The ultimate DIY? A Stax SRM-T2!

[JoaMat]

Thanks, simmconn, you are right about R53 and 300B. Table above changed.

 I’ve no idea of limitation of the output swing. What I know though is that 300B pleases me.

Edited October 8, 2023 by JoaMat

[jokerman777]

😵 Spend the last few days trying to adjust the battery and no luck

I first tried the method of setting voltage across R42 to 6.55V, keeping RV1 centered and adjust RV2. As I slowly bring things into conduction, voltage across R42 starts slowly decreasing till it get to around 6.8V then it starts jump around 6.55V, as I adjust both RV2 back and forth, the voltage across the resistors seems always likes to stay a few hundred mV apart around 6.55V, and the best I can do is something like 6.4V and 6.6V.

In this case negative battery voltage stays at -548V, while I can adjust RV1 to bring positive voltage to ~200V, output DC is wild (somewhere around 80 to 130V) and no small adjustment can bring it close to 0, only can be reduced by bringing positive voltage back up from 200V with either pot.

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

I then reset all the pots back to center position and check the overall behavior.

When first power up batteries are not conducting with many of the LEDs not lighting up. output voltage is simply 250V/-560V. While keep RV1 (2k) centered, turning RV2 (10k) I get the following behavior (time aligned):

Negative voltage: -560V -> -543V ->-549V (stays)

Positive voltage:    250V ->   245V -> keep decreasing

offset:         both positive -> closest to 0 (<+-3V) -> keep increasing as negative voltage (~-100V as + voltage approaches 200V).

RV1 didn't really do much IMO, I can use it to trim down positive voltage too just way less change per turn and offset behaves the same as when I turn RV2 to do it. 

 

So offset wise it really likes -543V/245V the best and trimming it any closer to 740V it just keep getting worse in negative direction. I measured R42 voltage in this condition and it's 6.9V/7V, which makes sense bc below these values it starts jumping around 6.55V as I observed previously.

I did this measure on left channel but I briefly tried right channel as well and it seems to have similar behavior, so likely I made the same mistake in both channels.

I feel the servo is working as +/- output offset follows each other closely as long as neither is too wild (>100V).

Parts I used are 2sa1413/ksa1156 for 2sa1486s on board and on heatsinks respectively, tubes are supposedly matched JJ E88CC and new production Mullard EL34.

 Is there any insight on what could be the problem or how to track it down? Many thanks!

 

 

Edited October 10, 2023 by jokerman777

[Pars]

I've never built a T2, so take this with a grain of salt, but anytime I see DC levels bouncing all over the place, one of the first things I would suspect is oscillation somewhere.

[justin]

or use some gentle compressed air and see if you can locate a spot of thermal instability

[jokerman777]

Thanks! So to elaborate none of the things I measured are bouncing themselves all over the place when leaving the pots alone.

Output DC offset simply follows battery voltage and goes + -> 0 -> - as I trim the pot but only likes -543V/245V with the lowest offset near 0.

As for voltages across R42 - they start off well above 7V when battery is not conducting. Trimming RV2 will continuously reduce this voltage, until I get to around 6.8-7V ( which is also the -543V/245V; 0 offset point) then there seems to be a discontinuity where further trimming the pot will make it jump to around 6.3V. Only back and forth trimming both batteries I can sort of get 6.4V and 6.6V, it almost seems like V R42 really doesn't want to go to 6.55V specifically.

Oscillation yes definitely happens as turning the pot makes all kinds of squeaking noise coming in and out from the tubes.

I'm assuming batteries from the 2 channels are independent as they only share the PSU, so I only tried to bring batteries in one channel to conduction at a time.

I guess another way to look at it is, is it normal that when both batteries are conducting, I can never trim the negative voltage below -543V? Passing that point it just likes to go back up towards -549V and stay there.

 

[JoaMat]

Set R42 voltage to 6.55V with RV2.
Set battery voltage to 740V.
Same for all for batteries.
Do not try to use batteries to zero out offsets.

Tell us the offset measurements for all outputs.

[jokerman777]

9 hours ago, JoaMat said:

Set R42 voltage to 6.55V with RV2.
Set battery voltage to 740V.
Same for all for batteries.
Do not try to use batteries to zero out offsets.

Tell us the offset measurements for all outputs.

I just gave my best shot on left channel, don't know if it's normal at all but setting voltage across R42 isn't easy, when either voltage is below 7V trimming the pot don't make them go down continuously and they like to jump down a few hundred mV and hanging around there. Also trimming either pots affect both voltages across R42s and in general seems like they like to stay on different side of 6.55V.

