Hello
I once owned a Threshold Stasis 3 and Fet One pair, which I loved, but had to sell due to financial difficulties.
This set worked well with my Ditton 66s, which I also had to sell along with the Threshold set.
I was able to buy my Ditton 66s again, 20 years later, but the Threshold set had long since been resold.
I started looking for a new set, but these devices are rare and still too expensive for me.
I was lucky to find a Stasis 3 and an NS10 separately, both in France.
The NS10 was 110V, so I easily switched it to 220V and replaced all the chemical capacitors. I'd still like to find a Fet One, but they are very rare, especially in Europe.
I haven't done anything on the Stasis 3 since I got it. I recently decided to replace the RCA sockets because the plugs don't hold well.
I knew this amp had been professionally repaired, but what I didn't know and didn't see was that the job hadn't been done very well: some wires were melted and the circuit boards were damaged.
At that moment, I regretted opening it!
The RCA cables had been replaced, but they were too stiff and had bent the leads of an MPSA42, making contact with each other. Luckily, these leads were already in contact on the circuit board, so it didn't change anything!
So I completely disassembled the amp to rewire it.
While cleaning the circuit boards, the leads of a 2SA1112 were cut off flush with the case. I was lucky here too, because since I listen very loudly, they could have broken from the vibrations.
So I was right to take it apart, without knowing it. I replaced the 2SA112 and 2SC2592 with MJE15033G and MJE15032G, which I had in stock.
I rewired it, changing all the ring terminals and lengthening the wires so I could keep it open without unwiring anything. I also changed the banana plugs.
I was very nervous about powering it up again. I first used a 100W bulb in series and measured the output voltage. There were 2x16V on the filter capacitors and over 100mV negative on the outputs.
I finally mustered the courage to connect it directly, and it didn't catch fire! The output offset is lower but still high: -48mV on one channel, -60mV on the other.
But, after a while, the offset increases on the channel that was at -48mV, going beyond -70mV after only a few minutes, while it decreases a little on the other channel to around -50mV.
The transformer is a bit noisy, I'd never noticed it before.
I noticed that it heats up much faster, and that the temperature seems higher than before.
Perhaps the fact that the MJE transistors have a much higher gain than the original ones means the quiescent current is higher?
I'll have to adjust the quiescent current potentiometers; it scares me.
I'll first open it up and measure the voltage across one of the emitter resistors on each channel to get the actual value of this current.
I would need to replace the two filter capacitors, but my finances do not allow me to do it at the moment, I will do it later, perhaps at the same time as replacing the two printed circuits of the input stage.
Here's some photos before my intervention:
After my intervention
I once owned a Threshold Stasis 3 and Fet One pair, which I loved, but had to sell due to financial difficulties.
This set worked well with my Ditton 66s, which I also had to sell along with the Threshold set.
I was able to buy my Ditton 66s again, 20 years later, but the Threshold set had long since been resold.
I started looking for a new set, but these devices are rare and still too expensive for me.
I was lucky to find a Stasis 3 and an NS10 separately, both in France.
The NS10 was 110V, so I easily switched it to 220V and replaced all the chemical capacitors. I'd still like to find a Fet One, but they are very rare, especially in Europe.
I haven't done anything on the Stasis 3 since I got it. I recently decided to replace the RCA sockets because the plugs don't hold well.
I knew this amp had been professionally repaired, but what I didn't know and didn't see was that the job hadn't been done very well: some wires were melted and the circuit boards were damaged.
At that moment, I regretted opening it!
The RCA cables had been replaced, but they were too stiff and had bent the leads of an MPSA42, making contact with each other. Luckily, these leads were already in contact on the circuit board, so it didn't change anything!
So I completely disassembled the amp to rewire it.
While cleaning the circuit boards, the leads of a 2SA1112 were cut off flush with the case. I was lucky here too, because since I listen very loudly, they could have broken from the vibrations.
So I was right to take it apart, without knowing it. I replaced the 2SA112 and 2SC2592 with MJE15033G and MJE15032G, which I had in stock.
I rewired it, changing all the ring terminals and lengthening the wires so I could keep it open without unwiring anything. I also changed the banana plugs.
I was very nervous about powering it up again. I first used a 100W bulb in series and measured the output voltage. There were 2x16V on the filter capacitors and over 100mV negative on the outputs.
I finally mustered the courage to connect it directly, and it didn't catch fire! The output offset is lower but still high: -48mV on one channel, -60mV on the other.
But, after a while, the offset increases on the channel that was at -48mV, going beyond -70mV after only a few minutes, while it decreases a little on the other channel to around -50mV.
The transformer is a bit noisy, I'd never noticed it before.
I noticed that it heats up much faster, and that the temperature seems higher than before.
Perhaps the fact that the MJE transistors have a much higher gain than the original ones means the quiescent current is higher?
