumb mistake happened and took up your time.  Lesson learned.

1/ Defintely don’t hook up the LED (any kind) directly into the Coil Supply.  The regulator won’t work without load anyways, so better hook LED into the 12V output

2/ Defintely Have to have the Delay Circuit working before hooking up your amp board.......otherwise, the freaky light show happen, cooking tubes, and possibly giving you a heart attacks and possibly collateral damages.  I cross my fingers that nothing is wrong as I measured and read around on board, and they turned out “ok”.....until next power up.

 

* for all that said * my PSU is working correctly once again, relay clicked on at 60 seconds timed.  Though strangely my High Bias is at 563 and low Bias is at 273 which is lower than last time measured.

the 7912 definitely overshoot the regulated negative rail without anything hooked up, it read -16.40.  Now, when hooking LED into it, it read -11.96V

I'm kinda surprise with your posts regarding "purity of the build", that you went that route to begin with. Glad it is sorted, as the T2 seems to be a well documented build regarding "Do.s/Dont's).

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5 hours ago, Skooby said:

Thanks for this head-up...

"Coil supply" = filament AC supply?  Keep all filament supplies floated, no ground connection/leave center tap open?

Wonder it's worth to swap 78xx/79xx with TPS7A470x/330x?       

The relay coil is running 12V DC into ground.  If you want to run any LED indicator, tap directly into the regulated supply +12 and -12 output.

4 hours ago, Pars said:

I'm kinda surprise with your posts regarding "purity of the build", that you went that route to begin with. Glad it is sorted, as the T2 seems to be a well documented build regarding "Do.s/Dont's).

Luckily it was not “pure catastrophic”.

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So, my active battery on one channel is perfectly responsive.  I could adjust it from 700 to 740+ with every turn on 10k pot.  However, the other channel has 1 stuck battery, no pot turning can affect it at all.  All LED in the battery is on, but not the output 4 LED, the other LED out of the battery are dim.  If I adjust the one responsive battery on that channel to 740, the stuck battery is at 780.  Or if I turn the other one to 699, the stuck one goes to 740.

it seems the stuck battery is 40V off and away from the responsive battery.  What do I look for :(

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I was thinking that dialing while measuring the 22k resistor in the battery was just another way to do it ?

 

I tried to set 2k pot in the mid value of 1k before soldering on, and then adjusting to 740V with 10k pot.  This way I was able to set all 3 batteries, and the Left channel is fully lit up.  Only the right channel that has one battery refuse to respond regardless of what I do.  Unless I turn the pot on the battery next to it, then it will be affected together.  This is the Channel I had the input tube arced very sparkly before.  I am trying to chase down collateral somewhere, but it seems I am chasing a ghost ATM

The stuck battery differences to the one responsive one is around 40V.  

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So, after thinking about why arcing happened for a while and sweating my brain out for no reason.  Pulling parts, and measuring with all kind of tests, I was chasing a ghost.  Thanks to George and I was getting the battery responding to reading the 22k resistor.  It was simply 8.5V to 9.5V prior, and now 10k pot turned it down to 6.5V, then the other battery is the same reading across 22k.  The output LED clicked on.

 

Now, another ghost appeared, the 6CA7 would arc when HV is kicked on, and will only continue to flicker together with it LED after the HV is off for another second :(.  It appeared that this Tube-arcing problems persists, and has Now attached itself to the 6CA7

black square is where the 6ca7 would arc and the led would be blinking, and Red square is the stuck battery.  

*there is 1 Led now on the other side of the board due to pad fell off*

49870547-ECB2-4601-B6DA-FC9B1CC677CD.jpeg

 

The method of dialing 2k and 10k pot to mid value prior to soldering, then using 10k pot to dial in 740V was working.  On my working channel, the 22k resistor measured 6.85V which dialed into 740V.  However, on the stuck battery, the 22k resistor had 8.5V while the battery next to it had 6.85V or so, which is similar to the good working 2.  

After proceeding to adjust the 10k pot to get 6.5V across the R22k, my 6ca7 Started Arcing.  Spent many hours to diode and short measures all the active parts, and I believe that now I am still chasing a ghost....a pretty good one at haunting my brain

Edited by Whitigir
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I am getting very close! Thanks god that @Kevin Gilmore , @Georgep , @Soren, Birgir, Mike, and the whole community here.  Especially thanks to Kevin for taking up your time and advises, and thanks George for the chassis, some obsolete sands and sharing hard to find Xicon.

I finally realized that adjusting the batteries with 10k pots is wrong, and it will in the end make 1 battery stuck.  Have to measure 22k resistor to 6.5V and then dialing with 2k pot to 740V .  Thanks George for the method and it worked to unstuck my battery

Now, I can have all 4 batteries at 740V, O+/O- is -100mV to +50mV on right channel.  -200mV to +12mV on Left channel.  This is cold start

O+ to Ground References is R (2.2V and dropping steadily) and L (4.5V and dropping steadily)

I think my next step is to plug in the 009 and try to observe noises

**my Tube arcing problem turned out to be that JJ 6ca7 was pretty fragile, and couldn’t handle the T2.  I was sweating and checking everywhere to realize it was a ghost, until KG confirmed that JJ are junk.  I stuck in Groove tubes EL34, and it got to this result.

 

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Edited by Whitigir
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Yay ! I got music, and no background noises even at max volume.  No Hums, no hiss !!

Oh my gawd! I am blessed by Dr. Kevin Gilmore!.  I appreciate it so much sir Dr. Gilmore!  Finally, I have reached my “ultimate dream goals!” With very limited knowledges.

Thank you to George for making this Build a possibility!!

Thank you so much to the community as a whole.  The best Stax community out there !

Finally, some pictures, herself and in all her glory.  Pure solid silver for wires :D . I am big fan of it

I am smiling like a maniac! Yayyyyy!

stacking up, and Amp is 130F, the PsU is 141F after 2 hours.  Pretty warmth ! Can’t imagine the original 

The most beautiful amp I have ever had! Period.  Time to “burn-in” :D 

 

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Edited by Whitigir
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