Hi everybody!
I've just drawn the schematics for the power management board of our beloved ML-9 amplifier. I was wondering why I had no delay at all when switching the amp, and was worried about the inrush current flowing in the big caps every time I put the amplifier on. I dismantled it once again and had a more serious look at the board to try to understand how it worked. Thanks to @Hans Polak who put the ML-2 schematics online, and having a thorough look at the n°29 circuit, I was able to reverse engineer (sort of...) the schematics from my own board.
I replaced both the relay (I had bought one long ago in the USA since they're not easily obtainable) and the TIP125 Darlington, both of which showed no evident sign of malfunction, but the relay click was then clearly audible. Then I replaced the original 6.8µF/50V cap to the Darlington base with a 10µF/63V as shown on the n°29 circuit. The delay after switching on increased slightly to around 2s which is probably enough to avoid the big inrush current I could observe before, letting the 3 x 10R resistors doing their job.
I hope I didn't make any mistake with the schematics (double checked everything) and couldn't draw all transformer's secondaries outputs because I didn't want to take it out this time, so it's a "partial" view of that board which stands behind the front plate. It is connected to the rear DCO protector board at the back and with the 2 x 36000µF/100V caps to the amplifier boards (on each side).
I wish it can be useful for ML amps owners of that period of time, since I think the basic principles remain nearly the same for a number of ML amplifiers.
Cheers to all.
I've just drawn the schematics for the power management board of our beloved ML-9 amplifier. I was wondering why I had no delay at all when switching the amp, and was worried about the inrush current flowing in the big caps every time I put the amplifier on. I dismantled it once again and had a more serious look at the board to try to understand how it worked. Thanks to @Hans Polak who put the ML-2 schematics online, and having a thorough look at the n°29 circuit, I was able to reverse engineer (sort of...) the schematics from my own board.
I replaced both the relay (I had bought one long ago in the USA since they're not easily obtainable) and the TIP125 Darlington, both of which showed no evident sign of malfunction, but the relay click was then clearly audible. Then I replaced the original 6.8µF/50V cap to the Darlington base with a 10µF/63V as shown on the n°29 circuit. The delay after switching on increased slightly to around 2s which is probably enough to avoid the big inrush current I could observe before, letting the 3 x 10R resistors doing their job.
I hope I didn't make any mistake with the schematics (double checked everything) and couldn't draw all transformer's secondaries outputs because I didn't want to take it out this time, so it's a "partial" view of that board which stands behind the front plate. It is connected to the rear DCO protector board at the back and with the 2 x 36000µF/100V caps to the amplifier boards (on each side).
I wish it can be useful for ML amps owners of that period of time, since I think the basic principles remain nearly the same for a number of ML amplifiers.
Cheers to all.
Hi @perelman ,
recently I had my ML-9 partially recapped on the power supply board and also replaced the resistor that you noticed as broken the first you saw my pictures. The relay now works again and I also can hear a clear click within a couple of seconds after pushing the mains switch.
Happy to notice yours works fine as well
Gaetano
recently I had my ML-9 partially recapped on the power supply board and also replaced the resistor that you noticed as broken the first you saw my pictures. The relay now works again and I also can hear a clear click within a couple of seconds after pushing the mains switch.
Happy to notice yours works fine as well
Gaetano
Hi @gaetanov , yes I remember very well the problem you had. Good to know that all is working well.
By the way, I've got a question for you, and to all the people who own or know that amplifier. On my own amp, I can hear music as soon as I switch it on. No wait till the click of the relay (1,5-2s). If music is playing through the preamplifier when I switch the amp on, it is delivered to the speakers immediately. It's quite different from the delayed start procedures I've seen on many (if not all) amplifiers.
That's the main reason why I reopened the amp recently, to understand how it works. But this is beyond my skills... maybe someone could elaborate on this?
I put the picture again (jpg format) so it's not necessary to download it...
By the way, I've got a question for you, and to all the people who own or know that amplifier. On my own amp, I can hear music as soon as I switch it on. No wait till the click of the relay (1,5-2s). If music is playing through the preamplifier when I switch the amp on, it is delivered to the speakers immediately. It's quite different from the delayed start procedures I've seen on many (if not all) amplifiers.
That's the main reason why I reopened the amp recently, to understand how it works. But this is beyond my skills... maybe someone could elaborate on this?
I put the picture again (jpg format) so it's not necessary to download it...