So the best I got is I got both stay between 6.5-6.6V, this is when battery voltages are： 216V/-542V and 231V/-547V

Starting from there I use RV1 to bring down the positive voltage and it ended up at: 200V/-548V and 200V/-549V

Remeasuring voltage across R42 they be what they wanted to be: 6.3V/6.7V

Output offset is L+ = -103V and L- = -135V

Also there is likely some oscillation going on in this state as I can hear the output tube squeaking.

is setting voltage across R42 not suppose to be this hard and trimming RV2 can continuously bring it from 7V down to 6.55V and they will comfortably stay around there?

[JoaMat]

@jokerman777

Well, remote diagnosis isn’t easy.

So far, I haven’t had any difficulties to set R42 to 6,55V. Knock on wood.

+200V/-548V and +200V/-549 seem somehow all right but can’t you set the voltage of the batteries to 740V. Do you adjust RV1 to set positive battery voltage to +200V?

[jokerman777]

30 minutes ago, JoaMat said:

@jokerman777

Well, remote diagnosis isn’t easy.

So far, I haven’t had any difficulties to set R42 to 6,55V. Knock on wood.

+200V/-548V and +200V/-549 seem somehow all right but can’t you set the voltage of the batteries to 740V. Do you adjust RV1 to set positive battery voltage to +200V?

Thanks for the help!

Yes I only used RV1 to adjust positive battery voltage after trying to set voltage across R42.

And yes I can set overall battery voltages to 740V too: because the negative voltage is just staying there at -549V I just keep further trimming RV1 to bring down another 10V on positive voltage, that gives both battery voltage -549V/191V.

Now output offset just increase to L- = -123V and L+ = -150V, measure R42 again it's 6.35V and 6.6V

[JoaMat]

Thank you, @jokerman777

Try to do same thing with the right channel. 6,55V and 740V and see what battery potentials and offsets you get.

[jokerman777]

5 minutes ago, JoaMat said:

Thank you, @jokerman777

Try to do same thing with the right channel. 6,55V and 740V and see what battery potentials and offsets you get.

thx, I will do that in the next few days (I remember last time I tried the behavior was similar). When I do that should I leave left channel in this current state?

[JoaMat]

Leaving left channel as it is now might be wise. Than you can compare it with the right channel later.

[jokerman777]

😑Right channel is exactly the same story - negative voltage likes to bounce back from - 544V to -549V and stay when trimming RV2, with RV1 bringing positive voltage down output offset just keep increasing, and is -125V/-151V near -549V/200V

I found one thing though - if I start off setting RV1 wiper to the upper R32 (9.1k) side, trimming RV2 I can easily set 6.55V across the R42 independently and they stay there, until I trim RV1 to bring down the positive voltage, they drift to something else between 6.2V-6.6V. This happens to all 4 batteries.

 

 

[JoaMat]

What transistor do you have for Q16 -Q19 and Q23?

[jokerman777]

6 hours ago, JoaMat said:

What transistor do you have for Q16 -Q19 and Q23?

Q16-Q19 are 2sa1413, I also have a bunch of 2sa1627 as well, least both of their datasheet looks like a straight copy and paste from A1486 datasheet.

Q23 is 2sc3675.

I believe they are all genuine from reliable sources.

[JoaMat]

2sa1413 is to-252 package, isn’t it? How do you solder them in – adapter board? Also, what kind of resistors do you use in the batteries.

[jokerman777]

17 minutes ago, JoaMat said:

2sa1413 is to-252 package, isn’t it? How do you solder them in – adapter board? Also, what kind of resistors do you use in the batteries.

Mine from Bdent looks like this. If anything my batch of 1413 are labelled as "K" grade and my batch of 1627 are labelled as "L" grade, on my toy transistor tester from Amazon the hfe measures are 100 and 60 respectively which kinda checks out.

From my mouser list all resistors are rated 350V, 0.5 watt, most are Xicon except a few. I will double check on board resistor values from color code tonight though. 🙂

[simmconn]

Watch the power rail voltages as you see strange things happen, such as a low battery bottom voltage (-549V) coincide with negative output offset. Depending on your 10M90s in the PSU, you may need to reduce the current setting resistor values if they get into current limiting condition.

It’s a good practice to load-test every power rails at 20% more than the rated current.

Spend some time doing measurements across the board, mark the voltages at the key nodes in the schematic and verify the voltage and current that are marked in the pdf. Those are very valuable info.

[simmconn]

The difficulty setting the V(R42) to 6.55V could be due to something else. I assume that you observed different pin out when substituting 2SA1486 with 2SA1413. As long as the LED strings are lit stably and not flickering, the RV2 should adjust smoothly for 6.55V across R42. The idea is to set the 2SK246 ccs to about 297uA with the majority of that going through R42 and only a tiny bit from the base of Q16. If you have some spare 2SK246 from the same batch, resistors and pot, breadboard a circuit and test with a bench DC power supply. See if the pot hits the bottom or something wrong with your 2SK246. 
It’s a good idea to measure around the active battery circuit, using the top node as the reference point, mark voltages and currents on the schematic. The active battery does not take rocket science to troubleshoot. You just need to know 1) Ohm’s law, 2) Transistors have hfe, and 3) A forward-biased silicon junction has a voltage drop about 0.6V.

