Hi Gang,
I just pulled out the Stasis II power amp from storage, about 10 years now. It was brought up to date with new caps and poly bypasses etc a few years before I put it in storage. So when I plugged it in, I powered it up through the big 200W light bulb, the light was bright at first, then dimmed to a moderate level, and the started to pulse slightly at about 1Hz. Thought it must just be that the bias wasn't quite at the level to keep it stable so I threw the switch to bypass the bulb and bang, blew the left rail fuse and popped a 1 ohm emitter resistor and 4 output devices. Oh darn I exclaimed!
Today I fixed yesterdays damage and powered it up the same way, and the same result, pulsing light bulb at about 1Hz. I used the variac this time to bring it up slow, monitoring the rail current in the fuse holder, and when the rails hit 30VDC, it jumps from .3A up to about 3.9A and the rails drop to about 25VDC. Output offset goes from .05 to about .5VDC. Any increase in rail voltage, results in drastic increase in rail current, up to about 5A is where I stopped. I checked all the small signal transistors while I had the board loose and everything looked fine. This amp has been modified for fixed bias, someone has removed the pot and put in a precision resistor to make the bias about .6A. Worked great 10 years ago when I used it regularly. I changed the electrolytics on the driver board, no difference.
So, have any of you experienced a similar problem to this? I will wait to hear from you before I tear it apart again. Any thoughts on the possible problem? I checked all the output devices and emitter resistors. Right channel is fine so I could start by comparing L-R resistances etc., swap the driver card etc..
Cheers
Paul
I just pulled out the Stasis II power amp from storage, about 10 years now. It was brought up to date with new caps and poly bypasses etc a few years before I put it in storage. So when I plugged it in, I powered it up through the big 200W light bulb, the light was bright at first, then dimmed to a moderate level, and the started to pulse slightly at about 1Hz. Thought it must just be that the bias wasn't quite at the level to keep it stable so I threw the switch to bypass the bulb and bang, blew the left rail fuse and popped a 1 ohm emitter resistor and 4 output devices. Oh darn I exclaimed!
Today I fixed yesterdays damage and powered it up the same way, and the same result, pulsing light bulb at about 1Hz. I used the variac this time to bring it up slow, monitoring the rail current in the fuse holder, and when the rails hit 30VDC, it jumps from .3A up to about 3.9A and the rails drop to about 25VDC. Output offset goes from .05 to about .5VDC. Any increase in rail voltage, results in drastic increase in rail current, up to about 5A is where I stopped. I checked all the small signal transistors while I had the board loose and everything looked fine. This amp has been modified for fixed bias, someone has removed the pot and put in a precision resistor to make the bias about .6A. Worked great 10 years ago when I used it regularly. I changed the electrolytics on the driver board, no difference.
So, have any of you experienced a similar problem to this? I will wait to hear from you before I tear it apart again. Any thoughts on the possible problem? I checked all the output devices and emitter resistors. Right channel is fine so I could start by comparing L-R resistances etc., swap the driver card etc..
Cheers
Paul
Last edited:
Hello Nelson,
Thank you so much for the reply. So you think I may still have a flakey output device in there eh? (Canadian accent noted?) Okay, will do. A quick check of voltage across the emitter R's outa do it. I would eventually like to load both channels with all new MJ15023/24 but for now, all I have in stock are MJ15003/4 so that is what I have replaced the damaged devices with. I know they are only 2Mhz parts.
I'll let you know how it goes.
I have been away from designing and playing with high end audio for 9 years now, while raising my son, but now I have been re-bitten by the audio bug. I seem to have missed a lot in those years, as far as design and semis go...SIC jfets?? I am just starting to read old posts and get caught up. A whole lot has progressed.
Cheers and thanks again for sustaining this wonderful hobby with fresh ideas!