I'll have to adjust the quiescent current potentiometers; it scares me.
I'll first open it up and measure the voltage across one of the emitter resistors on each channel to get the actual value of this current.
I would need to replace the two filter capacitors, but my finances do not allow me to do it at the moment, I will do it later, perhaps at the same time as replacing the two printed circuits of the input stage.
Here's some photos before my intervention:
After my intervention
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I assume the transistors you mentioned you replaced are on the pcb's.
Yes, measure the Iq on some of the emitter resistors, or all of them, record and compare results. If all output transistors are OK and reasonably well gain matched, then the Iq should be fairly close on each pos/neg output transistor. This will not be the case however if you have dissimilar output transistors. All N types should be the same type and all P types should be the same type.
The DC output offset volts should be less than 50 mV.
Start from there and report your findings, also record what transistors you have fitted in the output stage for each channel. It will be hard to identify the FE board transistors compared to the schematic because the pcb is not silk screened.
Yes, measure the Iq on some of the emitter resistors, or all of them, record and compare results. If all output transistors are OK and reasonably well gain matched, then the Iq should be fairly close on each pos/neg output transistor. This will not be the case however if you have dissimilar output transistors. All N types should be the same type and all P types should be the same type.
The DC output offset volts should be less than 50 mV.
Start from there and report your findings, also record what transistors you have fitted in the output stage for each channel. It will be hard to identify the FE board transistors compared to the schematic because the pcb is not silk screened.
Thank you.
I didn't replace any output transistors, but some had been replaced before my intervention.
The only ones I replaced were the two TO220s mounted on heatsinks, on the FE board. (the pre drivers ?)
During the alleged intervention, the technician had noted the gains of each transistor on the heatsinks.
I should have noted them before erasing them during cleaning, because I cleaned the solder flux off all the boards.
On one channel, only one TO3 was replaced, while on the other, four were.
I only removed the heatsinks on that channel to see which transistors were installed (MJ15024 and MJ05025) .
I didn't replace any output transistors, but some had been replaced before my intervention.
The only ones I replaced were the two TO220s mounted on heatsinks, on the FE board. (the pre drivers ?)
During the alleged intervention, the technician had noted the gains of each transistor on the heatsinks.
I should have noted them before erasing them during cleaning, because I cleaned the solder flux off all the boards.
On one channel, only one TO3 was replaced, while on the other, four were.
I only removed the heatsinks on that channel to see which transistors were installed (MJ15024 and MJ05025) .
I have find the equivalent for the originals capacitors: https://www.mouser.fr/ProductDetail/Cornell-Dubilier-CDE/CGS153U100W5L?qs=ioz7Wz2k%2BQwG5RB90/CMpQ==
I just checked the voltage across the emitter resistors.I was at about 130mV, I lowered this value to 110mV on both sides, or 169mA per transistor.
But I can't measure this value on all the resistors, since it's wound up and few are safely accessible.I'm now waiting for it to stabilize.
If it doesn't heat up enough, I'll raise this value.
Without the top cover, the amp seems to heat up less quickly
But I can't measure this value on all the resistors, since it's wound up and few are safely accessible.I'm now waiting for it to stabilize.
If it doesn't heat up enough, I'll raise this value.
Without the top cover, the amp seems to heat up less quickly
The amp has been on for over an hour already, the two radiators are at 46°C and the quiescent current has risen well, it is at 160mV so 235mA, but it continues to rise, I will have to lower it.
I got it down to just under 100 mV, and the temperature seems to be stable at 44-45°C.
I'll try again tomorrow to confirm this setting.
One more thing: I can't find the schematics for the power supply and display. Do they exist?
I saw, on the 400A power supply schematic, that there is a secondary winding used to power the display.
I suppose it must be the same on the Stasis 2 and 3. I'd like to use this surge to power a speaker protection kit.
What do you think of this?
I'll try again tomorrow to confirm this setting.
One more thing: I can't find the schematics for the power supply and display. Do they exist?
I saw, on the 400A power supply schematic, that there is a secondary winding used to power the display.
I suppose it must be the same on the Stasis 2 and 3. I'd like to use this surge to power a speaker protection kit.
What do you think of this?
It's good, I found the diagram of the display, it is powered by 15V AC, so it is perfect for powering a speaker protection module.
Good work so far, yes you need to do an adjustment and wait on the heatsinks to stabilize in temp with the lid sitting on the chassis. Final Iq should give a temp on the heatsinks for 45 to 50 deg C. On the original FE pcb you have there is no adjustment for DC offset, the amp relies on hfe matched output transistors within 15% or so to give a minimum DC offset. Looking at the hfe numbers written on the heatsink, the gain on the devices shown has a very wide spread. The newer transistors that have been replaced are a good replacement, but even then whoever did the job has used 2 different manufacturers.