There is no need to be paranoid about the battery not being spot-on at 740V. Since it is transparent to the audio signal, they can even be purposely set slightly differently from each other to compensate unbalances elsewhere in the circuit. But if your amp don’t like them being near 740V at all, that means some other parts of the amp is quite different from what the designer has envisioned.

There are quite a few feedback loops in the works that maintain a near-zero offset. Let go of the active battery voltage temporarily in exchange for zero offset can set those loops at their normal operating point and help you find out who’s not behaving correctly.

Mark any voltage reading with a ‘*’ if you hear oscillation when you get the reading, as it may not be reliable.

[jokerman777]

Many thanks @simmconn! 

Things I know so far:

PSU voltages sits still as I tried to trim the batteries closer to 740V, none is loaded down. Also unlikely that I'm running on the boundary of current limiting because I trimmed one channel at a time while leaving another channel with batteries not conducting? monitoring total power consumption it is 30-40W lower than bringing both channels to be on.

All resistors seems fine, no visible damage and ones below 100kohm I can measure on board and values are correct.

Measured two C3675s in same channel on board with toy tester, hfe are both around 48

When I set RV1 to R32 side, I can comfortably set voltage across R42 to 6.55V, this is what I get:

 

Then I leave RV2 alone and turn RV1 to reduce positive voltage to ~200V, I got:

I remember reading somewhere that LEDs need to be 2.1V, mine with handheld meter all measured around 1.7V, but I guess around below 1ma bias current in circuit this sounds about right? 

I unfortunately don't have any spare k246, best I can do is pop one out and find replacement if they are malfunctioning.

Gonna measure more nodes over the wknd

 

 

[simmconn]

It looks like most of the voltage adjustment comes from the wandering reference, no wonder you said RV1 doesn't have much effect.

Assuming your Q16 path works okay, comparing the two diagrams, V(R42) dropped (6.22-6.56)/6.56=-5.18%, almost the same as the LED string voltage drop (11.4-12)/12=-5%, which means the 2SK246 ccs didn't have much effect at all. To give you a data point, my mocked-up 2SK246 ccs changes V(22k) from 6.5496V to 6.5446V (less than -0.1%) when the supply changes from 12V to 11.4V.

Your LED string seems not great, either. When the current drops from 703uA to 663uA (-5.7%, ignoring the Q16 base current), the LED string voltage drops -5% as well. Not much of regulation, isn't it? I grabbed a random red LED from my stash and in similar situations its forward voltage changes from 1.7914V to 1.7881V, only -0.18%. Your LED probably has 700uA right around the knee on its IV curve. It may not be a bad part, just not suitable for this location.

The above assumes the Q16 and the rest of the battery circuit works okay. Oh well, let's fix one problem at a time.

[jokerman777]

19 hours ago, simmconn said:

It looks like most of the voltage adjustment comes from the wandering reference, no wonder you said RV1 doesn't have much effect.

Assuming your Q16 path works okay, comparing the two diagrams, V(R42) dropped (6.22-6.56)/6.56=-5.18%, almost the same as the LED string voltage drop (11.4-12)/12=-5%, which means the 2SK246 ccs didn't have much effect at all. To give you a data point, my mocked-up 2SK246 ccs changes V(22k) from 6.5496V to 6.5446V (less than -0.1%) when the supply changes from 12V to 11.4V.

Your LED string seems not great, either. When the current drops from 703uA to 663uA (-5.7%, ignoring the Q16 base current), the LED string voltage drops -5% as well. Not much of regulation, isn't it? I grabbed a random red LED from my stash and in similar situations its forward voltage changes from 1.7914V to 1.7881V, only -0.18%. Your LED probably has 700uA right around the knee on its IV curve. It may not be a bad part, just not suitable for this location.

The above assumes the Q16 and the rest of the battery circuit works okay. Oh well, let's fix one problem at a time.

This is really helpful, thank you so much!

I'm way too ignorant on this so I wanna verify if I understand things correctly.

More probing around the battery with Vgs on 2SK246 now, "/" is before and after trimming RV1 to bring down battery voltage to around 740V.

So in theory assume base current of Q16 is negligible, while the regulation on the LED string is not ideal, Q22 shall work as a current source fine despite the Vds drop from 4V to ~2.65V, that means regardless the LED is not the source of my problem right?

