I should add that along with the burnt boards and components ( resistors and 2 faulty transistors) three output transistors were found to be faulty, these were the original Motorola A6/A8 ones and the these were all on the one channel, on the other channel we could see that all the transistors are the newer Motorola mj15022/23 ( before the G versions )
I will be replacing all of the A6/A8’s .
I will be replacing all of the A6/A8’s .
Don't worry, we will get there with time and patience and meticulous care.
You and Gary have done a great job combing through this project! Very good work. Yes, from where you started, there was a pretty big learning curve to getting this project towards the finish line. I am impressed with the progress that you have made.
Are the outputs that you are getting going to be matched? I don't know how close the matching was on these old amps.
Here is the heatsink for the front end in case you were unable to track it down:
https://www.digikey.com/en/products.../240980?s=N4IgTCBcDaIBIGUCMBOAzAWgHIBEQF0BfIA
Cheaper at Newark if you can bundle it with other things:
https://www.newark.com/boyd/551002b...184?CMP=e-email-sys-orderconfirmed-NA-Product
I had a hard time finding it when I built my front end.
The good news is that this project is far less straight forward than building one of these DIY clones. So if you end up liking this amp, try your hand at building something else on this forum. 🙂
Are the outputs that you are getting going to be matched? I don't know how close the matching was on these old amps.
Here is the heatsink for the front end in case you were unable to track it down:
https://www.digikey.com/en/products.../240980?s=N4IgTCBcDaIBIGUCMBOAzAWgHIBEQF0BfIA
Cheaper at Newark if you can bundle it with other things:
https://www.newark.com/boyd/551002b...184?CMP=e-email-sys-orderconfirmed-NA-Product
I had a hard time finding it when I built my front end.
The good news is that this project is far less straight forward than building one of these DIY clones. So if you end up liking this amp, try your hand at building something else on this forum. 🙂
Hi Mike, good to hear from you.
I have those heatsinks ordered and due to arrive tomorrow along with all the bits for the new boards , including new thermistors ( I couldn’t find ANY on the old boards or in the amp itself ! ) . New thermostats also on their way.
I bought the On Semi mj15023g’s last weekend and then hit a wall when trying to buy the mj15022g’s so I placed a wanted ad on here and a member has a bunch of Motorola mj15022’s which hopefully I will buy from him ( waiting to hear from him now ) , I’ve learned the Motorola became On Semi so they should work out as a good match.
You say about a DIY build which I’ve wanted to do for many years but as I lacked the knowledge or know how I never did but now things have changed considerably ( confidence mainly) so guess what I’m going to need for this power amp ? 😁
Yes I will need a preamp ( surely much easier)
Any ideas ?
I intend to use my Counterpoint SA-2000 , opinions from members on that as a pre with the Stasis would be appreciated.
It has its own issues such as crosstalk , and the mute doesn’t really work .
Andy
I have those heatsinks ordered and due to arrive tomorrow along with all the bits for the new boards , including new thermistors ( I couldn’t find ANY on the old boards or in the amp itself ! ) . New thermostats also on their way.
I bought the On Semi mj15023g’s last weekend and then hit a wall when trying to buy the mj15022g’s so I placed a wanted ad on here and a member has a bunch of Motorola mj15022’s which hopefully I will buy from him ( waiting to hear from him now ) , I’ve learned the Motorola became On Semi so they should work out as a good match.
You say about a DIY build which I’ve wanted to do for many years but as I lacked the knowledge or know how I never did but now things have changed considerably ( confidence mainly) so guess what I’m going to need for this power amp ? 😁
Yes I will need a preamp ( surely much easier)
Any ideas ?
I intend to use my Counterpoint SA-2000 , opinions from members on that as a pre with the Stasis would be appreciated.
It has its own issues such as crosstalk , and the mute doesn’t really work .
Andy
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Nothing wrong with the Counterpoint SA-2000.
I say listen to the amp and see how you think it sounds. Once you have some listening impressions, then I would see what you think the sound needs. A bit more sugar, then try the Korg Nutube B1. If you want a clean sound, maybe try Zen Mod's Iron pre. Something in between would be the BA2018 or the BA-3
Honestly, for a first build, I think the Korg Nutube B1 kit would be the way to go. It has a sweet sound but adds this harmonic structure that I really believe adds to the imaging. It sounded really nice with my Threshold amplifier. With the kit, you have everything in front of you as well as a great build guide (courtesey of member 6L6) to go with it. There is also a terrific support thread for it with people who know it top to bottom. I have built I think 4 for myself and friends. My dad has one, I have a modified one, a few of my friends have benefited from me building them... There is not a single person I know who isn't elated with that little preamp. You can build the stock unit and then tweak it as you learn more about it.
