Here is a wrap up of the restoration for those interested.
The old very damaged FE boards were replaced with the new Nelson Pass FE design.
Component changes necessary due to the high Stasis 2 DC rails of +/-73VDC were:
R6 changed to 27K
R7 changed to 8K2
R29 changed to 8K2 1 watt
ZTX transistors changed to higher voltage ZTX457/550 types
New 22,000uF 80VDC power supply caps installed
New higher current rating diode bridges installed
New higher current rating silicon insulated wiring installed.
New 3 watt resistors installed on both OS
One channel OS already had original transistors replaced with MJ15022/23
The other channel OS still had original Motorola A6/A8 transistors - however 3 of these were short circuit, so all were replaced with new MJ15023G/24G
On powering up, it was found the OS current sharing was terrible, so all transistors were removed for simple Hfe testing.
It was found that the originally replaced MJ15023 transistors were showing an Hfe that was more than twice as high as the max datasheet spec - so these were obviously fake.
More MJ15023G On Semi were purchased to replace these.
Groups of 6 transistors were matched for each channel.
Some tips:
I reached out on several occasions by email to Jon Soderberg - Vintage Amp Repair and he responded admirably to my emails with advice to help Andy with his restoration in the UK - his suggestions were:
OS transistors need to be matched better than 20% max difference (10% is preferred), N to N and P to P. It is not possible to match N to P as the P complement always measures at a higher gain value to the N. So match each group of six as they show in the horizontal lines in the Stasis 2 schematic.
Do not mount the 2K NTC thermistor on the heat sink ( we did) as this will overcompensate. The original FE had the thermistor mounted on the board as a component monitoring the amp internal air temp. So we moved it to be the same.
The bias cannot be adjusted for final accuracy until the amp is all bolted together with the lid sitting in place Having the OS loose on the bench will cause the bias to drift all over the place - we found that out the hard way.
Jon does final adjustments and testing over a 6 hour period as he says it sometimes takes that long for the amp to reach thermal equilibrium, during that time he would monitor and adjust the bias every 20 mins or so till the amp locks in at a stable setting.
Back in the good old days of Threshold, Jon said the factory spec for the Stasis 2 was 85mA transistor bias (85mV over the 1R0 emitter resistor), less than +/- 50mVDC offset (the original amp did not have an adjustable trimpot for this - they had to do a resistor change if over 50mV) and a final heat sink temp of 45 to 46 deg C max.
So, thanks to Zen Mod for technical advice and new component values on the FEB, thanks to Mike in the US for his thoughts and comments, both in the thread and by PM, also other members , thank you and finally a big thanks to Jon Soderberg for his considered replies to my emails and suggestions which ultimately helped a hell of a lot.
Andy, it has been a pleasure to help you on this journey, I suppose the title of your thread got my attention initially, and even I have learnt out of this. I know you have learned a lot over the last month or two and I hope you get many happy years of enjoyment out of your new Stasis 2 - now 44 years old and looking brand new, together with those speakers you have.
Finally, many thanks as usual around here goes to Mr. Nelson Pass for the initial Threshold design all those years ago and then his updated new FE board, without which, the restoration would not have happened. Even Jon Soderberg mentioned he would be using the new FE on future work as the original boards and active components are now non existent.
All of this highlights why this forum and its members are a great place to be part of.
Great work Andy, you persevered very well, with the knowledge you had at the time, and you inspire us all around here.
Cheers to all involved,
Gary.
The old very damaged FE boards were replaced with the new Nelson Pass FE design.
Component changes necessary due to the high Stasis 2 DC rails of +/-73VDC were:
R6 changed to 27K
R7 changed to 8K2
R29 changed to 8K2 1 watt
ZTX transistors changed to higher voltage ZTX457/550 types
New 22,000uF 80VDC power supply caps installed
New higher current rating diode bridges installed
New higher current rating silicon insulated wiring installed.
New 3 watt resistors installed on both OS
One channel OS already had original transistors replaced with MJ15022/23
The other channel OS still had original Motorola A6/A8 transistors - however 3 of these were short circuit, so all were replaced with new MJ15023G/24G
On powering up, it was found the OS current sharing was terrible, so all transistors were removed for simple Hfe testing.
It was found that the originally replaced MJ15023 transistors were showing an Hfe that was more than twice as high as the max datasheet spec - so these were obviously fake.
More MJ15023G On Semi were purchased to replace these.
Groups of 6 transistors were matched for each channel.
Some tips:
I reached out on several occasions by email to Jon Soderberg - Vintage Amp Repair and he responded admirably to my emails with advice to help Andy with his restoration in the UK - his suggestions were:
OS transistors need to be matched better than 20% max difference (10% is preferred), N to N and P to P. It is not possible to match N to P as the P complement always measures at a higher gain value to the N. So match each group of six as they show in the horizontal lines in the Stasis 2 schematic.
Do not mount the 2K NTC thermistor on the heat sink ( we did) as this will overcompensate. The original FE had the thermistor mounted on the board as a component monitoring the amp internal air temp. So we moved it to be the same.
