The ultimate DIY? A Stax SRM-T2!

[audiostar]

Was just looking for a version of the Amphenol MIL connector with crimp contacts and indeed there is. The widely known solder-type connectors are

PT02A-14-15S for the socket type (chassis side), as used in the PSU
PT02A-14-15P for the pin type (chassis side), as used in the amp

the equivalent but crimp type box-mounting receptacles would be 

PT02SE-14-15S for the socket type (chassis side)
PT02SE-14-15P for the pin type (chassis side)

20 AWG crimp pins: M39029/31-240
20 AWG crimp sockets: M39029/32-259
16 AWG crimp pins: M39029/31-228
16 AWG crimp sockets: M39029/32-247

Straight plugs for the cable side are available in crimp as well.
Just replace the PT06A-xx-xxx with PT06SE-xx-xxx

Full specs here.

[simmconn]

The solder type is a lifesaver for DIYers, unless you want to deal with the uber-expensive crimper, the crimping dies, and the go/no go gauges.

Having said that, I found the power rails considerably cleaner at the PSU board than at the amp board.

[jokerman777]

I just got the crimping contact version of the connectors earlier this week. Yeah the tools are very expensive, gotten some rusty secondhand one from ebay and hope it will work out...

All that is just bc I'm pretty sure I don't want to deal with soldering 8*14 closely spaced soldering cups while also worry about insulation 😅

[s_r]

Figured I may as well add a pic of the other side of the board. A few of the K216s have 500Ω resistors attached to the gate. The 3675's in the batteries all have an hfe of about 64-67 too.

[demonkuro]

Yesterday I did a continuous 5-hour test. I found that the voltage of the battery will decrease, the voltage from -549/+206 at 30 minutes, and then to -549/+202 after 5 hours, only the positive voltage will decrease, and as the positive voltage decreases, the output voltage to GND will also change from about -5V to -20V。 why is only the positive voltage not stable? At the same time, I found that the output of the R channel to GND must be above -549/+208 to be stable, is it okay to have such a high voltage?

[s_r]

Good news, everything appears to be working normally now. With some help from georgep and joamat I managed to track down what seems to have been the culprit: one dead 2SC3675 in one of the triplets of 2SC3675's. Probably the reason two of the LEDs nearby were dead too. I just need to plug in some stax now and have a listen...

All batteries now measure +200V/-539V, and R42 measures 6.55V just as it should.

[Kerry]

Congrats!  Awesome news

[jokerman777]

Should signal ground meet chassis/earth ground in the PSU box?

I found that the center stand-off pad near the output terminals on the PSU board is connected to PSU board circuit ground and mounting a standoff there will make the chassis gnd to signal gnd connection, otherwise the two grounds will be separated in both boxes.

I found Kevin's old post in this thread mentioning "everything floats" so should I leave out this stand-off?

On 11/7/2009 at 7:14 AM, kevin gilmore said:

Everything floats inside both boxes to get rid of any ground noise.

[jamesmking]

My Stax builds have always had the signal ground completely isolated from the chassis/protective ground - which is connected to the earth pin of the mains plug.

amp chassis is earth grounded via a lead going to a star grounding point in the psu chassis - which is where the psu chassis and mains earth meet.

 

[simmconn]

I agree that metal chassis should connect to the earth ground from the power cord/line input module. In general, I also connect the transformer electrostatic shield ground and the DC power supply ground (which is electrically connected to the amplifier's signal ground) to the chassis/earth ground. That seems to give me the lowest measured hum level. Sometimes grounding the input signal ground at the connector entry point to the chassis works best for me, but YMMV. 

In terms of DIY T2, since its in two separate chassis, the only way to ground the amp chassis is through the power supply ground wires in the umbilical cords. The center standoff near the back on the PSU board becomes the most convenient star-ground point. The wires going to the grounding point should be as short as possible and stay clear of the transformer leakage flux. Unfortunately that's not easy to do in the DIY T2 PSU chassis. 

[jokerman777]

Thank you both for the input!

Yes I thought of if chassis/earth ground need to meet signal ground best place would probably be at the signal input, so for now in the PSU Chassis I leave signal GND floating and mains earth/chassis gnd/transformer shields meets at one point and then will be passed to amp chassis ground via umbilical.

-------------------------------------

So yesterday I put the PSU together, did connectivity test for shorts and then power on with no load, all output voltages measures fine.

Today I thought I want to mount the power transformer tighter to see if it helps with HV transformer buzz so I replaced bolts with longer ones and added lock washer. Power on again with no load (and I didn't do connectivity test as I assume things were same as yesterday), everything was fine until I done measuring the negative voltages and about to go measure the positive voltages, something suddenly sparkled (in the area of the red circle in the picture, too fast I didn't catch which component) and then a bad smell came out, I powered it off right away and nothing looks burnt visually for now.

