The ultimate DIY? A Stax SRM-T2!

[Inu]

I think I've been the only one with 2SC3675 troubles - and that is restricted to the one in the voltage source batteries. Formally that part is sailing very close to the wind - the spec sheet says that at 5mA collector current second breakdown happens for Vce greater than 600V. Depending on the LED in the current source Q30 the current is closer to 4mA or less. At 3mA it should just about do 750V.

Now spec sheets are always conservative, but the statistical spread of characteristics must have put my batch just about where the spec sheet says. They went short in both channels, which is classic second breakdown failure. This happened spontaneously once in my class A Krell - that generated a hell of a lot of smoke! Failure of the 3675 took out two LED's in the anode current sources, a pair of 100V zeners (they glowed briefly!), and all four 2SK216's. That it happened on both channels identically indicates that it is not a stuffing issue (I checked that first), dry joint, or probing mistake - which points the finger at the 2SC3675.

So once I get the hot stick out, I'm going non-authentic 2SC4686A (KG's suggestion). They should be good for 1200V at up to 8mA. We'll see if that cures the problem.

FYI

I measured hfe for all transistors before use on the T2.

2SC3675

Measured: 74pcs

@Ic=10mA

Lowest = 43

Highest =74

Average = 58

I used hfe=57@10mA for 4xQ23

2SC4686A

Measured: 50pcs

Lowest = 22

Highest =31

Average = 27

I did not use because of low hfe.

[blubliss]

What are you guys doing for the feet?

[livewire]

This.

[ATTACH=CONFIG]4089[/ATTACH]

[nattonrice]

Justin had a group buy a little while back on some feet from china.

Now the feet are awesome but the configureable packing foam cubes that he boxed mine in are a new level.

[wink]

Yep, they were great feet - not a sign of tinea or ingrown toenails amongst them.......

[Craig Sawyers]

FYI

I measured hfe for all transistors before use on the T2.

2SC3675

Measured: 74pcs

@Ic=10mA

Lowest = 43

Highest =74

Average = 58

I used hfe=57@10mA for 4xQ23

2SC4686A

Measured: 50pcs

Lowest = 22

Highest =31

Average = 27

I did not use because of low hfe.

That is very helpful information. The low hfe of the 2SC4686A is certainly a concern, but I need to find a robust solution for the fact that my 2SC3675's die through second breakdown. So I either need to use a darlington configuration, or a higher current in the differential pair (by reducing the emitter resistor).

Not very original, of course - but in the batteries I'm not massively concerned.

[pedefede]

Hi All

If others europeans also are sourcing parts - I can recommend to also look at the danish site:

PC ELEKTRONIK

(Site is in danish, but send an email

... or use "s

[blubliss]

Amp section done, just waiting on the transformers now.

[ATTACH=CONFIG]4116[/ATTACH][ATTACH=CONFIG]4115[/ATTACH][ATTACH=CONFIG]4114[/ATTACH][ATTACH=CONFIG]4112[/ATTACH]

[ATTACH=CONFIG]4111[/ATTACH][ATTACH=CONFIG]4113[/ATTACH][ATTACH=CONFIG]4117[/ATTACH]

[luvdunhill]

Nice! Might as well pop in the ICs into the PSU sockets and do the jumpering on the PSU as well. You might find some solderable test points to be useful, or just make some loops with wire for the grabber probes to attach to.

[spritzer]

Looks nice Andy!!

[n3rdling]

[Pars]

x2, looks great. You've been a busy boy

[Torpedo]

Nice? Great? That looks foockin' awesome Andy, I wish I had the time, place, knowledge and skills to do that. Congratulations

[blubliss]

Thanks all, the hard stuff was done by my friend steve (stv1756). I helped a bit; it has been fun.

The transformers are at my local post office but i missed the closing time by 1 hour today. I coulda had it running by now maybe , or fried

[Voltron]

Very cool Andy! Get it running and bring it up to Mayberry. Not sure when Terry will have time to work on mine.

[blubliss]

I would love to come, and I will make an effort.

[Kerry]

Nice job on the bulid Andy. It's always very thrilling to power up for the first time. Good luck and enjoy!

I'm looking forward to getting this build moving I've got a reasonably good feeling for the circuit and once I have an inventory of parts I can start moving on pulling everything together.

[Craig Sawyers]

The 2SC3675 problem is solved. I hauled my Tek 577 curve tracer out with the aim of measuring the hFE of my batch of 2SC4686A. The answer was 25 +/-10%.

So I thought I'd try out a 2SC3675 just for comparison. The one I connected up gave the answer ... 10. Say what? So I grabbed another and this was was 70. Another was 10.

Then I spotted that there were two batches that looked quite different. One lot has an hFE of 10, the other was in the range 50 to 70. A picture of the two types is here http://www.tech-enterprise.com/tekstuff/2SC3675.jpg . If you zoom in you will see the markings on the packages. The one on the left gives the correct high value hFE, and the one on the left has a puny hFE of 10. All remaining ten samples of the right hand one have a gain of 10. The remaining four of the left hand one have gains of 50 to 70.

Of course I have a random selection stuffed into my T2. And the ones that have failed in the batteries are (surprise surprise) the low gain ones.

In forty years of designing and building electronics, this is a first for me. The right hand ones in the picture are either a faulty batch, or counterfeit. The duff ones are marked iSC, and may come from this outfit isc,inchange Semiconductor Company Limited Welcome You , and clearly Chinese crap.

I'm going to order another batch of these sods, and check every last one before I replace the defective ones in my T2 lock stock and barrel.

[justin]

i have a bunch of 2SC3675 that measure a hFE of 4. They're the only ones I've had that say Sanyo on them..

[Craig Sawyers]

i have a bunch of 2SC3675 that measure a hFE of 4. They're the only ones I've had that say Sanyo on them..

Bloody hell. Your and my experience with this should sound warning klaxons to all T2 builders - measure any 2SC3675 that you are planning on using!

[Craig Sawyers]

And just to add to eveyone's 2SC3675 paranoia, Sanyo are disconintuing the part:

SANYO Transistor: 2SC3675

Only 1200 left in Sanyo's inventory. After that - nothing.

Anyone in the US up for a group buy of 200 or more (200 is the MOQ) at 86c each? You could certainly count me in for a T2's worth of them (34) plus a bunch of spares.

[spritzer]

I've got some dodgy C3675's as well but I haven't tested them. They did come from Farnell or rather their distributor here.

Some of us already had a group buy for the now obsolete parts (2SA1968 and 2SC3675) so count me out. I have enough to last me a long, long time.

[Voltron]

Nice job on the bulid Andy. It's always very thrilling to power up for the first time. Good luck and enjoy!

I'm looking forward to getting this build moving I've got a reasonably good feeling for the circuit and once I have an inventory of parts I can start moving on pulling everything together.

That honestly wasn't meant as a hint Kerry. I just assume it will be awhile before you are able to even get started. Plus I just noticed my iPhone auto-corrected your name to Terry.

Andy's pics also reminded me that I need an RK50 so if anyone has a good source please let me know.

[spritzer]

Justin should be able to help with the RK50.

[digger945]

Justin should be able to help with the RK50.

Yea, his website is the first place i looked. Can't remember if I saw one there.