OK, we have our first problem.
The MPSA42 when looking at the flat on the body has pins left to right of E B C
The BC550C is opposite, being C B E.
I am assuming the BC550's have been fitted where there should be MPSA42's. What does the other working channel board look like where these are?
Are these 2 transistors clearly marked as BC550C's?
As I said before, the big issue here is the fact that the pcb is not silk screened with the components. I think the next step for you if you want to be in a position to repair these 2 pcb's is to print out the schematic and label the components from left to right, ie:
Resistors R1 to Rx
Capacitors C1 to Cx
Transistors Q1 to Qx
etc....
Then you need to identify the resistor values by being able to read the colour bands, google will help with that.
Then you need to carefully draw out the pcb an a large sheet of paper, and identify the components and label them as you have marked on the schematic.
By doing this you can then identify each transistor, its type and polarity (NPN or PNP), once each transistor position is identified, you can check what each pin is, by looking at the circuit to see what components it is attached too. Familiarize yourself with the transistor symbol to identify if NPN or PNP.
An NPN transistor symbol has the arrow on the emitter pointing away from the base, and a PNP transistor has the arrow on the emitter pointing towards the base. (the arrow identifiers direction of current flow when the transistor is turned on).
You really need to be able to identify each component on the pcb and be able to match it by value and component number to a marked up schematic to be able to trouble shoot and check each transistor and which way around it should be inserted.
I dont know why whoever repaired the amp before has used BC550 transistors which have the C-E pins reversed. These should be inserted 180 degrees form the original MPSA42's for the amp to work. I would get rid of those and get hold of new MPSA types which are still available from Mouser , Farnell, RS etc...
I hope that all makes sense - will be a steep learning curve.
If it were me and looking at the state of your existing original FE boards, I would pull them out and fit the newer NP designed FE boards. Mike from the US can then probably help with marrying the new board into the Stasis chassis - which he has done.
As I said before, the big issue here is the fact that the pcb is not silk screened with the components. I think the next step for you if you want to be in a position to repair these 2 pcb's is to print out the schematic and label the components from left to right, ie:
Resistors R1 to Rx
Capacitors C1 to Cx
Transistors Q1 to Qx
etc....
Then you need to identify the resistor values by being able to read the colour bands, google will help with that.
Then you need to carefully draw out the pcb an a large sheet of paper, and identify the components and label them as you have marked on the schematic.
By doing this you can then identify each transistor, its type and polarity (NPN or PNP), once each transistor position is identified, you can check what each pin is, by looking at the circuit to see what components it is attached too. Familiarize yourself with the transistor symbol to identify if NPN or PNP.
An NPN transistor symbol has the arrow on the emitter pointing away from the base, and a PNP transistor has the arrow on the emitter pointing towards the base. (the arrow identifiers direction of current flow when the transistor is turned on).
You really need to be able to identify each component on the pcb and be able to match it by value and component number to a marked up schematic to be able to trouble shoot and check each transistor and which way around it should be inserted.
I dont know why whoever repaired the amp before has used BC550 transistors which have the C-E pins reversed. These should be inserted 180 degrees form the original MPSA42's for the amp to work. I would get rid of those and get hold of new MPSA types which are still available from Mouser , Farnell, RS etc...
I hope that all makes sense - will be a steep learning curve.
If it were me and looking at the state of your existing original FE boards, I would pull them out and fit the newer NP designed FE boards. Mike from the US can then probably help with marrying the new board into the Stasis chassis - which he has done.
Take your time - no rush and double check and document everything,
I will get on it tomorrow mate , I’ve ordered all the transistors now ( I think) I just need to draw up the diagram from the schematic which I’m sure I’ll figure it out once I get my head around it.
You are a legend mate
You are a legend mate
To draw up the actual pcb with components and tracks, I find it is easiest to get some squared or graph paper. I then usually draw the pcb on the paper twice as big as the actual board. Then I measure in from the edge of the board where the component leads are and transfer that to the paper (remembering to double the measurement). I draw a small circle in blue pen for all the component lead positions, then looking at the back of the board I join up the component leads with the tracks as they appear on the back of the pcb (remember when you flip the pcb the tracks are in reverse sort of.) I draw all the tracks on the rear of the board in black pen and then I do the same for the tracks on the front of the board in red pen. You can also then mark the input and other connections to the output stage where they attach to the FE board, along with the N/C thermostat and thermistor that should be in contact with the heat sink somewhere.
Once you have done all the board and double check everything you have done ( good job for a rainy day) then you get the schematic and follow that through and number the components on the board you have drawn to line up with the numbering you have done on the schematic.
As you get to each transistor then write the EBC (CBE) pins on the board layout with their part number, that way you will know for sure which way around they are to be inserted. Google the data sheet for each transistor type to check the correct pin out details.
You will also know what the value is of the blown resistor(s) by tracing out the circuit to do a pcb layout.
Hope all of that is clear for you.
Once you have done all the board and double check everything you have done ( good job for a rainy day) then you get the schematic and follow that through and number the components on the board you have drawn to line up with the numbering you have done on the schematic.
As you get to each transistor then write the EBC (CBE) pins on the board layout with their part number, that way you will know for sure which way around they are to be inserted. Google the data sheet for each transistor type to check the correct pin out details.