I say listen to the amp and see how you think it sounds. Once you have some listening impressions, then I would see what you think the sound needs. A bit more sugar, then try the Korg Nutube B1. If you want a clean sound, maybe try Zen Mod's Iron pre. Something in between would be the BA2018 or the BA-3
Honestly, for a first build, I think the Korg Nutube B1 kit would be the way to go. It has a sweet sound but adds this harmonic structure that I really believe adds to the imaging. It sounded really nice with my Threshold amplifier. With the kit, you have everything in front of you as well as a great build guide (courtesey of member 6L6) to go with it. There is also a terrific support thread for it with people who know it top to bottom. I have built I think 4 for myself and friends. My dad has one, I have a modified one, a few of my friends have benefited from me building them... There is not a single person I know who isn't elated with that little preamp. You can build the stock unit and then tweak it as you learn more about it.
How much testing/matching should Andy perform with his new transistors? Should the emitter resistors (1 ohm?) be enough to handle part variations?
Given some of the output transistors are broken, should the emitter resistors be tested?
Re preamp: I don't have any experience with the BA3 or the Korg B1, but I can heartily recommend both the BA2018 line stage and ZM's IronPre.
Given some of the output transistors are broken, should the emitter resistors be tested?
Re preamp: I don't have any experience with the BA3 or the Korg B1, but I can heartily recommend both the BA2018 line stage and ZM's IronPre.
In my opinion, what you have bought was a piece of junk, I hope you did not pay much. No one in his right mind leave transistors floating in mid-air like that, anyway, and asks for trouble. You will have to carefully go through the board and check if any of those leads are close enough to touch or if debris had got stuck in it. When you bought that, what did the seller claim to be wrong? Have you been able to identify the burnt component on the PCB. This design is marginal and anything could have caused it, judging by the poor built quality.
All output transistors now fitted and the new boards are complete but not yet mounted.
I screwed up by not ordering enough 1 ohm resistors so I now have to wait for those .
New 22,000uf power supply caps arrived today ( Nichicon )
I had a go at setting the trim pots before going to work this morning and discovered that one of them wasn’t altering when I turned the little screw so that meant yet another email to my mentor ( Gary ) and as always he pointed me in the right direction ( I sweated all solder joints again) so they’re now working on both boards and so the boards are ready for installation.
I’m probably going to tidy up the rats nest of wires this weekend and I’ll use those silicone insulated ones that someone kindly recommended.
There’s still something that I’m not 100% sure about despite drawings, Gary’s help and even copying the original boards before I started and it’s the Resistors , I’m not confident that I’ve got them in their correct place on the heatsinks.
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As long as the 1R8 is on the neg side and the 1R5 is on the pos side, then all other resistors are correct. Looks good to me.
Very nice! Good progress! 🙂
So if you can, it may make it easier to adjust the trim pots from the top if you place them on the side facing the capacitors. Side adjust pots are also key. These will get you started so that you can confirm that everything is smoke free.
In the pic below, the trim pots are facing down because the whole channel is upside down. When installed in the amp, the trim pots will face up. It is also helpful to use a fine tipped sharpie to put a "O" for offset and a "B" for bias.
So if you can, it may make it easier to adjust the trim pots from the top if you place them on the side facing the capacitors. Side adjust pots are also key. These will get you started so that you can confirm that everything is smoke free.
In the pic below, the trim pots are facing down because the whole channel is upside down. When installed in the amp, the trim pots will face up. It is also helpful to use a fine tipped sharpie to put a "O" for offset and a "B" for bias.
If you do that make sure the bias pot is still at max resistance for start up - if you flip by mistake it it will be zero in the circuit - you don't want that. Also make sure the 2 heat sinks are clear of the OS. Also, Mikes amp is an S500 , not a Stasis 2.
To make your self 100% sure on the OS resistors, get the DMM on resistance range and measure each one as follows:
From the RF end you have marked - the pos side is on the left vertical row and neg on the right vertical row - there are 13 resistors on each side. Starting at the top end you have marked RF.
POS.
1R
1R
1R
1R
1R
10R
1R
1R5
1R
1R
1R
1R
1R
NEG.
1R
1R
1R
1R
1R
10R
1R
1R8
1R
1R
1R
1R
1R
To make your self 100% sure on the OS resistors, get the DMM on resistance range and measure each one as follows:
From the RF end you have marked - the pos side is on the left vertical row and neg on the right vertical row - there are 13 resistors on each side. Starting at the top end you have marked RF.
POS.
1R
1R
1R
1R
1R
10R
1R
1R5
1R
1R
1R
1R
1R
NEG.
1R
1R
1R
1R
1R
10R
1R
1R8
1R
1R
1R
1R
1R
yes, checking the clearance from the heatsinks and the OS is a good idea. I had to add washers to space mine out from the OS. I had forgotten about that...
The reason for the trim pots being out is so you have a clear path to them. Looking at the boards from the top of the chassis when the channel is installed, there will be components in the way.
I have sent you the top adjust trimpots Andy - to do what Mike has done you will need the side adjust trimpots - Bourns 3296X-1-501LF, so you can mount it so the adjust screw faces the top of the amp.