Thanks @willcycle for this simple but clever response. I hadn't thought of the second part of your answer, "no output relays"..., which becomes obvious after you wrote it 
Then, would the inrush current to the capacitors benefit from (little) higher values resistors? Is there a specific difference there between 120 & 230V mains (USA and Europe), that would make that increase in resistor values beneficial or necessary? As I=VA/A and so on with Ohm's law, we could think that near 40ohms of total resistance would be more adapted to our 230V mains here in France?
And I guess you're right about the (some) design similarities between ML amplifiers. Particularly about this power board, which is nearly identical to the one in the 29 if I'm right.
Then, would the inrush current to the capacitors benefit from (little) higher values resistors? Is there a specific difference there between 120 & 230V mains (USA and Europe), that would make that increase in resistor values beneficial or necessary? As I=VA/A and so on with Ohm's law, we could think that near 40ohms of total resistance would be more adapted to our 230V mains here in France?
And I guess you're right about the (some) design similarities between ML amplifiers. Particularly about this power board, which is nearly identical to the one in the 29 if I'm right.
I have the impression that things are not right in the area around K1, Q1 etc.
I fail to see how the transformer will be energized because there is always a blocking C5 in the mains signal path and I would expect K1 to short circuit R1-R3 when switched on.
Hans
Hi @Hans Polak and thanks for your valuable impression. I know you're savvy about these amps!
I drawn the schema from the real board, as precisely and close to real I could. But of course 1) something may have been not seen or falsely reported and 2) As I didn't have an easy access to the wires going to and getting out of the transformer, this part is let a little bit unknown.
I somewhat think I could confirm a few things on the schema I drawn with the help of the ML29 schematics as below:
It seems to me that they are pretty close, or am I wrong? (not impossible)
What I can say is that the relay clicks after 1,5-2s, that music is listenable as soon as the switch is set on, and that nothing is hot (even warm) on this board. And the amp behaves very well.
What I wanted to test is the inrush current flowing through the 3 resistors, but I didn't do it (yet).
Is the n29 schematics more suitable to you?
Thanks again !
I drawn the schema from the real board, as precisely and close to real I could. But of course 1) something may have been not seen or falsely reported and 2) As I didn't have an easy access to the wires going to and getting out of the transformer, this part is let a little bit unknown.
I somewhat think I could confirm a few things on the schema I drawn with the help of the ML29 schematics as below:
It seems to me that they are pretty close, or am I wrong? (not impossible
)What I can say is that the relay clicks after 1,5-2s, that music is listenable as soon as the switch is set on, and that nothing is hot (even warm) on this board. And the amp behaves very well.
What I wanted to test is the inrush current flowing through the 3 resistors, but I didn't do it (yet).
Is the n29 schematics more suitable to you?
Thanks again !
You're right, @willcycle , except that it is not Madrigal but Mark Levinson the original (T. Colangelo design, RIP) in 1985.
And, but this you can't know, I bought this amp in the US (originally 120VAC mains). It was adapted to 230VAC mains by a French techie, former employee of the original ML importer in France, back in the 70's. I'm fully confident in his work (it was done in 2009 just after I had it delivered). And I've seen pictures and heard of original-made-for-Europe-ML9s with these very same resistors. That's the reason of my question...
But as it is working, I shouldn't worry about this. I just wanted to understand how it works, and as you explained, the lack of speakers relay justifies the immediacy of music playing when switching on!
And, but this you can't know, I bought this amp in the US (originally 120VAC mains). It was adapted to 230VAC mains by a French techie, former employee of the original ML importer in France, back in the 70's. I'm fully confident in his work (it was done in 2009 just after I had it delivered). And I've seen pictures and heard of original-made-for-Europe-ML9s with these very same resistors. That's the reason of my question...
But as it is working, I shouldn't worry about this. I just wanted to understand how it works, and as you explained, the lack of speakers relay justifies the immediacy of music playing when switching on!
Hi @perelman and all,
I can't check right now but I think it should work exactly as you describe, based on my direct experience. In mean, before fixing the relay trigger, I was anyway able to listen to the music because the power path was established through the 10Ohm resistors, which is what happens before the relay is armed.
There is one more thing to consider, in this topic. For what I have found during some research in the past months, there are at least three "versions" of this board and I think that yours @perelman is the first in time. The second one should be the one in my amp which is missing the diodes D1 and D2 as well as the two capacitors besides them. A third one has the resistor arming the relay (R4?) placed on the higher side of the PCB, in the same place where you see the D1,D2, C5 and C6 on your board. This was probably due to thermal impact of the resistor R4 on the relay case
Thanks a lot @Hans Polak for your redrawing work and time for making this more understandable.