Paul
Thank you so much for the reply. So you think I may still have a flakey output device in there eh? (Canadian accent noted?) Okay, will do. A quick check of voltage across the emitter R's outa do it. I would eventually like to load both channels with all new MJ15023/24 but for now, all I have in stock are MJ15003/4 so that is what I have replaced the damaged devices with. I know they are only 2Mhz parts.
I'll let you know how it goes.
I have been away from designing and playing with high end audio for 9 years now, while raising my son, but now I have been re-bitten by the audio bug. I seem to have missed a lot in those years, as far as design and semis go...SIC jfets?? I am just starting to read old posts and get caught up. A whole lot has progressed.
Cheers and thanks again for sustaining this wonderful hobby with fresh ideas!
Paul
Well I finally got back to it today...shorted out the bias xstr, MPSA42, and brought it up slowly on the variac, and there was no current draw in the suspect channel. Rails went right up to +/- 45 VDC......then as an insentive to really dig into this amp, something in the right channel failed and THANKFULLY, I had the light bulb in series with the variac, the bulb came on full brightness and the rails dropped to +/- 5VDC. GREAT! I give up.
Nothing went pop or emitted smoke, but definately a new problem in the right channel, confirmed by pulling the rail fuse. Bloody hell..
I will have to pick up on it again later. Totally frustrating.
Anyway, that's my report. Any thoughts?
Cheers
Paul
I give up. Nothing went pop or emitted smoke, but definately a new problem in the right channel, confirmed by pulling the rail fuse. Bloody hell..
I will have to pick up on it again later. Totally frustrating.
Anyway, that's my report. Any thoughts?
Cheers
Paul
Finally got back to this amp yesterday and put in 4 hours of troubleshootiung. Good news is that I fixed the right channel...it was one output shorted. What makes these random failures happen??
Resoldered all connections on the original problem left channel and cleaned up the PCB. Tried powering it up again and same thing. Won't go past +/- 28VDC rail voltage before the output stage starts drawing 3 or 4 amps. I checked the output thinking maybe osc. but it's clean. The wires are so darn short in there it's hard to power it up and get any access to the outputs, but I'll give it another try and see if I can find a flakey part. I checked all the devices when I had the channel out and they all check fine on my DVM.
I'll keep trying.
Paul
Resoldered all connections on the original problem left channel and cleaned up the PCB. Tried powering it up again and same thing. Won't go past +/- 28VDC rail voltage before the output stage starts drawing 3 or 4 amps. I checked the output thinking maybe osc. but it's clean. The wires are so darn short in there it's hard to power it up and get any access to the outputs, but I'll give it another try and see if I can find a flakey part. I checked all the devices when I had the channel out and they all check fine on my DVM.
I'll keep trying.
Paul
Are new driver boards available for this amp?
Thanks Nelson,
The cranium has a little rust on it these days.... and the troubleshooting skills have faded slightly...but they are getting better as I go. I was hoping to just replace ALL the outputs with new MJ15023/24 but On-Semi lost my order. Waiting for a new shipment.
In the meantime, I'll hone my skills.
One other question, are there new driver boards, i.e. newer rev. that I could install? My amp is from 1979. I was also thinking of maybe trying to work the A40 boards into this...but I haven't looked at the A40 schematics in a while. I have 2 driver boards, stuffed and ready to use. May need higher voltage parts?? Just thinking that if the trouble is not output devices, and these input/driver boards have been modifed with new caps etc over the previous years owners, that maybe a fresh set of input boards with new parts might be an improvement. Your thoughts?
Thanks again,
Paul
Thanks Nelson,
The cranium has a little rust on it these days....
and the troubleshooting skills have faded slightly...but they are getting better as I go. I was hoping to just replace ALL the outputs with new MJ15023/24 but On-Semi lost my order. Waiting for a new shipment. In the meantime, I'll hone my skills.