The bias cannot be adjusted for final accuracy until the amp is all bolted together with the lid sitting in place Having the OS loose on the bench will cause the bias to drift all over the place - we found that out the hard way.
Jon does final adjustments and testing over a 6 hour period as he says it sometimes takes that long for the amp to reach thermal equilibrium, during that time he would monitor and adjust the bias every 20 mins or so till the amp locks in at a stable setting.
Back in the good old days of Threshold, Jon said the factory spec for the Stasis 2 was 85mA transistor bias (85mV over the 1R0 emitter resistor), less than +/- 50mVDC offset (the original amp did not have an adjustable trimpot for this - they had to do a resistor change if over 50mV) and a final heat sink temp of 45 to 46 deg C max.
So, thanks to Zen Mod for technical advice and new component values on the FEB, thanks to Mike in the US for his thoughts and comments, both in the thread and by PM, also other members , thank you and finally a big thanks to Jon Soderberg for his considered replies to my emails and suggestions which ultimately helped a hell of a lot.
Andy, it has been a pleasure to help you on this journey, I suppose the title of your thread got my attention initially, and even I have learnt out of this. I know you have learned a lot over the last month or two and I hope you get many happy years of enjoyment out of your new Stasis 2 - now 44 years old and looking brand new, together with those speakers you have.
Finally, many thanks as usual around here goes to Mr. Nelson Pass for the initial Threshold design all those years ago and then his updated new FE board, without which, the restoration would not have happened. Even Jon Soderberg mentioned he would be using the new FE on future work as the original boards and active components are now non existent.
All of this highlights why this forum and its members are a great place to be part of.
Great work Andy, you persevered very well, with the knowledge you had at the time, and you inspire us all around here.
Cheers to all involved,
Gary.
Hello gents .
Thought I’d update the thread .
The important thing is that the amp sounds fantastic, I’ve now had it powered on continuously for 3 days, after the first 24hrs I adjusted the bias ( again! ) I can’t get the temperature over 46c and that was with the bias at 115mv and 112mv , at 5am this Saturday morning I checked the temperature again ( at the back screw hole ) and it’s now 41c and that’s on the warmer right side, left side never even barely gets warm 🤷♂️
Dc offset remains nice n low ( 005mv )
So it looks like I’ll have to get the DMM’s out yet again to see if the bias has dropped.
Another thing that I’m not sure about is the fact that with either of the 3 preamps I have I’m having to turn the volume to around 10pm ( 6pm being zero ) before I get any sound and if I want it loud but not blasting out then the volume is at around 10 past or quarter past , maybe it’s the 4ohm speakers causing that ?
Anyway I’ve had many many amplifiers over the years and I’ve always thought that where they were great in one area they failed or were lacking in another until now 😁
Also on day 2 of it running constantly I came home from work having left it with fm playing I heard a difference in the sound, it was as if I had added some super tweeters, bloody lovely sound this beast gives .
Thought I’d update the thread .
The important thing is that the amp sounds fantastic, I’ve now had it powered on continuously for 3 days, after the first 24hrs I adjusted the bias ( again! ) I can’t get the temperature over 46c and that was with the bias at 115mv and 112mv , at 5am this Saturday morning I checked the temperature again ( at the back screw hole ) and it’s now 41c and that’s on the warmer right side, left side never even barely gets warm 🤷♂️
Dc offset remains nice n low ( 005mv )
So it looks like I’ll have to get the DMM’s out yet again to see if the bias has dropped.
Another thing that I’m not sure about is the fact that with either of the 3 preamps I have I’m having to turn the volume to around 10pm ( 6pm being zero ) before I get any sound and if I want it loud but not blasting out then the volume is at around 10 past or quarter past , maybe it’s the 4ohm speakers causing that ?
Anyway I’ve had many many amplifiers over the years and I’ve always thought that where they were great in one area they failed or were lacking in another until now 😁
Also on day 2 of it running constantly I came home from work having left it with fm playing I heard a difference in the sound, it was as if I had added some super tweeters, bloody lovely sound this beast gives .
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Thanks Zen,
I had read previous posts from members including yourself where it said that temperature is key , I can’t get the amp to said temperature ( 45c let alone 50c )
I have read the thread that you linked to ( cheers for that)
Can I establish what is my ultimate goal with the Stasis ? Higher voltage at the bias which gives a higher temperature ( but more noise ? ) or a lower bias which means much lower temperatures which I read that you don’t want either 🤷♂️
So bloody confusing
If I set the bias at the recommended level ( old boards and NOT the newer FE boards ) of 85mv then the heatsink temperature is in the low 30c’s .
I had read previous posts from members including yourself where it said that temperature is key , I can’t get the amp to said temperature ( 45c let alone 50c )
I have read the thread that you linked to ( cheers for that)
Can I establish what is my ultimate goal with the Stasis ? Higher voltage at the bias which gives a higher temperature ( but more noise ? ) or a lower bias which means much lower temperatures which I read that you don’t want either 🤷♂️
So bloody confusing
If I set the bias at the recommended level ( old boards and NOT the newer FE boards ) of 85mv then the heatsink temperature is in the low 30c’s .