Things I know so far:

-Chassis/earth ground is now shorted to on board signal ground, either through the case of 7812 or I might have a short somewhere else.

-The anodes of the 10m90s at the +500V/Bias supply side are shorted to gnd, so either one or more of these might have the case shorted to the bracket or I have dead diodes causing it.

-On the other side where the sparkle happens the cases of all the transistors are not shorted to the bracket.

-Disconnecting the HV trafos power on again +-12V supply still works

I know this is way too little information to find out what went wrong other than I have one or more things shorted, I will take it apart further inspect this wknd

When testing without load is it ok to run the PSU without heatsinks installed? As that will save quite some effort since I likely need to do multiple iterations of trouble shooting.

 

[sorenb]

@jokerman777looks like you did not use stth512's ?

[jokerman777]

11 minutes ago, sorenb said:

@jokerman777looks like you did not use stth512's ?

Oh I got parts referencing a BOM where it says stth1512D that is obsolete and I used STTH15S12D instead, I suppose not going to make a difference?

[jamesmking]

1 hour ago, jokerman777 said:

 

Things I know so far:

-Chassis/earth ground is now shorted to on board signal ground, either through the case of 7812 or I might have a short somewhere else.

-The anodes of the 10m90s at the +500V/Bias supply side are shorted to gnd, so either one or more of these might have the case shorted to the bracket or I have dead diodes causing it.

-On the other side where the sparkle happens the cases of all the transistors are not shorted to the bracket.

-Disconnecting the HV trafos power on again +-12V supply still works

I know this is way too little information to find out what went wrong other than I have one or more things shorted, I will take it apart further inspect this wknd

When testing without load is it ok to run the PSU without heatsinks installed? As that will save quite some effort since I likely need to do multiple iterations of trouble shooting.

 

if the +-12V rails work (with no load some voltage regulators will not regulate properly and most have current and thermal  limiters built in) then its highly unlikely the 7812 or 1912 voltage regulators are shorted to ground.

with no load I don't see why a working psu cannot be run without heatsinks for short periods of time... however if you have a short on the output side of a HV rail then the pass mosfet/transistor could be trying to output a lot of current and be running hot even with a heatsink...

if you have a thermal probe you could CAREFULLY measure the temperatures of the heatsink transistors and see if any are getting excessively hot.

when the psu has fully discharged you could run your multimeter in diode check mode on each of the diodes in the psu (both directions).

I assume your negative HV rail is working ok. If your psu is symmetrical then the negative HV rail is basically exactly the same as the positive rail in basic topology then you can start making comparative tests between the two.

(for my T2 build I did not use the same pcbs or psu as you have so I don't have any direct experience of your psu)

 

Edited July 18, 2023 by jamesmking

[jokerman777]

10 minutes ago, jamesmking said:

if the +-12V rails work (with no load some voltage regulators will not regulate properly and most have current and thermal  limiters built in) then its highly unlikely the 7812 or 1912 voltage regulators are shorted to ground.

with no load I don't see why a working psu cannot be run without heatsinks for short periods of time... however if you have a short on the output side of a HV rail then the pass mosfet/transistor could be trying to output a lot of current and be running hot even with a heatsink...

if you have a thermal probe you could CAREFULLY measure the temperatures of the heatsink transistors and see if any are getting excessively hot.

when the psu has fully discharged you could run your multimeter in diode check mode on each of the diodes in the psu (both directions).

I assume your negative HV rail is working ok. If your psu is symmetrical then the negative HV rail is basically exactly the same as the positive rail in basic topology then you can start making comparative tests between the two.

(for my T2 build I did not use the same pcbs or psu as you have so I don't have any direct experience of your psu)

STTH512FP is fully encapsulated and so has no exposed metal LIVE tab so no change of spark between the tabs of the diodes (since there is a rather high voltage difference between the tabs and they are spaced fairly close together)

Thanks for the suggestions!

I will take it apart and do connectivity & diode test first, then maybe reconnect rails one by one for measure.

7812 the case is ground pin so if that's shorted to the bracket it will short signal ground to chassis ground and circuit will still work, this is the only reason I can think of rn that can short the two GNDs.

Negative rails was working fine until I see the spark which happened on the negative supply side so I'm not sure now, something might have gone up to smoke.

great tip that if I have one leg of the rails working I can use it as reference 🙂

 

[sorenb]

59 minutes ago, jokerman777 said:

Oh I got parts referencing a BOM where it says stth1512D that is obsolete and I used STTH15S12D instead, I suppose not going to make a difference?

STTH512FP is ready available and fully insulated.
If you look through the KGSShv thread, some had troubles with un-insulated rectifier tabs shorting.
 

