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Vintage Stax repair and maintenance - Page 6

post #76 of 196
Originally Posted by kevin gilmore View Post

yes I own one, have not used it in over 35 years. Was very good in its
Time. Battery is a problem

The later ECP-1 battery packs weren't sealed in a plastic box - they were sealed in shrink wrap, so they were slightly more user replaceable, as long as you had someone who could make up a pack in that fashion. 

post #77 of 196
Originally Posted by nemomec View Post



I have here a vintage Stax SRD-X Professional amp with the Stax 12V 400mA power supply. The amp works well with normal bias phones but with pro phones it works only well about that two minutes then the sound gradually fades to become inaudible.


Where is the problem and how can i fix them?




The bias supply is simply a voltage doubler off the flyback transformer.  Check the diodes and the caps for damage. 

post #78 of 196



i was done the ECC99 mod on my SRM-T1S. Now I have a big problem the amp work for a hour without any issues with superb sound and after that I had problems on the left channel with sound distortion and fail. :mad:


Reason: the bias setting on left channel was drifting and was to high but i can´t set this under 60V!


In the first step i switched the tubes without success. Then i checked the new 30K resitors with ohm meter no problems all ok. Then I done some voltage measurements on the resistors between the channels and there was a big difference on R27 and R28 (100V difference incomming, outgoint between R28 and R27 0V!!!).

I need some help to solve the problem :confused_face(1):



Edited by nemomec - 11/15/13 at 9:11am
post #79 of 196

It's probably a component failing under load or a partial short somewhere.  Not a whole lot to this circuit (the output stage) and all the cathode stuff is shared between the + and - phase so that rules out any issues there. 

post #80 of 196

I think the active components can be damaged first under the new higher load like the thyristors NEC A1156 :confused_face(1): 


I test now some things. The main problem of the defective left channel is on resistor R27 they have on one side a negative voltage of 315V and on R28 is a positive voltage of 315V between this two resitors the voltage falls to near 0V, the current goes up and the two resistors get very hot. On the resitors of the full working right channel there are all postive voltages! This problem goes directly to pin 6 on the tube socket there is on the defective left channel -315V and on the full working right channel +315V.


Is there a problem of the rewiring of the tube sockets?


Here are my rewire of the tube sockets:

(pin 4 and 5 bridged, trace to pin 5 cut, pin 9 wired to the inner circle)



and this was the template from a T1 found here in the forum:



I found also now a picture of T1S with a little different rewiring. The pin 9 cable on left channel (in the picture right side) goes to a other solding point  :confused_face_2:


Edited by nemomec - 11/17/13 at 11:06pm
post #81 of 196

There is no higher load presented by the ECC99's except on the heater supply.  They draw more current but I always test the transformers to see if they can handle that. 


The 2SA1156 is a BJT and should not suffer any damage. 


I that pic I can see copper in the scratched out area so it might be shorting.  The heater supply has high voltage DC floating on it so it could be arcing. 

post #82 of 196

Hello spritzer,


thanks for your information. I have made today a new recabling and removed the rest of cooper without success. :mad:


On R27 the voltage is -315V and the pin 4 on the left socket is sometimes a little burning and smoking :eek: I have break the bridge to pin 5 and pin 9 with no effect only when i sold out a leg of R27 the pin 4 is normal with -315V. I have also changed the resitor.


Its crazy i can´t find the problem!


post #83 of 196

Problem solved :D


There was a short between the trace of pin 4 and pin 6 they can´t see from outside. I isolated the trace and was made a new connection for pin 6 and now the T1S works on both channels!


Thanks spritzer for your help!

post #84 of 196

That's excellent news indeed.  Those crappy PCB's Stax used back in the day can be a bloody pain to deal with.  :)


So do you like what the mod did to the sound? 

post #85 of 196

Yes the soundstage was increased and the bass detail is better, I haven´t heard a better stax amp! The T1S with ECC99 tubes plays with the lambdas wonderfull a perfect match. :gs1000smile: 

Edited by nemomec - 11/19/13 at 11:10pm
post #86 of 196

Now you need to try a 007t modded to take 6S4A's.  :)  That is a starting change to the better. 

post #87 of 196

In my knowlege the 006t, 006tA and the 006ts are based on the same circuit then the SRM-T1.


Its possible to take the ECC99 tubes in this models?

I like the new front of the 006tA and ts and i think there are good amps with the ECC99 tubes :gs1000smile: 

Edited by nemomec - 11/25/13 at 6:11am
post #88 of 196

Yes, T1, T1S, T1W, 006t, 066tA/II and 006tS are all the same design and all can be configured to use ECC99's. 

post #89 of 196

After reading this thread I kinda realized you can't really do much about possible dust/hair stuck in the driver.. So depressing.

post #90 of 196
Originally Posted by davidsh View Post

After reading this thread I kinda realized you can't really do much about possible dust/hair stuck in the driver.. So depressing.

You can and you'd need to go extreme lengths to pry off the internal dust filter stuck on the stator. If you do rip the PVC film on the dust filter you need to re-film not drum tight but securely from a PVC food bag wrap. You must do it in a dust free environment. Even using special equipments and a dust free room (room where we open up hard drives and recover sensitive data off the platters of faulty drives), it was a very tedious task to retain originality and I would not do it again.

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