Thanks for correcting my mistake about C8/C10 value making the delay time for the relay. I had changed the numbers of some components to fit better the logic of the schema, but had forgotten this change and didn't modify accordingly. This is corrected on my schema.
Then, on your drawing, you make a different arrangement concerning the connections between the two 10000µF caps (not really sure of this because you don't write the polarity of them). But I confirm that the way I drawn it is correct. As one can see on the image below, the 2 pos (+) are each connected to a negative (-). I was surprised to see it like that, and a friend of mine too, so I double checked. I don't understand the meaning of that arrangement, but I couldn't draw it another way.
Then, here's the full board, component side :
I don't see what you mean by "missing resistor". As far as I can see, no such resistor is visible between D3 and R4 (on my drawing).
Here's the full board copper side so you'll see better:
And what helped me to track the links between components (it's an old version maybe some mistakes). A kind of mix between the two pictures above.
Maybe this can help to understand better why I had the drawing made like I did...
I don't know if this changes anything in your understanding of my drawing, and if it shows evident mistakes I could have made...
And to @gaetanov , my amp is from 1985. As this model has been produced from 1981 to the Madrigal period, I don't know which version it is...
Thanks to all.
Thanks for correcting my mistake about C8/C10 value making the delay time for the relay. I had changed the numbers of some components to fit better the logic of the schema, but had forgotten this change and didn't modify accordingly. This is corrected on my schema.
Then, on your drawing, you make a different arrangement concerning the connections between the two 10000µF caps (not really sure of this because you don't write the polarity of them). But I confirm that the way I drawn it is correct. As one can see on the image below, the 2 pos (+) are each connected to a negative (-). I was surprised to see it like that, and a friend of mine too, so I double checked. I don't understand the meaning of that arrangement, but I couldn't draw it another way.
Then, here's the full board, component side :
I don't see what you mean by "missing resistor". As far as I can see, no such resistor is visible between D3 and R4 (on my drawing).
Here's the full board copper side so you'll see better:
And what helped me to track the links between components (it's an old version maybe some mistakes). A kind of mix between the two pictures above.
Maybe this can help to understand better why I had the drawing made like I did...
I don't know if this changes anything in your understanding of my drawing, and if it shows evident mistakes I could have made...
And to @gaetanov , my amp is from 1985. As this model has been produced from 1981 to the Madrigal period, I don't know which version it is...
Thanks to all.
I added the “missing resistor” because C8 has to be charged by some current in someway
However when C8 is small, loading can also be done through Q1’s base current.
I’ll have a look at your PCB tomorrow.
Anyhow, when the electrolytics are not connected with their minus sides together, it is definitely wrong.
Someone must have replaced them in the past in the wrong way.
Hans
However when C8 is small, loading can also be done through Q1’s base current.
I’ll have a look at your PCB tomorrow.
Anyhow, when the electrolytics are not connected with their minus sides together, it is definitely wrong.
Someone must have replaced them in the past in the wrong way.
Hans
It's curious because there are small marks on the PCB for positive leads (whether caps or diodes) and the marking is correct and corresponds with the reality. I'll investigate more about this. There were Frakos before I replaced them with Vishay caps (same value, voltage... and position).
Here's how it was when I bought it:
Thanks once again for your attention, you are amazing.
And these capacitors are exactly the ones missing from my power board, along with the diodes. There are no traces of the electrical lines witnessing that they have never been there. The serial of my item is 2260 and I guess it was already in the Madrigal epoch or right before. Do we have any serial numbers table providing a time reference? May I ask you which is the serial of your amp?
Thanks to all of you guys for the great information available in this thread.
Thanks to all of you guys for the great information available in this thread.
Hi Gaetano, mine is SN#2130, so not too far from yours probably.
Normally, there's a (very) small tinned dot on the component side of the PCB for positive lead of caps and cathode side of diodes. That's why I can confirm that the caps C5 & C6 on my drawing are soldered as intended by the marks on the PCB. And I can add that I'm pretty sure that the Frakos were there when the amp left the factory since the solder traces on the PCB when I replaced the caps some time ago were like new or absolutely untouched.
This makes me wonder why such a disposition. Maybe Hans will have an answer...
Jerome
Normally, there's a (very) small tinned dot on the component side of the PCB for positive lead of caps and cathode side of diodes. That's why I can confirm that the caps C5 & C6 on my drawing are soldered as intended by the marks on the PCB. And I can add that I'm pretty sure that the Frakos were there when the amp left the factory since the solder traces on the PCB when I replaced the caps some time ago were like new or absolutely untouched.
This makes me wonder why such a disposition. Maybe Hans will have an answer...
Jerome