One other question, are there new driver boards, i.e. newer rev. that I could install? My amp is from 1979. I was also thinking of maybe trying to work the A40 boards into this...but I haven't looked at the A40 schematics in a while. I have 2 driver boards, stuffed and ready to use. May need higher voltage parts?? Just thinking that if the trouble is not output devices, and these input/driver boards have been modifed with new caps etc over the previous years owners, that maybe a fresh set of input boards with new parts might be an improvement. Your thoughts?
Thanks again,
Paul
Paul, did you ever get that Stasis 2 running?
It would sure be nice if someone would make some bare driver board copies for the Stasis 2/3 amps. These board takes beating everytime you work on them. It's so easy to lift a land if you are not super careful. My Staisis 2 boards both have more than one repaired land - they could look a lot better!
The Stasis 2 is a lot of labor to work on, but these great amps deserve all the attention you can give them. You will spend many hours on the resoration, but it will be worth the effort. My guess is that once you pull the at left-bank off, if you haven't done so already, you are going to find a short somewhere - either a shorted output device, a sneeky little solder bridge, or some other "tech" mistake made in the past.
Also, ditch that non-adjustable bias fixed-resistor for a 10-turn or better trim pot. Once you have your new outout devices installed and everything is just right, you will need to tweak the bias to make it perfect.
I have restored three of these to date (two Stasis 2s and one Stasis 3) all with age related and abuse issues, two of which required all new output devices on both channels. I had great success and recommend ON MJ21195/96 devices. With the new output devices, be sure to go for some nice new emitter resistors. The old carbon-comps will be out of spec by now. Panasonic ERX series are darn good and resonably priced, and are available in all the values that you will need: ERX-3SJ1R0 Panasonic - ECG | P1.0W-3BK-ND | DigiKey.
Best of luck and keep us posted on the progress!
It would sure be nice if someone would make some bare driver board copies for the Stasis 2/3 amps. These board takes beating everytime you work on them. It's so easy to lift a land if you are not super careful. My Staisis 2 boards both have more than one repaired land - they could look a lot better!
The Stasis 2 is a lot of labor to work on, but these great amps deserve all the attention you can give them. You will spend many hours on the resoration, but it will be worth the effort. My guess is that once you pull the at left-bank off, if you haven't done so already, you are going to find a short somewhere - either a shorted output device, a sneeky little solder bridge, or some other "tech" mistake made in the past.
Also, ditch that non-adjustable bias fixed-resistor for a 10-turn or better trim pot. Once you have your new outout devices installed and everything is just right, you will need to tweak the bias to make it perfect.
I have restored three of these to date (two Stasis 2s and one Stasis 3) all with age related and abuse issues, two of which required all new output devices on both channels. I had great success and recommend ON MJ21195/96 devices. With the new output devices, be sure to go for some nice new emitter resistors. The old carbon-comps will be out of spec by now. Panasonic ERX series are darn good and resonably priced, and are available in all the values that you will need: ERX-3SJ1R0 Panasonic - ECG | P1.0W-3BK-ND | DigiKey.
Best of luck and keep us posted on the progress!
Last edited:
Are you sure all the driver transistors are good?? I have seen many times that when an output device fails, it will sometime take a driver transistor out with it too! If you are drawing heavy current from the AC line when you are below 30 volts on the variac, that is usually a sign of a blown output device. if you can get past 30vac and it starts drawing heavy current when you get near or just past 1/2 line voltage, that typically points to an driver transistor that has failed. and granted this is a generalization and assumes 120V unit. but it helps point to where to look.
Also watch for oscillation. I have seen amps that will start to oscillate and draw heavy line current.
Zc
Also watch for oscillation. I have seen amps that will start to oscillate and draw heavy line current.
Zc
Jeepers creepers!
It's been a year since I worked on this amp....now to get back to it. Been a busy year for me work wise.
So I have the rest of the MJ15024/25s on order and they should be here soon. There is a 1/8" of dust that needs to be removed from the amp and then I will start rebuilding the driver boards, reinstalling the 10 turn bias pots etc. Repair any previous damage by previous owner and maybe update some of the wiring so I can work on it easier.