You will also know what the value is of the blown resistor(s) by tracing out the circuit to do a pcb layout.
Hope all of that is clear for you.
As Mike has mentioned in post #9, use his link to identify resistor values.
Another trick is to shine a light from the back of the PCB board. The shadow of the traces from the back should show through. You know the PCB is Pass's work when you see primarily 90-degree traces.
Great advice Gary! 🙂
Great advice Gary! 🙂
That is a neat trick Mike, well done with that advice. Might be a bit tricky where the pcb is burnt.
This is just brilliant guys , at this moment it feels a little intimidating but like I said in the original post “ I am determined to get this beast working” just hope it’s worth it !
Your pcb layout once drawn should look like this as an example - which is the new NP FE pcb. Where Nelson has used green for the tracks on the rear of the pcb and red for the front tracks.
As you chip away at it, you will become more and more familiar. It will take a bit of time though.
Wow! Just had to say, you guys ROCK! Reading through these posts has taught me so much, unfortunately, as I have said previously, I've learned how little I know 😉
Hi Mike,
I am just getting some parts, the NP FE boards and documentation together to send over to the member in the UK, just in case the original boards are beyond repair.
As you have shown, you have implemented the new NP FE pcb into your stasis amp - and I have not, can you confirm or correct the following as to how to connect the new FE to the Stasis 2 output stage, thanks.
Looking from left to right on the NP FE connections at the bottom of the pcb:
V- = connects to the OS negative voltage rail ( does this wire go to the OS board or PSU caps?
-C = connects to the base of the first transistor marked A6 (PNP) left hand end on the Stasis schematic on the bottom row of 6 output transistors.
-D = connects to the base of the first transistor marked A6 (PNP) left hand end on the Stasis schematic on the second bottom row of 6 output transistors.
OUT = connects to the pos speaker output connection on the output stage pcb.
GND = connects to power supply ground - is this on the OS pcb or direct to PSU capacitors?
+D = connects to the base of the first transistor marked A8 (NPN) left hand end on the Stasis schematic on the second row from top of 6 output transistors.
+C = connects to the base of the first transistor marked A8 (NPN) left hand end on the Stasis schematic on the top row of 6 output transistors.
V+ = connects to the OS positive voltage rail (does this wire go to the OS board or PSU caps?
Also Nelson has shown a zobel RC on the pcb (R22/10R C6/0.1uF), but I notice that there is one located on the OS board (5R1 / 0.1uF). Did you leave out R22/C6 on the FE pcb?
Did you wire in a new 2K NTC thermistor attached to each heat sink, as the original Stasis shows 1K in that position?
Also did you use the existing N/C thermostat (NC thermal breaker on the original Stasis schematic) where it is labelled T on the new FE board?
Anything else to add to help the member to implement the new FE board if he chooses?
Biasing info would help a lot. what quiescent current (mV across the 1R emitter resistors did you settle on). NP says to adjust till the heastsinks settle at 50 degC ( but at what ambient temp is this for)?
Hope you can give the member some answers here, many thanks again.
Cheers,
Gary.
I am just getting some parts, the NP FE boards and documentation together to send over to the member in the UK, just in case the original boards are beyond repair.
As you have shown, you have implemented the new NP FE pcb into your stasis amp - and I have not, can you confirm or correct the following as to how to connect the new FE to the Stasis 2 output stage, thanks.
Looking from left to right on the NP FE connections at the bottom of the pcb:
V- = connects to the OS negative voltage rail ( does this wire go to the OS board or PSU caps?
-C = connects to the base of the first transistor marked A6 (PNP) left hand end on the Stasis schematic on the bottom row of 6 output transistors.
-D = connects to the base of the first transistor marked A6 (PNP) left hand end on the Stasis schematic on the second bottom row of 6 output transistors.
OUT = connects to the pos speaker output connection on the output stage pcb.
GND = connects to power supply ground - is this on the OS pcb or direct to PSU capacitors?
+D = connects to the base of the first transistor marked A8 (NPN) left hand end on the Stasis schematic on the second row from top of 6 output transistors.
+C = connects to the base of the first transistor marked A8 (NPN) left hand end on the Stasis schematic on the top row of 6 output transistors.
V+ = connects to the OS positive voltage rail (does this wire go to the OS board or PSU caps?
Also Nelson has shown a zobel RC on the pcb (R22/10R C6/0.1uF), but I notice that there is one located on the OS board (5R1 / 0.1uF). Did you leave out R22/C6 on the FE pcb?
Did you wire in a new 2K NTC thermistor attached to each heat sink, as the original Stasis shows 1K in that position?
Also did you use the existing N/C thermostat (NC thermal breaker on the original Stasis schematic) where it is labelled T on the new FE board?
Anything else to add to help the member to implement the new FE board if he chooses?
Biasing info would help a lot. what quiescent current (mV across the 1R emitter resistors did you settle on). NP says to adjust till the heastsinks settle at 50 degC ( but at what ambient temp is this for)?
Hope you can give the member some answers here, many thanks again.
Cheers,
Gary.
No problems Mike, when you can will be fine. Not sure if repairs to the existing damaged pcb's will work out, so covering all bases with the new FE option as well for the member.