[jamesmking]

8 minutes ago, sorenb said:

STTH512FP is ready available and fully insulated.
If you look through the KGSShv thread, some had troubles with un-insulated rectifier tabs shorting.
 

yep, there is a lot of voltage differential across those tabs. I only use STTH512FP so far zero failures zero issues with them.

[JoaMat]

I believe the space around the rectifiers makes it risk free to use the uninsulated version on the original Kevin T2 PSU board.

[audiostar]

9 hours ago, jokerman777 said:

Thank you both for the input!

Yes I thought of if chassis/earth ground need to meet signal ground best place would probably be at the signal input, so for now in the PSU Chassis I leave signal GND floating and mains earth/chassis gnd/transformer shields meets at one point and then will be passed to amp chassis ground via umbilical.

And this is exactly how you don't do it.
You pass both signal and PE grounds (separately) to the amp's chassis. PE connects there to the chassis only. Signal ground is obvious. And absolutely no other connections between both. 

In the PSU case you connect to the chassis (near the mains power entry):
1. PE ground,
2. transformer shield (if any) and
3. signal ground (best through a circuit breaker, at least a 10 ohm resistor)
in a star manner.
And this is the only connection where PE ground connects to signal ground (better called circuit ground as a differential signal doesn't use ground).

Edited July 18, 2023 by audiostar

[jokerman777]

9 hours ago, audiostar said:

And this is exactly how you don't do it.
You pass both signal and PE grounds (separately) to the amp's chassis. PE connects there to the chassis only. Signal ground is obvious. And absolutely no other connections between both. 

In the PSU case you connect to the chassis (near the mains power entry):
1. PE ground,
2. transformer shield (if any) and
3. signal ground (best through a circuit breaker, at least a 10 ohm resistor)
in a star manner.
And this is the only connection where PE ground connects to signal ground (better called circuit ground as a differential signal doesn't use ground).

Mind explain why it should be this way? My naive thought was in the case of single box amp, source and amp have their circuit/signal ground connected at signal input jack of the amp and then both meets mains earth in their own box, thus having amp circuit ground meeting earth at input jack minimizes the ground loop between amp and source, for two box T2 if earth meets circuit ground within the PSU box this ground loop would be extended by the umbilical cable?

[jokerman777]

I took the PSU apart today for simple checkups.

On the positive supply side it was the case of 7812 and Q3 10m90s both shorted to the bracket, burning up the insulation. I think I initially had applied force to drive the screws incorrectly such that they bite into the insulation washer and damaged it. They been measuring fine for shorts until I mount the board into chassis and heatsinks and also move it around which probably eventually break the limit. 

Diodes all measured fine after eliminating shorts but I also ordered 512s for backup.

Think I'm going to replace all the 10m90s at the 500V/bias rail later on and see how it goes. Still don't know what was with the spark on the other side of the board. 

[sorenb]

@jokerman777
The thermal compound looks kind of grey'ish ... do you use some exotic compound?
The screw needs to be able to slide into the 7721 without any force, otherwise it is too big.

Maybe you found the cause of the HV transformer buzzing with "no load" ...


 

Edited July 19, 2023 by sorenb

[jamesmking]

57 minutes ago, sorenb said:

@jokerman777
The thermal compound looks kind of grey'ish ... do you use some exotic compound?
The screw needs to be able to slide into the 7721 without any force, otherwise it is too big.

Maybe you found the cause of the HV transformer buzzing with "no load" ...


 

personally I use Arctic Silver Ceramique 2 it is non conductive is not metal based and I have had no issues with it so far.

 

[jokerman777]

1 hour ago, sorenb said:

@jokerman777
The thermal compound looks kind of grey'ish ... do you use some exotic compound?
The screw needs to be able to slide into the 7721 without any force, otherwise it is too big.

Maybe you found the cause of the HV transformer buzzing with "no load" ...


 

Thermal compound I used this: https://www.amazon.com/dp/B09VDLH5M6?psc=1&ref=ppx_yo2ov_dt_b_product_details

I believe pretty generic kind for CPU mounting in PC.

For those 4/40 screws yeah fitting is probably marginally fine, most of them slide in and out of the 7721 freely, some minority samples I feel resistance try to slide them in and I don't use those, I suppose excessive thermal compound filling the gap doesn't help neither..

What did you suggest that could potentially be the cause of trafo buzz? I'd think it's not bc of shorting since over the wknd everything was fine with no shorting and I still heard a buzz. (my trafos are at smaller size and I was given a VA rating of 84, I think SumR/toroidy uses 100VA rated bigger core). Thx!

[Craig Sawyers]

You also need to be careful on tightening torque. For correct assembly, the torque is surprisingly low. I bought a torque screwdriver for exactly this purpose.

What insulating bushes and washers have you used?

Have a look at  https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Basically overtightening can distort the semiconductor package and compromise thermal transfer, and that you have split the insulating bush suggests you are massively overtightening the fixing screw.

Craig