Yes, changing all drivers and outputs, thanks! Too much work to just change out a few then have it fail again. Also will change the emitter resistors as well. I have loads of .68 ohm 2W metal films. Do you see any problem with using these instead of 1 ohm?
That's the long awaited update.
Cheers for now.
Paul
It's been a year since I worked on this amp....now to get back to it. Been a busy year for me work wise.
So I have the rest of the MJ15024/25s on order and they should be here soon. There is a 1/8" of dust that needs to be removed from the amp and then I will start rebuilding the driver boards, reinstalling the 10 turn bias pots etc. Repair any previous damage by previous owner and maybe update some of the wiring so I can work on it easier.
Yes, changing all drivers and outputs, thanks! Too much work to just change out a few then have it fail again. Also will change the emitter resistors as well. I have loads of .68 ohm 2W metal films. Do you see any problem with using these instead of 1 ohm?
That's the long awaited update.
Cheers for now.
Paul
It is great to hear that you're back on the the Stasis 2 project. The Stasis 2 and 3s are just wonderful amps to rebuild, but I'm not too sure that I would use those 0.68 ohm emitter resistors.
If I recall correctly, the Stasis 3 uses 0.68 ohm emitter resistors but the Stasis 2 uses 1.0 ohm. Though at first glance it doesn't seem to be a big difference between 1.0 ohm and 0.68 ohm, you know the 0.68-ohm resistors are going to have to pull more bias current over the 1.0 ohm emitter resistors. I know, you always want to use the stuff that you already have, but as inexpensive a the Panasonic ERX metal-film resistors are mentioned above, and I can attest from over three years of listening to my rebuilt Threshold Stasis 2 amps (bi-amped Apogee Duetta Sigs), you just can't go wrong with these resistors and Nelson's original design values. The MJ15024/25 can probably handle the extra current flow just fine, but I say why risk it? It's a lot of labor to do a good job of shaping the leads and soldering in all those emitter resistors correctly so why have to do it twice? Worse would be to find that the the extra bias-current flow caused some unexpected overload up the line.
Anyway, keep us in the loop on the entire rebuild.
If I recall correctly, the Stasis 3 uses 0.68 ohm emitter resistors but the Stasis 2 uses 1.0 ohm. Though at first glance it doesn't seem to be a big difference between 1.0 ohm and 0.68 ohm, you know the 0.68-ohm resistors are going to have to pull more bias current over the 1.0 ohm emitter resistors. I know, you always want to use the stuff that you already have, but as inexpensive a the Panasonic ERX metal-film resistors are mentioned above, and I can attest from over three years of listening to my rebuilt Threshold Stasis 2 amps (bi-amped Apogee Duetta Sigs), you just can't go wrong with these resistors and Nelson's original design values. The MJ15024/25 can probably handle the extra current flow just fine, but I say why risk it? It's a lot of labor to do a good job of shaping the leads and soldering in all those emitter resistors correctly so why have to do it twice? Worse would be to find that the the extra bias-current flow caused some unexpected overload up the line.
Anyway, keep us in the loop on the entire rebuild.
Last edited:
And this is why I love this site....good solid advice. You're right, I'll stay with the 1 ohm and order the R's you described. I'll also replace the drivers which I think are the 2SC2592 and 2SA1112. Is there anything special about these drivers, has anyone subbed the xstrs? I would like to find something from On-Semi. Just a quick look on their site...how about MJE15034/35?
Cheers
Paul
Cheers
Paul
Last edited:
Taking a brain break...reading all 42 pages of on old thread by Barchetta on fixing a 400a. There is lots of good troubleshooting info in that thread and I think the 400a is similar to the Stasis II. Also found some info on emitter R's in that thread. Seems the value wasn't critical as Nelson didn't comment on the question of .22 vs .68 vs 1 ohm (at least not up to page 33 )
Cheers
Paul
)Cheers
Paul
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.