Now the current running thru R42 before and after are 298uA/284uA, current thru RV2 before and after are 303uA/287uA, while not super accurate but it seems the base current of Q16 is on order of magnitude of a few uA and had changed about 2uA, this can cause Vgs of Q22 to change about some tens of mV depending on RV2 value, enough to say that whatever Q16 is doing here, it's effect on the CCS is indeed negligible?

That lead to the conclusion the first thing I should do is replace the 2sk246 right? And breadboard test them with a bench supply varying input voltage around 10-12V with R42 and RV2 in place.

Is there a recommendation on readily available replacement of this part?

Thanks again! 🙂

 

 

 

[simmconn]

Yes the 2SK246 looks suspicious. Its |Yfs| should be around 1mS around Id=300uA, so changes in Vgs of 1.51V-1.43V=80mV should generate 80uA of change in Id. Apparently that's not the case here. You could use a 2SK373 or even a 2SK117 as a substitute. The 2SK208 is also a close sub, albeit in SOT-23 package.

Also, Q16 Ib should not be that high. The current thru R39 is about 100uA, split between Q16 and Q17. If Q16 has an hFE >100, it's Ib should not be over 1uA even in the extreme case.

[jokerman777]

I went on and measured more things outside battery while setting battery voltage to 750V:

Seems like everything is a little off. I suppose the key to get correct low offset at output is to get the output tube operating point in place (without considering the feedback shenanigans that I don't quite understand😵), with grid sitting on the -500V supply it should come down to getting the plate load CCS and cathode voltage in place.

Other than the previously mentioned battery adjustment weirdness, measuring voltage across D1 and D23 I got 1.57V-1.67V, definitely on the low side similar to what I found in the battery. Also battery seems to be not outputing any current with little to no voltage drop across R99 and R100.

LEDs

With current noted on the schematic, at least for D1 and D23 they be running at about 0.5mA with a forward voltage drop of 1.8V (if all base current can be ignored). I took D1 and D23 and another one from the battery for a measure.

LTL4213_old is taken off from the board, only plotting one as they all measured similarly, LTL4213_new is another batch I bought recently, along with the Cree C566D also a 2.1V red LED with nominal forward current of 20mA. These all measured pretty much the same, on the lower side of 1.8V but strangely never went down as low as around 1.6V as I measured in amp when operating. Only thing I can think of is derating due to the negative temperature coefficient?

3RDL-S is a 2.1V red LED with nominal forward current of 2mA, as expected the curve shifted to the right side. Seems to be a shot in the dark but I shall replace all LEDs with 3RDL-S and sufficiently elevate them from the board? (which I didn't do with the current LTL4213s on board..)

2SK246

I didn't take out the ones on board but gotten a few backup ones from ebay, hook them up on the breadboard and set voltage across 22K resistor to around 6.55V at 12V supply voltage from the bench. Then sweeping supply voltage nothing surprising and they regulates fine...

Resistors

I replaced R33, R34, R43, R44 with 500V rated resistors - as I just realized voltage across them will always be a little above 350V, and this doesn't change anything.

 

So I think I'm going to replace all the LEDs this weekend, although it seems everyone else are using LTL4213 just fine, possibly still not the cause of my problem...

 

[simmconn]

LED:

The numbers are all over the place:

22 hours ago, jokerman777 said:

measuring voltage across D1 and D23 I got 1.57V-1.67V

 

22 hours ago, jokerman777 said:

With current noted on the schematic, at least for D1 and D23 they be running at about 0.5mA with a forward voltage drop of 1.8V

Your IV curve shows the LTL4213 should have Vf between 1.725V and 1.750V with 0.5mA of current. You've got three different voltage ranges for the LED. Which one is correct? You need to find out where the dependencies come from. Your previous LED chain voltage measurement shows 12.1-10.7=1.4V of voltage variations with not much If change. Is the 1.4V evenly distributed across 7 LEDs? If you suspect Vf change due to the temperature coefficient, that's easy to verify. Just keep monitoring the LED voltage with the amp powered on from cold.

2SK246:

Your test indicates that the 'backup ones from ebay' are probably good. Unless the ones on the board are from the same seller and same batch, the test result probably doesn't help much more than that.

Rather than replacing parts shotgun style. I'd take voltage measurements shotgun style, then sit down and analyze the data. Replacing parts without knowing why and what would be a very inefficient way of troubleshooting.

For example, when looking into the active battery, measure voltage on all nodes. Beware of the burden by the DMM input impedance (usually 10M Ohms but YMMV), so plan carefully when you measure across high-value resistors. When looking into the final stage, collect enough voltage data within the CCS so you can calculate the current supplied by the CCS. Measure the voltage at Output+/Output-, then determine if the tubes are in the right operating point.

If you know very well how the circuit works, you can strategically take only a few measurement at key spots; otherwise just do more leg work and collect all data. Don't worry if some could be redundant. They often end up helping you where least expected.